The Atmosphere and Weather

Air seems empty because we cannot see it. Yet it fills a balloon, has mass, and pushes against surfaces. Gravity holds this mixture of gases around Earth as the atmosphere.

Compare the gases

Scientists compare clean, dry air with water vapor, dust, and pollution removed. Its main gases have these shares by volume:

Tiny amounts of other gases complete the mixture. Rounded to whole percentages, the mixture is 78% nitrogen, 21% oxygen, and 1% other gases. Air is mostly nitrogen, even though oxygen is the gas we use in breathing. Nitrogen does not support breathing or burning in the way oxygen does.

For a 100-liter sample, each percentage gives the number of liters. There are about 78 L of nitrogen and 21 L of oxygen. Carbon dioxide fills about 0.04 L, which equals 40 mL. A small share can still have a large effect. Plants use carbon dioxide in photosynthesis, and it absorbs heat released by Earth's surface. Its amount has risen from about 280 parts per million before industrialization to over 420 parts per million.

Water and particles change the weather

Water vapor is invisible water gas. Its share ranges from almost zero in very cold, dry air to about 4% in warm tropical air. This changing share explains why water vapor is left out of the dry-air percentages. Evaporation adds it to the air; later it can become droplets or ice.

A cloud is visible because it contains tiny liquid droplets or ice crystals. The white cloud above a kettle also contains liquid droplets. The water vapor itself remains invisible.

Aerosols are tiny particles suspended in air. Dust, sea salt, smoke, and pollen are examples. Many particles act as condensation nuclei, surfaces on which cloud droplets form. Weather depends on these tiny particles even though they occupy little space.

Ozone has three oxygen atoms per molecule. Most ozone lies in the stratosphere, where it absorbs harmful ultraviolet radiation. Near the ground, ozone is a pollutant in smog. Its location changes its effect.

Explain thin mountain air

The weight of the air above squeezes the lower atmosphere. About half the atmosphere's mass lies below 5.5 km, and about three-quarters lies in the troposphere. Pressure and density decrease upward.

On a mountain, oxygen still makes up about 21% of the air. The gas shares stay nearly the same up to about 80 km. A breath at high altitude contains fewer molecules of every gas, including oxygen. A lower number of oxygen molecules per breath explains why the air feels thin.

A trip upward passes through air that cools, warms, cools again, and warms again. Scientists use these temperature trends to divide the atmosphere into layers. Pressure and density decrease through all of them.

Follow the layers from the ground

The troposphere is about 8 km deep near the poles and 16–18 km deep near the equator. It contains about three-quarters of the atmosphere's mass and almost all its water vapor. The Sun warms the ground, and the ground warms the air above it. This helps explain why the layer generally cools upward.

The stratosphere warms upward because ozone absorbs ultraviolet radiation. Most ozone lies about 15–35 km high. The ozone layer is still mostly ordinary air, with only a few ozone molecules per million air molecules. Large thunderstorm tops spread into an anvil near the tropopause.

The mesosphere cools toward about −90 °C (−130 °F) at its top. Above it, the thermosphere absorbs high-energy radiation. Its temperature can exceed 1,000 °C. Its molecules are so far apart that they transfer very little heat to an object. The International Space Station orbits within this thin layer, about 400 km up.

The exosphere gradually merges with space. Hydrogen and helium are common there, and molecules rarely collide. The agreed start of space at 100 km is called the Kármán line. It is not the end of the atmosphere.

Estimate a mountain temperature

The average tropospheric temperature decrease is about 6.5 °C per kilometer. Suppose the surface temperature is 25 °C and the height is 3 km.

1. Find the cooling over 3 km: $3\times 6.5=19.5$ °C.
2. Subtract that cooling: $25-19.5=5.5$ °C.

The estimate is 5.5 °C. The actual rate changes with weather. In US units, the average decrease is about 3.6 °F per 1,000 ft. A climb of 14,000 ft from an 80 °F starting point gives about 30 °F.

Read a temperature–height graph

Temperature goes on the horizontal axis; height goes on the vertical axis. Follow the line upward. A turn from cooling to warming marks the tropopause. The next changes mark the stratopause and mesopause. At 30 km, warming with height identifies the stratosphere. A height clue helps distinguish it from the thermosphere, which also warms upward.

A sunny sidewalk can feel much hotter than nearby grass. A lake warms more slowly than either surface. These differences begin a chain of changes in the air above them.

Trace three ways heat moves

Energy reaches Earth as radiation, mostly visible light, with ultraviolet and infrared radiation too. Much sunlight passes through the atmosphere. About half is absorbed by land and oceans. The warm ground then transfers heat to air touching it by conduction.

Warmed air expands. At the same pressure it becomes less dense than surrounding cooler air, so it rises. Cooler, denser air moves in below. This movement transfers heat by convection and forms a convection current. A rising bubble of warm air is a thermal. Birds and gliders can ride thermals above warm ground.

These processes have different jobs: radiation brings energy from the Sun, conduction warms air touching the surface, and convection carries warmed air upward. The troposphere receives much of its heating from below.

Compare sunlight at different latitudes

Near the equator, midday sunlight often strikes steeply. Its energy is concentrated over a small area. Near the poles, sunlight strikes at a lower angle, spreading the same energy over more ground. The rays also pass through more atmosphere.

Try the idea with a flashlight. Point it straight down at a table to make a small bright patch. Tilt it without changing the light output. The patch spreads into a larger oval and becomes dimmer. Each part of the table now receives less energy. Low-angle sunlight has the same spreading effect.

Compare land, water, and surface color

Water needs about four to five times as much energy as dry sand or soil to warm the same mass by 1 °C. It also mixes, lets light penetrate below the surface, and loses energy through evaporation. Land heats and cools quickly; large bodies of water change slowly.

A beach can therefore stay cooler than an inland town on a summer afternoon. In winter, nearby water can keep coastal nights milder. Compare places at a similar latitude to separate this effect from the sunlight-angle effect.

Light surfaces reflect much incoming sunlight. Fresh snow reflects about 80–90%. Dark asphalt and forests absorb more energy. A snow-covered field can stay cold even in sunshine because little of the arriving energy is absorbed.

Explain nighttime cooling

After sunset, the ground gives off infrared radiation. Clouds absorb some and send part back downward. Clear, calm nights cool fastest and often favor frost or fog.

High mountain air is usually colder despite being slightly closer to the Sun. A climb of a few kilometers is tiny beside the Sun's distance of about 150 million km. Heating from the ground explains the temperature trend. Uneven surface heating also helps create density and pressure differences, setting air in motion.

Air pushes against your skin, a window, and the inside of a balloon. Air pressure is the force on each unit of surface area. The weight of the air column above and molecules hitting surfaces produce this pressure.

Measure pressure

A barometer measures air pressure. A mercury barometer balances pressure against a column of mercury. An aneroid barometer uses a sealed metal capsule that changes shape. A barograph records pressure through time.

Average sea-level pressure is about 1013.25 mb, 29.92 inHg, or 101.3 kPa. For a rough conversion from mb to inHg, divide by 33.86. Thus 1,000 mb is about 29.53 inHg, while 1,030 mb is about 30.42 inHg. Typical sea-level weather values lie near 980–1040 mb. Very strong storms can fall much lower; Typhoon Tip reached 870 mb in 1979.

Compare a mountain with sea level

Pressure falls with height because less air remains above you. It roughly halves over the first 5.5 km. Near that height, pressure is about 500 mb. Mount Everest's summit pressure is about one-third of sea-level pressure.

A sealed chip bag carried uphill can puff outward. The air trapped inside now pushes against lower outside pressure. Your ears can also respond to pressure changes during a climb or flight.

Weather stations correct their readings to sea level before putting them on one surface map. Otherwise, a mountain station would always look like a deep low just because of its altitude.

Connect pressure with moving air

Warm air expands and can rise; cooler, denser air can sink. Rising air and surface lows commonly occur together, while sinking air is common under highs. Surface pressure depends on how much air is in the column above. Temperature alone is not enough to identify a high or low.

In a low, rising air expands and cools. Clouds and precipitation often develop. Under a high, sinking air compresses and warms, helping cloud droplets evaporate. A falling barometer often accompanies approaching unsettled weather; rising pressure often follows clearing weather.

Humid air is less dense than dry air at the same temperature and pressure. Water molecules are lighter than the nitrogen and oxygen molecules they replace. A muggy feeling does not mean the air has become denser.

Compare nearby pressure values

High and low are relative labels. A center of 1008 mb is a low if surrounding air is at 1020 mb. Wind strength depends on pressure change over distance. A high reading alone cannot tell you that the wind is strong.

Wind is air moving horizontally. A pressure difference pushes air toward lower pressure. Its path also depends on Earth's rotation and friction near the ground.

Read direction and speed

Winds are named for where they come from. A north wind moves south. A westerly wind comes from the west. Directions can also be degrees clockwise from north: east is 90°, south is 180°, and west is 270°.

A wind vane points into the wind. A cup anemometer measures speed from how quickly its cups turn. A windsock gives a visible guide at an airport. Speed may be in mph, km/h, m/s, or knots. One knot is about 1.15 mph or 1.85 km/h. A 20-knot wind is about 23 mph or 37 km/h. The Beaufort scale estimates wind strength from visible effects, from force 0, calm, to force 12, hurricane force.

The pressure gradient is the pressure change over a distance. A larger change across the same distance generally makes stronger wind. Closely spaced isobars, lines of equal pressure, show this steep change on a map.

Build a sea breeze

On a sunny day, land warms faster than the sea.

1. Air over land warms, expands, and rises.
2. Surface pressure over land becomes lower than over the cooler sea.
3. Air flows from sea to land near the surface.
4. Higher up, air returns toward the sea and sinks, completing the loop.

This sea breeze often grows strongest in mid-afternoon. At night, land cools faster. A weaker land breeze can then blow offshore. Mountain slopes have a related cycle: valley breezes move uphill by day, while cold mountain air drains downhill at night.

Allow for Earth's rotation

Over large distances, Earth's rotation deflects moving air. This Coriolis effect acts to the right in the Northern Hemisphere and left in the Southern Hemisphere. It is zero at the equator and stronger toward the poles. It matters for weather systems crossing countries; it does not determine a sink's swirl.

Near the surface, friction slows wind, allowing it to cross isobars toward lower pressure. Around a Northern Hemisphere low, wind spirals counterclockwise and inward; around a high, clockwise and outward. Southern Hemisphere rotation reverses: clockwise into lows and counterclockwise out of highs.

Buys Ballot's law gives a quick check. Stand with your back to the wind: lower pressure lies to your left in the Northern Hemisphere and right in the Southern Hemisphere.

The prevailing wind is the direction wind most often comes from. Westerlies prevail across much of the United States, southern Canada, Europe, southern Australia, and New Zealand. Their weather systems generally move from west to east.

Water continually moves between the surface and the air. Evaporation adds water vapor from wet surfaces. Plants release it through transpiration. Humidity describes this water vapor in the air.

Compare water vapor with saturation

At higher temperatures, more water can remain as vapor before saturation is reached. Approximate saturation amounts are 5 g/m³ at 0 °C, 9.4 at 10 °C, 17.3 at 20 °C, and 30.3 at 30 °C. Saturated air has reached this temperature-dependent level.

Relative humidity, or RH, compares the amount present with the saturation amount at the same temperature:

Suppose air at 30 °C contains 15.2 g/m³. Divide 15.2 by 30.3, then multiply by 100. The RH is about 50%. If the air cools to 20 °C with the same vapor amount, divide 15.2 by 17.3 instead. RH rises to about 88%.

The air gained no water in this comparison. Cooling reduced the amount needed for saturation. This helps explain high RH around dawn and lower RH on warm afternoons. Equal RH values at different temperatures need not mean equal vapor amounts: 50% RH at 10 °C is only about 4.7 g/m³.

Use the dew point

The dew point is the temperature at which cooling air becomes saturated without changing its water vapor content. The closer air temperature gets to dew point, the higher the RH. When they match, RH is 100%.

For the 15.2 g/m³ example, the dew point is about 18 °C (64 °F). Summer dew points above about 65 °F feel muggy because they indicate much moisture. Dew point is useful when comparing moisture in warm and cool air.

Explain dew, frost, and fog

A cold glass cools the air next to it below its dew point. Vapor condenses into liquid dew on the outside. The drops came from the surrounding air.

Frost forms when vapor deposits directly as ice on a sufficiently cold surface below freezing. A dew point below freezing alone does not make frost appear; the surface must cool enough too. Fog consists of suspended droplets at ground level. It can form as air cools on a clear, calm night or as moist air crosses a cold surface.

Evaporation takes in heat. Condensation returns that energy as latent heat, which helps power rising storm air. Deposition changes vapor directly into ice.

Measure humidity

A hygrometer measures humidity. A psychrometer compares dry- and wet-bulb thermometers. Evaporation cools the wet bulb. Drier air allows faster evaporation and a larger temperature gap. Equal readings indicate saturation. A table converts the readings to RH.

Rain can fall through unsaturated air below a cloud. Ground-level RH therefore need not reach 100% for rain to reach the ground.

A growing cloud marks a change in water's state. Invisible vapor becomes visible liquid droplets or ice crystals. Rising air often starts this change.

Follow rising air

Pressure decreases with height. A rising parcel of air expands in the lower pressure and cools, without needing to lose heat to its surroundings. Unsaturated air cools about 10 °C per kilometer. Its dew point drops more slowly, about 2 °C per kilometer.

When the air reaches its dew point, vapor condenses onto tiny particles called condensation nuclei. A typical cloud droplet is only about 0.02 mm wide. Condensation releases latent heat, so saturated rising air cools more slowly, about 6 °C per kilometer. In cold clouds, ice forms by droplets freezing or by vapor depositing directly as ice.

Estimate the cloud base

The temperature–dew point gap closes by about 8 °C per kilometer. Each 1 °C of surface gap therefore represents about 125 m of rise before a cloud forms.

For a surface temperature of 25 °C and dew point of 17 °C:

1. Find the gap: $25-17=8$ °C.
2. Estimate the height: $8\times 125=1000$ m above the ground.

In US units, divide the °F gap by 4.4, then multiply by 1,000 ft. A 14 °F gap gives about 3,200 ft. These rules estimate the height; real conditions vary. The flat bases of nearby cumulus clouds often mark a shared condensation level.

Identify the lifting process

Air rises through convection over warm ground. Relief lifting forces wind up a mountain. Frontal lifting raises warm air over denser cold air. Convergence brings surface air together and forces it upward, such as where sea breezes meet.

Read the cloud names

The prefix alto- means middle level. Nimbus or nimbo- indicates precipitation. Low clouds have bases below about 6,500 ft (2 km); middle clouds about 6,500–20,000 ft; high clouds above about 20,000 ft (6 km). Cumulonimbus can extend through several levels.

Cirrus thickening into cirrostratus, then lower altostratus, can signal a warm front. Growing cumulus towers can signal afternoon showers. Cloud cover may be measured in oktas, eighths of the sky: 0 is clear and 8 is overcast.

Droplets are denser than air. Their tiny size makes them fall slowly, and rising air can keep them suspended. They are not floating because liquid water is lighter than air.

A cloud droplet is far too small to fall quickly through rising air. Precipitation begins when droplets or ice crystals grow large enough to fall and reach the ground.

Grow a raindrop

In warm clouds, droplets of different sizes fall at different speeds. They collide and join, a process called coalescence. In colder clouds, ice crystals grow as vapor transfers to them while nearby supercooled droplets remain liquid below freezing. Falling crystals can reach the ground as snow or melt into rain.

A 2 mm raindrop is about 100 times wider than a 0.02 mm cloud droplet. Volume increases in three dimensions: $100\times 100\times 100=1000000$. About a million such droplets provide the water in one raindrop.

Follow the temperature layers

Here, sleet has its US meaning, ice pellets. In Britain and Ireland, the word means a mixture of rain and snow. Hail grows inside strong thunderstorm updrafts; its growth is different from sleet's refreezing below a cloud.

A rain gauge measures water depth in millimeters or inches. Snow can be measured by its depth and its melted water. A 10-to-1 snow-to-water ratio is an average, with wet snow nearer 5-to-1 and fluffy snow sometimes 20-to-1 or more.

Explain why air rises to make rain

Convectional rainfall starts with sunshine heating the ground. Moist air rises in thermals, cools, and forms clouds that can grow into thunderstorms. Short, heavy afternoon showers are common results.

Relief rainfall forms as moist wind climbs a mountain. The windward slope faces the wind and receives much rain. Air descending the leeward slope warms, cloud droplets evaporate, and less rain falls. This drier area is a rain shadow. The wet western Cascades and drier land to their east show the pattern. New Zealand's Southern Alps show it too.

Follow air across a 2,000 m mountain. It starts at 20 °C with a 12 °C dew point. The 8 °C gap gives a cloud base near 1,000 m. Cooling 10 °C to that height leaves it at 10 °C. It then cools about 6 °C over the next kilometer, reaching the summit at 4 °C. If the cloud has lost water and the descending air is unsaturated, it warms 20 °C over 2 km and reaches 24 °C. Such warm, dry downslope winds are called chinook or foehn winds.

Frontal rainfall forms when warm air rises over colder, denser air. Warm fronts tend to bring broad areas of steady rain; cold fronts often bring short heavy bursts. One place can receive several rainfall types during different weather systems. A rain shadow is drier, not rain-free.

A change from muggy warmth to cool, dry air often means that a different air mass has arrived. An air mass is a huge body of air with fairly similar temperature and humidity across it at the same height.

Connect air with its source

Air takes on the properties of a broad surface where it stays for days or weeks. This is its source region. Air over a warm ocean gains moisture and warmth. Air over a cold, snowy continent becomes cold and dry.

The word similar describes conditions from side to side. Temperature still changes upward inside an air mass. Large air masses can stretch more than 1,600 km across.

Two letters describe the source. Lowercase m means maritime, formed over water and moist. Lowercase c means continental, formed over land and dry. The second letter describes temperature: T is tropical, P polar, and A arctic.

These are starting properties, so the source helps explain the weather an air mass may bring. Warm, humid mT air can feed thunderstorms. Dry cP air can bring clear, cold winter weather.

Recognize the same idea elsewhere

Around Ireland and Britain, polar maritime air usually comes from the west or northwest and brings cool, showery weather. Tropical maritime air from the southwest is mild and moist. Arctic maritime air from the north can bring winter snow showers. Polar continental air comes from the east and can be very cold in winter. Tropical continental air arrives from southern Europe or North Africa and can be hot and dry in summer.

Account for the journey

Air changes as it travels. This modification can be important. Cold air crossing warmer water gains heat and vapor from below. Rising air then builds showers or snow showers. Warm, moist air crossing cold ground can cool to make fog or stratus.

Read temperature with dew point

A temperature of 90 °F and dew point of 75 °F indicates hot, moist air, consistent with mT. A temperature of 95 °F and dew point of 30 °F indicates hot, dry air, consistent with cT. A high dew point shows abundant moisture. A large temperature–dew point gap shows low RH.

An air mass is the body of air. A front is the boundary between different air masses. A high or low describes a pressure pattern. These labels describe different features of the same weather system.

When different air masses meet, they do not quickly blend into one. Their boundary is a front. Denser cold air stays lower, while warmer air rises over it.

Compare four front types

A front is named for the advancing air. A cold front can still bring rain on a warm day. The temperature layers, not the front's name, decide whether water reaches the ground as snow.

Watch a warm front approach

A warm front slopes gently upward over cold air ahead of its surface position. High cirrus may appear first. The cloud sheet thickens into cirrostratus, then altostratus, then nimbostratus. Precipitation can begin hundreds of kilometers before the front at ground level arrives.

After the surface front passes, the air usually becomes warmer and more humid. The steady rain eases. In a typical Northern Hemisphere passage, a southeast wind may turn southwest. Southern Hemisphere wind changes follow the opposite circulation around pressure systems.

Watch a cold front pass

Cold air pushes steeply under warm air, lifting it quickly. Tall cumulus and cumulonimbus can form in a narrow band. Cold fronts often travel around 20–30 mph (30–50 km/h).

Before a sample front, a town records 75 °F, a 66 °F dew point, a south wind, and falling pressure. A short burst of heavy rain marks the passage. Behind it, temperature falls to 58 °F and dew point to 40 °F; pressure rises and skies begin clearing. The drop in dew point shows that the new air is drier, as well as cooler.

Read symbols without guessing motion

Triangles or semicircles on cold, warm, and occluded fronts point along the direction of travel. At a stationary front, symbols alternate on opposite sides. The triangles face warmer air; the semicircles face colder air. They describe the air on each side, not a moving front.

Occlusion develops as a faster cold front catches a warm front. The warm sector is lifted off the surface, and the low is mature or beginning to weaken. A stationary front can keep rain near the same place for days because neither air mass makes much progress.

Large areas of rising and sinking air organize the weather over several days. In the middle latitudes, these pressure systems often travel from west to east.

Follow a developing low

A mid-latitude low, also called a depression or cyclone, often forms where cold polar air meets warm tropical air. A ripple in the boundary develops into warm and cold fronts. The warm sector is the wedge of warm air between them. Surface air flows inward and rises, favoring clouds and precipitation.

A low can span 1,000–2,000 km and last several days. As a Northern Hemisphere low passes north of a town, the town may experience this sequence:

1. Pressure falls and high cirrus appears.
2. Clouds lower and thicken; steady rain arrives ahead of the warm front.
3. The warm sector brings milder, humid air, often with low cloud or drizzle.
4. The cold front brings a shorter burst of heavy rain and a wind shift.
5. Cooler air follows, pressure rises, and skies clear between showers.

This order follows a route through both fronts. A town on a different side of the low may experience a different sequence.

Explain a high's weather

A high, or anticyclone, has sinking air. Compression warms the descending air and helps evaporate cloud droplets. Winds near the center are often light, and dry weather may last for days.

Summer highs can bring hot, sunny days. In winter, clear nights cool the ground strongly and can produce frost or fog. Low cloud or fog can stay trapped under a high, so high pressure does not always mean sunshine.

Northern Hemisphere winds turn counterclockwise around lows and clockwise around highs. Southern Hemisphere winds turn clockwise around lows and counterclockwise around highs. Surface air generally moves inward toward lows and outward from highs.

Compare isobar spacing

Isobars connect equal sea-level pressures. Tight spacing means pressure changes quickly across a short distance, generally producing strong wind. Wide spacing means a weaker gradient. The edge of a strong high beside a low can be windy too. Read spacing, not just the central pressure number.

Estimate when weather arrives

The jet stream is a narrow band of fast wind near the top of the troposphere, about 9–12 km high. It helps steer and strengthen middle-latitude lows. A southward dip can allow cold polar air into Canada and the eastern United States.

Suppose a rain band's leading edge lies 450 miles west and moves toward a town at 30 mph. Divide distance by speed: $450÷30=15$ hours. From 5 p.m., that is about 8 a.m. tomorrow. This times the rain band's edge. The low's center could arrive later, because warm-front rain extends well ahead of it. Updated observations help revise the estimate if the speed or path changes.

A useful weather record describes a place at a known time. It measures temperature, pressure, humidity, wind, cloud, precipitation, and visibility. Consistent methods let observations from different days and stations be compared.

Match instruments to measurements

A maximum–minimum thermometer keeps the highest and lowest readings since it was reset. Cloud type and cover can be observed by eye. Automatic stations record continuously and send readings to weather services.

Choose a fair location

Place a thermometer in a white, slatted, ventilated instrument shelter, about 1.25–2 m above grass. Shade stops direct sunlight heating the instrument. Ventilation lets outside air flow through. Keep it away from walls and pavement that can add heat.

Put a rain gauge in an open place. Trees and buildings can block rain or add drips. An anemometer needs open air above obstacles; the standard height is 10 m. A sheltered corner gives a poor guide to the wind over the wider area.

Read a school station at the same time each day. Record the date, time, values, and units immediately. A table with units in each heading prevents mixing rainfall totals with wind speeds or temperatures.

Calculate a weekly pattern

A week's daily highs are 58, 63, 66, 61, 54, 52, and 59 °F. Add them to get 413 °F, then divide by seven: the mean high is 59 °F. The range is the highest minus the lowest, $66-52=14$ °F.

Rain totals are 0, 0.10, 0, 0.32, 0.50, 0.16, and 0 inches. Add the four wet days to get 1.08 inches. Dividing by seven would answer a different question, the mean daily rain.

Graph and check the evidence

Use time on a line graph's horizontal axis for temperature or pressure. Bars suit separate daily rainfall totals. A wind rose shows how often wind came from each direction.

An unusual reading is an anomaly worth checking. Compare it with nearby official observations and review the instrument location. If a thermometer was known to be in direct sun, repeat the reading correctly or mark the faulty value. Keep an unusual but valid observation.

Two cool, rainy days suggest a weather pattern, but more evidence is needed to identify its cause. One week's weather also tells little about climate, which describes patterns over many years, usually 30.

A surface weather map shows many places at one moment. It combines pressure patterns, fronts, and local observations. Read the map's key, time, and units before interpreting the weather.

Read the large-scale pattern

Isobars join equal sea-level pressures, often at 4 mb intervals. Values falling toward a closed center identify a low, labeled L. Values rising inward identify a high, labeled H. H means high pressure, not hot air. A place between the 1008 and 1012 mb lines has a pressure between those values.

Tightly packed lines show a strong pressure gradient. In the Northern Hemisphere, wind turns counterclockwise around lows and clockwise around highs, crossing slightly toward lower pressure near the surface. In the Southern Hemisphere, the turning directions reverse.

Cold fronts have blue triangles; warm fronts have red semicircles. An occlusion has both in purple on one side. A stationary front has alternating symbols on opposite sides. Symbols point along travel for moving fronts. On a stationary front, triangles face warm air and semicircles face cold air.

Locate station-model values

A US station model arranges information around a circle:

• Upper left: temperature in °F.
• Lower left: dew point in °F.
• Upper right: coded sea-level pressure.
• Circle shading: cloud cover.
• Wind barb: wind source direction and speed.

A rain dot, snow asterisk, or fog symbol may show present weather. Three horizontal lines indicate fog; two indicate mist. A filled circle means overcast and an empty circle means clear.

The wind barb extends toward the direction the wind comes from. One long feather means 10 knots, a short feather 5 knots, and a pennant 50 knots. One long and one short feather give 15 knots. A circle around the station circle without a barb means calm.

Decode pressure one step at a time

The code keeps the last three digits and removes the decimal point. For the usual sea-level range:

1. If the code is below 500, put 10 before it. Otherwise, put 9 before it.
2. Place the decimal before the last digit.

Code 186 becomes 10186, then 1018.6 mb. Code 982 becomes 9982, then 998.2 mb. Code 024 becomes 10024, then 1002.4 mb.

A plot with 68 above 54 on the left, code 186, a half-shaded circle, and a southwest barb with 15 knots describes 68 °F air, a 54 °F dew point, 1018.6 mb, half cloud cover, and wind from the southwest. The 14 °F temperature gap shows air below saturation.

Compare radar and satellites

Radar shows falling precipitation and its intensity. Doppler measurements also show motion toward or away from the radar. Satellites show clouds across large areas. Infrared images measure cloud-top temperatures; colder tops usually indicate higher clouds. A radar-free area can still be cloudy.

A forecast joins observations with physical explanations. Forecasters use the atmosphere's present state to estimate how it will change. They communicate both the likely weather and its uncertainty.

Follow the forecast process

Weather services collect observations from land stations, ships, buoys, aircraft, satellites, and radar. Balloons carry radiosondes, instrument packages measuring temperature, humidity, pressure, and winds high above the ground. Many sites launch balloons at matching times twice daily, rising to about 30 km.

Computers organize observations into a three-dimensional grid. Numerical weather prediction uses physical laws to calculate changes in air movement, heat, and water within the grid, step by step.

A forecaster compares models, studies local conditions, adjusts the likely outcome, and communicates it. An ensemble is a group of model runs with slightly changed starting conditions. Close agreement increases confidence; large differences indicate more uncertainty.

Combine several clues

Falling pressure, high clouds that thicken and lower, and a wind turning south or southeast can indicate a warm front approaching in a typical Northern Hemisphere pattern. Rising pressure, a northwest wind after rain, and clearing skies suggest a cold front has passed. Southern Hemisphere turning around pressure systems is reversed.

Suppose a cold front is 200 miles west and moves east at 25 mph. Distance divided by speed gives $200÷25=8$ hours. Warm, humid mT air ahead can feed thunderstorms near the front. Behind it, cold, dry cP air brings lower temperature and dew point. Pressure often rises and skies clear. The timing estimate must be updated if motion changes.

Interpret a rain chance

A 40% chance of precipitation means a 40% probability that a given point in the forecast area receives measurable precipitation during the stated period. In US forecasts, measurable means at least 0.01 inch, about 0.25 mm, of rain or melted snow.

Over many comparable 40% forecasts, measurable precipitation should occur about four times in ten. The percentage does not say that rain lasts 40% of the day or falls on exactly 40% of the area. A rainy day after a 30% forecast is one possible result of that forecast.

Judge confidence and alerts

Small errors in starting observations grow as models run farther ahead. Forecasts generally become less reliable with longer lead times. Confidence also depends on the weather feature: a broad temperature trend can be easier to forecast than the exact location of an afternoon shower.

A US watch means hazardous weather is possible because conditions are favorable. A warning means the hazard is occurring or expected soon. Other services use colors, often yellow, orange or amber, and red, to show increasing danger. Always read the hazard, place, and time named in an alert.

A thunderstorm grows when moist air is lifted and keeps rising. Its tall cumulonimbus cloud contains strong air currents, liquid droplets, and ice. These explain its rain, lightning, and hail.

Supply three ingredients

A storm needs moisture, unstable air, and a lifting trigger. Unstable air keeps rising after a push because it remains warmer and less dense than surrounding air. Heating, a front, a mountain, or meeting sea breezes can provide the lift.

As rising vapor condenses, it releases latent heat. This energy helps keep the rising air warm enough to continue upward. The cloud grows into cold layers high above the ground.

Follow one cell's life

A single cell often lasts about 30 minutes to an hour. Groups of cells and rotating supercells can persist for hours. The anvil spreads where the tallest rising air reaches the tropopause.

Explain lightning and thunder

Collisions among ice crystals, soft hail, and supercooled droplets separate electrical charge. The cloud top usually becomes positive, while its lower region becomes negative. Lightning is a large electrical discharge within a cloud, between clouds, or between cloud and ground.

Lightning heats its narrow path to about 50,000 °F (28,000 °C). Rapid expansion of the heated air makes thunder. Light arrives almost at once, while sound takes about five seconds per mile or three seconds per kilometer. A 15-second delay gives about three miles or five kilometers. If you hear thunder, go indoors.

Distant storm flashes without audible thunder are often called heat lightning. They come from ordinary storms whose sound does not reach you. Lightning can strike the same structure repeatedly.

Grow a hailstone

Strong updrafts lift ice particles into regions with liquid droplets below freezing. These supercooled droplets freeze onto the particles. Hailstones grow in layers while the updraft supports them. They fall when they become too heavy or the updraft weakens.

Hail can reach warm ground in summer because it forms high in freezing air and falls before melting completely. Sleet forms differently: drops refreeze in cold air below a cloud during winter weather.

A thunderstorm is called severe by the US National Weather Service if it produces hail at least 1 inch across, winds at least 58 mph, or a tornado. Thunder and lightning alone do not meet this particular severe-storm definition.

A tornado is a violently rotating column of air connecting a cloud base with the ground. The visible condensation funnel may stop above the ground while rotating air continues beneath it. Swirling dust or debris can reveal that contact.

Build a rotating storm

Most strong tornadoes develop from supercells, thunderstorms with a deep, lasting rotating updraft called a mesocyclone. Supercells need the usual thunderstorm ingredients and strong wind shear, a change in wind speed or direction with height.

Imagine light southeast winds near the ground and stronger west winds above. Shear can create horizontal rotation, like a rolling tube of air. A strong updraft tilts that rotation toward vertical. The storm's updraft then rotates. In some storms, a tornado forms beneath it. Most supercells never produce one, so a rotating storm is evidence of risk, not certainty of a tornado.

Warm, humid air from the Gulf, cold Canadian air, dry southwestern air, and fast winds aloft often meet over the central United States. These ingredients help explain its frequent tornadoes, especially in spring and early summer. Tornadoes also occur in Canada, Europe, Australia, Bangladesh, and other regions. Formation is not limited to land; tornadoes over water can occur as tornadic waterspouts.

Look for rotation with radar

Doppler radar measures motion toward or away from the instrument. Nearby areas moving in opposite directions can indicate storm rotation. A hook-shaped precipitation pattern can also signal a storm capable of producing a tornado. Forecasters combine radar with observations when issuing warnings.

Read the damage rating

The Enhanced Fujita scale estimates a tornado's three-second gust from surveyed damage after the event.

A damage estimate of 150 mph lies within 136–165 mph, so it gives EF3. These are estimates from damage, since a wind instrument is rarely directly in the tornado's path. The width or darkness of a funnel alone does not give its EF rating.

Compare a tornado with a hurricane

A tornado is usually less than a mile wide and lasts minutes. It belongs to a much smaller part of a storm than a hurricane does. A hurricane is a rotating tropical low, often hundreds of miles across, powered over warm ocean water for days. Hurricanes can also produce tornadoes as they approach or cross land.

A tropical cyclone is a large rotating low without fronts. It develops over warm tropical ocean water. Its name depends on location: hurricane in the Atlantic and northeast or central Pacific, typhoon in the northwest Pacific, and tropical cyclone in other regions.

Supply the conditions

Formation needs a starting disturbance, moist air, and warm water about 26.5 °C (80 °F) or higher extending roughly 50 m deep. It also needs weak wind shear. Strong changes in wind with height can disrupt its upright structure.

Coriolis helps organize the spin, so formation generally occurs at least about 5° from the equator. The Atlantic season runs June 1 through November 30, though storms can form outside it.

Trace the energy loop

1. Warm ocean water evaporates, adding vapor to inflowing air.
2. Moist air rises in thunderstorms and condenses.
3. Condensation releases latent heat, warming the storm's core.
4. Surface pressure falls and more air flows inward.
5. Turning inflow picks up more ocean moisture and feeds further storms.

Warm water and weak shear allow this loop to strengthen. The water supplies energy; the storm does not draw its main fuel from land.

Locate the eye and eyewall

The eye is the relatively calm center, often clear because air sinks there. It is usually about 20–40 miles across. The surrounding eyewall contains the strongest winds and tallest thunderstorms. Spiral rain bands stretch outward across a system often several hundred miles wide.

The lowest pressure lies in the eye, but the strongest wind lies in the eyewall. A calm eye passage is followed by the other eyewall, with wind coming from the opposite direction. Northern Hemisphere hurricanes turn counterclockwise; Southern Hemisphere cyclones turn clockwise.

Classify the sustained wind

A tropical depression has sustained winds below 39 mph. A tropical storm has 39–73 mph winds and receives a name. A hurricane begins at 74 mph, or 64 knots.

Categories 3–5 are major hurricanes. A sustained wind of 120 mph gives Category 3. This wind scale does not rate rainfall or the rise of seawater, so a category cannot describe every part of a storm.

Explain weakening

Cooler water reduces the energy supply. Crossing land cuts off warm ocean moisture and adds friction. Strong wind shear disrupts the storm. A weakening tropical cyclone can later develop fronts in the middle latitudes and still bring heavy rain inland.

Extreme weather can grow from strong temperature contrasts or from a weather pattern that stays in place. Temperature layers shape winter precipitation. Persistent highs can shape heat waves and drought.

Distinguish heavy snow from a blizzard

Winter lows can bring snow, sleet, or freezing rain. A nor'easter tracks along the US East Coast, with northeast winds driving Atlantic moisture into cold air over land.

In the United States, a blizzard requires winds or frequent gusts of at least 35 mph and falling or blowing snow frequently reducing visibility below one-quarter mile for at least three hours. Snow depth is not part of that definition.

Ten inches of snow with 15 mph winds is heavy snow, but those winds do not meet blizzard criteria. Two inches with 45 mph gusts and one-eighth-mile visibility for five hours does. A ground blizzard lifts existing snow, so no fresh snowfall is required.

Make lake-effect snow

Very cold air moving across an unfrozen lake gains heat and water vapor. The warmed, moist air rises into clouds and snow bands. Heavy snow falls on the downwind shore. Westerly flow across a Great Lake, for example, can load the eastern shore with snow.

This is most common in late fall and early winter, when cold air arrives while lake water remains relatively warm. It is an example of an air mass changing as it crosses a new surface.

Make an ice storm

Snow from a cold cloud can melt in a warm layer aloft. If the near-ground cold layer is shallow, liquid drops survive to the surface and freeze onto roads, branches, and wires. This is freezing rain. A deeper cold layer refreezes the drops before landing, giving US sleet. The difference is where freezing happens.

Very cold air can still produce snow, although it contains little water vapor and often gives light snowfall. Heavy snow is common when ground-level temperatures are close to or a little below freezing.

Link persistent highs with heat and dryness

A heat wave is several days of unusually hot weather for a place. Local definitions differ. Under a persistent high, sinking air warms as it compresses and suppresses cloud growth. Sunshine heats the ground day after day. High humidity slows sweat evaporation; the heat index combines temperature and humidity to describe how hot conditions feel.

Drought is a lasting shortage of precipitation compared with a place's usual amount. It can develop over weeks, months, or years as highs steer rain-bearing systems away. Some rain can fall during a drought.

A desert's usual dryness is not itself drought. Compare its rainfall with its own normal amount. Dry soil also leaves less water to evaporate, so more incoming energy heats the ground and air. That links drought with stronger heat.