Weather and the Atmosphere

Weather is one of the more generous topics in General Science: the naming conventions are systematic, so once you know what the word parts mean you can work out a cloud you have never been taught.

The layers

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Troposphere first, and weather lives there. "Tropo" relates to change, which is what weather is.

Two questions the layers reliably produce:

The ozone layer is in the stratosphere, not the troposphere. It absorbs most ultraviolet radiation, which is why its thinning is dangerous.

The mesosphere is the coldest layer, and the thermosphere is technically the hottest - though it is so thin that "hot" there means fast-moving particles rather than anything that would feel warm.

Temperature does not fall steadily with altitude. It falls through the troposphere, rises through the stratosphere as ozone absorbs UV, falls again through the mesosphere, and rises in the thermosphere. A question asking whether it simply gets colder with height is testing that.

Composition

Dry air is about 78 percent nitrogen and 21 percent oxygen, with argon, carbon dioxide and traces making up the rest.

Nitrogen is the majority gas, which surprises people who expect oxygen. That is a question on its own.

Air pressure and wind

Air pressure falls with altitude, because there is less air above pressing down.

Wind is air moving from high pressure toward low pressure. The greater the difference, the stronger the wind.

Warm air is less dense and rises; cool air is denser and sinks. Rising air creates low pressure at the surface and generally means clouds and precipitation; sinking air creates high pressure and generally means clear skies.

Low pressure, bad weather. High pressure, fair weather. That association answers a good number of questions. A falling barometer therefore warns of stormy weather approaching.

Wind does not blow in a straight line from high to low. Because the Earth spins, moving air is deflected - to the right in the Northern Hemisphere, to the left in the Southern. That is the Coriolis effect, and it is why hurricanes spin.

Local winds come from uneven heating. Land heats and cools faster than water, so on a sunny day the land warms, air rises over it, and cooler air flows in off the water: a sea breeze. At night the pattern reverses into a land breeze. A monsoon is the same thing on a continental scale and a seasonal timetable: winds that reverse between summer and winter, bringing a wet season.

The jet streams are narrow bands of very fast wind high in the troposphere that steer storms across the continent.

The instruments

Air masses

An air mass is a huge body of air that takes on the temperature and moisture of where it formed, and it is named for both:

Combine them: continental polar is cold, dry air from Canada; maritime tropical is warm, humid air from the Gulf of Mexico. Fronts are where two of these meet.

Fronts

A front is the boundary between two air masses of different temperature.

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The difference in weather comes from the angle. A cold front has a steep boundary, so warm air is forced up fast, and fast-rising air builds tall cumulonimbus clouds and violent short storms. A warm front has a shallow slope, so the lift is gradual and the cloud is a long flat sheet giving hours of steady rain.

That is worth understanding rather than memorizing, because it predicts the cloud type as well as the rain.

Clouds

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The names are built from parts, so learn the parts rather than the clouds.

So cumulonimbus is a heaped cloud producing rain, which is a thunderstorm. Nimbostratus is a rain-producing layer, which is steady gray drizzle. Altocumulus is puffy cloud at middle height.

Three to know outright:

• Cirrus: high, thin, wispy, fair weather, often the first sign of change coming.
• Cumulus: puffy with a flat base, fair weather, the cartoon cloud.
• Cumulonimbus: the towering thunderstorm cloud with an anvil top, spanning all three altitude bands, and the one that produces severe weather.

Severe weather

Thunderstorms need warm moist air rising rapidly, which is why they come with cumulonimbus clouds and cold fronts. Lightning is a discharge of static built up within the cloud, and thunder is the sound of the air it superheated expanding.

Light travels much faster than sound, which is why you see the flash before you hear the thunder, and counting the gap estimates the distance - about five seconds per mile.

Tornadoes form from rotating thunderstorms, over land.

Hurricanes form over warm ocean water, need water above about 80 degrees Fahrenheit, and weaken over land because their energy source is gone. A hurricane is the same phenomenon as a typhoon and a cyclone, named differently by region.

Humidity is water vapor in the air. Relative humidity is how much it holds against the most it could hold at that temperature, which is why warm air can feel less humid at the same water content - it could hold more.

Measuring the storms

Rain, sleet, hail and frost

Seasons and climate

The seasons are caused by the tilt of Earth's axis, about 23.5 degrees, not by how close the Earth is to the Sun. When the Northern Hemisphere is tilted toward the Sun, sunlight hits it more directly and for longer each day, and it is summer - even though the Earth is actually nearest the Sun in January.

Weather is today; climate is the long-run average of a region's weather over decades. Oceans moderate climate: coastal places have milder temperatures year-round than places far inland, because water heats and cools slowly.

The atmosphere and pollution

The greenhouse effect is natural and necessary: gases such as carbon dioxide, water vapor and methane let sunlight in but trap the infrared heat the ground gives off. Without it the Earth would be frozen. Adding more of those gases traps more heat.

Two kinds of ozone, and the test asks which is which:

• Stratospheric ozone is the ozone layer, and it is good - it blocks ultraviolet light. It was thinned by CFCs, chemicals once used in aerosols and refrigerators.
• Ground-level ozone is a pollutant, the main ingredient of photochemical smog.

Acid rain comes from sulfur dioxide and nitrogen oxides released by burning fossil fuels. A temperature inversion - a warm layer sitting over cool air - acts as a lid that traps pollution near the ground, which is how smog builds up over a city in a valley.

What you can skip

Across all 2,104 General Science questions in our bank, these come up rarely or never:

• Specialist cloud and storm types. Mammatus clouds and derechos never appear, and virga and supercells come up once each. The cloud-name parts above cover the clouds that recur.
• Hurricane category thresholds beyond the 74 mph line. No question asks a category's wind range.
• El Nino and La Nina never appear.
• Named wind belts. The westerlies never appear and the trade winds once.

Where people lose points

Putting weather in the stratosphere. The troposphere.

Putting the ozone layer in the troposphere. The stratosphere.

Saying oxygen is the main gas in air. Nitrogen is, at about 78 percent.

Swapping the front types. Cold is brief and violent; warm is long and steady.

Saying temperature falls steadily with altitude. It reverses twice.

Associating high pressure with storms. Low pressure brings them.

Work one in under a minute

The sky fills with towering clouds, a short violent downpour follows, and afterward the temperature drops noticeably. What passed through?

Towering clouds are cumulonimbus, the storm was brief and violent, and the temperature fell behind it.

A cold front.

Each clue points the same way, and the temperature change after the passage is the single most reliable indicator.

Where this leads

The water condensing in these clouds is the same water the hydrologic cycle moves, and the pressure and density ideas reappear in fluids.