Home / Courses / Earth and Space Science / Unit 3

Earth and Space Science Unit 3: Weather and climate

Weather is the short-term state of the atmosphere; climate is the long-term average. Both come from the sun heating Earth unevenly.

Covers: Atmosphere, water cycle, air pressure and fronts, severe weather, climate zones, climate change

Study guide

The atmosphere is a layer of gases, about 78% nitrogen and 21% oxygen. Weather happens in the lowest layer, the troposphere, where temperature drops with altitude. Above it, the stratosphere holds the ozone layer. Weather is the short-term condition of the atmosphere, while climate is the long-term average pattern for a region.

The water cycle moves water through evaporation, transpiration from plants, condensation into clouds (when rising air cools to its dew point), precipitation, runoff and infiltration into groundwater. Humidity is the amount of water vapor in the air.

Air pressure is the weight of the air above. High-pressure systems bring sinking air and usually clear skies, and low-pressure systems bring rising air, clouds and storms. Wind blows from high to low pressure and is deflected by the Coriolis effect. Air masses take on the temperature and moisture of where they form. A front is the boundary between air masses: cold fronts push under warm air and cause quick, strong storms; warm fronts slide over cold air and bring longer, gentler rain.

Severe weather includes thunderstorms (from rising warm, moist air), tornadoes (rotating columns of air from severe thunderstorms), hurricanes (powered by warm ocean water and rotating around low pressure), and blizzards.

Climate zones depend mainly on latitude, which sets how much sunlight an area receives, plus elevation, ocean currents, prevailing winds and distance from water. Oceans moderate coastal climates. Climate change is the long-term shift in these patterns, currently driven largely by greenhouse gases from human activities, raising temperatures and sea levels and changing precipitation.

Relative humidity compares the water vapor in the air to the most it could hold at that temperature. A psychrometer measures it with a dry-bulb thermometer and a wet-bulb thermometer, which is cooled by evaporation. The bigger the difference between them, the drier the air. When the readings are equal, relative humidity is 100% and the air is at its dew point. The Coriolis effect deflects winds and ocean currents to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. So in the Northern Hemisphere, surface winds spiral counterclockwise into a low and clockwise out of a high.

On weather maps, a cold front is drawn with triangles and a warm front with semicircles, both pointing in the direction the front moves. Mid-latitude low-pressure systems usually have a cold front trailing to the southwest and a warm front extending to the east. They move generally from west to east across the United States with the prevailing westerlies. Dark, rough surfaces absorb more sunlight, while light, smooth surfaces like snow reflect more (higher albedo). Water has a high specific heat, so it warms and cools slowly, which gives coastal areas smaller daily and yearly temperature ranges than inland areas.

Evaporation absorbs heat energy from the surroundings, and condensation releases that energy back into the air. The heat released by condensation in rising air is what powers thunderstorms and hurricanes. Large eruptions put ash and sulfur aerosols into the stratosphere, which block sunlight and can cool global temperatures for a year or two. Paved urban surfaces stop water from soaking in, so runoff and flood risk increase. A tornado is usually less than a kilometer wide and lasts minutes, while a hurricane is hundreds of kilometers wide and lasts days.

Key ideas

• Water cycle: evaporation, condensation, precipitation, runoff, infiltration.
• Warm air rises (low pressure); cool air sinks (high pressure).
• Wind blows from high pressure to low pressure.
• Cold fronts bring quick storms; warm fronts bring steady rain.
• Climate depends on latitude, altitude, ocean currents and distance from water.

Formulas and rules

Common mistakes

• Confusing weather (today) with climate (decades).
• Thinking clouds form from evaporation. They form when vapor condenses.

Words to know

Weather vs climate

Weather is short-term; climate is the long-term average.

Cloud formation

Water vapor cools and condenses.

Wind

Air moving from high pressure toward low pressure.

Cold front

Cold air pushes under warm air, causing quick storms.

Maritime climate

Coastal climate moderated by the ocean: milder summers and winters.

Atmosphere composition

About 78% nitrogen and 21% oxygen.

Humidity

The amount of water vapor in the air.

Dew point

The temperature where air becomes saturated and water condenses.

Air pressure

The weight of the air above.

High-pressure system

Sinking air and usually clear skies.

Low-pressure system

Rising air, clouds and storms.

Air mass

A large body of air with uniform temperature and moisture.

Warm front

Warm air slides over cold air; long, gentle rain.

Hurricane

A storm powered by warm ocean water rotating around low pressure.

Tornado

A rotating column of air from a severe thunderstorm.

Climate zone

A region with a typical climate, set mainly by latitude.

Sample practice questions

13 more questions like these, from level 1 to 5, are in the practice set.