What Causes Weather?

Summary

Weather is driven by differences in heating that change air temperature, density, and pressure. These differences set air in motion, while moisture in the atmosphere produces clouds and precipitation. Thunderstorms can also generate lightning and thunder.

Main Ideas

• Uneven heating drives atmospheric circulation. The Sun’s energy is concentrated near the Equator and spread over a larger area near the poles, producing temperature differences.
• Warm air rises and creates lower pressure at the surface. As air near the Equator warms, it expands, becomes less dense, and rises. Air from higher-pressure areas moves toward the resulting lower-pressure area; this movement of air is wind.
• Global circulation is more complex than a single loop. In a simplified model, rising air moves poleward at high altitude, sinks near the poles, and returns toward the Equator. Earth’s rotation, through the Coriolis force, divides this circulation into three atmospheric cells that help shape prevailing winds.
• Land and water create local breezes. Because land heats and cools faster than water, differences between their temperatures produce sea and land breezes on a smaller scale.
• Moisture forms clouds and precipitation. Evaporation and transpiration add water vapor to the atmosphere. Rising air cools, and its water vapor condenses into clouds. Because cooler air can hold less water vapor, cooling can also produce dew. When cloud droplets or ice particles grow large enough, they fall as precipitation—such as rain, snow, sleet, or hail—depending on the conditions they pass through.
• Lightning and thunder arise from electrical charge separation. In thunderstorms, collisions between ice crystals and soft hail separate electrical charges: heavier hail becomes negatively charged and sinks, while lighter ice crystals become positively charged and rise with updrafts. When the resulting electric field overwhelms the air’s insulating ability, lightning occurs. Lightning rapidly heats the surrounding air, causing it to expand and compress in a shock wave heard as thunder.

Speakers and Sources

• Speaker: One unnamed narrator.
• Other speakers or cited sources: None identified in the subtitles.