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Weather Basics
Rain, Sun, Wind, and the Atmosphere
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Weather is the short term condition of the atmosphere, including temperature, clouds, wind, and precipitation. It matters because it affects travel, farming, ecosystems, and daily human activities. The main energy source for weather is the Sun, which heats Earth's surface unevenly. That uneven heating drives air movement, cloud formation, and the water cycle.
Sunlight warms land and water, but land usually heats and cools faster than water. Warm air becomes less dense and rises, while cooler air sinks and moves in to replace it, creating wind. Water evaporates, rises, cools, and condenses into clouds, and when droplets grow large enough they fall as rain. These connected processes explain many common weather patterns students observe every day.
Key Facts
- Weather is driven mainly by uneven solar heating of Earth's surface.
- Warm air rises because it is less dense, and cool air sinks because it is more dense.
- Wind is air moving from higher pressure to lower pressure.
- Evaporation + condensation + precipitation = the basic water cycle in weather.
- Relative humidity = (actual water vapor in air / maximum possible water vapor at that temperature) x 100%
- As rising air cools to its dew point, water vapor condenses into cloud droplets.
Vocabulary
- Atmosphere
- The atmosphere is the layer of gases surrounding Earth where weather happens.
- Evaporation
- Evaporation is the process in which liquid water changes into water vapor.
- Condensation
- Condensation is the process in which water vapor cools and changes into tiny liquid droplets.
- Air pressure
- Air pressure is the force exerted by the weight of air on a surface.
- Precipitation
- Precipitation is any water that falls from clouds to the ground, such as rain, snow, sleet, or hail.
Common Mistakes to Avoid
- Thinking the Sun heats Earth evenly, which is wrong because different surfaces absorb and release energy at different rates and different places receive different amounts of sunlight.
- Assuming wind is created only by moving trees or storms, which is wrong because wind is caused by pressure differences in the atmosphere even when the air motion seems gentle.
- Believing clouds are made of gas only, which is wrong because most visible clouds are made of tiny liquid water droplets or ice crystals.
- Saying rain falls as soon as a cloud forms, which is wrong because droplets must grow large enough through collisions and collection before gravity can pull them down as precipitation.
Practice Questions
- 1 A puddle contains 2.0 L of water after rain. By the next day, 0.5 L has evaporated. What percentage of the original puddle water evaporated?
- 2 Air can hold 20 g of water vapor per cubic meter at a certain temperature, but it currently contains 15 g/m^3. What is the relative humidity?
- 3 Land heats up faster than a nearby lake during the day. Explain how this temperature difference can create a local wind and describe the direction the air moves near the surface.