Heated unevenly
Show on a big screenA rice field is warm at midday. The poles stay cold.
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The rule
A slanted beam spreads over more ground. Snow reflects most of the sunlight that lands.
A slanted beam is tilted from straight down.
- Sunlight arrives as parallel beams.
- At the equator, a beam lands straight down on a short patch.
- Near the poles, the same beam lands at a slant, on a long patch.
- The long patch shares the same energy across more ground.
The idea
Earth is a ball. Latitude is the distance north or south of the equator. Near the equator, the Sun is nearly overhead at midday. Sunlight lands almost straight down. Near the poles, the same beam lands at a slant. It spreads over more ground, so each patch gets less.
The ground itself matters too. Albedo is the fraction of sunlight a surface reflects. Snow reflects 0.80 to 0.90 of it. Take a dark field that reflects only 0.15. A surface keeps what it does not reflect. Snow keeps 0.10 to 0.20. The dark field keeps 1 − 0.15 = 0.85. So the field keeps far more of the Sun's energy.
Winds and currents
Uneven heating makes pressure differences. Where warm air rises, ground pressure is lower. Wind flows from high pressure to low pressure.
By day, hill slopes heat faster than the valley floor. Warm air rises. Cooler valley air moves up the slope to take its place. This is a valley breeze. At night the air flows the opposite way, down the slope. This is a mountain breeze.
Winds drag surface water. This helps start ocean currents moving.
Watch out!
The equator is not warm because it is closer to the Sun. The angle of the sunlight is the reason.
Try the lab to finish this lesson.