Why some places are hotter

Why do you feel hotter in a black shirt than in a white one? Dark surfaces absorb more sunlight; light surfaces reflect more and stay cooler. Dark roads get hot quickly for the same reason. That's why school shirts are often white in summer.

Albedo (Latin for "whiteness") is the fraction of sunlight a surface reflects:

• high albedo: reflects a lot, stays cool;
• low albedo: absorbs a lot, heats up.

Albedo of some surfaces (Table 13.1):

Snow and ice reflect most sunlight, which keeps the polar regions very cold. Black soil and ocean water have low albedo, so they absorb more and are warmer.

Why a concrete house is hot at night. All objects give off heat. Concrete soaks up heat all day and gives it back at night, so the room stays hot. Traditional houses with thick mud and wooden walls give back less, and stay cool even in hot summers.

The urban heat island. Cities are full of steel, concrete, brick and asphalt, which absorb and hold heat. So cities are warmer than the countryside around them, especially at night. Villages and forests have plants, which keep them cool with shade and by giving off water vapour (transpiration). Hotter cities then use more air-conditioning, which adds to the problem.

Why the equator is hot and the poles cold. Shine a torch straight at a ball: you get a small, bright patch. Tilt it: the same light spreads over a bigger, dimmer patch. Or roll out an aloo paratha bigger: the same filling, but less in each bite. The Earth is a sphere:

• near the equator, the Sun's rays hit almost straight on and are concentrated on a small area;
• near the poles, they hit at a slant and spread over a large area.

So the equator stays warm and the poles cold.

Together with the tilt of the Earth's axis, this gives us seasons and changing day lengths. (Seasons are not caused by the Earth moving nearer to or farther from the Sun.) This uneven heating is what drives the winds and ocean currents.