What is the weather?

What is your weather like today? Is it sunny, cold, wet or dry? What causes these different kinds of weather? Heat and light from the sun travel through the earth's atmosphere. The sun's rays affect the air in four ways. They can change its temperature, or the amount of heat it contains. They can change its humidity, or the amount of water it contains. They can change the air pressure, or the force of the atmosphere pushing on the earth. And they can change the wind, or the way the air moves.

Temperature

Most of the heat given off by the sun is lost in space. Then about one-third of the sunlight that does reach the atmosphere is reflected back into space by the clouds. Less than half the sunlight that gets to the earth's atmosphere actually reaches the planet's surface. This sunlight warms the ground, oceans and lakes, which then reflect warmth back into the air.

Humidity

Humidity is the amount of water vapor in the air. When sunlight warms oceans, lakes and rivers, some of the water turns into water vapor. The vapor rises into the air, condenses, and forms tiny drops of water that float in the air as clouds. These drops fall to earth as precipitation. That's what scientists call rain, snow, hail or sleet.

Air pressure

What difference does the weight of the air make? Warm air weighs less than cool air, which is why hot-air balloons stay up in the sky. Lighter weights put less pressure on whatever is underneath them. So where there is warmer air, the air pressure is lower. Cool air weighs more, so where the air is cooler, the air pressure is higher.

Wind

What is the wind? The wind is air moving from one place to another. Air always moves from where the pressure is high to where the pressure is low. The greater the difference between the levels of high pressure and low pressure, the stronger the wind movement.
Which way does the north wind blow? It blows from the north towards the south. Winds are always named after the direction from which they blow.