Weather
The weather-sunshine, fog, rain, clouds, wind, heat, cold—is always chang- ing in most parts of the world. These changes are caused by what happens in the atmosphere, the layer of air above the earth. The atmosphere is always moving, driven by the sun's heat. Near the equator the sun's strong rays heat the air . At the North and South Poles the sun's rays are weaker and the air is colder. This uneven heating means the atmosphere is never still. Huge masses of warm and cold air flow round and round between the tropics and the polar regions. As these wandering air masses meet, rise and fall, heat and cool, they cause weather. When cold and warm masses meet, the air whirls in a giant spiral called de- pression. Depressions bring clouds, wind, rain, and summer thunderstorms. They can also cause violent tornadoes and hurricanes. a The meeting line between two air masses is called a front. When cold air push- es up behind warm air, it forms a cold front; when a warm air mass catches up with a cold mass, it creates a warm front. Fronts usually bring changes in the weather.​