Free
The things that affect daily weather – like storm clouds and cold fronts – are always changing.
That’s why weather predictions are only good for about a week ahead of time – if you’re lucky. But climate is relatively steady over decades or even centuries. Why? Because climate is influenced by factors that change very slowly, if at all. Such factors include mountains, oceans, the landscape, and closeness to the equator. Their steady influence makes for regular kinds of weather patterns, and therefore steady climate.
Winds and Climate
The Earth has consistent global winds that follow general patterns. Air warmed by the sun rises, and then cold air takes its place. (Warm air is lighter than cold.) The result is a circular pattern of air, which creates winds.
◀ The trade winds, westerlies, and polar easterlies are surface winds. Trade winds are winds from the subtropics that blow toward the equator. They are called northeasterlies north of the equator and known as southeasterlies to the south of the equator. Winds blowing toward the poles from the subtropics in midlatitudes are westerlies. The doldrums are ocean regions near the equator where the winds are light and unpredictable.
Patterns of rising and sinking air are called Hadley, Ferrel, or polar cells. Here’s how a Hadley cell works. Intense sunlight at the equator makes the air very warm, and that hot air rises, then spreads out north and south of the equator. The air cools and sinks back down at around 30° latitude north and south. As the air sinks, it piles up, increasing the air pressure. High air pressure brings dry weather, so places around 30° latitude are some of the driest on Earth. As you can see on a world map, deserts are often located in these regions. ▶
Sunlight and Climate
1. The closer to the poles you go, the less direct the sunlight is. Indirect sunlight delivers less heat to the Earth’s surface, so climates near the North and South Poles are cold.
2. In equatorial areas, the sun is almost directly overhead. Direct sunlight is concentrated, delivering a lot of heat to the Earth’s surface, so climates near the equator are hot.
Mapmakers use imaginary lines to mark areas of Earth. Latitude lines go around the planet. The equator marks 0° latitude. The other lines of latitude mark the distance between the equator and the North Pole or the South Pole. ▶
Oceans and Climate
▲ Coastal climates are moderate because oceans change temperature more slowly than land does. In the fall, inland temperatures drop. Warm ocean breezes keep coastal areas warmer for longer. In the spring, cool ocean breezes keep coastal areas from warming up as quickly.
Mountains and Climate
This moist ocean air carries water vapor.
When the air rises, the water vapor forms clouds, then falls as rain or snow.
The now-dry air sinks and gets warmer.
▲ Mountains force air to rise over them. Often, the rising air is associated with cloud formation and rain. Most of the rain falls on the windward side (the one the wind blows on) or on the peaks, so the lee (sheltered) side is often dry – an effect called a “rain shadow.”
The windward (French) side of the Pyrenees is moist.
The leeward (Spanish) side of the Pyrenees is dry.
Altitude has an effect on climate. As air rises, it becomes thinner and gets cooler. For every 1,000 feet you climb, the temperature drops 5.5°F. A mountain with large, leafy trees near its base may have only scrubby plants at the top. It may even have a snow line, above which snow never melts – and little, if any, vegetation can grow.
Jet Streams and Climate
▲ Jet streams are fast-flowing rivers of air in the upper atmosphere that can bring stormy weather. This photo shows a jet stream carrying clouds over Egypt. (The Nile River is near the middle of the picture.) Jet streams are more pronounced in winter than in summer. But they tend to flow in regular patterns and affect the weather in fairly regular ways. Therefore, they help to determine climate.
