Science 1.3: Global Winds and Local Winds

The Doldrums

An area around the equator where the trade winds of both hemispheres meet. There are very little winds there because the rising, warm air creates an area of low pressure. "Doldrums" means dull or sluggish.

The horse latitudes

Areas at about 30 degrees north and 30 degrees south where sinking air creates an area of high pressure. The winds there are very weak. Most of the world's deserts are located in them because the sinking air is very dry.

Sea and land breezes

During the day, air over the ocean is cooler and produces an area of high pressure. That cool air flows to the land, producing a sea breeze. During the day, air over the land is warmer. As the warm air rises, it creates an area of low pressure. At night, air over the ocean is warmer. As it rises, it forms an area of low pressure. At night, air over land is cooler and forms an area of high pressure. It moves toward the ocean, producing a land breeze.

Local winds

Local winds usually move short distances and can blow from any direction. They can be caused by local geographic features. During the day, the land heats up faster than the water, so the air above the land becomes warmer than the air above the ocean. The warm land air rises, and the cold ocean air flows in to replace it. At night, the land cools faster than water, so the wind blows toward the ocean.

Mountain breezes and valley breezes

Mountain breezes and valley breezes are other examples of local winds caused by an area's geography. Those in mountainous areas may feel a warm afternoon quickly change into a cold night soon after the sun sets. During the day, the sun warms the air by the mountain slopes. That warm air rises up the mountain slopes, creating a valley breeze. At nightfall, the air along the mountain slopes cools. That cool air moves down the slopes into the valley, producing a mountain breeze.

Jet streams: atmospheric conveyor belts

Narrow belts of high-speed winds that blow in the upper troposphere and lower stratosphere. They can reach maximum speeds of an astounding 400 kilometers per hour. They don't follow regular paths around the earth. They affect the movement of storms, so meteorologists can track a storm if they know the location of a jet stream.

Trade winds

The winds that blow from 30 degrees latitude to the equator in both hemispheres. The Coriolis Effect causes them to blow east in the Northern Hemisphere and west in the Southern Hemisphere. Early traders used them to sail from Europe to the Americas. That is how they got their name.

The Coriolis Effect

The apparent curving of the path of a moving object from an otherwise straight path due to Earth's rotation. In the Northern Hemisphere, winds that travel north curve east. In the Southern Hemisphere, the winds curve west.

Global winds

The combination of convection cells found at every 30 degrees of latitude and the Coriolis effect.

Air rises at the equator and sinks at the poles

The uneven heating of the Earth causes differences in air pressure. At the equator, warm air, which is less dense, rises, causing a low pressure. This is because the equator receives more direct solar energy than any other latitudes. Cold air, which is more dense, sinks, causing a high pressure. The polar air then flows down toward the equator.

Pressure belts are found every 30 degrees

They separate convection cells, which are large, circular patterns of air. Pressure belts are bands of high pressure and low pressure. As warm air rises over the equator and moves toward the poles, it begins to cool. At about 30 degrees north and 30 degrees south latitude, some of the cool air begins to sink. Cool, sinking air causes high pressure belts near 30 degrees latitude in both hemispheres. The cool air flows back to the equator where it warms and rises again. At the poles, cold air sinks and moves toward the equator. Air warms as it moves away from the po0les. Around 60 degrees latitude in both hemispheres, the warmer air rises, which creates a low pressure belt. This air flows back to the poles.

Westerlies

Wind belts found between 30 degrees and 60 degrees latitude in both hemispheres that travel from west to east. They carry moist air that can result in rain and snow.

Polar easterlies

Wind belts that go from the poles to 60 degrees latitude in both hemispheres that travel from east to west. They carry cold air that produces snow and freezing weather.

Why air moves

Wind causes air to move because of the differences in air pressure.