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Winds- Now with even better explanations!
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Reminder: Air masses are masses of air with similar temperature and moisture The leading edges of air masses are called fronts Air masses are classified based on where they formed Polar masses =formed near poles, cold Tropical masses =formed near equator, warm Maritime= formed over water, moist Continental= formed over land, dry
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Winds are caused when air moves from high pressure areas (cold) to low pressure areas (warm)
Remember that pressure exists in a gradient, meaning that it gradually changes in pressure Winds are names after the direction from which they blow So, from East to West = Easterly West to East = Westerly
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Coriolis Effect The rotation of the Earth causes an interesting effect
Objects in the Northern Hemisphere are deflected to the right, while objects in the Southern Hemisphere are deflected to the left. The key to the Coriolis effect lies in the Earth’s rotation. The Earth rotates faster at the Equator than it does at the poles. This is because the Earth is wider at the Equator. A point on the Equator has farther to travel in a day.
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In the Northern Hemisphere, wind from high-pressure systems pass low-pressure systems on the right. This causes the system to swirl counter-clockwise. Low-pressure systems usually bring storms. This means that hurricanes and other storms swirl counter-clockwise in the Northern Hemisphere. In the Southern Hemisphere, storms swirl clockwise.
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Subtropical Highs Each of the major oceans contains a semi permanent high pressure. These high pressures are almost always present, and have a significant impact on the wind patterns present in Earth’s atmosphere. Within these highs weather is usually warm, calm, and free of wind. These subtropical highs are the major cause for two types of global wind flow. These are the trade winds and the westerlies
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Wind flows outward down the pressure gradient away from the subtropical highs. As it does so, it encounters the coriolis effect caused by the rotation of the Earth. This force causes the winds in the Northern Hemisphere to move from the east towards the west below the subtropical high, and from the west towards the east above the subtropical high. The opposite is true in the Southern Hemisphere. Above the subtropical high winds move from east to west, and below the subtropical high winds move from west to the east. Note that the subtropical high is located roughly between 30N and 30S
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Thus, in the Northern Hemisphere below the subtropical high, which is located just above the equator, we find winds blowing from east towards the west. At the same time, in the Southern Hemisphere above the subtropical high, which is located just below the equator, we also find that winds are blowing from the east towards the west. The result is that between 30 ° North latitude and 30 ° South latitude winds usually blow from the east towards the west. This wind is referred to as an easterly wind. These easterly winds are known as the trade winds.
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Above the subtropical highs in the Northern Hemisphere, and below the subtropical highs in the Southern Hemisphere, winds blow from the west towards the east. These winds are thus called westerly winds, after the direction from which the winds blow The westerlies generally blow between 30 ° and 60 ° latitude in both the Northern and Southern hemispheres. The higher one travels into the atmosphere, the more noticeable these westerly winds are.
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At the Equator there is little wind
At the Equator there is little wind. This region is often referred to as the doldrums.
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At the core of the westerly winds lies what scientists call a jet stream. Jet streams are super high-speed winds. Each hemisphere’s westerly has two main jet streams. Closer to the poles we find the polar jet stream. At a slightly lower latitude we find the subtropical jet stream.
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Located above each of the poles we find a semi-permanent low pressure
Located above each of the poles we find a semi-permanent low pressure. This low pressure is almost always present, and is the cause for many of the storms that follow the westerlies into the continents of the Earth. The subpolar lows generally cause the winds above 60 ° latitude to move from the east towards the west. We call these winds the polar easterlies.
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In areas where there are mountains and valleys we see a type of wind pattern known as mountain breezes and valley breezes. During the day, the surface of the mountain heats the air high up in the atmosphere, quicker than the valley floor can. As the warmer air expands a low pressure is created near the top of the mountain. This attracts the air from the valley, creating a breeze that blows from the valley floor up towards the top of the mountain. This is a valley breeze.
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In the evening, the mountain slopes cool the surrounding air more quickly than the air found lower in the atmosphere. This creates a high pressure as air becomes more densely packed. The resulting high pressure causes winds to blow down the mountain towards the valley floor. This type of wind pattern is known as a mountain breeze.
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Sea Breeze Sea and land breezes are caused by the different heating and cooling rates of water and land. In the summer, the land is heated by the sun. Air above the land will rise as it is heated. This creates low pressure. Cooler, higher-pressure air from over the water will rush in to replace the rising warm air. This creates a wind from land to water, called a sea breeze.
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Land Breeze At night, the land cools more quickly that the water.
Warm air over the ocean rises and the cooler air from over the land moves to replace it. This creates wind blowing from land to ocean, called a land breeze. Because the warm, rising air is less hot than the land was during the day, the land breeze is less powerful than a sea breeze.
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