Differences in atmospheric pressure are what cause the wind to blow. Pressure differences—also called pressure gradients—can happen at many scales. They can be global or local, but no matter what size, they develop because of differences in heating and cooling of the Earth's surface. It is the heating/cooling cycles that happen on a daily basis that cause many of the local wind systems.
Air flows from high pressure to low pressure. This pressure gradient is set up when an area of land receives more sunshine than another area and begins to heat up faster. As evidenced by a hot air balloon, warm air rises. The Earth's surface warms the air directly above it through the process of conduction and convection. This forms an area of high pressure, which will then naturally flow to an area of low pressure, because a pressure gradient has been set up. An upper air low pressure may exist because of the presence of clouds, keeping the ground beneath it cooler than the section of ground beneath the high pressure.
As the upper air high pressure air flows to the upper air low pressure, this air then sinks to the Earth's surface, which creates a high pressure on the ground. Along the ground, this high pressure air flows to the low pressure area under the upper air high, where it is lifted, creating a circulation cell as the air travels in a circular pattern from one pressure gradient to another as shown in the illustration on page 96.
The presence of land also makes a difference in weather patterns and seasons. Seasons are much more greatly defined over land than they are over the oceans. Because of this, there is a significant difference between the seasonality of climate in the Earth's Northern Hemisphere than in the Southern Hemisphere. In the Northern Hemisphere, where most of the Earth's landmasses are located today, weather differences are much more dramatic. This is because land heats up and cools down more rapidly than water, making the resulting temperature ranges much greater. Thus, the impacts of global warming will be felt more strongly in the Northern Hemisphere.
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There are various atmospheric circulation patterns that affect local and regional wind systems, such as (a) high and low pressure systems as a result of heating the Earth's surface; (b) development of a sea breeze due to the more rapid heating of land during the day; (c) development of a land breeze due to the more rapid cooling of land during the night;
(d) development of a valley breeze during the day; (e) development of a mountain breeze at night; (f) typical monsoon patterns of winter and summer creating a pronounced dry and wet season; and (g) orographic precipitation, creating a windward and leeward side of the mountain with distinct vegetation patterns. (modeled from USGS)
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The jet stream refers to a channel of fast-flowing westerly air that resides high in the atmosphere—at a height of 20,000 feet (6,096 m). Usually existing in the stratosphere, it forms at the boundaries of warm and cold air masses. There are two principal jet streams—one in the polar region in each hemisphere between 30° and 70°N. These wind channels extend over long distances around the Earth. Sometimes they can split. They play a critical role in determining the weather because they steer storms and control the positions of high and low pressure regions. These winds can travel 50 knots (58 mph) or faster and exert a significant control over the weather in North America. In the winter, when the temperatures stay mild, the jet stream remains at the high (polar) latitudes. Conversely, when it becomes bitter cold, even down into the southern reaches of the United States, the jet stream has dipped far south into states such as Utah, Colorado, and New Mexico. In some cases, variations in the jet stream bring on greater than average rainfall, which causes flooding. Climatologists also believe that jet streams play a role in the creation of storms that spawn tornadoes. If global warming affects the temperature gradients on the Earth's surface and the atmosphere, it could cause changes in the jet streams that could change the weather in local or regional areas.
Midlatitude cyclones are the cause of most of the storms in the midlatitudes—such as the United States. These are the storm systems that bring snowy blizzards, flooding rains, lightning storms, and other severe types of weather to the latitudes between 30° to 60°N. They are huge moving systems of low pressure caused by the interaction of warm tropical and cold polar air.
These cyclones (rotating storm systems) generally move toward the east. It is these weather systems that are most commonly carried by the jet stream. Therefore, by understanding the movement of the jet stream, climatologists will be able to predict the weather for an area.
Midlatitude cyclones can generate a wide variety of storms, such as hail, sleet, freezing rain, and rain, snow, and ice pellets. Precipitation is most common at the center of the low pressure and along the fronts where the air is being quickly uplifted. A slower warm front of air usually leads, followed by a faster-moving cold front. When the cold front catches up with the warm front (both pivoting around the low pressure) the warm air mixes, a process called occlusion. The warm, uplifted air condenses and forms cumulus, then cumulonimbus clouds, which can develop into severe thunderstorms.
The occurrence of severe weather is directly related to global warming. Scientists believe continued warming will cause increases in severe weather events. Although these storms do not typically cause as much damage as tropical cyclones (hurricanes) they can still cause severe damage from wind destruction and flooding. Global warming can alter weather patterns, change surface climate, change atmospheric circulation, and change extreme events. According to the IPCC in their 2007 global warming assessment report, cyclone activity over both hemispheres has changed since 1950. General features include a poleward shift in storm track location, increased storm intensity, and a decrease in total storm numbers. Some scientists have reported that the North Atlantic storm track has shifted 112 miles (180 km) northward in the winter, which could be related to reduced midlatitude winter precipitation. If this is because of global warming, it could permanently affect ecosystems. The IPCC has also documented a gradual reduction of the number of days with frost and an increase in the number of warm nights in the midlatitudes in recent decades.
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