The winds at the surface of the Earth vary depending on the location. In the stratosphere, the winds are steady. The air is clear and dry there. The consistency of the airflow allows airplanes to fly on regular courses. Wind speed is measured using an anemometer. The direction is shown by a wind vane that points into the direction that the wind is coming from. This is how winds are named. On the station model, the wind speed is recorded in knots (nautical mi/hr). A knot is about 1.15 mi/hr (1.85 km/hr).
The differences in pressure cause air to move. The larger that this gradient is, the stronger the air flow. The air wants the pressure to be equal, but different factors cause the air pressure to differ. The main cause is unequal heating of the Earth. As air is heated, the pressure decreases.
The unequal heating on the Earth causes air to rise and sink in different regions. If the Earth didn't rotate, the air would rise along the equatorial regions and sink at the poles. Because the Earth rotates, the circulation of air around the Earth is more complex. The Coriolis Force causes the winds to bend to the right in the Northern Hemisphere and to the left in the Southern Hemisphere. The strength of the Coriolis Force depends on latitude. As the latitude increases, so does the force. The amount of land and water in each hemisphere also causes more differences in heating. All of these factors cause the global circulation to be made up of several cells. These are seen in Figure .
Figure 1 Planetary wind and moisture belts in the troposphere.
Along the 0° and 60° latitude lines are areas where the air is rising. These are areas of low pressure, which leads to more rainy conditions. The 30° and 90° latitudes are regions where the air is sinking. This creates areas of higher pressure and drier air. These bands move north and south as the concentration of the Sun's rays migrates over the course of a year. The surface winds along each of the bands just mentioned are light, whereas the areas between these zones have constant winds.
Some of these areas have been named. The area along the equator has the same weather conditions for most of the year and is called the doldrums. Along the 30° latitude line, the winds die off. This caused problems for sailors in the Northern Hemisphere who were bringing horses to the New World across the Atlantic Ocean in the 1700s. These horses were considered dead weight and thrown overboard to lighten the ships, allowing them to sail through to their destinations. Thus, these are known as the Horse Latitudes. Between 0° and 30° are the trade winds. These winds constantly blow from the northeast. Most of the United States falls in the area between the 30° N and 60° N band. The winds here blow from the southwest and are called the prevailing westerlies. These winds help to guide weather systems.
A local effect of uneven heating is the sea breeze. Daytime heating along a beach area warms the land and water at different rates. The land heats up much faster than the water does. The land then heats up the air above it. The air becomes less dense and rises. The cooler air over the water moves in to take its place. The heated air eventually cools and moves to take the place of the air that was over the water. This convection cell that is created causes a sea breeze for anyone who is on the beach. At night, the land cools off faster than the water and the air reverses direction. A land breeze is formed. The land breezes are generally weaker than sea breezes due to the fact that water cools off more slowly than land heats up. Remember that the winds are named for the direction that they came from. The sea breeze and land breeze situations are shown in Figure .
Figure 2 Circulation patterns for sea breezes and land breezes.
For the same reason that sea and land breezes form, larger‐scale winds are created between continents and oceans. In the summertime, the oceans are cooler than the continents. The areas of cooler, sinking air over the oceans cause high pressure to be formed. Areas of lower pressure are formed over the land. Along the east coast of the United States, a Bermuda High forms, creating the hot, humid conditions that are experienced. The reverse happens in the wintertime, when the land is colder than the ocean. In some areas of the world, seasonal winds are created. These monsoon winds cause wet and dry seasons. In India, the winds coming off of the Indian Ocean create extended periods of heavy rain. When the winds shift and come down from the Himalayan Mountains, much drier conditions are experienced. Near the top of the troposphere, about 6 to 12 km (3.5 to 7 mi) up from the surface, is a band of very fast‐moving air. This is the jet stream, and it moves at about 300 to 500 kmph (180 to 300 mph). This river of air helps to move weather systems in the lower parts of the troposphere. The jet stream also affects the courses of airplanes.