Winds and the Pressure Gradient Force

Wind is the movement of air across the Earth’s surface and is produced by differences in air pressure between one place to another. Wind strength can vary from a light breeze to hurricane force and is measured with the Beaufort Wind Scale. Winds are named from the direction from which they originate. For example, a westerly is a wind coming from the west and blowing toward the east. Wind speed is measured with an anemometer and its direction is determined with a wind vane. Since wind is produced by differences in air pressure, it is important to understand that concept when studying wind as well. Air pressure is created by the motion, size, and number of gas molecules present in the air. This varies based on the temperature and density of the air mass. In 1643, Evangelista Torricelli, a student of Galileo developed the mercury barometer to measure air pressure after studying water and pumps in mining operations. Using similar instruments today, scientists are able to measure normal sea level pressure at about 1013.2 millibars (force per square meter of surface area). The Pressure Gradient Force and Other Effects on Wind Within the atmosphere, there are several forces that impact the speed and direction of winds. The most important though is the Earth’s gravitational force. As gravity compresses the Earth’s atmosphere, it creates air pressure- the driving force of wind. Without gravity, there would be no atmosphere or air pressure and thus, no wind. The force actually responsible for causing the movement of air though is the pressure gradient force. Differences in air pressure and the pressure gradient force are caused by the unequal heating of the Earth’s surface when incoming solar radiation concentrates at the equator. Because of the energy surplus at low latitudes for example, the air there is warmer than that at the poles. Warm air is less dense and has a lower barometric pressure than the cold air at high latitudes. These differences in barometric pressure are what create the pressure gradient force and wind as air constantly moves between areas of high and low pressure. To show wind speeds, the pressure gradient is plotted onto weather maps using isobars mapped between areas of high and low pressure. Bars spaced far apart represent a gradual pressure gradient and light winds. Those closer together show a steep pressure gradient and strong winds. Finally, the Coriolis force and friction both significantly affect wind across the globe. The Coriolis force makes wind deflect from its straight path between high and low-pressure areas and the friction force slows wind down as it travels over the Earth’s surface. Upper Level Winds Within the atmosphere, there are different levels of air circulation. However, those in the middle and upper troposphere are an important part of the entire atmospheres air circulation. To map these circulation patterns upper air pressure maps use 500 millibars (mb) as a reference point. This means that the height above sea level is only plotted in areas with an air pressure level of 500 mb. For example, over an ocean 500 mb could be 18,000 feet into the atmosphere but over land, it could be 19,000 feet. By contrast, surface weather maps plot pressure differences based at a fixed elevation, usually sea level. The 500 mb level is important for winds because by analyzing upper-level winds, meteorologists can learn more about weather conditions at the Earth’s surface. Frequently, these upper-level winds generate the weather and wind patterns at the surface. Two upper-level wind patterns that are important to meteorologists are Rossby waves and the jet stream. Rossby waves are significant because they bring cold air south and warm air north, creating a difference in air pressure and wind. These waves develop along the jet stream. Local and Regional Winds In addition to low and upper-level global wind patterns, there are various types of local winds around the world. Land-sea breezes that occur on most coastlines are one example. These winds are caused by the temperature and density differences of air over land versus water but are confined to coastal locations. Mountain-valley breezes are another localized wind pattern. These winds are caused when mountain air cools quickly at night and flows down into valleys. In addition, valley air gains heat quickly during the day and it rises upslope creating afternoon breezes. Some other examples of local winds include Southern California’s warm and dry Santa Ana Winds, the cold and dry mistral wind of France’s Rhà ´ne Valley, the very cold, usually dry bora wind on the eastern coast of the Adriatic Sea, and the Chinook winds in North America. Winds can also occur on a large regional scale. One example of this type of wind would be katabatic winds. These are winds caused by gravity and are sometimes called drainage winds because they drain down a valley or slope when dense, cold air at high elevations flows downhill by gravity. These winds are usually stronger than mountain-valley breezes and occur over larger areas such as a plateau or highland. Examples of katabatic winds are those that blow off of Antarctica and Greenland’s vast ice sheets. The seasonally shifting monsoonal winds found over Southeast Asia, Indonesia, India, northern Australia, and equatorial Africa are another example of regional winds because they are confined to the larger region of the tropics as opposed to just India for example. Whether winds are local, regional, or global, they are an important component to atmospheric circulation and play an important role in human life on Earth as their flow across vast areas is capable of moving weather, pollutants, and other airborne items worldwide.