Wind is the movement of air. To move the air a force is required. The initiating factor that develops wind is due to horizontal temperatures differences in the troposphere. Air masses (and air with different temperature and moisture properties on smaller scales) will have different densities and these density differences cause pressure differences between the regions the air masses or smaller scale air regions meet. The force responsible for moving the air is the Pressure Gradient Force. There are other factors that influence the wind speed and direction (Coriolis, friction, centrifugal, etc.) but the Pressure Gradient Force starts it off. Since temperature differences occur on all scales (global, synoptic, mesoscale, and smaller), wind patterns are also on all of these scales. Below is a wind glossary that covers many of the various winds and terminology important to wind in meteorology. Advection- The horizontal motion of air (parallel to sea level) Anemometer- An instrument used to measure wind speed Backing wind- A wind that shifts in a counterclockwise direction with height. For example, an east wind at 850 mb and a north wind at 700 mb would be a backing wind. Also can be a wind shift at the surface or a particular pressure level in which over time the wind shifts in a counterclockwise direction at a point location. Bora Wind- A cold and dry northeasterly wind that blows down from the mountains along the eastern shores of the Adriatic Sea. Calm- A wind speed of less than 3 knots. Chinook Wind- It is termed a snow eater wind since the warmth and lower humidity rapidly melt and evaporate snow. A Chinook is produced by a downsloping wind. As air downslopes it warms adiabatically and decreases in relative humidity. The Chinook is common in the northern plains of the United States in winter. Convection- A vertical motion of air. The transfer of energy by mixing a fluid. Coriolis- Coriolis is the deflection a parcel of air takes relative to the Earth's surface due to Earth's rotation. Country Breeze- The city tends to be warmer than the surrounding countryside. During period of clear weather with weak synoptic wind a breeze will blow from the countryside toward the city. The city air is warmer and thus rises and is replaced by countryside air. Cyclonic flow- Rotating in the same sense as low pressure. A cyclonic flow is counterclockwise in the Northern Hemisphere and clockwise in the Southern Hemisphere. Derecho- A derecho is a long line of convective wind gusts originating from evaporationally cooled high momentum air aloft that accelerates to the surface due to negative buoyancy. Downsloping Wind- Air that sinks with the terrain as it moves toward lower elevations. Downsloping air warms adiabatically and decreases in relative humidity. Eddy- A rotation or spin-off that is embedded within a fluid. Convection will have eddies at the edges of where air with different temperature and moisture properties is mixing. Foehn Wind- A warm and dry wind that warms by adiabatic compression as it moves toward a lower elevation. The process is the same as that that produces a Chinook wind. Gale Wind- A wind speed ranging from 39 to 54 mph. Geostrophic Wind- A wind that is balanced by the Pressure Gradient Force and Coriolis. To remain in geostrophic balance the wind needs to occur in the middle or high latitudes (since Coriolis is strong enough there) and needs to flow at a constant speed and direction (to prevent ageostrophic accelerations and centrifugal accelerations). Gust Front- A boundary separating downdraft air from a thunderstorm and the surrounding environmental air. Air tends to be cool and gusty behind the gust front. Haboob- A severe sandstorm or dust storm with violent winds, occurring chiefly in Arabia, North Africa, and India during period of dry weather. Isotach- A line of equal wind speed. Jet Stream- The Jet Stream is a global upper tropospheric wind belt that separates mid-latitude air from polar air. Katabatic Wind- Air that flows under the influence of gravity from higher toward lower elevations. A air is initially very cold thus it has a high density and is thus negatively buoyant. Land Breeze- A wind caused by the differential heating between land and water. During the night the land cools more than the water. This promotes higher pressure on the land surface and lower pressure on the water surface. Since air flows from higher toward lower pressure the wind flow is toward the ocean. Meridional flow- Meridional flow is a wind flow through highly amplified troughs and ridges. Mistral- Extremely cold air in high elevations that blows into the western Mediterranean basin from the French Alps. Even though the air warms by adiabatic compression, it starts out so cold that it still ends up being a cold wind. Mountain Breeze- A wind that blows from the mountains at night due to differential cooling. The mountains slopes cool more at night than the valley. The cooling air becomes more dense and then flows toward the valley. Prevailing Wind- The dominate wind direction at a location for a particular season. Santa Ana- A warm and dry wind in southern California. Low pressure to the south of the area and high pressure to the north of the area set up a dry and adiabatically compressed wind that comes out of the higher elevations. Sea Breeze- A sea breeze is a wind caused by the differential heating between land and water. During the day the land heats more than the water. This promotes lower pressure on the land surface and higher pressure on the water surface. Since air flows from higher toward lower pressure the wind flow is from the ocean. Trade Wind- An easterly band of wind in the tropics. The easterlies are equatorward from the subtropical highs. Upsloping Wind- Air that rises with the terrain as it moves toward higher elevations. Upsloping air cools adiabatically and increases in relative humidity. Valley Breeze- A wind that blows from the valley toward the higher elevations during the day. The mountain slopes warm more than the valley during the day due to more direct exposure to the sun. This produces lower pressure along the mountain slopes and thus the flow is from the higher pressure air in the valley toward the lower pressure air on the mountain slope. Veering wind- A wind that shifts in a clockwise direction with height. For example, a south wind at 850 mb and a west wind at 700 mb would be a veering wind. Also can be a wind shift at the surface or a particular pressure level in which over time the wind shifts in a clockwise direction at a point location. Wind- The movement of air. Wind when measured is usually measured as the horizontal component of the wind speed (parallel to Earth's surface). Wind Vane- A device used to measure wind direction. Zonal flow- A zonal flow is a large scale wind flow that is fairly parallel to the lines of latitude. |