Planet Earth/4h. What Makes Wind?

High pressure → Low pressureEdit

Wind is fundamentally the flow of air from high pressure to low pressure across the surface of the Earth. Weather reports often discuss barometric readings, indicating regions of the Earth experiencing high or low pressures at any point in time. Air rushes toward low pressures, so regions experiencing low pressure readings on barometers are going to be windier than regions experiencing high atmospheric pressures. These pressure gradients result in much of the weather experienced on the surface of the Earth.

A typical weather map showing low pressure (L) and high pressure (H) regions.

Weather reports will often highlight high pressure and low-pressure regions on a weather map to indicate the barometric recordings taken from various weather stations. Lines of equal atmosphere pressure are draw on weather maps to indicate isobars. Isobars are lines of equal atmospheric pressure where there is no change in pressure readings. Theoretically, air masses will typically flow from high pressure to low pressure regions perpendicular to isobars, but are also influenced by the rotation of the Earth’s orbit; Coriolis force. Closely spaced isobars indicate a very high-pressure gradient, and will produce the strongest winds, while widely spaced isobars will produce weaker winds, due to a gentler pressure gradient. Meteorologists map barometric readings from a wide network of weather stations to generate maps of isobars to identify developing strong wind storms.

Isobars showing the changing atmospheric pressures. Each line represents equal atmospheric pressure as recorded on the surface of Earth..

Geostrophic windEdit

A geostrophic wind is the theoretical wind resulting from an exact balance between the pressure gradient force and the Coriolis force, called a geostrophic balance. The geostrophic wind is initially directed perpendicular to isobar lines, but with motion also dependent on the Coriolis force, the wind turns resulting in geostrophic wind following directions that are parallel with isobars as they flow toward low air pressure zones. Geostrophic winds occur higher in the troposphere, where friction and changing elevations due to topography and terrain do not interfere with the motion of air in the atmosphere.

A weather vane in Massachusetts.

There are different ways to measure wind direction and wind speed. Weather vanes often mounted to roofs pivot on a rod to indicate prevailing wind direction. Wind direction is expressed in terms of the direction it originates from, so westerly wind, is wind that blows west to east, while northerly wind is wind that blows north to south. Wind socks are also used to indicate wind direction and relative wind speed, since they are more visible from a long distance, they are used at airports and helipads. Weather stations often have an anemometer, which are rotating propellers or cups that can determine wind speed more acurately.

An anemometer that measures wind speed.

Wind Speeds, and the Beaufort ScaleEdit

On Earth wind speeds average about 10 miles per hour (16 km/hr), when they are often described as either calm or a gentle breeze. Record high wind speeds on Earth have exceeded 200 miles per hour (322 km/hr), with strong wind storms ranging from 31 miles per hour (50 km/hr) to 73 miles per hour (117 km/hr). Higher wind speeds between 75 miles per hour to values over 200 miles per hour are observed during powerful hurricanes and tornadoes, which cause considerable damage. The Beaufort scale is a scale of wind speeds used by sailors that range from 0 to 17, with values 12 and above indicative of high wind speeds observed in cyclones, and other storms. The scale is 0-Calm, 1-Light air, 2-Light breeze, 3-Gentle breeze, 4-Moderate breeze, 5- Fresh breeze, 6-Strong breeze, 7-Moderate gale, 8-Fresh gale, 9-Strong gale, 10-Whole gale, and 12-Storm, with 12 through 17 indicating cyclone strength winds.

Cyclone and AntiCyclone wind directions in low and high pressure weather systems.

Upward winds are a result of heated air masses rising, causing low pressure directly below the rising air. Air moving into this low-pressure space below rising air, results in observed high winds. Likewise, downward winds are a result of cooling air masses sinking, causing high pressure directly below the sinking air. These motions result in convergent and divergent flows, or anticyclones or cyclones. Winds will form a spiral motion due to the Coriolis effect. In the Northern Hemisphere inflowing low-pressure systems (cyclones) will spiral counterclockwise, while outflowing high pressure systems (anticyclones) will spiral clockwise. This is similar to the flow of liquid water in drain experiments conducted in large kiddie pools.

Cyclones (Hurricanes) in the Gulf of Mexico and Atlantic Ocean.

Both cyclones and anticyclones can be either large or small, but with low-pressure cyclones wind converges and form large storms that produce significant accumulations of rain and snow. This precipitation results because air rushes upward through the spiraling cyclone cools as it expands crossing the saturation pressure of water vapor, resulting in rain or snow falling. In anticyclones, air will diverge from the center of the high-pressure system which dries the surface of the Earth because the air is undersaturated in respect to water vapor. High pressure anticyclones do not result in rain or snow, but often represent clear and dry weather, and are associated with heat waves during the hot summer.

As low-pressure systems, cyclones, because of their significant accumulations of rain and snows as well as high wind speeds, frequently cause damage. Supercells are cumulonimbus clouds that cause wind to rise within a thunder cloud formation, producing a mesocyclone (a middle-sized cyclone). Frequently these storms will result in tornadoes, whirlwinds, and dust devils, as they move across the landscape. Larger cyclones are given local names depending on where they form. Hurricane is a term used for large cyclones in the North Atlantic Ocean (and sometimes in the Northeast Pacific), while Typhoon is used for large cyclones in the Northwest Pacific Ocean, along the coasts of Asia and Japan. In the Indian Ocean these storms are referred to as simply tropical cyclones.

The Effect of TopographyEdit

The Rain Shadow Effect.

Topography, such as mountains, valleys and canyons can result in unique interactions between winds and the terrain, resulting in friction and changes in flow. One of the most important effects happens during orographic lifting of air masses over large mountain ranges. This particularly affects the climates, as mountains can restrict the movement of large air masses. Rain shadows result when moist air over ocean basins rises over mountains top. The adiabatic lapse rate results in the air expanding and cooling, with cloud formation and rain as the saturation pressure of water vapor is reached. These slopes of the mountains facing ocean waters can become very wet from the frequent rainstorms and cloud coverage. The Pacific Northwest is particularly lush because of the rain shadow effect, however these air masses as they pass over the mountain range will sink, due to adiabatic lapse rates the air will condense and heat, resulting in undersaturated air in respect to water vapor on the far side. These dry winds result in sides of the mountains that are significantly drier than mountains facing ocean basins. These dry winds are called Chinook “Snow eater” winds in North America or Föhn winds in Europe, which are undersaturated and warmed by their decent from neighboring mountains. These winds melt and evaporate snow from the drier side of mountain slopes.

Wasatch winds occur in Utah, caused by dry westerly winds blowing from the anticyclones or high-pressure zones that develop in the dry Great Basin Desert into narrow canyons along the Wasatch Mountain front. These are more globally referred to as a jet-effect winds, and they can reach high speeds in mountain canyons.