Understanding Wind

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Wind is air in motion relative to the earth's surface. This motion is caused by differences between various forces in the atmosphere. These forces are the Pressure Gradient Force, Coriolis force, friction, and centrifugal force. Wind velocity is a vector quantity. It has both magnitude and a direction. The magnitude of the of the wind velocity is called the Wind Speed and the direction the Wind Direction. Surface wind velocity is subject to rapid fluctuation and can be effected by surface features.

Pressure Gradient Force

The difference in MSL pressure between two locations dictates that the wind will flow from the high to the low. The wind speed is determined by the spacing of the isobars (lines of equal MSL pressure) on the weather chart. The amount of force that is caused by the pressure gradient on a parcel of air is known as the Pressure Gradient Force. A greater force equates to a greater wind speed and vice-versa. The direction part of the velocity imparted in a parcel of air by the force is directly across the isobars from the high to the low.

Coriolis Force

We are all used to seeing water spiral down a drain when we empty a sink. This spiral motion is caused by the Coriolis Force and is imparted into a parcel of air in the same way that it is imparted into a column of water. This force arises from the rotation of the earth and is imparted into any particle in motion on the earth's surface or in the atmosphere. As the earth has different rotation speeds at different latitudes, any particle moving to the north or south will carry in it's velocity a component from where it started. This means that as it moves to a different latitude where the rotation speed is different; in the northern hemisphere it will curve to the right and in the southern hemisphere it will curve to the left. This imparts a force with a direction parallel to the isobars and a speed that varies with latitude.

Friction

Friction is a force imparted into a parcel of air as it moves over the ground or another layer of air. Rough terrain tends to slow down the surface wind more than flat terrain such as smooth ice or water. As one moves up into the atmosphere away from the effects of friction, the wind speeds tend to increase and veer (turn clockwise). As friction slows down a parcel of air it tends to make the parcel of air turn in towards the low pressure area.

Centrifugal Force

As isobars are at times not straight we have to consider the effect of the wind 'going around a corner'. Like the old game of crack-the-whip or a ball at the end of a string the wind curving creates a force that moves it out away from the low pressure area. The strength of this force is proportional to the wind speed and inversely proportional to the curvature of the isobars.

Putting it all together

All the above mentioned forces influence the speed and direction of the real wind observed in the atmosphere. The stronger the Pressure Gradient force the stronger will be the wind speed. If only the Pressure Gradient force existed the wind would flow directly from the area of high pressure to the area of low pressure.

The Coriolis force causes the wind direction to deflect to the right in the Northern hemisphere. Also for a given pressure gradient the wind speed is less in polar regions than nearer the equator as Coriolis force varies with latitude.

The curvature of isobars produces a centrifugal force which in turn causes a decrease in wind speed around a low pressure system and an increase in wind speed around a high pressure system for a given pressure gradient. Generally the pressure gradient around a low is stronger than around a high. It is thus difficult to see the effect of curvature on wind speed on day to day weather maps.

The friction force causes air motion to be slowed down and deflected in the boundary layer or in the bottom kilometer of the atmosphere. Locations near hills and valleys or close to the sea often experience marked variations in the direction and speed of the surface wind. This effect undergoes diurnal (daily) variations.

1 The wind, acting only under the Pressure Gradient force will have the wind blowing directly from an area of high pressure to an area of low pressure	2 The Coriolis Force will turn the wind to the right in the northern hemisphere trying to get the wind to flow parallel to the isobars
3 At the surface friction slows the wind down which has the tendency to turn the wind back towards the low	4 Adding up these forces leaves the resulting wind flowing around the low in a counter-clockwise rotation turned slightly into the low.
Winds above the surface Due to the reduction in friction the wind above the surface veers (turns clockwise) and increases in speed

Wind and Constant Pressure Charts

A constant pressure level chart looks like a surface chart. It has it's highs and lows, ridges, troughs and other features. The difference is that a surface chart shows the pressure at a given location on the surface of the earth while a constant pressure chart shows the height above mean sea level of the pressure level that the chart is drawn for. In Canada you'll be most familiar with the 850mb, 700mb, 500mb and 250mb charts that roughly equate to 1000m, 3000m, 5000m and 10000m.

Just as a lower level wind follows the topography of the lane, an upper level wind follows the surface of a particular pressure level. The isopleths of height (contour lines) are usually analyzed every 60m. As with a pressure gradient at the surface, a steeper gradient in the upper pressure levels means a stronger upper wind. Although the Pressure Gradient force remains the driving force in producing upper wind, it can be substituted by the height gradient to infer upper wind. The closer the contour lines, the steeper the gradient and therefore the stronger the winds. At these levels surface friction plays little in determining wind. Coriolis force on the other hand tends to turn the wind along the contour lines. Determine wind speed from the spacing of the contour lines and the direction will be parallel to the lines due to Coriolis.

Understanding Precipitation
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