1. Wind blowing below height of 3000 feet or below 1 Km are called surface winds. Generally, wind is reported with respect to True North on Charts and graphs, but surface wind is reported with respect to Magnetic North and not true north. Surface wind is measured by an Instrument called Anemometer which measures wind speed and direction. This instrument is placed at a height of 10 mtr/33 feet above ground level and in an obstruction free area. At airport, It is mandatory to have at least one wind sock which is easily visible from ground and from air for visual assessment of surface wind direction and speed by pilots. Plotting of wind speed vs time would be called Anemograph. Wind direction is always reported as the direction from which wind is coming from..and not where it is going to…Wind is said to be Veering when its direction changes clockwise & Backing when wind its direction changes anti clockwise. Gust is local increase in wind speed by more than 10 Kts lasting for more than 1 min.
2. Geostrophic wind blows above friction layer, that is above 3000 feet AGL straight and parallel to Isobars under influence of PGF and CF and Gradient wind blows above friction layer parallel to curved isobars under influence of PGF, CF and centrifugal force. Mathematical modelling adopted for calculation and prediction of Geostrophic wind will not work for the surface wind since surface wind at a given place would be altered/modified/affected by presence of terrain, obstructions, buildings, relief features, water bodies etcetera. Wind blowing at ground level up to the friction layer which is 3000 feet is called surface winds. Surface wind would be weaker as compared to Geostrophic wind at higher altitude since friction with terrain features, reliefs and buildings will reduce the speed of wind.
3. Reduction in wind speed will also reduce Coriolis force since Coriolis force is directly proportional to wind speed. This will cause PGF and CF to be out of balance. Net effect of this reduction in CF is that CF will not be able to turn the wind blowing from high pressure area to low pressure area by 90 deg and will not be able to make it blow exactly parallel to isobars. It will blow across isobars at some angle, some resemblance would be maintained for the basic understanding that wind should blow from high to low pressure area.
4. Let us understand it with the help of an example. You need to keep three basic facts in mind. First is that Wind start to move from High Pressure area to low pressure area and the differential force between pressure values between two places is called PGF. Closer the spacing between isobars, higher the PGF and stronger will be the winds. It is the Coriolis force which makes the wind turn through 90 deg to the right in northern hemisphere and finally make it blow straight and parallel to isobars. So reduction in CF will result in smaller degree of turning of winds and thereby allow winds to blow from high pressure to low pressure area at some angle. Increase in CF will tend to make the wind blow along isobars and reduction in CF will tend to make the wind blow across isobar from High to Low Pressure area.
5. Surface friction has reduced the wind speed for surface winds. Reduction in speed causes reduction in CF. and reduction in CF results in backing of winds in northern hemisphere. In Northern hemisphere, Surface winds over land is backed by 30 deg and speed is reduced to 50%. Over sea, the friction is less, so speed is reduced to 70% of original value and backing is only by 10 deg as compared to Geostrophic wind. Opposite effects occur in southern hemisphere. Backing will change to veering and rest of the figures will remain same. Remember these figures for the purpose of examination.
6. Daily Variation of surface Winds: During day time, earth gets heated up, in turn it heats up the air in contact and air rises upwards and meet the faster moving Geostrophic wind above friction layer. By coming in contact with faster wind, surface wind also gets accelerated. So surface wind will speed up during daytime when temperature rises. Max strength of surface wind is at 1500 Hrs local time when temp is max. Now with Increase in wind speed, CF will increase which will make the wind turn more to get it parallel to isobar; so surface will veer in Northern Hemisphere during day as the day progresses towards 1500 Hrs.
7. In night, there is cooling of earth and subsidence so no mixing of surface wind with Geostrophic wind above friction layer, so surface wind will slow down as compared to daytime. Min speed occurs at dawn (half an hour after sunrise) when temp is Min. And reduction in wind speed means reduction in CF and so wind tend to blow across isobars, and so it will back. Max strength of surface wind is at 1500 Hrs local time when temp is max and Min speed during dawn (half an hour after sunrise) when temp is Min.
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