Surface Wind Modifiers
In previous blogs we have described how the surface winds we measure with wind instruments are created by large area pressure differences, synoptic winds, and by local temperature and pressure differences that create phenomena like sea breezes, thunderstorm winds and katabatic winds, local winds. These winds are often modified by surface irregularities and obstacles that can significantly impact surface wind speed and direction.
As wind flows over irregular surfaces from forests, to buildings, to hills, and mountains both its speed and direction can be change by these surface wind modifiers. Sailors know that wind flowing over a forest before reaching a body of water will reduce wind by up to ½ the wind speed in open water well away from the forested shoreline. Mountain climbers know that wind speeds through mountain passes can often be much higher than surrounding wind speeds. Pilots know that the wind speed measured at the top of a hangar may be significantly different than runway level wind as it is distorted by the uplifting effect of the building on the horizontal component of the wind. Anyone that has walked the streets of Chicago when the wind blows off Lake Michigan knows that wind flowing between two buildings is squeezed into a smaller area with a consequent increase in velocity. To properly site wind instruments each of these surface wind modifiers must be carefully considered.
Point to Ponder: What wind measurements are of importance at a rooftop helicopter landing pad, are these measurements useful to a weather forecaster?