Wind Flow between Buildings
As wind flows between buildings the mass of the gas is compressed with subsequent increase in velocity to wind speeds that may be several times the speed of the wind on lee side of the buildings. In addition to creating increased wind velocity turbulence is also likely to occur on the leeward side of the building. It may require a distance of 7-10 times, (depending on the wind speed and amount of compression), the width of the compressing building before the effect of compression on air flow is minimized.

Wind Flow over Tall Buildings
When wind hits a tall building it will be deflected upward as well as around the building.  This can create turbulence and backflow of wind near the windward surface and vertical deflection of the wind up the windward side of the building. The upward motion of the wind can create a substantial decrease in horizontal wind speed at the top of the building, a good reason to be suspicious of any wind instrument located on a building top.  The wind immediately to the leeward side of the building will be turbulent and significantly reduced in speed until it is measured at a distance that is 7-30 times the height of the building on the leeward side.

Point to Ponder: When making wind measurements on or near buildings for rooftop Helicopter landing pads, where should you position the measurement instruments? Should you place a wind instrument on a rooftop edge or at its center?

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?

Katabatic and Anabatic Winds:
Local Surface winds are sometimes more a function of Temperature Differences between mountain tops and lower elevations than overriding Synoptic winds.  These winds are sometimes called Mountain Winds as they occur most frequently in mountainous areas, meteorologist call them Katabatic or Anabatic Winds

Anabatic Winds are upslope winds driven by warmer surface temperatures on a mountain slope than the surrounding air column.  Katabatic winds are downslope winds created when the mountain surface is colder than the surrounding air and creates a down slope wind.   Katabatic wind may range over fairly large areas as in the case of the Santa Anna winds experienced throughout southern California during certain times of the year.  They can produce winds to 80 miles per hour and dominate local weather patterns for extended periods of time (weeks).  As shown in figure 2.4 below, they are initiated when cold air atop higher land masses begins to flow down hill (remember cold air is heavier than warm air) displacing the warm air below it and warming adiabatically and often gaining speed in the process.   When the lower elevations are hot desert areas the temperature differences can be quite substantial on the order of 60 to 70 degrees F.  The greater the temperature difference the stronger the wind.  They are often so well-known that they are given names like California’ Santa Anna as mentioned above, the Chinook of the pacific northwest or the  Fohn in Switzerland.

Figure 2.4 Katabatic Wind

As you can see wind can be derived from a number of different meteorological phenomena that are either caused by large scale synoptic pressure and temperature differences or by local temperature and pressure differences.  Once generated, however, there are many small scale surface structures that can modify the wind direction and speed and distort the accuracy of the observing instrumentation.  We call these wind modifiers and will talk about them in future blogs.

Point to Ponder: Why do hot air balloonists like sea/land breezes and katabatic wind flow?