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.
Winds 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.