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The Health of Southern Forests, USDA Forest Service, Forest Health Protection, Southern Region


Other Stresssors


Weather

Weather has been associated with forest processes for many years. For example, the Europeans began to record their thoughts on how weather and the forests interacted by the early 1200's. Weather is occasionally directly responsible for devastation of forests through flood, high wind, lightning, or severe drought. It is a major driving force behind wildfire, insect, and disease outbreaks, as well as seed dispersion and tree growth.

Tree growth is the result of complex interactions among many physical, chemical, and biological processes. Weather is the major uncontrollable or unknown factor influencing growth. Unfortunately, there is no definitive model that uses weather to predict tree growth. There is, however, general agreement that precipitation, relative humidity, and temperature are the major driving weather variables, with wind perhaps a major indirect influence.

The weather data contained in the GIS used by Forest Health were purchased from the National Climactic Data Center in Asheville, NC. Data were decoded, edited and summarized by James T. Paul, forest meteorologist (retired), and James K. Forbus, systems analyst, both of the USDA Forest Service, Macon, Georgia. The process required several years to complete. These data are unique because considerable effort was expended to detect and correct errors and because the monthly averages and deviations from a long-term average (1951 through 1990) are available in gridded format.

Climactic variables included in the GIS are: precipitation, maximum and minimum daily temperatures, relative humidity, wind speed, and freezing precipitation. Data from 98 weather stations in the South were used to summarize relative humidity and wind speed. Precipitation records from 994 stations were summarized, as were maximum and minimum temperatures from 537 stations. After error processing, monthly means and deviations from the monthly mean were computed by year. These data were interpolated to a 0.5 degree grid, using the seven nearest stations.

The plotted data in the following examples are typical of weather data summaries available. They include March precipitation for Macon, Georgia (Figure 25); March maximum temperatures for Lexington, Kentucky (Figure 26); March relative humidity for San Antonio, Texas (Figure 27); and freezing precipitation for all three locations, based on the 1951 - 1990 mean (Figure 28).

Figure 25. March precipitation for Macon, Georgia, from 1951 throught 1990.
Figure 25. March precipitation for Macon, Georgia, from 1951 throught 1990.


Figure 26. March maximum temperatures for Lexington, Kentucky, from 1951 through 1990.
Figure 26. March maximum temperatures for Lexington, Kentucky, from 1951 through 1990.


Figure 27. March relative humidity for San Antonio, Texas, from 1951 through 1990.
Figure 27. March relative humidity for San Antonio, Texas, from 1951 through 1990.


Figure 28. Average hours of freezing precipitation per month from 1951 through 1990 at Lexington, Kentucky; Macon, Georgia; and San Antonio, Texas.
Figure 28. Average hours of freezing precipitation per month from 1951 through 1990 at Lexington, Kentucky; Macon, Georgia; and San Antonio, Texas.


March was chosen because new growth is being initiated for many tree species, it is a high-fire-risk month, and various fungal spores and seeds are disseminated by the wind during March. Freezing precipitation was plotted for all 12 months.

When interpreting data like the variables shown in the four figures, or extrapolating to other situations, the reader should be aware of several factors.

Precipitation associated with fronts is usually fairly uniform over large areas. The average thunderstorm cell is about 10 miles in diameter, and this relatively small size results in very localized precipitation patterns. Heavy rain may occur under a thunderstorm cell, but just a few miles away none may be observed. March is a month when both thunderstorms and frontal precipitation are common in Macon. Consequently, one should be cautious in extrapolating precipitation.

Maximum temperature commonly occurs in the middle or late afternoon when strong horizontal and vertical mixing usually are present. As a result, local site factors are minimized, and the maximum temperature at a point can be used to represent conditions over an area of 10 to 20 square miles with acceptable accuracy.

By using data from surrounding stations, relative humidity can usually be interpolated to an unknown point with an accuracy of about plus or minus 5 percent. This is acceptable accuracy for many purposes.

Freezing precipitation occurs when (1) the temperature of the earth's surface and objects such as trees are at or below freezing, and (2) when falling water drops are supercooled before impacting the earth's surface or other objects. These conditions occur when a cold layer of air near the surface is underneath a warmer precipitation-producing layer aloft. Damage to trees can be substantial, especially if the precipitation is heavy, of long duration, or accompanied by strong winds. In addition to breakage and uprooting, the damaged trees are more susceptible to disease and insect infestation. If the breakage is substantial and widespread, there is an increased potential for subsequent damaging wildfire.