Example and Definitions

The WETS program summarizes temperature and precipitation, growing season lengths, and last and first freezing dates. The table provides the normal range for monthly and annual precipitation and growing season dates required to assess the climatic characteristics for a geographic area over a representative time period.

The table can be generated from any NWS Cooperative Climate Station with 20 or more years of data. The user has control over the starting and ending year and growing season threshold temperatures. An example follows.

Average Daily Maximum Temperature for a Month and Yearly Average (Column 2)

The WETS table uses daily maximum (TMAX) and minimum (TMIN) observations to calculate average daily maximum and minimum temperatures for each month.

Average daily maximum temperatures are calculated by summing the daily maximum temperatures for an individual month and dividing by the number of values used in the summation for that month. The monthly averages are then summed and divided by the number of months used in the period (years) selected.

The yearly average is calculated by summing the monthly average maximums and dividing by 12. The value represents the average over the period selected.

Average Daily Minimum Temperature for a Month and Yearly Average (Column 3)

Average daily minimum temperatures are calculated by summing the daily minimum temperatures for an individual month and dividing by the number of values used in the summation for that month. The monthly averages are then summed and divided by the number of months used in the period (years) selected.

The yearly average is calculated by summing the monthly average minimums and dividing by 12. The value represents the average over the period selected.

 

Average Daily Temperature for a Month and Yearly Average (Column 4)

Average daily temperature for a month is calculated by adding the individual monthly average daily maximum temperatures and average daily minimum temperatures shown in columns 2 and 3 and dividing by two.

Average yearly temperature is calculated by summing the monthly averages shown in Column 4 and dividing by 12. The value represents the average over the period selected.

Average Monthly and Annual Precipitation (Column 5)

The WETS Table uses daily precipitation to determine average monthly precipitation. Monthly precipitation is calculated by summing the daily precipitation for each month. All monthly amounts are then summed and divided by the number of months used in the period (years) selected. The Yearly Total is the sum of the averages for individual months shown in Column 5.

"30% Chance Less Than" Values for Monthly Precipitation and Annual Precipitation (Column 6)

This value represents the threshold for which 30 percent of precipitation amounts will be less than or equal to the value shown. Viewed inversely, 70 percent of all precipitation amounts can be expected to exceed this value. These thresholds are calculated from the fitted two-parameter gamma distributions (Soil Conservation Service, 1985).

It should be noted that the annual threshold shown in Column 6 is not the sum of the individual monthly thresholds. Individual monthly, (e.g. all January totals) and annual precipitation totals possess different statistical distributions which must be modeled separately with the gamma distribution. Accordingly, monthly totals are used to calculate the monthly threshold values and annual totals are used to calculate the annual threshold values.

"30% Chance More Than" Values for Monthly Precipitation and Annual Precipitation (Column 7)

This value represents the threshold for which 30 percent of precipitation amounts will be greater than or equal to the value shown. Viewed inversely, 70 percent of all precipitation amounts can be expected to be less than this value. These thresholds are calculated from the fitted two-parameter gamma distributions. The monthly and annual thresholds are calculated in the same manner as described in the "30% Chance Less Than" (Column 6).

Average and Total Number of Days with .10 Inch or More of Precipitation (Column 8)

The monthly average value is calculated by summing the number of days with precipitation greater or equal to .10 inches for a individual month over the period (years) selected and dividing by the number of months used in summation. The yearly average is calculated by summing the 12 monthly average values.

Average Total Snowfall (Column 9)

Snowfall is the incremental depth of snow that has fallen since the last snow depth observation. The time between snowfall observations is usually 24 hours for the NWS Cooperative Network. The monthly average value is calculated by summing the observed daily snowfall values greater than or equal to 0.1 inch for an individual month and dividing that sum by the number of months used in the selected period (e.g., 1971-2000). The yearly average is calculated by summing the 12 monthly average values.

Precipitation Data Listing

At the end of the WETS Table is a listing of the precipitation data used to create the table. It displays monthly and annual totals summed from daily observed precipitation. Months with at least one missing daily observation are annotated with an "M". Months containing no daily observations are shown as a blank.