Climate/Almanac Data Summaries

How to read the Preliminary Climate Data Report (Form F-6)

The Preliminary Climate Data (Form F-6) has 18 columns of data and one row for each day of the month. There are several summaries and definitions at the end of the report. Many of the data elements are self explanatory.  If a column is marked as either M or N/A that means that the data could not be collected and is missing.

Column Explanations -- Column numbers are in parentheses ().

(1) - Day of the month ("calendar day"). Note that the time period is different between Standard Time and Daylight Saving Time. The calendar day is midnight to midnight Standard Time, but 1 a.m. to 1 a.m. DST.

(2) - Maximum temperature. This is the highest temperature (°F) recorded for the calendar day.

(3) - Minimum temperature. This is the lowest temperature (°F) recorded for the calendar day.

(4) - Average temperature. The sum of the previous two columns, divided by 2, and rounded, gives the value for this column.

(5) - Departure from normal. This value is derived by subtracting the "normal" temperature from the "average" temperature (column 4). The "normal" temperature is the 30-year smoothed average for the date, supplied by the National Center for Environmental Information. 

(6a) - Heating Degree Days. The average temperature (column 4), subtracted from 65, yields HDD. The amount of energy used for heating is almost directly proportional to the number of heating degree days.

(6b) - Cooling Degree Days. Similar to (6a) above, CDD are derived in the reverse manner. Sixty-five is subtracted from the "average" temperature (column 4). Again, energy usage is the main application of this value.

(7) - Precipitation (Water Equivalent). This is the amount of liquid precipitation, in inches, to the nearest hundredth, that has fallen during the calendar day. If frozen or freezing precipitation (snow, sleet, freezing rain, etc.) falls, it is melted and added to the total of any liquid precipitation.

(8) - Snowfall. This total, where measured, is the amount of snow, hail, or ice pellets, in inches, to the nearest tenth, that falls during the calendar day. In this case, the frozen precipitation is not melted before measurement. Typically, snowfall is about 10 times its water equivalent (see column 7), but this ratio can vary dramatically when snow falls at temperatures above freezing - or well below freezing.

(9) - Snow depth. The depth of frozen precipitation (whether snow, ice, or hail) on the ground at 6 a.m. Standard Time (7 a.m. DST), in whole inches, is shown here. The value is the actual depth of the snow and ice, without including such things as grass underneath.

(10) - Average wind speed. The calendar-day average wind speed is shown here, in miles per hour, to the nearest tenth. This value is normally derived by dividing the total "distance" of the wind (as measured by an anemometer) and dividing by 24. Note that wind speeds during the daylight hours tend to be substantially stronger than those that occur at night.

(11) - Fastest 2-minute speed. The strongest 2-minute average wind speed that occurs during the calendar day is identified here. The units are miles per hour. By definition, this value must be less than the peak gust (column 18) and more than the daily average (column 10).

(12) - Fastest 2-minute direction. (See column 11 for information on the "fastest 2-minute wind.") This is the prevailing direction of the strongest 2-minute wind, expressed in tens of degrees. North is 360, east is 90, south is 180, and west is 270.  All other directions are in-between. Northeast, for example, would be shown as either 40 or 50, since it is 45 degrees clockwise from north.

(13) - Total minutes of sunshine. Where available, this value is the approximate number of minutes that the sun is detected as shining, according to a sunshine indicator.

(14) - Percent of possible sunshine. Where available, this is the ratio of the value in column 13 with the number of minutes between sunrise and sunset, expressed as a percent.

(15) - Sky cover.  This is the number of tenths of cloud cover, as an hourly average through the daylight hours (sunrise to sunset, S-S).

(16) - Weather occurrences. The numbers presented in this column are described in a chart on page 2 of the report under the headline "Symbols Used in Column 16".

(17) - Peak gust direction. This is the direction associated with the strongest gust of wind measured during the calendar day. See the description of column 12 for details on the coding.

(18) - Peak gust speed. The strongest wind gust measured during the calendar day is recorded here, in miles per hour.