Daily Weather Data Description
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Data is in the following comma delimited format:

Site ID, Date, Max Temperature (F), Min Temperature (F), Average Temperature (F), Heating Degree Days, Cooling Degree Days

The daily data file contains a 3 letter site ID instead of a 4 letter site ID.  
The 4 letter site ID for all cities in the daily data file is "K" + 3 letter ID
So, ORD would be KORD if the 4 letter ID is needed.

The average temperature is calculated by adding the high and low temperature together and dividing by 2.
It is not an hourly or other average.

Heating Degree Days (HDDS) are calculated as follows:

If Average Temperature is < 65
HDDS=65-Average Temperature

If Average Temperature is >=65
HDDS=0

Cooling Degree Days (CDDS) are calculated as follows:

If Average Temperature is > 65
CDDS=Average Temperature - 65

If Average Temperature is <=65
CDDS=0

Note that NWS/NOAA rounds the daily average temperature BEFORE calculating the HDDS and CDDS.
The HDDS and CDDS in this file are calculated off the unrounded daily average and thus can contain fractional values.