Changing Averages

More on the average temperature changes we talked about last Thursday.   The average highs and lows didn't change much in Philadelphia but they reflected the trend higher in monthly average temperatures overall. You can click on the graphic below to see the projected average high and low for the 21st of each month throughout the year.

How are "averages" for each day of the month calculated and figured out?

Average temperatures are based on a 30 year period of time, with the newest averages based on 1981-2010's temperatures.   The daily highs and lows for each day in that thirty year period are added up, divided by 30 to get a rough average temperature.  Since the rough average from day to day can vary somewhat (the average high on January could be 41.4, on January 2nd be 41.5, and January 3rd be 40.9 in an example), climatologists take the rough average and smooth it out, providing you what are typically referred to as "normal" highs and lows on a daily basis.   Those "normals" or what I like to refer to as average highs and low climb steadily from late January to July and fall from the end of July to January.   The averages of each day in a month (highs and lows) are taken together to provide a monthly average temperature, which is used to get a sense of how much warmer or colder to what is typical for that time of the year.