Saturday, January 26, 2008

Weather 101: Calculating Normal

Now that Philadelphia has passed the coldest time of the year based on "normal" high and low temperatures the temperature trend is going nowhere but up. Or is it?

You may already know that what is classified as "average" in the temperature department is based on a three decade average of high temperatures and low temperatures on any given date. In the case of Philadelphia, the current "normal" period that is used are temperatures from 1971 to 2000. Each day's high and low temperature over 30 years are averaged together to get an "average." What you may not know is that those average temperatures that we see on TV and elsewhere are a smoothed curve that the National Weather Service develops in order to maintain a consistent trend of temperatures warming or cooling throughout the course of the year.
The graph above shows four lines regarding Philadelphia's temperature over the last 30 years. The dark red and purple lines represent the official normal temperature in Philadelphia from January 1 to December 31st. The orange and dark blue lines represent the ACTUAL 30 year average temperature. Notice how the orange and blue lines have much sharper changes to them on a day to day basis while the red and purple lines are slowly rising, falling, or staying the same on a daily basis.

You can see that day to day variation in greater detail above. Notice that around January 20-25 (just after 19 on the x axis) there's a spike in the blue and orange lines followed by another bottoming out in early February (around #37 on the bottom line or x axis).

The table below shows the "smoothed" normal high and low, the actual 30 year averages, and the difference between the smoothed and 30 year. A positive departure means the "smoothed" normal is warmer while a negative means that the raw data average is warmer. Notice that the coldest day of the 1971-2000 time period is not in the 1/18-1/23 period but is actually in early February! The average high/low based on smoothed data on February 9th is 41/26. However, the actual raw 30 year data says the normals are 37.1/21.3.
I added a graphic with a full comparison between the smoothed and raw 30 year averages that shows the difference on a day to day basis throughout the year. When the National Weather Service computes the smoothed 30 year normal these day to day variations are factored in and smoothed out using statistics and higher level math in order to obtain the smoothest curve possible in order to provide consistency in normal temperature rises and falls. Normal monthly temperatures are based on smoothed curve data, not on the actual raw temperature data.