When you see the average high for a date in your community (or normal high) do you know where that data comes from?
The same applies to average precipitation or snowfall.
Climatological averages are calculated from 30 years of data. Currently, average highs and lows are calculated from weather records between 1971-2000.
They are recalculated every 10 years using the previous 30 years worth of data.
So in 2011 the average high and low for your area will come from 1981-2010.
Now don't confuse this with record highs, lows, rain or snow for your area. A record is over the entire climate record.
So if weather data has been collected since the 1800s where you live, the entire record is examined to determine when a new record is achieved.
This is a point of debate in some places where the weather station has moved. Here in Denver for example, our official weather station moved approximately 30 miles northeast of downtown Denver with the new airport in the 1990s.
Many people don't like when a new record is set for the city because the airport's location isn't representative of where people live.
Well -- for climate purposes, even if a weather station moves, it is still continued one continuous climate record. And the simple fact of that is that documenting weather data is still a new science. Most stations have only been around 100 or so years now.
If we didn't compare the current weather to what we have documented before the weather station moved, what would we use for a comparison? The answer is nothing -- because there were no weather records kept prior.
So remember -- average or normal highs, lows, rain or snow can change every 10 years because these parameters are recalculated.
But records are compared to the entire climate history available for a station, even if it has changed location within a city. There is probably always room for an exception and I cannot immediately think of one, but I wanted to say that just in case. ;-)