Boston experienced unusually warm weather this week, most notably on Wednesday when the record high of 86°F set in 1991 was smashed by a high of 90°F. Expect more seasonable weather to return for the weekend, which will be dry except for a chance of rain today.
The media often talks about weather averages and extremes, but often there is little discussion about just how extreme an event is. For instance, Wednesday’s high was 35°F above average, but how do we put these numbers into perspective? In a place with unchanging weather such as Los Angeles, such an event would be more profound than places where the weather swings drastically like the cities in the Great Plains. Those familiar with statistics are aware that the standard deviation is a measure of how scattered the data is. Using Boston high temperature data for the month of April from the years 1920 to 2009, the standard deviation is about 10°F. This makes Wednesday’s record over 3 standard deviations from the mean high of 55°F!
One day or week of particularly warm weather is not a clear sign of global warming. Weather in places such as Boston is highly variable, and climate scientists must average temperature data over many years to filter out these natural fluctuations.