It used to be said that when the dewpoint in Phoenix is 55 or more three days in a row, we are officially in monsoon. What does that mean? What is a dewpoint of 55?
All air contains water vapor. The dewpoint (or dew point) is a measure of the amount of moisture in the air. The dew point of humid air is higher than the dew point of dry air.
During most of the calendar year Phoenix dewpoint temperatures are well below 40 degrees (often in the single digits) and our relative humidities are very low. However, starting in June, our upper level wind, which is typically from a westerly direction for most of the year, begins to shift to an easterly or southeasterly direction. This wind shift is the simple definition of a monsoon: a seasonal shift in the wind.
Dewpoint is the temperature to which the air has to drop in order for the moisture in the air to condense. Since the amount of moisture in the air is continuously varying, so are the dewpoint temperatures. Historically, when the dewpoints in Phoenix gets to 55 degrees consistently, the intense surface heat of the desert, coupled with that higher level of moisture in the air generates the type of thunderstorm activity associated with the Arizona monsoon.
Why is it so complicated? Well, it isn't if you are a meteorologist. Scientists needed to come up with a means to measure when it was likely there would be abundant thunderstorm activity throughout the state. Research throughout past decades determined that if the average daily dewpoint temperature in Phoenix was at or above 55 degrees for three consecutive days, the likelihood of statewide thunderstorms was good.
In 2008 the National Weather Service decided to take the guesswork out of monsoon start and end dates. After all, monsoon is a season for us in Arizona, and most people should not be concerned with whether or not a particular dust storm was defined as monsoon storm or not. From now on June 15 will be the first day of monsoon, and September 30 will be the last day. Now we can be more concerned with monsoon safety and less concerned with definitions. Meteorologists will still track and report dewpoints and study monsoon weather patterns.
One more thing--keep in mind that the dewpoint at which summer thunderstorm activity occurs in different parts of Arizona are not all 55°F. That's just what it happens to be in the Phoenix area.
Special thanks to the National Weather Service in Phoenix for providing the material for this article.