Chance of Rain
It’s raining outside in Tucson today, which leads me to think about what they mean when they say there is an X percent chance of participation. This morning, Weather Underground says there is an 80% chance of precipitation in Tucson. Does that mean 80% of the area will get rain? That it will rain 80% of the day?
I did some research, and the answer is pretty simple : out of 100 days where the weather conditions were exactly or similar to how they are today, it rained 80 times. Pretty simple!
The questions becomes, then, how do they get that data to make such a calculation? The answer lies with the National Weather Service. Each day the NWS releases several balloons into the atmosphere from locations across the country. Those balloons, called radiosondes, are released twice a day and the information they collect is radioed to the ground where it’s collected by the National Meteorological Center near Washington, D.C, where it’s processed by computer. All the information the radiosonde collected during its rise in the atmosphere – pressure, humidity, wind speed and direction, temperature – is fed into the computer and used, in conjunction with data from ground sources, to create a 3D model of the atmosphere. That model is evaluated against various laws of fluid mechanics to predict future conditions.
Chance of precipitation for 22 February 2010.
Unfortunately, given the nature of precipitation, these percentages are really just educated guesses based on previous conditions. The atmosphere is a very fluid and dynamic entity, and many things can trigger an unexpected change in conditions. A wet day can suddenly turn sunny, just like a sunny day can suddenly turn severe. Weather forecasters have a difficult job at best, and providing a chance of precipitation number is just one way that they can help you plan your day.
By the way…a 100% chance of rain does not mean that it’s raining right this moment (unless, perhaps, you live in the Pacific Northwest). Again, it simply means that out of 100 previous days where conditions have been similar to those today, it rained every time. And of course that begs the next question…what is rain? We’ll save that for a later post!