2006_04_ir.jpgAnother warm spring day is underway today. Temperatures are much like yesterday. South facing shores are several degrees cooler because the wind will be blowing in off the cold ocean. Clouds move in this evening and we are likely to get a few showers after midnight through tomorrow morning's commute. The rain will probably not amount to much. After the rain moves out, tomorrow will be one of those paradoxical weather days when it gets warmer after a cold front.

The Weather Service is forecasting a 40 percent chance of rain Friday through Saturday. If you were walking to work, or going to spend your lunchtime outside, is a 40 percent chance of rain high enough that you would carry an umbrella? How about 60 or 80 percent? How likely does the promise of rain have to be for you to carry an umbrella? The answer, in large part, has to do with our personal level of risk aversion. In turn, our perception of risk depends on the probability of rain, what our loss might be if we get wet (higher if we're carrying a cake or wearing fancy shoes), and what choices we have in staying dry. Gothamist ran across a 1996 letter in the journal Nature in which the letter writer demonstrates that "unless one is particularly concerned about getting wet… disregard of forecasts of rain [is] the optimal strategy". The reason for this is because precipitation forecasts predict the probability of rain occurring over a many-hour time period. Our commute, or lunch hour, is but a small portion of the forecast period. The chance of rain in the specific hour that we might be outside is very low compared to the entire forecast period. This analysis is a little simplified, as a showery type rainfall is much more likely to miss us than widespread rain associated with a storm, but after carrying an unused umbrella around several times recently, it does encourage us to leave our umbrella home next time.

Cold front headed our way from the National Weather Service.