Continuing the theme of academic research into the effects of weather, we hope the following item may be useful in helping analyse your own behaviour when it comes to deciding whether to arm yourself with an umbrella in the morning.
The relevant paper, by Yoshiaki Nakajima and Hirohiko Ohta (Psychological Reports Vol 64, 1989) is entitled "Decision-making with probability forecasts of rainfall". The experiment involved asking 274 Japanese undergraduates whether or not they would take an umbrella with them under each of 16 different conditions. The variables that were altered between conditions were the time they were going to be away from home, the expected probability of rainfall, and the immediate weather conditions when they left home.
The results confirmed all their hypotheses: "The necessity of taking an umbrella tended to increase as probability of rainfall rose, as duration became longer, and when the outdoor weather was cloudy rather than clear."
In other words, pe ople are more likely to carry umbrellas if they think it's going to rain.