The psychological effects of a heatwave

This has been a summer to remember, not least because of the unusual and prolonged spells of heat and sunshine. But aside from the physical changes of tanned skin or freckles, how does unusually hot weather affect us mentally?

VM Sharma and colleagues at AIMIL Geotechnical Consultancy in New Delhi asked participants to take bouts of exercise, and then to perform tests of mental alertness, associative learning and problem solving in temperatures varying between 25-35 degrees. Performance dipped as the temperature rose.

They noted, however, that this effect was much more pronounced when the humidity was also high. Other research—for example a study led by Margaret Allen and Gloria Fischer at Washington State University—shows that temperature alone has less effect on cognitive performance than the combination of high temperature plus high humidity.

Perhaps we’re aware of the adverse effect of hot weather on our ability to think clearly, and as a result we avoid making challenging decisions on those days. To see if this might be true, Amar Cheema and Vanessa Patrick at the Universities of Virginia and Houston tabulated the sales of different types of lottery tickets in St. Louis, Missouri for one year.

They found sales of lottery tickets were unaffected by a rise in temperature, whereas sales of scratch tickets—which demand a number of decisions to be made—fell by $594 with every one degree (Fahrenheit) rise. Intrigued, they set up two experiments.

In one they asked participants to proofread an article, and in another they were asked to choose between two types of mobile phone, either a traditional model or a new, more innovative iteration. Under warmer conditions, participants made more mistakes when proofreading, and chose the phone they were familiar with over the one that required new learning.

Turning to mood, numerous studies show that warmer temperatures, particularly temperatures higher than expected, are associated with greater levels of happiness and wellbeing.

However, once again it’s not temperature alone that’s making a difference: both humidity and the number of sunshine hours matter at least as much. Studies in Canada, Taiwan, Australia and the UK have shown that higher humidity makes us feel gloomier, sleepier and more cynical, whereas more hours of sunshine are associated with greater happiness and optimism.

In field studies, Michael Cunningham at the University of Louisville, Kentucky noted that on sunnier days, people were more likely to assist him when he asked them for interviews, and when dining out, they were more likely to tip waiters generously.

The conclusions? When it’s hot and humid you’ll find it harder to learn new material or to think logically, and you won’t feel like making complex decisions. However, if it’s hot – especially if you didn’t expect it to be – and also sunny and dry, you’ll feel happier and more optimistic, and you’ll find it easier to be helpful and generous towards others.

• Linda Blair is a clinical psychologist and author of Siblings: How to Handle Rivalry and Create Lifelong Loving Bonds. To order for £10.99, call 0844 871 1514 or visit books.telegraph.co.uk