The weather supplies many metaphors for our changeable minds. Moods can brighten and darken, dispositions can be sunny, futures can be under a cloud and relationships can be stormy. Like the weather, our emotions sometimes seem like fickle forces of nature: unstable, enveloping and uncontrollable.
Weather provides a vivid language for describing our emotional atmosphere, but does it also influence it? Do grey days bring grey moods? When the mercury rises, does our blood boil?
Of the many aspects of weather, sunshine is the most intimately tied to mood. Although the link is weaker than many people imagine, sunlight has repeatedly been found to boost positive moods, dampen negative moods and diminish tiredness.
Anything that alters our moods can affect our behaviour. Happy people are more favourably disposed to one another, and accordingly people are more helpful when the sun is out. One study found that Minnesotan diners tipped more generously on sunny days. Investors may benefit in the same way as waitresses; American studies have observed better daily stock returns in sunny weather.
The sun may melt hearts as well. In a 2013 study by French psychologist Nicolas Guéguen, an attractive male confederate approached unaccompanied young women and solicited their phone numbers. "I just want to say that I think you're really pretty", he cooed. "I'll phone you later and we can have a drink together someplace". "Antoine" achieved an impressive success rate of 22 per cent on sunny days but only 14 per cent when it was cloudy.
Guéguen's finding of sun-assisted flirtation followed up his earlier studies on the effects of exposure to flowers (2011) and pastry aromas (2012) in priming women for seduction. Can we expect future studies on chocolate (2014) and puppies (2015)? Rarely have psychologists lived up to national stereotypes so well: the Americans study money, the French study romance.
And the Australians study shopping. Research by Sydney's Joseph Forgas shows that sunshine can also affect our mental sharpness. Shoppers exiting a boutique were quizzed about ten unusual objects - including a toy tractor and a pink piggy-bank - that had been placed in the check-out area. They correctly recalled seven times as many objects on cloudy days as on sunny ones.
This effect accords with other findings that negative moods induce careful and systematic cognition. Grey weather may similarly induce sober, grey-flannelled thinking. In a paper titled "Clouds make nerds look good", Uri Simonsohn showed that university admissions officers weighted the academic credentials of applicants more on overcast days, and their non-academic attributes more on sunny ones.
Temperature can also affect our mind and behaviour, independently of sunshine. The more it departs from an ideal of around 20°C the more discomfort we feel. One study found that rates of helping declined as temperatures dropped below or rose above this value.
In addition, the higher the temperature, the more people are likely to act aggressively. Rates of aggression are higher in hotter years, months, days and times of day, a pattern observable for murders, riots and car-horn honking. Baseball pitchers are more likely to hit batters on hot days, an effect that isn't merely a result of having sweat-slick fingers.
Heat may also increase verbal aggression. A recent study of news media coverage of the 2008 Beijing Olympics found that stories filed by American journalists contained more negative words on hotter days, even when they were writing about China in general rather than the Games in particular.
Aspects of weather beyond heat and sunshine have also been shown to affect mood. Humidity tends to make people more tired and irritable. Barometric pressure fluctuations can alter moods and trigger headaches, some studies finding a link between low pressure and suicide. On rainy days people report lower satisfaction with their lives.
Weather influences our psychology in myriad subtle ways. Why this might be the case is not entirely obvious. One possibility is that the effects of weather on mood are primarily physiological. Excess heat causes discomfort by taxing our capacity to thermoregulate, and this causes irritability and aggression.
Exposing skin to sunlight produces vitamin D, promoting the brain's production of serotonin, which lifts mood. Exposure to bright lights, a treatment for people affected with the Winter depressions of seasonal affective disorder (SAD), also enhances the mood of unaffected people.
However, the effects of weather on mood are not straightforwardly biological. They are also psychological and social. One reason why heat is associated with aggression is that people interact more in public in hot weather.
Indeed, the effects of weather on mood depend on our behaviour and on how we think. Most basically, weather will only influence us if we expose ourselves to it. On one estimate, people in industrialised societies tend to spend only 7 per cent of their time outside.
A study by US psychology researcher Matthew Keller and colleagues showed that beneficial effects of warm and sunny conditions on mood were only seen in people who had spent more than 30 minutes outdoors that day. Good weather even had negative effects on mood for people confined indoors, who perhaps gazed enviously outside at the solar fun they were missing.
It is hard to argue with Dr Keller's prescription: "If you wish to reap the psychological benefits of good springtime weather, go outside."