 The weather gives us some really good metaphors for talking about our mood. So you can talk about having a sunny disposition, you can talk about your relationship being stormy, you can talk about your future being under a cloud, but does the weather actually affect our mood? The aspects of whether that seem to affect mood most are probably sun and heat. Of course rainy weather will also tend to depress people's moods but the amount of sun and the amount of heat of the main determinants and of course sun primarily creates more positive moods and creates a sense of vitality. When it's sunny we tend to be more like to be active and to be going out and doing things and sharing experiences with others. So to some extent it's just a social phenomenon but also there's a biological side to it. So when you are exposed to some light that produces vitamin D and vitamin D promotes the production of serotonin, a brain chemical that helps to regulate and elevate our mood as well. Some people just don't have enough sun exposure and are prone to seasonal affective disorder which is a kind of depression that comes about mainly in the winter months especially if you're living a long way from the equator. One of the interesting things about effects of weather on mood is that anything that improves our mood tends to change our behavior. So for instance when people are in positive moods they tend to be more helpful and there are studies showing that on sunny days people are more likely to tip weighters and waitresses better. They're more likely to help strangers who are in need and they're more likely to respond positively to attempts to be picked up. Negative moods created by gloomy weather can have an effect as well. So a study by Sabrina Bryniel and colleagues in Belgium showed that people were more likely to play the lottery on gloomy days. So it seems when people are in a negative mood state created by gloomy weather they'll try to find a way out of it. There are very large differences between people and how susceptible they are to sun for instance in their mood and we don't really know what causes that. It doesn't seem to be related to people's general personality. It's not that introverts are happier in gloomy weather or extroverts are happier in sunlight. Some people just are more susceptible not only to everyday fluctuations in weather but also seasonal fluctuations in weather. The way to benefit from weather is to be out in it. So when it's good weather people are found that you actually don't reap any mood benefits from sunny days unless you spend at least 30 minutes outside. People tend to mistake their current emotional state for their general satisfaction with life. So on cloudy days they tend to think their life is worse than on sunny days. So it's important to stop and think is the way I'm feeling a reflection of my life or is it just the weather?