In The Happiness Advantage, Shawn Achor recommends spending time in the fresh air to improve your happiness:

Making time to go outside on a nice day also delivers a huge advantage; one study found that spending 20 minutes outside in good weather not only boosted positive mood, but broadened thinking and improved working memory...

This is pretty good news for those of us who are worried about fitting new habits into our already-busy schedules. Twenty minutes is a short enough time to spend outside that you could fit it into your commute or even your lunch break.

A UK study from the University of Sussex also found that being outdoors made people happier:

Being outdoors, near the sea, on a warm, sunny weekend afternoon is the perfect spot for most. In fact, participants were found to be substantially happier outdoors in all natural environments than they were in urban environments.

The American Meteorological Society published research in 2011 that found current temperature has a bigger effect on our happiness than variables like wind speed and humidity, or even the average temperature over the course of a day. It also found thathappiness is maximized at 57 degrees (13.9°C), so keep an eye on the weather forecast before heading outside for your 20 minutes of fresh air.

The connection between productivity and temperature is another topic we've talked about more here. It's fascinating what a small change in temperature can do.