Why are clear nights so cold?

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    Helen Czerski Helen Czerski | 15:00 PM, Wednesday, 9 November 2011

    Distance travelled ~ 804'321'600 km

    It's the season for baked potatoes, parkin, treacle toffee and bundling up to stay warm. I love the sharp, cold starry evenings and being able to see my breath - it's not every day that you get to make your own cloud! But in the past few days I've remembered that there's a price to pay for being outside on those fabulous clear evenings. It's cold. Frigid, frosty, freezing. Your knuckles go red and the inside of your nose feels like it's full of ice. Why can't we admire the autumn stars in comfort?

    The answer is to do with how energy gets from place to place, and how much clouds get in the way. We may associate clouds with bad weather, but when it comes to nighttime, clouds are our friends.

    Autumn clouds

    Image credit NASA

    Our planet's energy comes from the Sun, mostly as visible light. We know that - it lights up our world. Air is invisible, and by definition visible light travels straight through it. So on the way in, the Sun's energy is carried by all the colours of the rainbow, straight through the atmosphere and all the way to the ground. The ground absorbs that energy and warms up. Black tarmac absorbs more heat than white sand, but they all capture some.

    Next time you make some toast, watch the element in your toaster. As it gets hotter, it glows, first dull red, then bright red and then orange and yellow. Hot things give away their energy by glowing - it's a fundamental rule of physics - and the colour tells you their temperature. The ground under our feet, along with you and everything else around you also glows. But because those things aren't as hot as your toaster, they glow in the infrared, which we can't see directly.

    So the ground glows in the infrared all day and all night, constantly emitting invisible energy back upwards. Some of this energy heats the air near the ground, but some keeps going upwards. And here's where clouds matter at night. Clouds are really good at capturing that infrared radiation and sending it back down the Earth. They act like a blanket, trapping heat between the ground and the clouds. If there are no clouds, the energy from the ground just goes up, up, and away...

    Whenever it's a clear night and you can see lots of stars, there is nothing to trap all that infrared energy, so it's lost to space and we feel cold. If it's cloudy, there are no stars to see, but we have a nice warm blanket above us, keeping the heat in. The fact that Earth gains energy as visible light and loses it as infrared light is really important for the heat budget of our planet, not just for freezing astronomers.

    Sadly, this means that stargazing will always require extra layers. Happily, that means extra excuses in life for hot chocolate. In fact, just writing this has made me feel chilly. It might be hot chocolate time right now!

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