• Question: How do clouds form?

    Asked by to Edward, Ian, Mathew, Naomi, sakshisharda on 20 Jun 2014. This question was also asked by .
    • Photo: Naomi Osborne

      Naomi Osborne answered on 20 Jun 2014:

      clouds form when our air becomes saturated with water vapour which evaporates from the sea and lakes.

      As the water vapour gas rises into our atmosphere, the pressure drops, causing water vapour to expand and cool as it loses energy. When the temperature of the air drops, our atmosphere is unable to hold as much water vapour and this is when condensation occurs, forming clouds. Condensation only occurs when water vapour collides with dust or salt particles – when enough condenses we see clouds.

    • Photo: Ian Stephenson

      Ian Stephenson answered on 21 Jun 2014:

      Air has water in it all the time. However depending on the temperature it can hold different more or less. When we measure humidity of the air its typically recorded as “relative humidity”, form 0% to 100%. 0% would be completely dry, while 100% would be when the air is “full”. When the air is “full” then it can’t hold any more water, and if there’s any extra it gets kicked out. Cold air can hold less, so when air which is holding a lot of water (like in a kitchen or bathroom) is cooled (for example by hitting a window) the extra water condenses back into droplets which either steam up your windows, or forms clouds.

      Another thing that’s often calculated is the Dew Point. This is where you measure the relative humity and temperature, and then calculate how cold it would need to be for water to start condensing. At night in the winter, when it gets cold overnight water condenses and everything gets wet with dew, even though its not raining:

      There are lots of weather stats, including the dew point here: http://www.weathercast.co.uk/world-weather/weather-stations/obsid/3862.html Airport weather stations track the dew point because thats’ when its going to start getting foggy!

      You can measure relative humidity really using a Raspberry Pi, or Arduino. You need to get a “dht11” sensor (they’re about £1 an ebay) and hook it up to the computer (its only three wires to connect, and there are plenty of tutorials), and you can measure temperature and relative humity for yourself. Try measuring it in the bathroom with the shower on, and you’ll see that when it hits 100% things start to get “cloudy”.

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