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!

    Day 234: UK and World weather report - hurricane Irene and tropical cyclone harvey

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    John Hammond Met Office Forecaster John Hammond Met Office Forecaster | 16:00 PM, Monday, 22 August 2011

    Distance travelled ~ 601'177'600 km

    It was another week of mixed weather across the UK and around the world last week.

    During Thursday, very heavy rain with thunderstorms moved across parts of southern England. Across many parts of Dorset and Hampshire, rainfall totals were between 25 and 40 mm with flooding across Bournemouth and Poole. 58.4mm of rain fell at Portland on Thursday and under the cloud and persistent rain parts of the south and the Midlands recorded maximum temperatures of between just 12 and 14 Celsius.

    This heavy rain was associated with very warm and humid air over the near continent which also triggered violent thunderstorms over the 'Low countries'. One of these storms with high winds and torrential rain hit a music festival in Belgium, killing five people and injuring dozens.

    On the other side of the world, the first half of last week saw a cold blast from Antarctic bring snow to many parts of New Zealand for the first time in almost 40 years and Wellington airport was forced to close. Crews and airport staff worked round the clock to keep Christchurch Airport operating, clearing 25mm of new snow from the airport runways and re-opening the terminal.

    Further west strong winds and heavy rain affected southwestern parts of Western Australia. The Cape Leeuwin area was hit by the strongest winds, which gusted up over 60 miles per hour. Pemberton recorded 33mm of rain by Tuesday morning, their heaviest rainfall in 6 years for August and Albany recorded a daily total of 37mm. Further inland, Wiluna picked up 17mm and this was their highest for August in 13 years.

    Elsewhere around the world last week:

    • A flashflood hit a village in Polomolok, South Cotabato in the Philippines left 215 people homeless.

    • Pittsgrove Township in Salem County, New Jersey, USA received 11 inches of rain since a rain storm began on Sunday.

    • Rainfall records were shattered at John F. Kennedy International Airport, USA which had the wettest day on record with 7.80 inches, smashing the 1984 record of 6.27 inches.

    • Several planes received damage in a storm, which included golf ball to baseball sized hail at Eppley Airfield in USA.

    The week ahead...

    • Back in the UK heavy thundery showers are expected to move across southern England tonight. This should move northeast through Tuesday to affect eastern England and parts of southern Wales. Heavy rain on Monday and Tuesday gives a risk of localised surface flooding.

    • Hurricane Irene, currently near the Dominican Republic will move west and then northwest through this week and passing through the Bahamas later this week and close to Florida's east coast during Friday.

    • Tropical Storm Harvey will continue moving WNW'wards but stay as a weak system. Heavy rainfall, strong winds and flooding can be expected over parts of Central America as a result.

    tropical storms Irene and Harvey

    The GOES-13 satellite saw Tropical Storm Irene over Puerto Rico on Sunday, August 21, at 6 p.m. EDT. Credit: NASA/NOAA GOES Project

    NASA satellite GOES East 1 km Infrared Hurricane Sector Pictures

    • Windy conditions are expected across Alaska and NW Canada. Some warmer than average temperatures may also affect parts of northern Canada for a time. Some heavy rain/strong winds also affecting NE Canada at first.

    • A tropical cyclone is currently forming just east of the Philippines. This has the potential to affect southern Japan by the end of the week.

    Day 231: weekly roundup in pictures and video

    Distance travelled ~ 592'816'000 km

    Sunset SurpiseImage taken by Wayne Karberg, August 16 in the Laramie Valley in Wyoming. "Several minutes before sunset (about 8:00 PM MST). This is the view east from my driveway. Rainbows occur frequently here due to the vast open sky and broken weather that passes over the Great Laramie Plains."

    August Lunar
    Full moon August 14, 2207hrs. Image taken by Dave Armstrong, Durham England. "I had the dogs out for their final walk of the day and spotted how beautiful the moon was and amount of light that was coming off it, on arriving back at the house grabbed the camera and tripod and set up in the garden and took approximately half a dozen shots"


    perseid meteor shower Perseid meteor shower. Image taken by Mavroudakis Fotis, 13 August over the mountains of Drama, a city in the Northern part of Greece . "This image is a stack of images which I took from 01:00 am to 02:00 am."


    And lastly, this video which was uploaded by weathernut27 of the snow brought about by a strong Antarctic storm over New Zealand, spreading over northern areas such as Christchurch, Wellington and Auckland, where this extent of snow hasn't fallen in decades...

    Cloudbursts in Kullu-Manali India cause havoc...

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    Helen Czerski Helen Czerski | 15:00 PM, Wednesday, 17 August 2011

    Distance travelled ~ 588'206'400 km

    "Cloudburst" sounds dramatic, doesn't it? As though a cloud was a large balloon filled with water and someone had just arrived with a very large pin. Pop! And then what went up must come down and the one place that you don't want to be is right underneath.

    Although the real thing isn't quite like that, the people underneath can be forgiven for not caring about the difference. This week a cloudburst in the town of Kullu produced 176 mm of rain in three hours and drenched three villages. A cloudburst is defined as any rainstorm where the rain rate is greater than 10 cm each hour, and they are usually only a few km wide. Just stop and think about that... 10 cm of rain in one hour. That is enough to turn an entire village into a temporary river bed. And these are large damaging drops, accompanied by strong winds and thunder. Cloudbursts are very hard to predict, but devastating to the places that they hit.

    Not long ago the 23 Degrees team were in India to film the monsoon rains

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    A cloudburst is a hard thing to study, because it's tough to be in the right place at the right time. So we don't have a good description of exactly how they form or what causes them. They're relatively common during the monsoon in India, especially close to the mountains. Scientists think that these huge amounts of rain come from very tall clouds, reaching up to 15 km into the air, and so the droplets falling from the top of that huge cloud have been able to hoover up lots of smaller drops along the way. That's why the raindrops are so big when they get to the bottom.

    The trigger for the storm seems to be that something nearby (for example shape of the local mountains) starts a huge upside down fountain of air that's very strong but only covered a very small area. Warm moist air pours upwards, releasing energy as it rises, and this builds the storm cloud very quickly. Once it gets big enough and if the conditions are right, all the water that had been lifted up comes down. Very quickly. This is similar to the process that generates normal storms, but what makes cloudbursts different is that this whole process happens inside a small area and very powerfully.

    The problem for the areas that are vulnerable to these events is that accurate prediction of cloudbursts is hard to do and would require some really detailed weather monitoring. You would need to have a weather station every couple of kilometres all over those areas, and installing and maintaining that system would be very expensive. To understand and predict really local events, you need to have really local measurements.

    Hopefully, new technologies and new monitoring systems will solve some of these problems soon. But until then, cloudbursts are going to remain a dramatic reminder of the invisible complexity of the air we breathe, and how little control we have over it.

    Day 224: This week in pictures

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    Aira Idris Aira Idris | 16:00 PM, Friday, 12 August 2011

    Distance travelled ~ 575'449'600 km

    Laramie Sunset 8_6_11 Image taken by Wayne Karberg 6 August, The Buttes, Wyoming. "Dark clouds drop virga as they pass over the Snowy Range west of Laramie, Wyoming near sunset."


    Sunday morning mix of cumulus clouds taken by Fenwalker1, 7 August, Dereham, Norfolk. "A breezy sunny morning"

    Timeless. Cirrus clouds taken by Garry Prescott, 8 August, Moors Valley Country Park.

    If you would like to submit your weather/space photos for next week's photo blog or a feature, you can do this via the 23 degrees weather photography pool or email your photos to 23degrees@bbc.co.uk

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