Science Interviews


Sun, 28th Mar 2010

Contrails Creating Clouds

Dr Jim Haywood, Met Office

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Ben - Most of us are familiar with the long trails left behind by airplanes as they pass overhead.  But those trails, or contrails as they're called, can actually give rise to clouds.  Weíre joined by Dr. Jim Haywood whoís from the Met office.  Now thank you for joining us, Jim.  My first question really to get a broad idea is how were clouds normally formed?

Jim -   Well, clouds almost invariably form when the air that contains the water vapour cools.  Cooling is almost always initiated by a lifting action.  For example, air flowing over mountains can cause the air to cool to such an extent that the water in it condenses, forming a cloud.  Alternatively, you can have things like cool air undercutting warm air and forcing it to rise in frontal systems associated with low pressure systems in the mid latitudes.  So really, itís just a case of air being forced to cool, the relative humidity Ė as meteorologists call it - exceeds 100% and the water in it condenses.

Ben -   When we see these big lines up in the sky, these contrails from airplanes, what are they actually made of?

Jim -   Obviously, most clouds that we can see from down here are made up of water droplets.  Contrails tend to be made up of ice crystals that grow when the conditions are favourable.

Ben -   How do these then go on to create a cloud?

Jim -   What you can get is Ė under certain conditions, which is basically cold and moist conditions  - the ice crystals that are initially formed by the aircraft contrail can grow.  Itís a bit like a physics experiment that you probably did at school where you grow crystals of copper sulphate or things in supersaturated solutions.  Thatís exactly whatís happening here.  When the conditions are right, itís cold enough and moist enough up in our atmosphere then the crystals that are initially injected can just grow tremendously and spread with the metrological flow.

Ben -   So weíre not talking about lots of individual particles like we will be for a normal cloud.  This is actually a very large ice crystal that just happens to be light enough to float about.  Is that right?

Jim -   The ice crystals, when they're initially injected are about a 1000th of a millimetre typically in terms of size.  But as they grow, they can actually get many times that up to about 100 microns.  So, they do form very large crystals which are important in the earthís radiation budget.

Ben -   These clouds are a bit different than normal clouds.  How do they affect the weather?  Are they rain clouds?  Are we likely to see rain from them or just perhaps a bit of shading or will they not affect us much at all?

Jim -   Well they do affect us in terms of the amount of sunlight that they let through.  Thatís quite important.  They tend to reflect sunlight back out to space and lead to a cooling of the weather and consequently, the climate.  But they also trap out going long wave radiation.  So, heat radiation if you like, rather like the greenhouse effect.  So thereís these two competing effects.  You've got the reflection of solar radiation or sunlight back out to space and you've also got a greenhouse type of effect.  And whatís critical is really the balance between the cooling from the reflection of solar radiation and the warming due to the greenhouse type of effect of these crystals.

Ben -   So if they need particular conditions in which to form, does this mean that certain flight paths are actually more likely to create these clouds and therefore, weíre more likely to have this warming effect or this reduction of sunlight effect in certain areas?

Jim -   Yes, thatís right.  I mean, one of the area thatís particularly good for forming cirrus types of particles Ė these ice crystals Ė actually coincides with the air traffic corridors, particularly the one linking North America with Europe.  Thatís a particular area thatís good for crystal formation and crystal growth.

Ben -   Weíve had a very, very relevant question from Neil Briscoe.  He said that he heard a while ago Ė thanks to the 9/11 attacks Ė when they grounded all the flights, people were able to work out the contrails during daylight hours, helped to prohibit warming by reflecting light back out into space.  But during night time, they actually increased warming.  Is this the same stuff that weíre talking about here?  Does it matter whether itís day or night?

Jim -   Yes, it does indeed.  Itís exactly what that question is about.  Really, weíre talking about a cooling, if you like from the reflection of sunlight and a warming during the night time which can cause a reduction in whatís called the diurnal temperature range.  After 9/11, there was certainly evidence of a reduction in the diurnal temperature range from a particular study.  But itís quite difficult to disentangle that signal, if you like from the natural meteorological events that can occur.  And when they looked at it in a little bit more detail, it became almost impossible to distinguish from other events that had nothing to do with 9/11.  You still could see this signal in diurnal temperature range, just due to the natural variability of the atmospheric system.

Ben -   So it may have an impact that we can't see because itís no bigger than noise.

Jim -   Itís just difficult to detect.  Thatís right.

Ben -   And speaking of detecting it, how do we actually study these things?

Jim - Well, we study it by a number of ways.  Weíve been simulating these contrails and contrails induce cirrus in the state-of-the-science atmospheric models that we use at the Met office and the Hadley Centre for Climate Change.  Weíve also been having a look at various different aircraft measurement campaigns.  I'm involved with an aircraft measuring campaign where weíre trying to create contrails and actually measure the amount of reflected sunlight, and the amount of infrared heat energy that these contrails affect.  So, you can do it from a purely modelling perspective but itís always better to base it on objective measurements.

Ben -   And, just finally, these contrails obviously tend to stay in one place in the sky.  When they go on to create this cirrus type clouds, do they also stay where they were created or do we find that they drift across the country and have that sort of effect on a slightly wider area?

Jim -   Yes.  We studied one particular contrail that was formed by an Awacs aircraft over the north sea and what we found was although that aircraft had only travelled 1500 kilometres, the area of cloud that it created was actually over 50,000 square kilometres.  This area of cloud - this was created last March - you could see quite clearly being vectored over the UK and lasted for several hours.  It actually lasted for around about 18 hours before the ice crystals started to dissipate.

Ben -   So thatís certainly something to think about next time you're 30,000 feet above the UK.  You might actually be making it literally cooler for us down on earth in the daytime, possibly a little warmer at night.  That was Jim Haywood.  Heís from the Met Office.


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