Archive for the ‘Clouds’ Category

Classic clouds #3 – altocumulus lenticularis

December 23, 2011

Well, there’s a bit of a buzz about lenticular clouds at the moment as there were some wonderful photos taken of a lenticular cloud in West Yorkshire yesterday.

Lenticular cloud in West Yorkshire on 22/12/2011. Photo from Paul Hudson's blog.

These crisp and layered lenticular cloud are relatively rare in the UK as they form downwind of mountains or hills.

What happens is that the air flowing over the hill gets “knocked” upwards which results in a type of wave forming. The cloud forms on this wave at a point where the flowing air moves upwards and cools to a point where the water vapour condenses into a cloud. So, although the cloud is stationary, there is a constant flow of air going through it.

Quick sketch of lenticular cloud formation.

Why does it snow?

December 22, 2010

The Barometer guys have just published a new episode on snow. It’s another good one. I’m starting to really miss being involved with the show!

In this episode they talk for a bit about how snow forms and it reminded me of this really nice video showing the melting layer in a precipitating cloud (click to play):

In the higher levels of the cloud (where it’s really cold) the precipitation starts out as ice and snow, which falls much slower than rain. But if you look closely at about 2 km you can see the point where the ice and snow melts and becomes rain – this is known as the melting layer. The rains falls much quicker than the snow above.

It snows at the ground when the lower levels of the atmosphere are particularly cold and so the snow doesn’t melt. This is what’s been happening recently.

Classic clouds #2 – virga

June 23, 2010

This is the second in a series of posts covering my favourite clouds. The first post looked at Kelvin-Helmholtz billows.

I wasn’t planning on covering virga this soon in my series of posts but I was in my local park and these beautiful clouds floated past.

Virga aren’t particularly rare but they’re still nice looking. The white streaks beneath the clouds is precipitation that is evaporating before it hits the ground – this is the virga (which apparently means twig in latin). In these photos the little high clouds are probably precipitating ice crystals that melt and then re-evaporate.

Classic clouds #1 – Kelvin-Helmholtz billows

January 21, 2010

My last few posts have been about fairly meaty climate issues or on science policy.  These are important subjects but one of the things I love about atmospheric science is that there are some beautiful things in our skies.  This post is about one of my personal favourites, Kelvin-Helmholtz Billows:

These billows occur when a layer of cloud is found beneath a layer of warmer air and the two layers are flowing in different directions. When this situation occurs, the lower cloud layer is not bouyant so it can’t push up into the warm layer.¬† However, under the rare circumstances that the two layers are flowing just right, the interaction between the two layers will form an instability and the cloud layer will “break” into the upper layer in the wave-like pattern seen above.¬† These cloud formations are so rare that they rank as the highest scoring cloud in the Cloud Appreciation Society’s Cloud Collector’s Handbook.¬† (Confession: I have never actually seen one in the sky.)

But, I have seen one in the lab!  The amazing video below shows an experiment I did during a summer school at the University of Cambridge where the same conditions can be set up.  Enjoy.

[Thanks to Tor Smith (University of Leeds) for the footage and to the University of Cambridge Fluid Dynamics labs for the equipment.]

There are some nice observations of these cloud using the Chilbolton radar either in Chapman and Browning (1997) or on this University of Reading webpage.

UPDATE: I recently saw something that looked like KH billows in the sky where I live. They formed on an aeroplane contrail so maybe this pattern was formed more by turbulence from the plane than atmospheric shear that was already there. Either way, it made my day!

Reference:

ResearchBlogging.orgChapman, D., & Browning, K. (1997). Radar observations of wind-shear splitting within evolving atmospheric kelvin-helmholtz billows Quarterly Journal of the Royal Meteorological Society, 123 (541), 1433-1439 DOI: 10.1002/qj.49712354114