spiral low near Greenland, 8 Apr 16

Eyes in the sky, courtesy of Dundee Satellite Receiving Station bring this image of a spiral depression near Greenland.

Such perfectly represented spiral lows happen when the vortex is present through all layers of the atmosphere: surface streamlines as well as wind aloft trace out the same pattern circular pattern.  This is often not the case in the mid-latitudes.  See following images from Earthnullschool which show that the surface low is located at the same position as the upper low.

The trailing cold front has a marked wind veer on it.

UKMO analysis for same day follows, which shows the somewhat complicated structure of the frontal system.  Bands of airflow are sucked up from the south and gradually wind upwards and around the vortex thereby mixing warm northwards.  There is a noticeable change in air mass because behind the front we can see convective cells in the polar airflow which pours down from the Davis Straits between Greenland and Canada.


Spirals, and in particular logarithmic spirals often occur in nature.  The dynamics are complicated here so there is no simple equation; circular motion on the surface of a spinning sphere moving in an elliptical orbit.  There seems to be an undeniable approximation to the following.



Jet-stream Cirrus from warp and weft of Atlantic air currents illustrate traditional forecast rules of thumb

One of the tricky things about meteorology is that it happens in three dimensions above the surface of a rotating sphere.  In fact, if you add in time, it is four dimensional.  Yet we only have two dimensional graphics from which to build our understanding of it; its rather like building the picture of a garden viewed only through the narrow gaps in boundary fence.

This featured picture has a stable south westerly jet-stream flow aloft, marked by cirrus which weaves itself across a lower level unstable north westerly from the direction of Cape Farewell, the southern tip of Greenland.  Such differential motion of low and high level clouds give rise to traditional seagoing forecast rules of thumb and allow an observer to detect whether they are ahead or astern of a storm.  Of course, a peek at the ships log might tell them that.

Jet stream is shown here from Earthnullschool’s 250mb streamlines.  This matches the shape of the cirrus in the satellite picture above.

250mb stream lines, 29 Mar 16, ENS.
250mb stream lines, 29 Mar 16, ENS.

Whilst below the 1000mb streamlines are:

1000 mb flow, 29 Mar 16, ENS
1000 mb flow, 29 Mar 16, ENS

In this case, cumulus at lower levels moves from the north west whilst cirrus at high levels comes from the south west.  The area does lie to the south west of the complex Atlantic low in the aftermath of storm Katie and pressure is building heralding more settled weather to come.

Airbourne Desert Sands Mark Out Cold Front in Eastern Mediterranean

desert sands blow out of Africa on a conveyor belt of air ahead of a cold front



Cold fronts are normally identifiable from their cloud signature on satellite imagery.  Of course, air mass boundaries can exist without condensation: density changes and air flows can exist without clouds particularly in dry and hot air.  Summer front can cross the UK with just low strato-cumulus.  In the Mediterranean, cloudless density boundaries one day give potential for fierce thunderstorms the next.

The 23 Mar 2016 was a case in point.  Over the Eastern Mediterranean we note a conveyor belt of air dragging sand northwards from the African deserts.  No clouds but a cold front none the less.