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July 24 2008
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weather Weatherwatch with Irv Lee

Warm Fronts

Having examined the process which forms Atlantic low pressure systems (see Weather Watch #9), pilots need to recognise the conditions and behaviour of the associated weather fronts as they pass over the UK.

This article examines the features and dangers of the 'warm front'. The boundaries between two different air masses are given the terms 'warm' or 'cold' front purely by direction of their movement across the earth's surface. If the air behind a moving boundary is even slightly warmer than that ahead of it, we call it a warm front - in other words, it defines the leading edge of the warmer air arriving.

Weather charts such as Metform 215 show the estimated ground level position of weather fronts at a particular time of day. However, on the warm front, the boundary between the two air masses is sheared forwards with increasing altitude. This is due to the rising warm air behind the front being taken upwards and onwards by the high altitude wind, the jetstream, which propels the whole system. At high altitude, the warm front and its associated cloud extends hundreds of miles ahead of the indicated ground position. At the airfield, an approaching warm front shows these classic signs:

wispy cirrus cloud formed from ice crystals appears at high altitude

pressure decreases, and more noticeably the regional QNH setting (the lowest sea level pressure forecast ahead for the coming hour in the altimeter setting region) begins to show a large difference compared to the real airfield QNH

the wind strength increases slightly and direction backs.

With the medium level altostratus cloud and lower level Nimbostratus still a few hundred miles away, GA flying is still safe and fun at this stage.

How it develops
Over the next few hours:

cirrostratus covers the sky, but a weak sun remains visible

pressure drops rapidly medium level altostratus arrives and sun is obscured

visibility reduces slightly as light rain starts

Cumulonimbus cloud development is rare, but not unknown, on a warm front, as the extended slope of the warm front encourages more layered (stratus) cloud formation.

GA flying is still safe below the freezing level, although flights towards the warm front will meet rapidly deteriorating conditions. (Beware 'get-home-itis' after a land-away).

The pilot making such a flight, or waiting longer at the airfield for the arrival of the front, will experience:

pressure dropping further

lowering cloudbase with nimbostratus and stratus

wind strength increasing

rain and drizzle, with rapidly reducing visibility

Warm front arrives

The warm front finally arrives and is marked by:

an increase in temperature -

very low stratus cloud and poor visibility, usually in drizzle -

a sudden veer in the wind direction -

an increase in humidity and dewpoint -

a stabilisation (minimum) of pressure

The biggest danger
Consider a flight heading west towards an approaching warm front, at an altitude above the freezing level. The aircraft structure is therefore below zero degrees celsius. High above, cirrus has long since turned to cirrostratus, and ahead, altostratus releases light rain into the path of the approaching aircraft.

Because upward movements of air in low pressure systems tend to remove pollution particles from the atmosphere, droplets of rain can become 'supercooled'. This is a strange phenomenon when, without a nucleus of dust to crystallise around, water can remain liquid and fluid well below normal freezing temperature.

As the aircraft flies into the supercooled falling rain, each droplet freezes instantly to solid ice on contact with the cold aircraft. Unless the pilot turns back immediately, or rapidly descends to an altitude well below freezing level, the build up of ice on the aircraft can be catastrophic in hardly any time at all.

This article first appeared in FLYER magazine's March 2001 edition

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