Weather Fronts

In the last lesson we discussed Low Pressure systems. We are now going to talk about weather fronts that operate inside these low pressure systems. In the pictures above I have shown you a weather chart of the UK and part of the Atlantic. The top one is the actual chart, the lower picture (for which I apologies for making  a bad job of painting) will give you a breakdown . That said  I hope you get the idea of how to read the charts as to where the warm front is etc. the bottom one is identical to the top

The Weather Fronts over the UK normally moving from South West to North East. If we start over the right hand side of the charts you will see the high pressure is slowly slipping away towards the right. ( Eastwards) and the Low Pressure area is moving in from left to right.. The first part of the low system is the warm fronts on this occasion. This is followed by the cold front behind it over to the left we see near the top a warm front and then in purple an Occluded Front. These are the three types of front. Iwill go on and explain them in detail below. Each front produces a different type of weather. The signs you see are the ones the weather men use for showing the boundaries of where the different types of weather are. Learn these

The first front we will talk about is the Warm Front. For example the one in the top right hand corner of the pictures above.Below is a horizontal look at a warm front moving from left to right.

This is stage 1

In the picture above are the stages of a Warm front. You are now the observer sitting in your deck chair to the North East of this front. You are enjoying the lovely sunny day of the last of a High Pressure System. As you look into the sky to the South West you see at around 30000 ft little white clouds that look like horses tails. These are Cirrus Clouds they could be at least 500 miles ahead of the main part of the Warm Front. The barometer starts to fall . Later in the day the cloud starts to thicken and get lower.You can see rain falling but it is not hitting the ground ( The area just above the aircraft.) The cloud later starts to turn a dirty grey colour and continues to lower.  Eventually the cloud becomes very low and it starts to drizzle for hours. Sometimes this could be sleet and snow. You have now witnessed the approach of a warm front. As the lower edge approaches the cloudbase  could be less than 500 ft from the ground. ( instrument approach weather having had to let down right through the whole lot to an airport.) For a private Pilot in a light aircraft this is no a flying day. 

You will remember me discussing with you the Icing problem with temperature inversions. Now you can see why. Look above the aircraft and note warm air has got over the top of the freezing level. It starts to rain the rain falls through the freezing level and and freezes as super cooled water droplets that stick to an aircraft on impact. The pilot would have to either climb above the freezing level or turn round and go back the way he come from underneath the cloud. I hope this makes sense to you. It is therefore important before flying you look at the charts and avoid or do not fly. Airliners have de icing systems many light aircraft do not. This ice can cause loss of lift. Recently a Fokker 50 Airliner was circling in a hold and crashes as a result of this airframe icing. The Fokker 50 had de icing systems. Control surfaces can also malfunction. You have been warned now.

A warm front moves at about 20 MPH so you can see that you will have sometime after seeing the Cirrus Clouds before the rain gets to you. Bearing in mind the cirrus clouds could be 300-500 miles in front of the rain portion. That could be 15 hours time. Another 12 hours for the rain to pass. These clouds produce persistent light rain sleet or snow. As the last three hundred miles pass you will notice the wind veer rapidly to the South West and increase.. (Normally the wind at the start of the front passing will be from the South East and Moderate.)The air becomes very warm and muggy now. As the front passes, visibility to the North is normally good except when its raining. In this warm sector there is  mist and fog  could ppear  just after the rain. 

Now lets look at how the clouds got up there and the various stages.

Firstly the warm air has come from over the Atlantic from the South West. ( Tropical Maritime Air). It therefore holds a lot of water vapour. As it catches up with colder air it is forced up. As the air is almost at its dew point it does not take long to condense into clouds. This causes the Nimbostratus clouds. As the air continues to rise it becomes know as Altostratus clouds and then Cirrostratus clouds and finally between 25000 and 30000 ft Cirrus Cloud. Winds at upper level are very strong compared to the lower level this will move Cirrus forward of the nimbostratus. The climb gradient for the clouds is about 1 in 120. Eventually the clouds cool and the rain starts.

Cloud names for Clouds of the Warm Front

Nimbo means Rain bearing 

Stratus means Layer cloud.  so the names are joined together to make Rain Bearing Layer Cloud. (Nimbostratus)

Virga This is the rain that falls from a cloud and does not hit the ground.

Cirro means heights above 20000 ft 

Alto means heights above 7000 ft and less than 20000 ft.

How to identify Warm Front Clouds

Cirrus (CI) They  look like horses tails

Cirrostratus (CS) It looks looks like an aircraft condensation trail  but more bushy. it goes a long way across the sky joined compared to Cirrus. If near the sun can give a halo effect.

Altostratus.  These are fibre looking and are grey and blue in colour.

Nimbostratus  Put your umbrella up its the dark ragged cloud that is drizzling on you with persistent light rain.. And the one nearest the ground.

Stage 2 of the Warm Front

As the Warm front passes cloud will turn at the lower level from Nimbostratus to just low Stratus Cloud initially. This could mean hill fog if the cloud was over it at the time. You also have to be aware this cloud maybe hiding from view a big Thunder Cloud above and behind it, (The Cumulonimbus Cloud a full CV of this cloud will follow later in the school its not a good cloud to be in and very dangerous). The barometer pressure normally stops dropping or drops very slowly now. Behind the warm front is a Cold Front.  

The Cold Front

Colder air pushes in behind the Warm Front in a low pressure area. The result of this is the hot air in advance of the cold air becomes unstable and is forced up rapidly at a gradient of about 1 in 50. The result is these towering  Cumulus CU Clouds which can develop into Cumulonimbus Cloud CB. Rain from these clouds are very heavy but are showery and not persistent like the Warm Front Clouds are. They are very turbulent though and windshear can occur.. They look like large cauliflowers. A Cold front rain belt is normally about 50 miles wide when these clouds pass. After these clouds pass the observer will notice it get colder but the skies will clear up. They have caused accidents at airports so pilots may hold off if one of these clouds are right over the airport at the time of landing. They normally pass in about twenty minutes normally.

There is one other type of front and that is the Occluded Front. If you look at the first map you will see the symbols for marking the types of front and where they are. You will notice on the occluded front one that it shows the cold front and warm front labels together. (The top left.) What happens here is the Cold Front eventually overtakes the warm front near the centre of the depression. Eventually the warm front disappears . This causes a lot of cloud and rain  as everything is all mixed up together. Sometimes its more a cold front sometimes its more a warm front at first. It normally means its near the end of a depression and High Pressure is coming in again with some good weather.

As stated earlier if the isobars in a low are distorted into a V or a U shape then the weather is very severe there.

This concludes the briefing on weather fronts. We still have Winds and Local weather to talk about but hopefully now you have grasped the main part of pilot meteorology. Also Fog in more detail.

Please bear in mind that a pilot may pass the warm front in reverse. The pilot may be flying North East himself. In that case he will hit the cold front first and then go into the warm front and then into good weather. Please do not get it in your heads that it always starts off with Cirrus as a pilot. That is only if you are on the North East side of the front. Think what might happen if the pilot is flying North to South. Sometimes the Cirrus Cloud is well ahead of the wam front line on a chart. This should be remembered.

To revise then 

Warm fronts have vast areas of layer cloud which in the last 150 miles low cloud base poor visibility icing followed by mist and humidity and persistent drizzle rain sleet or snow.

Cold fronts are short in length but have massive air instability causing turbulence and heavy short shows visibility better than the warm front. Cold Fronts follow warm fronts. Cold fronts have thunderstorm potential.

Continue to

See this lecture with photos of the clouds