Learning Weather - Part Four - Fronts and Frontal Systems

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stephenprudence
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Learning Weather - Part Four - Fronts and Frontal Systems

Post by stephenprudence » Wed Nov 16, 2011 9:22 pm

This is one of the basics of Atmospheric science, quite straightforward too!

If had to define a front in a sentence it would be; the boundary between two airmasses.

This boundary is typically the marker between cold air and warm air.

There are three types of fronts;

- Warm front
- Cold front
- Occluded front

Warm fronts are the boundaries where colder air is leading and warmer air is trailing, warm fronts therefore bring warm air in their wake. The type of weather you may expect to find on a warm front is generally stable, stratus cloud is fairly common, with drizzle, mist and murk.

Cold fronts often follow on behind warm fronts, these fronts introduce colder, more unstable air. The type of weather that occurs is usually showery in nature and can bring about some hefty showers, downpours and thunderstorms.

Occluded fronts are less common but occur in complex systems, they can bring about the heaviest rains of all, due to the injection of warm air into a cold system. Often occluded fronts are responsible for tornadoes in the USA where occlusions may intersect a cold front.

Diagram of front icons/symbols:

Image

Complex Fronts

Complex fronts are systems, they comprise of fronts all adjoined to a frontal axis. A simple frontal system is either a warm front or a cold front, or an occluded front on its own, these single warm fronts are most common in tropical and subtropical areas, whilst single cold fronts are most common in Arctic areas. Because the UK is in an areas where airmasses are constantly fighting for supremacy, complex fronts are common. Complex frontal systems are a feature of low pressure, the the frontal axis is the centre of the low pressure system.

Below is a diagram of a complex frontal system.

Image

Frontal systems can be modified by the sea, for example a cold frontal system delivering snow to the east coast of North America may well have developed warm fronts thanks to sufficiently high sea temperatures, meaning by the time it gets to our shores it is a significantly warmer system, which only delivers rain. However it can work the other way around.. and we see this with Hurricanes.

Tropical storms and Hurricanes have no frontal systems as there is no airmass differential - in essence there is only one airmass involved. As the system tracks northwards into colder waters it starts to develop fronts, this is a reaction to the cooler waters, and so air often develops a warm front once the storm is a tropical depression. Once the storms enter British waters they become extra-tropical, this means the storm has fronts associated with it, which has developed as a result of the cool sea surface temperatures.

The main purpose of frontal systems is to maintain a equilibrium in the troposphere and so fronts are vitally important to our livelihoods and plants!
Heswall, Wirral, UK
USDA equivalent average temperature zone: 9a/RHS zone 3
AHS Heat Zone: 1
Last 5 winter minimums:
2007: -0.1C, 2008: -4.2C, 2009: -5.7C, 2010: -10.5 (record), 2011: -4.9C, 2012: -5.3, 2013: -4.5C (so far)


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Yorkshire Kris
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Re: Learning Weather - Part Four - Fronts and Frontal System

Post by Yorkshire Kris » Fri Nov 18, 2011 8:43 pm

Thanks for taking the time to do these. icon_thumleft icon_salut


Conifers
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Re: Learning Weather - Part Four - Fronts and Frontal System

Post by Conifers » Fri Nov 18, 2011 9:35 pm

stephenprudence wrote: Occluded fronts are less common but occur in complex systems, they can bring about the heaviest rains of all, due to the injection of warm air into a cold system. Often occluded fronts are responsible for tornadoes in the USA where occlusions may intersect a cold front.
I'd disagree at least in part there - occluded fronts are very common in Atlantic weather systems, I'd say the norm close in to the centre of any depression that is more than a day or two old (i.e., virtually all of them by the time they reach Britain). They happen when the faster-moving cold front catches up with the slower-moving warm front. And they don't usually spawn tornadoes in the North Atlantic!

Couldn't be bothered with drawing one myself so filched one off tomorrow's synoptic chart forecast. Note the cold front catching up with the warm front progressively from the centre outward, and forcing the warm sector off the ground so its moisture condenses (that's why occluded fronts produce heavy rain!).
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Conifers
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Re: Learning Weather - Part Four - Fronts and Frontal System

Post by Conifers » Fri Nov 18, 2011 9:50 pm

Vertical cross-sections with C-D through the occluded front, and A-B through the warm and cold fronts
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stephenprudence
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Re: Learning Weather - Part Four - Fronts and Frontal System

Post by stephenprudence » Sat Nov 19, 2011 10:00 pm

I meant occluding system are not as common in complex systems (aka triple point lows), occluded fronts are as common as any other frost otherwise. Triple point lows are texbook occurrences, but the one above is typical situation whereby a warm second is closing up, and allowing cold air to undercut (as I think you are suggesting).

of course colder air undercutting is the bane of our winters :roll:
Heswall, Wirral, UK
USDA equivalent average temperature zone: 9a/RHS zone 3
AHS Heat Zone: 1
Last 5 winter minimums:
2007: -0.1C, 2008: -4.2C, 2009: -5.7C, 2010: -10.5 (record), 2011: -4.9C, 2012: -5.3, 2013: -4.5C (so far)


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