Weekend Weather Word #4 – To the Front!

Hey and welcome to this week’s Weekend Weather Word! I do have to apologize for skipping last week’s edition, but I was out with a pretty bad throat infection that took some time to get rid of.  And believe me, blogging under the influence of antibiotics is no fun at all. But back to topic. Last time, we discussed the atmosphere’s highs and lows, and today, I’m going full frontal on you. It’s all about weather fronts, and what they mean. I can already see the first question hovering in the room.

What the hell is a front?

Basically, a front is a boundary between two air masses of differing densities. That’s all there is to it. In theory. But this would be a pretty cheap cop out of a longer article, there is of course much more to it, not least because there’s more type of weather fronts out there than you can shake a stick at. Cold fronts, warm fronts, squall lines, occluded fronts, stalled fronts, and so on. There are some common characteristics that most fronts have in common. The biggest one of these is that the weather rapidly clears up once a front has moved through. That doesn’t mean that it’s all clear, more often than not, the strongest winds associated with everyone’s favourite, a North Atlantic low, occur in the post-frontal zone, which can also be riddled with showers. But enough with the generalisation, time to get back to the front. For starters, I’ll write about a type of front that is all too familiar to us here on the emerald isle.

When you see this, you know that a weather front, a cold front in this case, is about to hit you.

Oh, and before I forget, I’ll also post examples of how to identify these weather fronts on weather charts, just to make it easier for you to identify just what the hell is going on on those charts I keep posting on the board in the office.

Cold Front

440px-cold_front_symbol-svg

A cold front is basically the leading edge of a mass of cool air that’s moving in to replace a previously present area of warmer air mostly in association with a low pressure area. If you don’t know what that is, hang your head in shame and read the last Weekend Weather Word. Due to the higher density of the colder air mass, they tend to not mess around and move pretty quickly. Think of it as a wedge pushing in along the ground, lifting the warmer, less dense air up as it moves along. This creates all kinds of turbulence in the atmosphere, and often leads to high winds, rain or even thunderstorms forming along this leading edge. Have a look at the rainfall radar on the Met Éireann web site the next time a storm moves through. More often than not, you’ll easily be able to spot the band of rain that forms along the leading edge of the associated cold front.

Warm Front

warm_front

With all I’ve just written, it’s pretty easy to say that a warm front is the polar opposite of a cold front. It forms along the leading edge of a mass of warm air that is replacing a previously present area of colder air. It moves more slowly than a cold front, quite simply because cold air is denser than warm air, and therefore a real bitch to remove. Due to this rather slower movement, warm fronts are rarely associated with severe weather like squalls or thunderstorms. That doesn’t mean that there’s nothing happening during the passage of a warm front. Apart from an obvious rise in temperature, hey, we’re talking about a warm front after all, light to moderate rain may develop along the leading edge of the front, which usually clears after the passage of the front. The clouds associated with a warm front can often extend all the way to the ground, appearing as fog, before clearing once the front has passed.

Occluded Front

image_preview

Okay, so warm fronts and cold fronts are actually pretty self-explanatory, but what the hell is an occluded front? The answer is surprisingly simple. Remember what I just wrote about how warm fronts move slower than cold fronts? Well, both kinds of fronts are usually associated with a low pressure area, and an occluded front basically occurs when a cold front catches up with the preceding warm front. I’m not going to go into details about the atmospheric dynamics that take place when this happens, however it basically boils down to the warm front being separated from the core of the low pressure area, both of which then start to decay. Early on, there may still be a significant amount of severe weather associated with the cold front, but the longer the occlusion persists, the more both weather fronts decay, causing any rain or thunderstorms to basically fizzle out.

Stationary Front

220px-stationary_front_symbol-svg

Well, it basically does what it says on the tin. A stationary front is the Cold War of air masses, with neither cold or warm air masses having enough “oomph” to clear out the other. This usually boils down to one simple phenomenon: rain. Lots of rain. Those situations where you get days and days of persistent rain here in Cork are usually due to a weather front becoming stationary right overhead, and dumping a LOT of water on the ground below. So basically, when you hear about a stationary front, you’d better get that kayak ready. You’re gonna need it.

Squall Line

Doppler radar image of a large system of squall lines moving across the southeastern US.

Oh boy, as a storm lover, this one is easily my favourite! A squall line is not a front per se, but a line of severe weather forming ahead of a front. With Irish weather outside the stormy seasons being as lethargic as an arthritic snail on tranquillisers, We don’t see a lot of them, but when a squall line does develop, it sure leaves a mark. A squall line is marked by strong to severe thunderstorms or rainstorms, often associated with strong and damaging winds, and possibly tornadoes, even in Europe. Interestingly enough, these lines develop from one single strong storm cell, before enlongating and thinning, almost as if they were hammered thin by the strong high altitude winds driving it. As these squall lines usually precede a cold front, they are usually found trailing behind strong low pressure areas, but can also form on their own. However they are formed, they should not be taken lightly.

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