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COLD FRONTS

Cold fronts are mid-latitude phenomena - meaning they are not tropical in nature and occur mostly south of 25S or north of 25N. The word "front" means "boundary" and a cold front is a boundary between cold and warm air, where cold air is under-cutting (moving under) warmer air ahead of it. Fronts usually form far south where the westerly windbelt meets the polar easterlies - somewhere near 60S. A disturbance may cause the cold easterly air to invade the warmer westerly air and since cold air is denser, will, move under the warm air. Warm air could also invade the cooler air and ride up over it since, as you should know, warm air is less dense and it is force to rise by the colder air beneath it, it will want to rise further and as it does, air that rises, cools and as it cools, any water vapor in the atmosphere will condense and form clouds. It is suggested that you familiarise youself with synoptic chart terminology before reading this article unless you know basic meteorological terms.


FIG.1 Formation of a cold front in the southern hemisphere.
Below is a cross-section through a cold (blue) and warm (red) front - clouds are Ns (nimbostratus), Ac (altocumulus), As (altostratus), Cs (Cirrostratus), Ci (Cirrus).



FIG.2 Formation of a cold front in the southern hemisphere.


HOWEVER, here is the BIG catch. Most times these cold or warm air invasions into the warmer westerlies die out and no low pressure cell forms etc...WHY ? Well, the mid-latitudes are controlled by what is know as a "top-down" process...a "what ?" I hear you ask....


FIG.3 The "top-down" process.

Well, I think most of you have heard of the "jetstream" - this is a strong current of air (like a river) in the upper atmosphere (10-13km altitude) that snakes around the globe usually between above the surface westerly wind belt (remember, you name a wind FROM where it blows, so westerlies mean the winds blow FROM west TO east). Now, these upper winds/jetstream are FAR stronger than the surface winds...think of a river - the surface waters flow faster than the bottom waters - why, well, friction slows the water at the bottom of the river - the atmosphere is NO different....now the jetstream is not a constant speed airflow or "river", rather, think of it like traffic on a highway - in some places the traffic speeds along and at other places the traffic slows down to perhaps to a crawl. The jetstream is no different - now where it really speeds along in the upper atmosphere, meteorologists refer to it as a "divergent" area, and where it slows down, a "convergent" area. Once again, think about the traffic analogy - when cars on a highway move away from a congested area, the cars start getting spaced further apart as they pick up speed - i.e. the cars are diverging from one another, and where they encounter a snarl up, they become closer together and "converge" on one another....

Where air in the upper atmosphere diverges it really means that air is leaving a certain area faster than it can be replaced, and somehow, it needs to be replaced ASAP - most of it is replaced from the upper atmosphere, but some of the air is replaced from the surface, almost like a giant vacuum cleaner in the upper atmosphere sucking up air from the surface. See figure 3 above.

Now, lets go back to the original scenario that I explained on the surface, i.e. the cold air from the easterlies undercutting the warm westerly air...the warm is forced upwards since it is less dense....NOW here is the connection - IF the upper atmosphere /jetstream is DIVERGENT, i.e. needing air to replace from the surface, it will AID the warm air at the surface (that has been forced upwards by the cold air) to RISE to the upper levels and the stronger the upper-level divergence the more air that is needed from the surface and the faster the air will rise and bingo - a LOW PRESSURE cell is formed on the surface along the boundary between the cold polar easterly air and the warmer westerly air. This connection between the upper and lower atmosphere is called the "top-down process" - since a low pressure will not develop sufficiently into a deep low pressure and cold front if the upper atmosphere does not aid it with strong divergence aloft.


FIG.4 The upper and low circulations super-imposed - dashed lines are surface isobars (equal pressure lines) and solid isobars are of the upper-level ciruclation.

Cold fronts with their low pressure cells move eastwards, since the air in the JETSTREAM moves eastwards and the surface features are "dragged" in the same direction. However, since the air in the Jetstream moves a lot faster than at the surface, the area of strong divergence aloft (and which is causing the low pressure at the surface to exist) sometimes OVERTAKES the surface low pressure and the connection between the upper level divergence and lower atmosphere low pressure is disrupted and the low pressure at the surface will weaken and eventually die out and the cold front weakens and dissipates.

Hence, sometimes the jetstream in autumn in the southern Atlantic is such that active cold fronts may be formed in the southern Pacific Ocean west of Chile and move eastwards and appear strong on satellite images/1STWX/old in the mid Atlantic but weaken and die out or move south of the country owing to the way the jetstream is aligned in the upper atmosphere.....So, to cut a long story short, at certain times of the year, the jetstream is further south (in summer) and most of the cold fronts move south of the country, and during winter, they occur further north and CT gets a lot more rain. However, sometimes for a few days or even weeks, the jetstream may align itself such that cold fronts will weaken as they approach CT frustrating forecasters to no end....

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