Fronts and Mid-latitude Cyclones Notes

A front is a boundary between two characteristically different air masses.
It is a region of significant horizontal gradients in temperature or humidity.
Fronts are typically 100 to 200 km wide, but very sharp transitions are possible over a few km or even hundreds of metres.
The term 'front' was coined during WW1 by the Norwegians to describe lines of conflict between different air masses, resulting in bad weather.
Fronts are a dominant feature of mid-latitudes and are particularly associated with low-pressure systems.
The movement of fronts is responsible for much of the day-to-day variability in weather conditions.
Northwest Europe receives many different air mass types, with frequent frontal passages, resulting in very variable weather.

In the 19th century, forecasters believed that cyclones were symmetrical (like tropical cyclones).
Margules (1905) first showed how a front could be sustained by the winds blowing along the frontal surface.
The 'frontal cyclone' model was pioneered by the Bergen school (Norway) around and just after WW I.
This was the beginning of major improvements in quantitative weather prediction, including the timing of events and prediction of their development.

Warm air flows up over denser cold air.
The inclination of the frontal surface is very shallow, approximately 0.5 to 1 degree.
The approach of a warm front is signaled by high cirrus or cirrostratus clouds, with the cloud base lowering as the surface front approaches.
Rain starts ahead of the surface front, is widespread, and persistent.
Skies clear quickly after the passage of the surface front.
Cloud sequence: cirrus, cirrostratus, altostratus, nimbostratus.

Dense cold air pushes forward into warmer air, which is forced upward.
Cold fronts are steeper than warm fronts, with a slope of approximately 2 degrees or $1/50$ .
Deep convective clouds (cumulonimbus) form above the surface front, resulting in heavy rain in a narrow band along the surface front.
Behind the front, the cloud base lifts, eventually clearing.
Near the surface, the cold air may surge forward, producing a very steep frontal zone.

Type 1: Dense cold air pushes forward into warmer air, and over-runs it.
Type 2: Deep convective clouds form ahead of the upper front, with heavy rain in a narrow band along the upper front. Between the upper and surface fronts, there is shallower cloud.

Cold fronts generally move faster than warm fronts and may catch up with a warm front ahead, resulting in an occluded front.
There are two types of occluded fronts: warm and cold, depending on whether the air behind the cold front is warmer or cooler than the air ahead of the warm front.
Occlusion is part of the cycle of frontal development and decay within mid-latitude low-pressure systems.

Cool air replaces surface warm air.
The wedge of warm air is associated with layered clouds and frequently with precipitation.
Precipitation can be heavy if warm moist air is forced up rapidly by the occlusion.

There is only a small difference between the air masses on either side of a stationary front.
The front is defined by the direction of motion only.
When the boundary between air masses does not move, it is called a stationary front.
The wind speed is not zero; the individual air masses still move, but the boundary between them does not.

Low-pressure systems (or mid-latitude cyclones) are a characteristic feature of mid-latitude temperate zones.
Low-pressure systems form in well-defined zones associated with the polar front.
These systems provide a strong temperature gradient and convergent flow resulting from the global circulation (jet stream).

Low pressure forms at the surface over the polar front due to divergence aloft.
As rotation around the initial low starts, a 'wave' develops on the polar front.
Flow near the surface is directed towards the low center due to friction.
Mass balance: inward flow is compensated by large-scale lifting, leading to cooling and cloud formation.
The surface low is maintained (or deepens) due to divergence aloft exceeding convergence at the surface.
The cold sector air pushes the cold front forward; warm sector air flows up the warm front (warm front moves slower than cold).
The cold front overtakes the warm front to form an occlusion, which works out from the center.
Depression usually achieves maximum intensity 12-24 hours after the start of occlusion.
The low starts to weaken as inflowing air 'fills up' the low pressure.
The low continues to weaken, and clouds break up.

Since the 1970s, coherent belts of low-level winds have been identified ahead of cold and warm fronts.
These are important in the transport (advection) of atmospheric properties such as heat, moisture, and trace gases.