Fronts and Pressure Systems

The chart below is an attempt to place on one chart all the examples of frontal and pressure-centre type that you might see on some sites on the Internet. It is an unrealistic chart of course, though I have tried to keep the faith with meteorological theory!

map of different types of fronts etc.



Cold Front: Cold air replaces warm air at either the surface (surface cold front) or at some level aloft (upper cold front). Cold fronts are usually well-defined at the surface, and can have either ana- or kata-characteristics. (See the Glossary for more on this topic). Upper cold fronts (and occlusions) are best defined in terms of satellite imagery, particularly well picked out by the contrast between IR and VIS channels (when available) [ see Split-frontal type in the Glossary and also the article on Over-running Troughs].

Warm Front: Warm air replaces cold air at either the surface (surface warm front) or at some level aloft (upper warm front). Warm fronts are often ill-defined at the surface, particularly over land areas in the summer half-year, and careful analysis of dew points, changes in low-level cloud structure etc., is often required to pick out the front. Upper warm fronts can sometimes be located (on analysis) by reference to rainfall radar imagery.

Occluded Front: A classical Norwegian occlusion occurs where the surface cold front catches up with the warm front, and the warm-sector air is lifted off the surface in a wedge. Occlusions can be warm or cold, though on modern analysis charts, the distinction is not often preserved in the symbology. Cold occlusions mark (on the surface) a change to colder post-frontal air; warm occlusions mark a change to warm (or less cold) post-frontal air. The former is the more typical type, the latter a feature of winter and early spring, particularly where cold, continental anticyclonic blocks are slow to give way.

Upper Front: A front that has more significance above the surface - very roughly above 3km (or 10000 ft / 700hPa). Found by using parameters such as ThetaW, ThetaE, partial or total thickness etc. Broadly, can be regarded as occurring at some level between 850hPa and 500hPa, i.e. in the lower troposphere, below the level of Non-Divergence.

Convergence Line/zone: Most fronts have some form of convergence associated with them (except if they are very weak), but sometimes, due for example surface heating, convergence zones form which are not deeply baroclinic, but which have the potential to trigger intense convection. Best found by drawing streamlines, but remember that these latter only show confluence, NOT convergence (see the Glossary for explanation of all these terms). You need to inspect the wind speeds as well as directions to determine if there is true convergence of mass.

Trough: analogous to valleys on contour maps - with lower contour or pressure values along the axis of the trough relative to adjacent regions. (see "What is a trough?")

Frontal movement: except for the quasi-stationary front (which of course stays almost stationary), the 'spikes' or 'bumps' point in the direction that the front has been moving over the recent past (analysis) or is expected to be moving at verification time (forecast).

List of features shown on chart above

[ You will increasingly see on these charts occlusions with the same symbology as for frontolysing cold and warm fronts: I have not shown such above, as in my view, an occlusion is weakening anyway (from the standard Norwegian frontal theory), and to show such as weakening is a bit of 'over-egging the pudding'. Also, it is not included in the standard frontal types for use on monochromatic picture facsimile as authorised by the World Meteorological Organisation.]


Over-running Troughs

In view "A", the mid-tropospheric trough (nominally around 500 hPa) is a fairly sharp, easily identifiable feature, with the trough axis to the rear of the surface location of the Occlusion/cold-front. Under PVA-maxima conditions, vertical (upward) motion is focussed just forward of the trough-axis, leading to thick cloud, high precipitation intensity (other factors being right). Given the location of the upper forcing relative to the surface trough, the front appears to be a rearward sloping/ana-frontal type.

trough not yet overrun

Rearward of the trough axis, lies the zone of negative vorticity advection (NVA) associated with descending air and the area of relatively low relative humidity conditions (at mid-tropospheric levels) shown.

In view "B", the primary forcing trough has now 'relaxed' away and is losing some of its shape, thus the dynamics (vorticity advection >> strong upward motion etc.) are also weakening, and for this reason alone, the frontal activity will start to fragment.

However, there is an element of 'de-coupling' also in play, as the upper activity moves away from the lower-tropospheric humid zone, and in satellite imagery, the cold topped cloud (seen via IR channels) will move well ahead of the low level frontal break (seen in VIS channels), and will appear to be divorced from surface discontinuities such as wind-shift, dew-point drops etc.

trough now overrun

The upper trough will still have vorticity forcing associated with it of course (albeit weaker), and may manifest itself as the upper cold front shown - this feature is found to the rearward of the IR cloud mass, with the major mid-tropospheric dis-continuity being the change from high relative humidity shown, and the now advancing dry/descending air associated with the region of NVA. The surface front, by and large, loses considerable activity. However, care should be taken in this case, as in spring/early summer in particular, as the drier air moves into what is effectively the warm sector, and over-runs humid low level air, destabilisation can lead to marked convective activity.