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!
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
- C1: Classical surface cold front
- C2: Upper cold front
- C3: Cold frontogenesis: i.e. cold front forming/intensifying at surface
- C4: Cold frontolysis: i.e. cold front weakening/dissipating
[ Strictly, the cross-stroke should be at an angle to the frontal line, not 'normal' to same: unfortunately, when the software for producing the fronts on the Bracknell (now Exeter) charts was developed, this error slipped through & we are now stuck with it! ]
- W1: Classical surface warm front
- W2: Upper warm front
- W3: Warm frontogenesis: i.e. warm front forming/intensifying at surface
- W4: Warm frontolysis: i.e. warm front weakening/dissipating.
[ See note above about the angle of the 'cross' line being erroneous. ]
- O1: Classical, recently-occluded front at surface
- O2: 'Graveyard' long-since occluded front
- O3: 'Wrap-around' occlusion
- O4: Upper occlusion. (I have seen this on a chart, but really it's a misnomer - the Canadian 'TROWAL' might be better here.)
- O5: 'Instant' occlusion.
(An instant occlusion [or pseudo-occlusion ] is the name that has been coined to label the cloud mass associated with an active trough in the cold air, that comes close to, and interacts with a pre-existing baroclinic zone, forming a pattern that looks superficially as if it was part of a traditional occlusion process.)
[ 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.]
- T1: Cold-air troughs
- T2: 'Thundery' (warm/humid plume-type) trough.
- Q: Quasi-stationary front (QSF): An air-mass boundary which is slow-moving over a period of roughly 24hr. The warm and cold symbols will becorrectly placed with respect to the respective air-mass: i.e. in the example given, warm air lies to the south of the front, and cold air to the north.
- L1: Thundery low (often heat-triggered).
- L2: Classical occluded low pressure area - finished deepening.
- L3: Developing wave-depression
- L4: Cold-air low, non-baroclinic
- H1: Cold air, transitory high pressure cell
- H2: Semi-permanent area of high pressure
- R: Ridge
- X: Col (region between areas of low and high pressure): Little weather over sea, but overland can be foggy (in winter half-year) or thundery (summer half-year)
- V1: Non-developing wave on trailing cold front (too close to parent low for significant development)
- V2: Developing wave on trailing cold front
- Z: Convergence zone.