Thickness Pattern Definitions
The chart shown here is intended to show some common features to be found on the 500-1000 hPa thickness chart over the North Atlantic & Europe. It does not represent any one particular situation, and is rather artificial as a result. Refer to the text below for an explanation of the features.
(last updated 29 JAN 2000)
A: COLD POOL (QUASI-STATIONARY, LONG-LASTING TYPE).
B: WARM DOME.
C: COLD POOL (TRANSITORY/MOBILE TYPE).
C': COLD POOL (TRANSITORY/SLOW-MOVING TYPE).
D: COLD POOL (LOW-LATITUDE TYPE).
E: BAROCLINIC ZONES.
There are at least three distinct types of cold pool:
1. Large (in areal extent), slow-moving vortices at latitudes poleward of roughly 50deg N & S (Example A). They are often over, or immediately downwind of the source regions of polar or arctic air masses, in such areas as the Canadian Arctic or Siberian Russia. They take up the characteristics of quasi-permanent features, often appearing on monthly, or even seasonal average maps. The column of very cold (dense) air associated with this version of the cold pool, is reinforced by net outgoing radiation which produces a highly stable, bitterly cold near-surface environment, with a surface ridge or anticyclone on MSLP charts.
2. Once cold air from the source region as detailed in (1.) above is caught up in the circulation of mid-latitude depressions, the characteristics of the air mass are altered. Warming from below (over relatively warmer sea surfaces for example), leads to instability developing, and with an accompanying injection of moisture (after a reasonable length of passage over the sea), cloudy convection (showers, thunderstorms) is triggered. Cold pools (Examples C, C') in these situations are often maxima of such activity, with surface charts showing west or northwesterly airflow, often with cyclonically curved isobars. The cold pool will move in the general synoptic flow, and will eventually warm out (disappear) due to sensible & latent heat exchanges within the environment of the cold pool. However, as at example C', sometimes the cold pool will transform into a slow-moving, longer-lasting entity, particularly if the feed of ex-polar air on its western flank is maintained - this occurs in highly meridional situations.
3. A pool of cold air can also become 'detached' at lower latitudes (Example D), i.e. away from the mid-latitude westerly zone, and drift slowly over relatively warmer seas, (e.g. the Mediterranean), and lead to intense convective development, often taking on marked cyclonic characteristics through the troposphere, and giving rise to locally severe condtions due to heavy rainfall, severe thunderstorms and squally winds. Remnants of these types of cold pool will sometimes drift polewards in summer and bring outbreaks of severe convective activity to mid-latitude regions, as these features will destabilise hot/humid airmasses.
This is a term that is not often heard nowadays, but is applied to the opposite case of the cold pool (Examples B) , where relatively warm (high thickness value) air is enclosed within a closed contour value. The associated low-level weather will be quiet, settled with little vertical development of cloud, if any at all.
Areas where there are marked contrasts between cold and warm air masses (Examples E). These can be determined on a thickness chart by a packing together of contours. Usually associated on a msl chart with classical fronts, and therefore an area for potential cyclonic development.