Water Vapour Imagery
(This note wouldn't pass muster as an answer to a question in a degree-atmospheric physics paper, so don't use it a such! It's simply an attempt to put into words for the intelligent layman the basic concepts of some powerful principles used in current operational and research meteorology)
The advent of high-quality water-vapour imagery (WVI), received from geostationary satellites, offer meteorologists a "new" way of visualising developments in the atmosphere as they occur ('real time').
At its most basic, viewing areas of moist (usually presented as white, or very light-grey shades) as distinct from dry (dark/black areas), is obviously useful. [ However, it is important to remember that the sensors are detecting radiance from a broad (altitude) band, with diffuse boundaries between 400 and 200hPa (circa 7 to 11km)]. Now, not only can tangible clouds of water droplets or ice crystals be observed and followed, but also the potentially cloudy areas can be observed - at least in the upper troposphere.
Once frequent images were obtained (at least hourly), then these moist and dry regions could be followed. One use is to monitor the advection (movement) of dry (dark) areas over the top of humid and potentially unstable lower tropospheric air-masses (detected by other traditional synoptic methods). Such situations can lead to "explosive" convective activity.
In the field of aviation meteorology, the sharp discontinuity (light vs. dark/black) between dry & moist regions are associated with jet-stream shear; not only useful to track the position of the jet of course, but also areas of potential clear air turbulence (CAT).
However, allied to theories associated with vorticity-forced cyclonic development, looped WVI really comes into its own.
In adiabatic, frictionless flow, potential vorticity (PV) can be defined as the product of two variables:
the [ absolute vorticity ] of the air (on an isentropic* surface) and ...
its [ static stability ];
or more loosely, a measure of the tendency of air to 'twist' as it flows downstream and the degree of 'damping' to vertical motion. The resultant variable is strictly called 'Isentropic Potential Vorticity' or IPV.
(* isentropic: a thermodynamic process not involving change of entropy - crudely equivalent to adiabatic in meaning. )
Wherever these two quantities have a high & positive value, then PV will be high.
So, how does all this fit in with the use of WVI?
Stratospheric air in association with a powerful jet (and less dramatically with short-wave troughs), have high stability (see the definition relating to the stratosphere), and high, positive absolute vorticity - provided the air is being sampled on the cold side of the strongest flow aloft, or in the area in or just ahead of the axis of an upper trough.
Stratospheric air is also dry. WVI detects such dry regions very efficiently. If we 'see' a dark/dry slot in the imagery, then we can infer that stratospheric air has descended to lower levels, dominating the 7 to 11km column that the satellite is monitoring; these must be areas of high (stratospheric) PV - and "PV anomalies" (discrete dark slots associated with marked cyclonic development) can be used to monitor the vigour, location, phasing etc., of developmental regions.
Why should such discrete areas of dry, stratospheric air be so important? One way of trying to understand what it going on is to understand that if stratospheric air has descended, (revealed as the dark slot on WVI), then there must be motions causing this descent, and where air is going sharply down, there must be air going up! The greater the 'vigour' of the air going down (sharply darkening dry slot over a relatively small area), the greater must be the effect of the air going up - leading to thick cloud cover, rapidly falling pressure, and associated developments involved in cyclonic development.
A more rigorous view, at least in terms of IPV development theory, is to regard well-formed upper-level IPV anomalies as the initiator of cyclonic circulations in the column below it. [(This is analogous to the effect you get when you stir the surface of an initially still bath of water - a small circulatory motion at the surface of the bath will eventually translate to motion deeper within the water: thus it is with the atmosphere (only another fluid after all)].
If this 'stirring' at high levels (the PV anomaly) is coincident with a low-level marked baroclinic zone (i.e. a classical frontal boundary), then, other factors being allowed for, the induced circulation will lead to poleward warm-air advection (and thus falling pressure), equator-ward cold-air advection, and atmospheric 'development' (formation or enhancement of low pressure areas) will occur. (On operational charts, the warm front moves north, the cold front sweeps south, (reverse directions if viewing in the Southern Hemisphere) and the wave depression deepens.)
If the PV forcing is marked and is co-incident with sharp baroclinicity (large gradient of temperature with horizontal distance), then 'explosive deepening' of a depression will occur.
Modern NWP diagnostics packages can output 'pseudo' WVI - this is overlaid on 'real' WVI and any mismatch in model analyses readily seen and allowed for. Forecasters have become used to using water vapour imagery to diagnose and monitor development in mid-latitudes - a powerful tool indeed.
If you want to know a little more about water vapour imagery, see here.
And a classic example of WVI associated with a major NE Atlantic autumn storm is shown here.