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Eclipse (of a geostationary satellite)

The earth's equator (and therefore a geostationary satellite's orbit) is inclined to the orbit of the earth around the sun. This inclination allows sunlight to power the satellite on-board systems for most of the year. However, there is a period of about 3 weeks either side of the vernal and autumnal equinoxes when a satellite will be in the earth's shadow for about 70 minutes each day (around local midnight). Because most of these platforms do not carry sufficient battery power to tide them over this gap, no imagery is generated and thus a local-midnight image is missing.


European Centre for Medium Range Weather Forecasts, located on the southern outskirts of Reading, Berkshire, UK. Visit their web site at

Eden Winter Snow Index

See Snow Index


See Environmental Lapse Rate


Embedded (in cloud, as in CB embedded in LYR cloud).


A collection of NWP runs (typically in excess of 15, many having 50 or more) from the same start time (t=0) and using the same model physics, but each run (or 'member') having a slightly perturbed (altered) set of initial conditions from the 'control' run (q.v). The alterations are constrained within limits which are calculated in various ways - one example being that of performing a separate short-range model run and identifying the errors that would grow most over a 48 hr period. These errors are then applied in varying amounts to the initial conditions before performing the operational ensemble run. Another technique is to use (known) errors from a previous run and apply these in small amounts to the initial conditions of the new run. [ NB: these output are in addition to (and run some time after) the 'operational' model output, i.e. the deterministic run which is the set of charts most often seen on web sites: it should not be assumed that the operational run (OP/OPER: q.v.) is close to the ensemble mean (q.v.) or mode - significant deviations can and do occur at longer lead times. Also note that a particular centre's operational model is often run at a higher spatial resolution than that used for the ensemble generation - the control.]

Ensemble mean

An average of the ensemble output from a particular computer run - this is usually more accurate than just following one of the individual forecasts that make up the average. Further, by comparing the individual members 'spread' about the mean, some estimate can be made of the reliability of the forecast: if there is strong agreement and therefore small divergence from the mean solution, then high confidence can be assigned to the average solution. Wide divergence, or clustering of groups of individual members well away from the mean will lead to considerable caution regarding using the output too slavishly and lower confidence in issued forecasts.

Environmental Lapse Rate

The actual plot of temperature against height (or equivalent) on a thermodynamic diagram.


The transformation of liquid water to water vapour - in the process 'absorbing' latent heat (of vapourisation).


(abbr)(also EWR) England and Wales Precipitation (or Rainfall). A data series combining the rainfall (and melted snowfall) amounts from a matrix of recording stations (well over 30) averaged to produce a single figure for an area taken to represent 'England and Wales'. The series runs from 1766 - maintained (separately) by the Hadley Centre (EWR) and the University of East Anglia (EWP), though I understand that a unified data-set is to be (has been?) produced. CLICK HERE FOR LATEST DATA FROM THE HADLEY CENTRE.

Explosive cyclogenesis

Sometimes, in an otherwise 'normal' cyclogenetic situation, factors are conducive to rapid falls of pressure leading to very tight isobaric gradients / extreme low pressure. These situations often give rise to 'damaging' or stormy / hurricane force winds: watch for 3-hourly pressure falls in excess of 10.0 mbar. (sometimes referred to as 'bombs', particularly in North American meteorological circles.)