What is the weather going to be like ?

Heterogeneous, multi-phase, compressible, viscous, diabatic, 'coriolised', turbulent and baroclinic, the atmosphere is a complex environment.  Consequently weather forecasting is a difficult exercise, the result of multiple combinations of different thermodynamic factors such as heat, humidity and air speed.  Covering periods of as long as 72 hours a single forecast may take a specialist more than half a day.  Beginning with the numerical model provided by Météo-France, each meteorologist then generates and refines his forecasts according to his own preferred reference parameters.

As an example, at CSG Bruno Jacquemin generally begins with the dew point temperature at 400 hectopascals, i.e. at an altitude of 7,500 m, which characterises how far the temperature of the air mass must fall in order to produce a cloud.  Bernard Dupont favours the undulations in the dynamic tropopause, the separation layer located at around 17 km in altitude.  Acting like a 'lid' which the large clouds come and bump up against, the tropopause represents the upper limit of meteorological phenomena.  Michel Rameau has a preference for anticyclones and depressions at ground level, especially undulating flow at around 12 km in altitude (the famous 'thalwegs').  Lastly, Philippe Lumineau likes conventional humidity measurements at 500 m and 800 m, as well as CAPE (Convective Available Potential Energy) data.  CAPE indicates the energy potentially available for the formation of unstable cloud masses, a phenomenon known as convection.

* Cumulus congestus, cumulonimbus…As far as clouds are concerned, the Latin identifiers and the names that precede them, are more expressive than we might think: a cloud may be capillatus (hairy, also known as anvil-shaped) or quite the opposite: calvus (bald).   Pileus refers to a hood and velum a veil.

* French Guiana has four times the rainfall of Paris, but the rain only falls for half the time.

This is partly due to the structure of the clouds, which are much thicker in French Guiana (up to 18 km compared with a maximum of 10 km on the French mainland).  In addition, the phenomenon of hygrometry plays an important role: at 0°C one kilogramme of air may contain 5 g of water vapour at most.  At 30°C (the typical temperature at French Guiana's latitude), this same kilogramme of air may contain as much as 25 g of vapour.  To put it briefly, a greater quantity of vapour in a colossal cloud mass generates intense – but not necessarily enduring – rainfall.

* In Guiana again, the top of the cumulonimbus, at nearly 20 km in altitude, can generate as much as one million volts per metre.  Best avoided!