Meteorology

Forecast showers

cloud, allowing you to forecast showers:
 

1. A deepening (growing taller) cumulus signals showers. So, check whether the distance from base to top larger is than the distance between the earth's surface and the base.
   The right cloud spells rain, the left one is harmless.
    
2. A solid, preferably top heavy cloud is much more likely to produce rain than a chimney cloud. So, check whether the top is as wide as the base.
   The right cloud spells rain, the left one is harmless.
    
3. A pileus (Latin: “cap”) is an eyebrow shaped cap over the top of a developing rain cloud. Eventually, the ascending cloud overtakes the icy pileus formations and leaves these behind. In general: clouds that generate pilei are those most likely to develop into full-blown cumulonimbus clouds. Pileus examples.
   Both clouds bring rain.
    

Further insights

• It is easy to tell the difference between ice and liquid in a cloud, because liquid has a very sharp cloud edge, yet ice has a diffuse and fuzzy cloud edge.
• Virga is the meteorological term for rain, snow or ice crystals visibly falling from a cloud in a streak, without reaching the ground due to evaporation. Virga examples.
• A cloud that starts to rain will affect the wind we sail in. Rain cools the air underneath the cloud through evaporation as it collects heat (similar to perspiration), resulting in a downwash of colder air. Expect gusts and a new wind direction which is a combination of the original wind direction and the cloud's direction. A big cumulonimbus will actually take over the local wind regime.
• Rain most often starts off as snow, so an high rising cloud is necessary to reach the colder altitudes. Hence the icy pilei and the height of the cumulus are good indications if showers are imminent.