Rain & clouds

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Forecast showers

An expedient guide to visually distinct between a fair weather cumulus and a rain bearing cumulonimbus 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.Forecast showers - deepening cloud
    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.A cloud should be solid if it is to produce rain
    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. Pilei examples. Two different clouds with a pileus. The pileus in the right example is left behind.
    Both clouds bring rain and quite possibly severe weather as these caps indicate strong updrafts within the clouds.


The pleasant pungence of the air after it started raining is called “petrichor”. It comes — like so many beautiful words do — from the ancient Greek: a combination of ichor ἰχώρ, the ethereal golden essence the Greeks believed flowed through the veins of their gods, and petros πέτρος, the stones that form the surface of the earth.

Especially during long periods of drought decomposed organic materials are blown airborn from dry soil landing on dirt and rock where it reacts with minerals, producing potent petrichor molecules. In fact without rain these molecules remain in the soil and signal plants to prefend roots from growing and seeds from sprouting.

The first bouncing raindrops will bring lovely smells upwards, and the absence of petrichor molecules in the soil will signal plants to start growing and flowering and seeds to start sprouting.

One noteworthy and ubiquitous petrichor molecule is geosmin, a metabolic by-product of bacteria and blue-green algae. Its name is again derived from the ancient Greek: a combination of earth γεω and smell (noun) ὀσμή.
Camels will home in on geosmin to find an oasis and the bacteria will then see their spores delivered elsewhere. But also our own human nose is extremely sensitive to geosmin and is able to detect it at concentrations as low as 5 parts per trillion. If fish, wine or vegetables (notably beets) are too strongly flavoured with geosmin, then boiling or acidity (vinegar, tomatoes, citrus fruits) will easily break this muddy taste down to pleasant levels.

The petrichor smell of summer rain brings us positive associations → fertility and kisses ☺


Already before a thunderstorm arrives we can smell ozone, O₃, because lightning splits oxygen molecules, O₂, into oxygen atoms which subsequently react with oxygen molecules to form ozone.
The name ozone derives from ozein ὄζειν, the ancient Greek word for smell (verb), referring to ozone's distinctive smell.

Further insights

Related pages:
Rain isn't teardrop shaped
Meltemi forecasting
Wind chill
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Diederik Willemsen | E-mail me
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