Precipitation

Meteorology · Chapter 11 · DGCA CPL and ATPL

How a cloud's tiny droplets grow big enough to fall, which cloud gives which kind of precipitation, and why Mumbai is drenched while Pune stays dry.

Remember

Drizzle 0.2-0.5 mm, rain 0.5-5 mm, hail 5-50 mm or more. Sleet = rain and snow together. Rainy day: 2.5 mm or more in 24 hours.
Cold clouds (tops well above the freezing level): ice crystals grow at the expense of supercooled drops, because SVP over ice is lower (Bergeron). Warm clouds: coalescence. Sea air: giant salt nuclei.
ST → drizzle. AS → rain or snow. NS → continuous. TCU and CB → showers and hail. Fair-weather CU, AC, CI, CC → nothing.
Tropics: most rain in the afternoon and in summer; coasts and NE valleys at night and early morning. Temperate latitudes and J&K: most in winter (WDs).
The leeward side of mountains is the rain shadow: east of the Western Ghats.

What decides whether it drizzles, pours or hails?

What falls from the sky

Four cards with real photographs. Swipe or tap Next.

From mist-fine drizzle to golf-ball hail Drizzle drops are 0.2-0.5 mm across, rain 0.5-5 mm, and shower drops can be larger. Hail is balls or lumps of ice 5-50 mm or more; stones of 1 kg have been reported. Snow is an assembly of ice crystals; ice pellets are small transparent ice particles; snow grains and ice crystals also count as precipitation.
The cloud decides A drop falls only when it is heavy enough to overcome the up-currents inside the cloud. Layer clouds have gentle currents, so their drops are small. Towering CB has violent updraughts that keep lifting drops and ice until they are big and heavy. So each cloud type has its own precipitation.
Ice that dents metal Hail grows in the strong updraughts of CB, where supercooled water freezes in layers on ice pellets carried up and down many times. It can fall from the storm and even from clear air under the anvil. This NASA external fuel tank was pitted by hail in a single Florida thunderstorm.
Cloud bursts and flash floods Very heavy rain or showers over a small area in a short time is a cloud burst. A sudden rise of rivers and streams is a flash flood. This satellite view shows the Indus valley before and during the 2010 monsoon floods; flood water shows as dark blue-green.

Takeaway: Precipitation is water drops or ice heavy enough to beat the cloud's up-currents. Strong up-currents (CB) grow hail and big shower drops; gentle ones (ST) let only drizzle out.

How big is a raindrop compared with drizzle and hail?

Drop sizes

Each picture is drawn to scale. Tap Next to grow the drops.

Liquid Drizzle (DZ) is made of very small drops, 0.2-0.5 mm across. Rain (RA) drops are 0.5-5 mm. Showers (SH) have drops (or solid bits) over 5 mm and fall from convective cloud, the heaped-up kind. For anything to fall at all, it must grow big enough to beat the updraughts in the cloud.
Hail Hail (GR) is balls or lumps of ice from 5 mm to 50 mm or more across. The smallest hailstone is as big as the biggest raindrop; the largest is ten times wider. A hailstone weighing 1 kg has been reported.
Other solid forms Snow (SN) falls as flakes of ice crystals. Sleet is rain and snow falling together. Ice pellets (PL) are clear balls of ice under 5 mm, often from thick nimbostratus at warm fronts. Snow grains (SG) and ice crystals (IC) are other, smaller solid forms.

Memory hook: Drizzle under 0.5 mm, rain to 5 mm, showers over 5, hail 5-50.

Takeaway: Precipitation is water or ice falling from cloud to the ground. Drizzle drops are 0.2-0.5 mm, rain 0.5-5 mm, shower drops over 5 mm, and hail 5-50 mm or more. Snow is ice crystals, sleet is rain and snow mixed, and ice pellets are small, clear balls of ice.