The five ways air is lifted
Cloud forms when air is lifted and cooled to its dew point. That height is the condensation level, and it is the height of the cloud base. There are five ways the lifting starts, and each one prints a different kind of cloud.
| Lifting agent | How | Cloud produced |
|---|---|---|
| Turbulence | Friction eddies carry air up within the friction layer, in stable air | St or Sc, spread flat, with an inversion above. Called turbulence cloud |
| Orographic uplift | Air forced up over high ground | Layer cloud if stable and moist, lenticular if drier, Cu or Cb if unstable. A rain shadow and a Fohn wind on the lee side |
| Convection | Surface heating lifts a parcel once the ground reaches the critical, or convective, temperature | Cu, then Cb. Fair weather Cu early, big Cu and Cb later in the day. Dissipates at night |
| Widespread ascent, frontal uplift | Warm air rising slowly over colder air at a front | Layer cloud at the warm front, heap cloud at the cold front |
| Convergence | Surface inflow into a low or a trough forces air up | St or Sc at a gentle circular low, Cu or Cb at a strong trough |
Convection cloud cannot form over the sea by surface heating, but cold air moving over a warmer surface, which is advection, makes the lower air unstable and can build Cu or Cb over the sea too. An inversion at any height stops cloud building through it.
How the raindrop is built
A cloud droplet is about 0.02 mm across and falls so slowly the updraught holds it up. To fall as precipitation it has to grow thousands of times heavier. Two processes do that, one for cold cloud and one for warm.
The ice crystal process, cold cloud
The Bergeron process works in cloud whose tops are above the freezing level, where supercooled water droplets and ice crystals sit side by side. The saturation vapour pressure is greater over water than over ice, so the droplets evaporate and the vapour deposits onto the ice crystals. The crystals grow at the expense of the droplets, then collect more supercooled water by collision, and fall out of the base as snow or rain depending on the temperature below.
The coalescence process, warm cloud
In the tropics, showers fall from cloud that never reaches the freezing level, so the ice process cannot be at work. The coalescence theory takes a spread of droplet sizes: the larger drops fall faster, sweep up the smaller ones in their path and in their wake, and grow until they drop out as drizzle or rain.
The precipitation types
| Type | Size | Notes |
|---|---|---|
| Drizzle | 0.2 to 0.5 mm | Visibility 500 to 3000 m, no perceptible impact. From stratus |
| Rain | 0.5 to 5.5 mm | Visibility 3 km or more, down to 1000 m in heavy rain |
| Snow | Crystals and flakes | Visibility 1000 m moderate, 50 to 200 m heavy. Drifting or blowing snow reduces it further |
| Sleet | Mixed | A mixture of rain and snow |
| Hail | 5 to 50 mm and more | Up to 1 kg. Grows by collision with supercooled water and deposition, only in a cumulonimbus |
| Soft hail, graupel | A few mm | Small rounded pellets from wintry showery cloud, an early stage of hail |
| Ice pellets | Under 5 mm | Transparent, spherical or irregular. Fall from layered cloud, often at a warm front |
Virga is precipitation that evaporates before it reaches the ground, a shaft trailing from the cloud base. A cloud burst is a very heavy shower over a small area in a short time, and can bring flash floods. A rainy day in Indian reporting is 2.5 mm or more in 24 hours.
Showers, continuous rain and the summary
Duration tells you the cloud
| Duration | Cloud |
|---|---|
| Showers, short lived | Always heap, convective cloud, Cu or Cb |
| Intermittent, from time to time, no clear break | Layer cloud |
| Continuous, no break for 60 minutes or more | Layer cloud, usually nimbostratus |
Cloud against precipitation
| Cloud | Precipitation |
|---|---|
| Cirrus, cirrostratus, cirrocumulus | None |
| Cumulus, small | Nil to light showers |
| Cumulonimbus | Moderate to heavy showers of rain, snow or hail |
| Stratus, stratocumulus, altocumulus, altostratus | Slight rain or snow, or drizzle |
| Nimbostratus | Moderate to heavy continuous rain or snow |
Intensity is reported as slight, moderate or heavy. A rainfall recorder gives the rate, the intensity. A rain gauge gives only the total amount collected.
When it rains, in India
- In the tropics, rain is mostly from afternoon convection, so it is heaviest in the afternoon and greater in summer. Coastal areas and NE Indian valleys, driven by the sea breeze and katabatic effects, get their maximum at night or early morning.
- In the temperate latitudes, cloud comes from winter depressions and fronts, so rain and snow are heaviest in winter. Over J and K and the western Himalayas, fed by winter western disturbances, the pattern is the same.
Cloud seeding injects nuclei such as silver iodide, common salt or dry ice to trigger the Bergeron process. It can start rain in a small cloud within 15 to 20 minutes and is used against drought, for fire fighting and to disperse fog.