Chapter 14, Precipitation 14-1
14 Precipitation
14.1 Introduction
Precipitation is any of the forms of water particles, whether liquid or solid, that fall from the atmosphere
and reach the ground. This chapter covers the necessary ingredients for formation, the growth process, and
the types of precipitation. Some precipitation types include drizzle, rain, freezing rain, freezing drizzle,
snow, snow grains, ice crystals, ice pellets, hail, and small hail and/or snow pellets.
Chapter 14, Precipitation 14-2
14.2 Necessary Ingredients for Formation
Precipitation formation requires three ingredients: water vapor, sufficient lift to condense the water vapor
into clouds, and a growth process that allows cloud droplets to grow large and heavy enough to fall as
precipitation. Significant precipitation usually requires clouds to be at least 4,000 ft thick. The heavier the
precipitation, the thicker the clouds are likely to be. When arriving or departing from an airport reporting
precipitation of light or greater intensity, expect clouds to be more than 4,000 ft thick.
14.3 Growth Process
All clouds contain water, but only some produce precipitation. This is because cloud droplets and/or ice
crystals are too small and light to fall to the ground as precipitation. Because of their microscopic size, the
rate at which cloud droplets fall is incredibly slow. An average cloud dropl et falling from a cloud base
at 3,300 ft (1,000 m) would require about 48 hours to reach the ground. It would never complete this
journey, because it would evaporate within minutes after falling below the cloud base.
Two growth processes exist that allow cloud droplets (or ice crystals) to grow large enough t o reach the
ground as precipit ation before they evaporate (or sublimate). One process is called the
collision-coalescence, or warm rain process (see Figure 14-1). In this process, collisions occur between
cloud droplets of varying sizes and different fall speeds, sticking together or coalescing to form larger drops.
Finally, the drops become too large to be suspended in the air, and they fall to the ground as rain. This is
thought to be the primary growth process in warm, tropical air masses where the freezing level is very high.
Most cloud droplets are too small and light to fall to the ground as precipitation. However, the larger cloud
droplets fall more rapidly and are able to sweep up the smaller ones in their path and grow.
Chapter 14, Precipitation 14-3
Figure 14-1. The Collision-Coalescence or Warm Rain Process
The other process is called the ice crystal process. This occurs in colder clouds when both ice crystals and
water droplets are present. In this situation, it is easier for water vapor to deposit directly onto the ice crystals
so the ice crystals grow at the expense of the water droplets. The crystals eventually become heavy enough
to fall. If it is cold near the surface, it may snow; otherwise, the snowflakes may melt to rain. This is thought
to be the primary growth process in mid-latitudes and high-latitudes.
14.4 Precipitation Types
14.4.1 Snow
The vertical distribution of temperature will often determine the type of precipitation that occurs at the
surface. Snow occurs when the temperature remains below freezing throughout the entire depth of the
atmosphere (see Figure 14-2).
Chapter 14, Precipitation 14-4
Figure 14-2. Snow Temperature Environment
14.4.2 Ice Pellets
Ice pellets (sleet) occur when there is a shallow layer aloft with above -freezing temperatures and with a
deep layer of below -freezing air based at the surface. As snow falls into the shallow warm layer, the
snowflakes partially melt. As the precipitation reenters air that is below freezing, it refreezes into ice pellets
(see Figure 14-3).
Figure 14-3. Ice Pellets Temperature Environment
14.4.3 Freezing Rain
Freezing rain occurs when there is a deep layer aloft with above -freezing temperatures and with a shallow
layer of below-freezing air at the surface. Ordinarily, air temperatures decrease with increasing altitude, but
freezing rain requires a temperature inversion, which can occur when a warmer air mass overlies a colder
air mass. This situation can occur along a warm front, where a warm air mass overruns a cold air mass. It
Chapter 14, Precipitation 14-5
can begin as rain and/or snow but becomes solely rain in the warm layer. The rain falls back into
below-freezing air, but since the depth is shallow, the rain does n ot have time to freeze into ice pellets
(see Figure 14-4). The drops freeze on contact with the ground or exposed objects, such as aircraft.
Figure 14-4. Freezing Rain Temperature Environment
14.4.4 Rain
Rain occurs when there is a deep layer of above freezing air based at the surface (see Figure 14-5).
Figure 14-5. Rain Temperature Environment
14.4.5 Hail
Hail is precipitation in the form of balls or other irregular lumps of ice produced by thunderstorms.
Thunderstorms that are characterized by strong updrafts, large Supercooled Liquid Water Content (SLWC),
large cloud-drop sizes, and great vertical height, are favorable to hail formation.
Chapter 14, Precipitation 14-6
Hail forms when supercooled water droplets above the freezing level begin to freeze. Once a droplet has
frozen, other droplets latch on and freeze to it, so the hailstone grows —sometimes into a huge ice ball.
Large hail occurs with severe thunderstorms with strong updrafts that have built to great heights.
Eventually, the hailstones fall and may be encountered in clear air several miles from the thunderstorm.
An individual unit of hail is called a hailstone. Hailstones can range in size from a pea (0.25 -in diameter)
to larger than a softball (4.5-in diameter). Hail competes with turbulence as the greatest thunderstorm hazard
to aircraft. Hailstones that are 0.75 in in diameter and larger can cause significant damage to aircraft and
make it difficult to control. A hailstone was collec ted at Vivian, SD, on July 23, 2010, that measured 8 in
in diameter, 18.62 in in circumference, and weighed 1.93 lb (see Figure 14-6).
As hailstones fall through air with temperatures above 0°C, they begin to melt, and precipitation may reach
the ground as either hail or rain. Rain at the surface does not mean the absence of hail aloft. Pilots should
anticipate possible hail with any thunderstorm, especially beneath the anvil of a large cumulonimbus.
Figure 14-6. Vivian, South Dakota, Record Hailstone
Hail is most frequently found in the interior of continents within the mid -latitudes and generally confined
to higher elevations within the tropics. In the U nited States, hail is most common across the Great Plains
region east of the Rocky Mountains. Hail is more common aloft and at higher elevations, because the stones
begin to melt when they fall below the freezing level, and the smaller stones may melt into raindrops before
they reach the surface.
When viewed from the air, it is evident that hail falls in paths known as hail swaths. They can range in size
from a few acres to an area 10 mi wide and 100 mi long. Piles of hail in hail swaths have been deep enough
to need a snowplow to remove them, and occasionally hail drifts have been reported.