Precipitation types & freezing layers

What does the code tell you about the material falling?

Use the international code terms rather than regionally ambiguous words such as sleet.

Liquid and frozen precipitation

DZ identifies drizzle, generally small closely spaced liquid drops; RA identifies rain. SN is snow, falling ice crystals or aggregates. SG means snow grains: very small opaque white grains, generally from shallow stratiform cloud or fog rather than a convective shower. PL identifies ice pellets, usually hard translucent particles formed by refreezing rain or substantially melted snow. A regional everyday word such as 'sleet' can refer to different mixtures, so PL is clearer in an aviation interpretation.

GR is hail when the largest hailstones are 5 mm or more across. GS covers small hail below that size and snow pellets, which are softer, opaque rimed particles rather than hard refrozen raindrops. IC identifies ice crystals, sometimes called diamond dust in very cold conditions. FZDZ and FZRA identify supercooled drizzle and rain that can freeze on suitable surfaces; they are not synonyms for PL, which is already frozen before impact.

How small is a cloud droplet compared with rain?

Keep micrometres, millimetres, diameter and volume separate.

Representative dimensions

An ordinary cloud droplet is often around 10–20 micrometres in diameter, or 0.01–0.02 mm, with a wider distribution in real cloud. Drizzle drops are usually below 0.5 mm, commonly of order 0.1 mm; rain drops are usually greater than 0.5 mm and can reach several millimetres before breakup limits very large drops. These are approximate size guides, not rigid identification by a ruler alone.

WMO also distinguishes the closely spaced, fine falling drops of drizzle from more scattered raindrops. Partly evaporated raindrops at a precipitation-zone edge can be as small as drizzle, so their distribution and source matter. For aviation hazard assessment, the mass and inertia of supercooled large drops are important because they can strike and freeze farther aft than typical small cloud droplets.

A 2 mm raindrop has 100 times the diameter and about 1,000,000 times the volume of a 0.02 mm spherical cloud droplet. Confusing radius with diameter or comparing their volumes linearly produces a very large error. The precipitation-growth model demonstrates the cube relationship with ideal equal-density droplets; it does not imply perfect collection in a real cloud.

When does falling water refreeze before it reaches the ground?

Read the full temperature profile and the particle's previous phase.

Melting followed by a cold layer

A common freezing-rain pathway begins with snow aloft, melting in an above-zero layer and then entering a shallow sub-zero layer near the surface. The liquid cools below freezing but can remain supercooled until it strikes a suitable surface. Warm frontal overrunning, with warm air above a retained cold surface air mass, often supplies this temperature inversion and precipitation arrangement.

If the lower cold layer is sufficiently deep and cold, liquid or partially melted particles can refreeze into ice pellets before reaching the ground. The depth, temperature and time of fall all matter: there is no universal single thickness separating every PL and FZRA event. Surface PL can indicate a freezing-liquid region higher up in the warm-nose system, so it does not prove a safe liquid-free aircraft environment aloft.

Snow may persist if no sufficient melting layer exists; rain can reach a warm surface after melting. The height of the 0 °C isotherm by itself is not the whole answer when a profile crosses zero more than once. Locate every warm and cold layer and assess the particle history. The sounding and precipitation forecast should support the interpretation rather than a rule based only on surface temperature.

Does freezing drizzle prove warm air exists above the aircraft?

Include liquid collection in an ice-poor cloud, not only the melting-snow pathway.

Collection without a melting layer

An entirely sub-zero stratiform cloud can retain abundant supercooled liquid water if ice formation is limited. Some droplets grow by collision–coalescence into drizzle-sized drops. They can remain supercooled while falling through the cloud and below it, producing freezing drizzle without an above-zero layer anywhere in the relevant cloud. A warm layer is therefore not a necessary condition for all freezing precipitation.

A shallow moist layer beneath drier air, weak ascent and cloud tops not cold enough to ensure extensive glaciation can support this situation, but temperature alone cannot diagnose its microphysics. A surface FZDZ report is a warning to examine the cloud liquid-water and large-drop environment; it is not evidence that climbing a small amount will necessarily reach warm air. Approved icing limitations and escape procedures depend on the actual aircraft and conditions.

The contrast with the classical warm-nose FZRA profile is important for both examination reasoning and flight planning. One mechanism melts ice before supercooling the liquid; the other grows liquid within an already supercooled cloud. Both can expose an aircraft to large drops that behave differently from ordinary small cloud droplets.

Which cloud process fits the precipitation pattern?

Use typical associations without making a cloud name a universal intensity forecast.

Stratiform and convective tendencies

Broad stratiform lifting commonly gives widespread continuous or intermittent rain or snow from As/Ns systems, while low stratus can produce drizzle, freezing drizzle or snow grains. A low Sc field may produce light drizzle or other weak precipitation when microphysics permits. The typical intensity is often light to moderate, but a thick moist stratiform system can sustain substantial precipitation and embedded convection can add much heavier bursts.

Cumulus and cumulonimbus give more localised showers with relatively abrupt onset, end and intensity changes. Deep Cu/Cb can produce heavy rain or snow showers, snow pellets, small hail or large hail; strong thunderstorm precipitation can have much poorer visibility than the surrounding route. Snow grains are associated with shallow stable cloud rather than a classic convective shower, whereas snow pellets often occur in a convective cold-air setting.

Cloud type, moisture content, growth efficiency and vertical motion all influence intensity. The SH descriptor identifies a showery character, while an intensity qualifier expresses the reported or forecast intensity under the applicable code rules. A shower need not be heavy, and a continuous rain region need not be light. Always use the complete message and weather structure rather than replacing it with one memorised cloud-to-code pair.

Why can dry powder snow be so difficult to see through?

Consider number, shape, concentration and suspension as well as total water.

No universal visibility from one flake size

Winter precipitation varies from tiny crystals or grains to large aggregates and wet flakes. Visibility depends on the number and projected area of particles in the optical path, their shape, fall speed and concentration, not simply the liquid-water amount or the diameter of one flake. Large wet aggregates can produce strong obscuration, and many smaller particles can also give very poor visibility. A fixed accumulation rate does not uniquely determine a cockpit view.

Dry fluffy snow has low bulk density, often falls relatively slowly and is readily lifted again by wind. Falling and blowing dry snow can therefore keep many particles suspended and reduce visibility severely, including after the main snowfall has stopped. Wet heavy snow bonds more readily and is less easily blown under otherwise similar conditions, although intense wet snowfall can still obscure the scene. Do not turn that tendency into a guarantee that one snow type is safe.

Compare the actual visibility report with snowfall intensity, snow character, wind and the route's exposed terrain. A transition from larger wet flakes to fine dry blowing snow can change the operational view even without a simple increase in liquid-water precipitation rate. Drizzle illustrates the same general optical issue in liquid form: many small droplets can impair visibility more than a smaller number of larger raindrops in a comparable situation.

Can you apply the ideas?

Four original questions. Decide from the facts, then review the explanation.