How can precipitation fall as snow, sleet, or freezing rain?

Picture the precipitation falling through stacked layers of air. It commonly begins as snow aloft; what matters next is whether it crosses air warmer than freezing, how much of the snow melts there, and how deep a freezing layer lies below it. [S1]

Here is a simplified three-column layer diagram. Read each column from top to bottom; “above freezing” means warmer than freezing, and “freezing” means at or below it.

SNOW                     SLEET                    FREEZING RAIN
Cloud                    Cloud                    Cloud
Snow forms               Snow forms               Snow forms
   ↓                         ↓                        ↓
Freezing air             Shallow above-freezing    Above-freezing layer
Snow stays frozen        layer; snow partly melts  Snow melts completely
   ↓                         ↓                        ↓
Freezing air             Deep freezing layer      Shallow freezing layer
Snow reaches ground      Drops refreeze            Liquid drops reach ground
   ↓                         ↓                        ↓
SNOW                     SLEET                    Freeze on contact

These are simplified temperature-profile paths, not a complete account of every precipitation situation. [S1]

Both paths include a warm layer that melts falling snow. The distinction is what happens below it.

For sleet, the snow melts only partly in a shallow warm layer. The partly melted flakes then pass through a sufficiently deep freezing layer and refreeze before reaching the ground. For freezing rain, the warm layer melts the snow completely, but the drops pass through only a shallow freezing layer. They remain liquid until they freeze on contact. [S1]

So “there was a warm layer” is not enough information to tell the two apart. You also need to know how completely the snow melted and whether the lower freezing layer was deep enough for refreezing before the drops reached the surface.

Use this three-step reading method, from the cloud downward:

1. Check the path for a warm layer. If the air stays at or below freezing all the way from cloud base to ground, the simplified path is snow.
2. If there is a warm layer, ask how much melting occurs. Partial melting followed by a sufficiently deep freezing layer points to sleet.
3. If the snow melts completely, check the lower freezing layer. A shallow one can let liquid drops reach the ground and freeze on contact: freezing rain. [S1]

For example, suppose a profile says “snow aloft; shallow warm layer; deep freezing layer below.” Applying the guide gives sleet, because partial melting is followed by enough freezing air for the drops to refreeze before reaching the surface. If the profile instead says “snow melts completely; only a shallow freezing layer near the ground,” the guide gives freezing rain. These are illustrative profiles, not observations of a particular place or time.

Treat the result as uncertain if a key layer is missing. A profile that mentions a warm layer but says nothing about how much melting occurs or the depth of the freezing air below it does not, by itself, establish whether the outcome is sleet or freezing rain. The simple paths above describe the distinctions in the cited explanation; they do not establish what is happening in any specific location or forecast. [S1]

A useful note to keep with a diagram is: “Follow the precipitation downward: stays frozen = snow; partly melts, then refreezes in a deep lower freezing layer = sleet; melts completely, then reaches the ground as liquid through a shallow freezing layer = freezing rain.”