The “Mares’ Tails” Signature

Cirrus clouds are distinct because they don’t look like typical clouds. They lack the flat base of a cumulus cloud or the gray uniformity of a stratus cloud. Instead, they appear as delicate filaments, hooks, or patches.

This unique appearance is due to their composition. According to NOAA’s JetStream research, cirrus clouds form in the upper troposphere where temperatures are freezing—often below -40°F (-40°C). Unlike lower clouds made of liquid water droplets, Cirrus clouds are made of solid ice crystals.

The “Fall Streaks”: When you see the cloud streak across the sky like a painter’s brushstroke, you are witnessing ice crystals falling from the cloud and sublimating (turning to gas) in drier air below. The curve of the “tail” shows the difference in wind speed at different altitudes. This is why sailors historically called them “Mares’ Tails”—they resemble the flowing tail of a horse in the wind.

Forecasting: The 24-Hour Rule

In our experience at The Weather, Cirrus clouds are the most reliable long-range visual indicator available to the naked eye. While a Nimbus cloud tells you it is raining now, a Cirrus cloud tells you it will rain later.

Here is the pattern we watch for:

1. The Setup: A warm front is approaching. Warm air is lighter than cold air, so it slides up and over the cold air mass sitting on the ground.
2. The Reach: Because the warm air is rising at a gradual slope, it reaches high altitudes (20,000+ ft) hundreds of miles ahead of the actual storm.
3. The Signal: The moisture in that rising warm air freezes, creating Cirrus clouds.

If the Cirrus clouds are isolated and not moving, the weather will likely stay fair. But if the sky begins to fill with them, and they are followed by lower, thicker clouds, a front is inbound. This progression is classic textbook meteorology, supported by National Weather Service classification guidelines.

The Sequence: From Ice to Rain

To truly understand Cirrus, you must see them as “Step 1” in a lowering ceiling event. We often advise hikers and mariners to watch for the following sequence:

• Step 1: Cirrus. High, thin, wispy. (24+ hours out).
• Step 2: Cirrostratus. The ice crystals spread out into a thin veil. This often creates a “Halo” around the sun or moon.
• Step 3: Altostratus. The ceiling lowers to the middle level. The sun becomes dim, like looking through frosted glass. It is no longer casting shadows.
• Step 4: Nimbostratus. The cloud layer hits the low levels. It is dark, thick, and the rain or snow begins.

By recognizing the Cirrus early, you buy yourself time to prepare for the inevitable wet weather at the end of this chain.

🔍 Visual Identification Guide

Identifying the transition: From isolated wisps to a “Mackerel Sky” (Altocumulus).

Watch: Cirrus Clouds in Motion

Video footage helps distinguish the drift of high clouds versus low clouds. In this clip, notice how the Cirrus clouds seem to move slower than lower clouds—this is an illusion caused by their immense distance from the ground.

Winter Context: The Jet Stream Indicator

In winter, Cirrus clouds take on a more critical safety role. Because they live at the level of the jet stream, they can indicate the movement of massive polar air masses.

Sometimes, what looks like Cirrus is actually the “anvil” top of a distant Cumulonimbus cloud that has been blown off by high winds. In winter, this can signal thundersnow squalls are nearby. Understanding this link is vital because cold weather is dangerous not just due to temperature, but due to the disorientation caused by sudden whiteouts associated with these systems.

Frequently Asked Questions