Anvil Tops

Part of Weather Forecasting

The flat, spreading top of a thunderstorm reveals storm severity, direction, and intensity at a glance.

How Anvils Form

A thunderstorm’s updraft pushes warm, moist air upward at speeds that can exceed 100 mph. This column of rising air cools, condenses, and forms the towering cumulonimbus. But it cannot rise forever.

At the tropopause — the boundary between the troposphere and stratosphere, roughly 35,000-45,000 feet in mid-latitudes — the air abruptly stops warming with altitude and starts getting warmer above. This temperature inversion acts as a ceiling. The updraft slams into this ceiling and has nowhere to go but sideways.

The cloud top spreads horizontally, forming the characteristic flat anvil shape. The anvil is made entirely of ice crystals carried outward by upper-level winds. It can extend 50-100 miles downwind from the storm tower.

Reading the Anvil

The anvil is the most visible part of a distant thunderstorm and contains critical information about what the storm is doing.

Anvil direction tells you storm movement. The anvil spreads downwind — pushed by the upper-level jet stream. If you see an anvil extending to your east, the storm is likely moving eastward. If the anvil points toward you, the storm is heading your way.

Anvil size indicates storm power. A small, thin anvil means a modest storm. A massive anvil stretching across a large portion of the sky means a powerful storm with a strong updraft. The bigger the anvil, the more energy the storm contains.

Anvil brightness matters. A brilliant white anvil catching full sunlight indicates a thick anvil with dense ice — a strong storm. A thin, transparent anvil you can partly see through suggests a weaker storm.

Overshooting Tops — The Severe Weather Signal

Sometimes the updraft is so powerful that it punches through the tropopause, creating a dome or bubble of cloud protruding above the otherwise flat anvil. This is an overshooting top, and it is one of the most reliable visual indicators of severe weather.

How to spot them:

• A rounded dome or knob rising above the flat anvil surface
• Usually directly above the main updraft column
• May appear and collapse repeatedly over minutes
• Best seen from a distance or in profile against the sky

What overshooting tops mean:

• The updraft is exceptionally strong (100+ mph vertical speed)
• The storm is producing or is capable of producing: large hail (golf ball or larger), damaging winds (75+ mph), and tornadoes
• This is a confirmed severe thunderstorm

Your action: If you see an overshooting top on an approaching storm, treat it as an emergency. This storm can kill. Get into the most solid shelter available. Interior rooms, basements, or sturdy buildings. If in the open, get to the lowest ground you can find and lie flat.

Back-Sheared Anvils

Normally, the anvil spreads downwind — the same direction the upper-level winds are blowing. But sometimes part of the anvil extends upwind, spreading backward against the upper-level flow. This is a back-sheared anvil.

How to identify:

• Part of the anvil extends in the opposite direction from the main anvil spread
• The back-sheared portion is often thinner and more ragged
• The storm tower appears tilted, with cloud material streaming upwind

What it means: The updraft is so powerful that it is pushing cloud material against the prevailing upper winds. This requires extraordinary vertical velocity. Back-sheared anvils indicate:

• Extreme updraft strength
• Very high probability of severe weather
• The storm has an exceptionally strong and organized internal structure

A back-sheared anvil combined with an overshooting top is the visual signature of a supercell thunderstorm — the most dangerous storm type, responsible for virtually all strong tornadoes and the largest hail.

Flanking Line

On the southwest side of many severe thunderstorms (in the Northern Hemisphere), you may see a line of towering cumulus stepping down in height from the main storm. This is the flanking line — new convective towers being fed into the storm’s updraft.

What it tells you: The storm is actively ingesting warm, moist air and producing new updraft pulses. A well-defined flanking line means the storm is self-sustaining and will persist. Storms without flanking lines are typically weakening.

Anvil Rollover

When the anvil edge curls downward on the upwind side, creating a smooth roll of cloud descending from the anvil, this is anvil rollover. It indicates the updraft is creating strong enough flow to pull the anvil ice crystals back downward. This is another sign of a powerful, well-organized storm.

Distance Estimation from Anvils

Anvils are visible from enormous distances because they sit at high altitude. Use this to your advantage:

If you can see the full anvil spread plus the dark tower beneath it: The storm is 20-50 miles away. You have 30-90 minutes depending on storm speed and direction.

If the anvil fills a significant portion of the sky and you can see the dark base: The storm is 10-20 miles away. You have 15-45 minutes.

If the anvil is overhead and the dark base is visible on the horizon: The storm is 5-10 miles away. You have 10-20 minutes. Shelter now.

If you hear thunder: The storm is within 10 miles. You are already in the danger zone for lightning.

Using Anvils for Multi-Day Planning

Observing anvil behavior over several days reveals patterns in the atmosphere:

Anvils all pointing the same direction, day after day: Upper-level wind pattern is stable. Storms will continue to move the same direction. Position your camp accordingly.

Anvil direction changing: Upper-level pattern is shifting. A new weather regime is arriving within 1-3 days.

No anvils for several days, then suddenly explosive anvil development: A strong disturbance has arrived. Expect an active severe weather period.

Anvils becoming taller and wider over successive days: Each day’s storms are stronger. Atmospheric instability is increasing. The risk of severe weather peaks on the day with the largest anvils.

Night Observation

You cannot see anvils at night with the naked eye, but you can infer them. Continuous lightning illuminating a large dome shape above the horizon reveals a cumulonimbus with an anvil. If the illuminated area is huge, the storm is severe. The rate of lightning flashes also indicates storm intensity — more than one flash per second indicates an extremely powerful storm.