THUNDERSTORM FAQs

• In addition to actually seeing them, how are they detected?
• Why do some thunderstorm have a greenish hue?
• Why does the sky sometimes turn orange after a thunderstorm?
• Why are some clouds darker than others?
• What is SKYWARN?

• Thunderstorms

• Climatology
• Detecting
• Forecasting
• Damage/Impacts
• Risks/Safety
• FAQs
• Related Links

• Tornadoes
• Floods
• Hail
• Lightning
• Winter Weather
• Damaging Winds

GLOSSARY

Thunderstorm Detection

What does a thunderstorm look like?

VISUAL EVIDENCE

Upper level features:
Thunderstorms can look like heads of cauliflower or they can have "anvils". An anvil is the flat cloud formation at the top of the storm. An anvil forms when the updraft (warm air rising) has reached a point where the surrounding air is about the same temperature or even warmer. The cloud growth abruptly stops and flattens out to take the shape of an anvil.

If the thunderstorm has a very strong updraft, a small portion of the updraft air will poke through the flat part of the anvil – looking like a bubble of cloud above the rest of the anvil. This bubble is called an overshooting top. Most thunderstorms will have an overshooting top for a short time, but if you see a storm with a large, dome-like overshooting top that lasts for more than 10 minutes, chances are good that the thunderstorm updraft is strong enough hand persistent enough to produce severe weather.

The anvil can provide other clues to the strength of the storm and how long it might last. If the anvil is thick, smooth-edged, and cumuliform (puffy, like the lower part of the storm), then the storm likely has a strong updraft and is a good candidate to produce severe weather. If the anvil is thin, fuzzy, and wispy like cirrus clouds, then the updraft is probably not as strong, and the storm is less likely to produce severe weather. If the anvil is large and seems to be streaming away from the storm in one particular direction, then there are probably strong upper-level winds in the storm's environment and the precipitation will be blown away from the updraft rather than fall through it.

Mid-level features:
Things you might notice in the middle levels of the storm are usually associated with the storm's main updraft tower. If the clouds in the main updraft area are sharply outlined and look like a cauliflower, then the clouds are probably associated with a strong updraft that could produce severe weather. If the clouds in the updraft area have a fuzzy, mushy appearance, the updraft is probably not as strong. If the updraft tower is almost perfectly upright, the storm probably has an updraft strong enough to resist the upper-level winds blowing against it. If the updraft leans downwind, then the updraft is usually weaker.

Thunderstorms with good storm-scale organization usually have a series of smaller cloud towers to the south or southwest of the main storm tower. These smaller towers are called a flanking line and usually have a stair-step appearance as they build toward the main storm tower.

Some supercells during their development will show signs of rotation in the updraft tower. You may see streaks of cloud material that give the storm tower a "corkscrew" or "barber pole" appearance (called striations) and strongly suggest rotation. A mid-level cloud band may also be visible encircling the tower like a ring around a planet. This is another sign of possible rotation within the storm.

As a storm grows in size and intensity, it will begin to dominate its local environment (within about 20 miles). If cumulus clouds and other storms 5-15 miles away from the storm dissipate, it may be a sign that the storm is taking control in the local area. Sinking motion on the edges of the storm may be suppressing any nearby storms. All of the instability and energy available locally may focus on that one storm which could result in its continued development.

Low-level features:
Some of the most critical cloud features to determine if a thunderstorm is severe and whether it could produce a tornado are found at or below the level of the cloud base. These features can be confusing and frustrating.

An easy feature to identify is the rain-free cloud base. It is an area of smooth, flat cloud beneath the main storm tower with little or no falling precipitation. The rain-free base is usually just to the rear of the precipitation area, and marks the main area of inflow where warm, moist air at low levels enters the storm. The rain-free base is sometimes called the "intake area."

Inflow bands are ragged bands of low cumulus clouds extending from the main storm tower to the southeast or south (usually). The presence of inflow bands suggests that the storm is pulling in low-level air from several miles away. If the inflow bands have a spiraling nature to them, it suggests the presence of a rotating updraft.

The beaver's tail is another significant type of cloud band. The beaver's tail is a smooth, flat cloud band extending from the eastern edge of the rain-free base to the east or northeast. It usually skirts around the southern edge of the precipitation area. The beaver's tail is usually seen with high-precipitation supercells and suggests rotation in the storm.

A wall cloud is an isolated cloud lowering attached to the rain-free base. A wall cloud forms as the storm intensifies, and the updraft draws in low-level air from several miles around. Some of the low-level air is pulled into the updraft from the rain area. The rain-cooled air is very humid, and the moisture in the rain-cooled air quickly condenses at a lower altitude than the rain-free base to form a wall cloud. The wall cloud is usually to the rear of the visible precipitation area. Wall clouds are usually about two miles in diameter and mark the area of strongest updraft in the storm. To determine if the wall cloud may be tornadic, it will have four basic characteristics. First, the wall cloud will be persistent – lasting for 10-20 minutes (it may change shape) before a tornado appears. Second, the wall cloud will rotate consistently, and often violently before a tornado develops. Third, strong surface winds will blow in toward the wall cloud from the east or south-east (inflow). Surface winds of up to 25-35 miles an hour are often found near tornadic wall clouds. Fourth, the wall cloud will show rapid vertical motion in the form of small clouds in or near the wall cloud and will quickly rise up into the rain-free base. However, not all tornadic wall clouds will have these characteristics, and some tornadoes do not form from wall clouds!

Shelf clouds or roll clouds are examples of other clouds that you may see beneath the cloud base of a storm. Shelf clouds are long, wedge-shaped clouds associated with the gust front. Roll clouds are tube-shaped clouds and are also found near the gust front. Shelf and roll clouds can form anywhere where there is outflow. Shelf clouds typically form near the leading edge of a storm or squall line. A shelf cloud can form under the rain-free base, and look like a wall cloud. A shelf cloud may also appear to the southwest of a wall cloud and is associated with phenomena called the rear-flank downdraft.

To tell the difference between wall clouds and shelf or roll clouds, remember a wall cloud 1) suggests inflow and an updraft, 2) maintains its position with respect to rain, and 3) slopes upward away from the precipitation area. In contrast, shelf clouds 1) suggest downdraft and outflow, 2) move away from rain, 3) slope downward away from the precipitation area.

SATELLITE EVIDENCE

Satellites show us pictures of the clouds before they become big enough to be thunderstorms. We can watch these pictures over an hour and notice that the clouds are growing rapidly. Satellites also can tell us the temperature of the clouds – and we can tell if a cloud has grown tall enough to be a thunderstorm. We can even see the thunderstorm anvil from satellites.

RADAR EVIDENCE

Doppler radar sends out pieces of energy that can be reflected back to the radar by things like rain and hail. The amount of energy that is reflected back can tell us how heavy the rain might be or give us an indication of hail. Doppler radar can also show us how the wind is blowing near and inside the storm. This is helpful in understanding what kinds of hazards the thunderstorm might have (tornado, microburst, gust fronts, etc) associated with it. It also helps us understand how the thunderstorm is feeding itself.