1.2.3. Updraft

Updraft

The updraft of the thunderstorm is the convectivelly rising moist warm air. It is invisible to the eye below the condensation level and becomes visible as the ‘cauliflower’ cloud above the condensation level.

The updraft varies a lot between different thunderstorms. Its strength, the air velocity within it (i.e. how quickly that air parcel rises), tilt and possibly rotation depends on:

  • how buoyant the warm rising air is
  • what the CAPE is like: is it short, fat or long, thin
  • how wind speed changes with height
  • how wind direction changes with height

We will work our way from the bottom up.


                    

1.2.3.1. Visual appearance and upward speed of the updraft

The visual appearance of the updraft can tell a lot about its strength and potential to produce a strong thunderstorm. As the updraft rises going through various phases of Cumulus and potentially Cumulonimbus , it cools and eventually looses buoyancy. In addition to the speed at which the updraft rises, the appearance of its edge is indicative of its strength.

Sharp, crisp updraft on a severe thunderstorm over north Italy. This is a strong updraft. Photo: Marko Korošec.

A sharp, crisp edge, that may resemble an explosion, indicates a strong updraft. It may, sometimes, form a pileus cloud cap on top of it. On the other hand, a soft, fuzzy edge is indicative of a weak updraft, that has lost its strength and upward speed and is likely not rising anymore.

Upward speed of the updraft

Fair weather Cumulus humilis and Cumulus mediocris clouds typically have gentle updrafts, with vertical speeds of about 2-3 m/s. Weak thunderstorms may have vertical updraft speeds of 6-12 m/s. At such a speed, a thunderstorm will vertically develop by 10 km in about 15 minutes.

On the other side of the spectrum are severe thunderstorms. A severe thunderstorm may have a vertical updraft going up at 30-35 m/s. In the most extreme cases, severe thunderstorm updrafts grow at up to 75-80 m/s! That means the updraft rises by a full kilometer in less than 15 seconds and can reach 10 km in 2-3 minutes – this is called explosive thunderstorm development.

Explosive updrafts on a severe thunderstorm over central Po plain, north Italy. July 17, 2009. This storm had an updraft strong enough to hold aloft tennis ball sized hail. Photo: Marko Korošec.

Explosive updraft on a severe thunderstorm over central Slovenia. Note the crisp edges of the updraft, indicating its strength. June 28, 2008. Photo: Marko Korošec.


                    

1.2.3.2. Updraft depending on the wind

As you watch cumulus and cumulonimbus clouds forming on different days, you will notice on some days the updrafts grow vertically upwards, while on other days they are tilted.

The tilt of an updraft depends on the strength, the upward speed of the updraft and how the wind changes with altitude. Change in wind speed and/or direction with increasing altitude is called wind shear. The more shear there is, the more the updraft is tilted.

We will take a look at three different situations:

  • There is little or no wind shear
  • There is wind shear present, wind changes speed with height, but not direction
  • Wind shear present, wind changes speed and/or direction with height

1.2.3.3. Pileus cloud caps

Often, a white wispy cloud cap forms above rapidly rising updraft – a pileus cloud. It forms as moist, but stable air above a rapidly rising updraft is pushed upward, causing the water to condense. It is a short-lived cloud, which either fades or is overtaken by the rising updraft, then forming a wispy collar cloud, called a velum cloud.

Stage 1: rising updraft still below the stable layer of moist air aloft.

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Stage 2: rising updraft pushes the stable layer of moist air upwards, causing it to condense and forming the pileus cloud (cap).

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Stage 3: rising updraft punches through the pileus cloud cap, forming a velum cloud.

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Pileus clouds are most often found above rapidly growing Cumulus congestus and Cumulonimbus calvus clouds.

Fun fact: pileus clouds can be found above other types of rapidly rising updrafts such as volcanic eruption columns and nuclear explosions.

On to examples!

1.2.3.4. Equilibrium level (‚anvil level‘)

Now we have seen why some updrafts are vertical, while others are tilted and some even rotate. Now we take a look at what the top of the updraft looks like and why.

We asked ourselves at the beginning of this section:

  • Why does a Cumulonimbus capillatus get its distinctive fibrous top?
  • Why does a Cumulonimbus capillatus incus have this form?
  • Why do many thunderstorms have the distinct anvil shape, but not all?

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We are going to see why Cumulonimbus capillatus clouds get the fibrous look and why Cumulonimbus capillatus incus spread out into their distinct anvil shape. It all has to do with equilibrium level.

1.2.3.5. Overshooting top

Overshooting tops develop on thunderstorms that have particularly strong updrafts. Overshooting tops typically develop on Cumulonimbus capillatus incus clouds. The updraft punches through the equilibrium level (and thus the anvil), rising higher due to its momentum, despite not being buoyant anymore. An overshooting top is usually short-lived, several tens of seconds, up to several minutes. In rare cases it may persist much longer.

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An overshooting top, particularly a persistent one, is an indication of a potentially severe thunderstorm.

An overshooting top is difficult to see, and can only be seen on distant thunderstorms. It is readily apparent also in satellite imagery, in particular at low Sun angles (i.e. morning and, more often, late afternoon).

EXAMPLES

Overshooting top on a severe thunderstorm in central Slovenia. Also note the pileus / velum clouds on the updraft in the foreground. June 28, 2008. Photo: Marko Korošec.

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Overshooting top on a severe thunderstorm near Ancona, north-central Italy on June 26, 2016. Note the massive updraft! Photo: Marko Korošec.


                    

1.2.3.6. Backsheared anvil – updraft strength vs. wind shear

Anvil vs. wind

As the anvil is forming, it is under the influence of upper level winds. Typically as you go higher in the troposphere, winds increase. Under stable conditions with a high pressure, winds in the upper part of the troposphere can be very weak, below 10 km/h. However, under more unstable conditions when a jetstream may be present aloft, wind speeds can exceed 200-250 km/h. The strength of upper level winds strongly influences the form of the anvil. We take a look how.

1.2.3.7. Mammatus clouds

Mammatus clouds form on the underside of a thunderstorm’s anvil. The name mammatus comes from the Latin word mamma, meaning “udder” or “breast”. They appear as pouch-like structures protruding from underneath the anvil. Mammatus clouds are gentle downdrafts – sinking cool air – descending from the anvil, that form, evolve and dissipate over a time span of about ten minutes to half an hour. While they are generally well-understood as downdraft features, the exact formation mechanism or mechanisms are not well constrained and a subject of research.

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Mammatus clouds vary in size and definition: from small, barely recognizable features to large, extremely well defined pouches. Even rather small thunderstorms with not particularly well-defined anvils (Cumulonimbus capillatus) often form some mammatus clouds. Intense thunderstorms that form extensive anvils (Cumulonimbus capillatus incus) can produce enormous ‘fields’ of mammatus clouds.

There are some common misconceptions about mammatus clouds. We talk about these next.

1.2.3.8. The highest ever measured thunderstorms

The strongest updrafts reach the tropopause and produce overshooting tops. The highest thunderstorms in the world have been recorded in the tropics, where the tropopause is the highest (15-18 km). Thunderstorms reaching 20-22 km high have been recorded there.

The July 23, 2010 Vivian, South Dakota severe supercell thunderstorm, which produced the world record hailstone (20 cm in diameter!) reached a maximum height of 17 km! This is all the more remarkable, since this storm was far from the tropics, with the tropopause at a lower altitude, indicating an exceptionally strong updraft. Recently, on May 26, 2016, a severe supercell thunderstorm on the Mexico-USA (Texas) border was measured at 20.7 km peak height; the storm was so intense it produced lightning up to 95 km from its core!

Keep in mind that the average cruising altitude of commercial airplanes is 11-12 km, so these thunderstorms are up to twice as high as you would normally go on your flights!

And with this example we finish our second part of the tutorial. There will be (much) more coming in the next days as we continue with downdrafts in thunderstorms, tune back in soon and follow us for updates on Severe Weather Europe on Facebook!