Thunderstorms

In this section we are going to take a general look at thunderstorms. What is a typical thunderstorm. We will take a thunderstorm apart into its basic components and how they make the thunderstorm work.

Typical thunderstorm – Cumulonimbus capillatus incus

 

The most distinctive and typical cloud shape associated with thunderstorms is Cumulonimbus capillatus incus: a convective tower (updraft) flattened into an anvil shape. As we have already seen in the previous chapter, it literally looks like an anvil. It is typically over 10 km high and sometimes reaches so high up, it turns day into night for anyone underneath it.

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If you see a cloud of this shape from a distance – that is a thunderstorm. While other varieties of cumulonimbus clouds also produce thunderstorms, the anvil shaped incus is the most distinctive – and associated with the strongest thunderstorms.

Why is it this shape? Let us see.

UPDRAFT AND DOWNDRAFT

 

A thunderstorm (or even a rain shower for that matter) is organized into two main components: updraft and downdraft. The updraft is the rising warm, moist air within the thunderstorm. The downdraft is the descending cooled air, containing precipitation.

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We first take a look at the 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.

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

Little or no wind shear

A typical setting with convective clouds developing in an environment with virtually no wind at any altitude is within a summer anticyclone. Generally the weather is very stable during anticyclones, as an anticyclone gradually weakens and/or temperatures near the ground become very high, typical summer heat thunderstorms develop. As they grow through Cumulus congestus phase into Cumulonimbus, these updrafts stay vertical. Thunderstorms that form in this environment are essentially a single updraft or pulse that dissipates in 20-30 minutes. As they dissipate, new updrafts may sometimes form on their flanks.

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Schematic of deep convective clouds in little or no vertical wind shear. The updraft rises vertically, with very little or no tilt.

Cumulonimbus capillatus incus in very little wind shear. Note the symmetric anvil. The updraft rising to the left is producing slightly tilted appearance, however, it is actually a new updraft, separate from the one that created the anvil.

Wind shear present, wind changes speed with height, but not direction

Environments with wind shear typically produce tilted updraft. Shear in which only the wind speed changes (usually increases) with height is called unidirectional or speed shear. The updraft is carried further downwind with increasing winds as it rises higher in the troposphere.

Thus tilted updrafts are a visual sign of a sheared environment. Typically, if thunderstorms form in this kind of environment they are multicell thunderstorms. We will go into much more details on multicells in the upcoming chapter “Severe thunderstorms”.


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Tilted Cumulus congestus, Cumulonimbus calvus and Cumulonimbus capillatus incus clouds in a moderately to strongly sheared environment.

Wind shear present, wind changes speed and/or direction with height

The environment that is particularly conductive for severe thunderstorms is an environment where the wind changes speed and/or direction with height. This is called directional shear. If the wind changes direction clockwise with increasing height, it produces a veering vertical wind profile.

A counterclockwise change in direction produces a backing vertical wind profile. Backing wind profiles are rarely associated with thunderstorms as they are typically associated with inflow of cold air at the surface. Veering profiles, on the other hand, are typically associated with inflow of warm air at the surface and often with severe thunderstorms.

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An environment where wind direction changes with height produces a rotating updraft. The entire updraft rotates around its vertical (tilted) axis. A thunderstorm with a persistently rotating updraft is called a supercell. Rotation often produces visually apparent rotational features, such as striations.

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The same thunderstorm from a somewhat larger distance and at a slightly different angle. The striated, rotational appearance of the updraft is apparent. The tilt is better revealed from this angle (the angle was still not optimal and the updraft had a larger tilt than apparent in this photo). Photo: Marko Korošec.

Recap: wind and wind shear have important effects on the way an updraft behaves. If there is little wind shear, updrafts rise vertically. If wind changes speed with height, but keeps the same direction (speed shear), updrafts become tilted. If winds also change direction with height (directional shear), updrafts may start to rotate.

On to some examples!

Example 1/5

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Very unstable and highly sheared environment: numerous strongly tilted Cumulus congestus clouds.

Example 2/5

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Strongly tilted Cumulus congestus.

Example 3/5

Another look at a rotating updraft (on a supercell): note that the only the front of the updraft shows distinct rotational features, while the back is cumuliform (“cauliflower” appearance). The storm is moving towards the right.

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Example 4/5

Another rotating updraft (supercell): here the view is more in the direction of approach of the storm. The front edge of the updraft, with distinct rotational features is visible. The cumuliform back of the updraft can just be seen.

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Rotational features readily visible.

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Example 5/5


Strongly tilted updraft (Cumulonimbus calvus) in a very strongly sheared environment.

Great! Next we find out why some updrafts have thin white caps.

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!

EXAMPLES:

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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.

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.

RISING AIR AND BUOYANCY

As the air in the updraft rises it cools. The cooling is adiabatic, meaning it cools due to expansion as the pressure drops with height. There is virtually no cooling due to heat transfer with the cooler surrounding air.

As long as the rising air (parcel) is warmer than the surrounding air it is buoyant. As it reaches ambient temperature it ceases rising, reaching the equilibrium level (EL). This level is also known as the level of neutral buoyancy (LNB) or the limit of convection (LOC).

In other words: equilibrium level is the altitude at which a rising updraft ceases to rise, its air having cooled down to the same temperature as the surrounding air.

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We will take a look at the two different situations:

  • The equilibrium level is the height at which the rising updraft cools to the temperature of the surrounding air
  • The equilibrium level is produced by a temperature inversion, which prevents the updraft from rising further

The equilibrium level is the the height at which the rising updraft cools to the temperature of the surrounding air

In this case the rising and adiabatically cooling updraft eventually ‘runs out of energy’. As the air in the updraft cools down to the temperature of the surrounding air, it looses buoyancy and stops rising. The height at which this happens can be anything from several kilometers to over 10 km high.

You can visually discern an updraft that has reached equilibrium level in this case: an updraft which has ceased rising (or is about to) looses its sharp edge and becomes soft. The strongest updrafts rise above the freezing level, and the water droplets rising with the updraft freeze into ice crystals. As the updrafts ceases to rise, the top of it spreads out in a fuzzy, wispy, fibrous edge, forming a Cumulonimbus capillatus.

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The updraft rises towards the equilibrium level through Cumulus congestus and Cumulonimbus calvus stages.

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The updraft reaches the equilibrium level. The top of the cloud becomes fibrous, as ice crystals form a wispy, fuzzy top. This is a Cumulonimbus capillatus.

The equilibrium level is produced by a temperature inversion, which prevents the updraft from rising further
In this case the rising updraft encounters a sharp temperature inversion, which acts as a ceiling. The updraft rapidly looses buoyancy in and above the inversion, effectively encountering a ceiling for its vertical motion. The updraft spreads laterally underneath the inversion, forming the distinctive anvil shape of a Cumulonimbus capillatus incus.

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The updraft rises towards the equilibrium level through Cumulus congestus and Cumulonimbus calvus stages.

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The tropopause is an important temperature inversion, controlling the vertical development of strong thunderstorms.

THE ROLE OF THE TROPOPAUSE

The tropopause is the boundary in Earth’s atmosphere between the troposphere and stratosphere. It is a strong temperature inversion: the temperature drops with height as you go upwards in through the troposphere, but begins rising again as you cross the tropopause into the stratosphere.

The World Meteorological Organization defines the tropopause as: “The boundary between the troposphere and the stratosphere, where an abrupt change in lapse rate usually occurs. It is defined as the lowest level at which the lapse rate decreases to 2 °C/km or less, provided that the average lapse rate between this level and all higher levels within 2 km does not exceed 2 °C/km.„ In other words, it is a boundary, a layer with a very small lapse rate, i.e. the temperature of the air changes very little with increasing altitude.

The height of the tropopause is not the same everywhere in the world. The tropopause height depends on the latitude: at latitudes above 60° (North or South) it is 9-10 km high, but it reaches up to 17-18 km close to the equator.
Strong updrafts, that form strong thunderstorms rise all the way up to the tropopause. The tropopause is a very strong temperature inversion that limits the growth of even the strongest updrafts. As the vertical temperature lapse rate in the stratosphere (above the tropopause) is positive, the updraft is not buoyant anymore and the tropopause is the ultimate limit of updraft upward development. Very little upward development is possible above this level. This is also why the strongest thunderstorms have large anvils that develop at the tropopause level.

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Diagram of a Cumulonimbus capillatus incus with equilibrium level in the tropopause. The updraft remains buoyant throughout the troposphere: it cools as it rises (at the moist adiabatic lapse rate), but does not cool to the temperature of the surrounding air. As it reaches the tropopause, the surrounding air does not cool with increasing height anymore and the rising air in the updraft soon cools to the temperature of the surrounding air, not rising anymore. In effect it reaches the ceiling and cannot rise anymore. The upward momentum is transformed into lateral (sideways) momentum and the updraft spreads into anvil shape.

Recap

The equilibrium level is the limit of upward development of an updraft. This is the level at which the rising air in the updraft cools down to the temperature of the surrounding air. Thus the air in the updraft loses its positive buoyancy and stops rising.

The height of the equilibrium level depends on the temperature of the air in the rising updraft and the temperature of the surrounding air (i.e. on the available convective potential energy of the rising air). Weak updrafts have low equilibrium levels, while stronger updrafts have higher equilibrium levels.

In thunderstorms the shape of the Cumulonimbus cloud depends on the equilibrium level. When the equilibrium level is not defined by a temperature inversion, the rising updraft forms a wispy, fibrous icy top, becoming a Cumulonimbus capillatus. When the equilibrium level is defined by a temperature inversion, it acts as a ceiling and the updraft spreads into ana anvil shape, becoming a Cumulonimbus capillatus incus. Updrafts in strong thunderstorms usually rise all the way up to the tropopause.

On to some examples.

EXAMPLE 1/3

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Large anvil of an intense thunderstorm over eastern Emilia Romagna (near Ferrara), north Italy. Very unstable day with very warm and hot air at the ground level (temperature over 35 °C, dew point around 25 °C) and cold upper levels. The updraft has obviously reached the tropopause and is spreading into a large anvil – forming a Cumulonimbus capillatus incus.

EXAMPLE 2/3

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Thunderstorm over the northern Adriatic sea, displaying a much different morphology. The updraft went through cumulus congestus (there is a cumulus congestus on the right edge of the photo) into cumulonimbus calvus and hit the equilibrium level (EL) significantly below the tropopause, probably around 7-8 km altitude.

The updraft developed a very weakly defined anvil, fibrous in apperance – cumulonimbus capillatus. Virga is visible in the downdraft around the fragmentary rainbow. Note the rainbow terminating at a high altitude, indicative of evaporation of precipiation (i.e. precipitation completely evaporates – virga).

There is a waterspout underneath the main updraft – can you spot it?

EXAMPLE 3/3

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Anvil of a supercell thunderstorm over the Friuli plain, NE Italy. Well developed anvil with a relatively sharp edge and some mammatus clouds developed. Tropopause was at 12 km altitude on this day (12Z 16080 LIML Milano sounding).