EOSC 114 The Catastrophic Earth - Natural Disasters

The Turbulent Atmosphere - Storms

A Vignette: Thunderstorms Saga: Humidity


  1. Vignette: Thunderstorms

  2. Saga: Humidity
    1. Saturation and Vapour Pressure

    2. Humidity Variables
      • Mixing ratio
      • Relative humidity
      • Dew-point temperature

    3. Measuring Humidity

  3. Cloud Identification

    1. Visual differences

    2. Types of Stratiform Clouds

    3. Types of Cumuliform Clouds

1. Vignette: Thunderstorms

Thunderstorms are cumulonimbus clouds that have lightning and thunder. These are deep clouds with strong updrafts, with a cloud base near the ground and cloud top near the top of the troposphere (roughly 11 km above sea level). Mature thunderstorms have a characteristic anvil or mushroom top, which can spread large distances (hundreds of km) downwind if blown by strong winds aloft.

Figure ST.1 Thunderstorm Basics.

On weather maps, the symbol:

represents thunderstorms (think of it as a T for thunderstorm, with a lightning bolt coming out the side). In text-based reports, cumulonimbus clouds are abbreviated as CB. So sometimes thunderstorms are called CBs.

Figure ST.2a Cumulonimbus clouds. Photos by D. Bush Missouri Skies (left) and from InFlightPhotos (right).

Normally, the anvil is at the top of the troposphere, at a location called the tropopause. You should learn how to recognize thunderstorms by the presence of their anvil. During daytime, you might not see the lightning or hear the thunder in distant storms, so the anvil is a good clue. For thunderstorms with strong updrafts, the cloudy air can overshoot above the anvil into the lower stratosphere, causing an "overshooting top". Thunderstorms can create many hazards to life and property: lightning, tornadoes, hail, downpours and local flooding, and downburst and gustfronts (we will learn about these in later vignettes).

Figure ST.2b A cartoon showing the parts of a thunderstorm. Drawing by R. Stull.

Weather radar transmits a beam of microwaves into the atmosphere, and "listens" for the faint echo of microwave energy that bounces back off of raindrops. Heavier rain causes a stronger echo, which is often coloured as yellow and red on radar displays. The lighter rain is shown with the blues and greens. The radar image below shows a number of thunderstorms producing spotty heavy rain, with no rain (shown as black in this image) in between

Figure ST.3 Radar reflectivity image from NOAA.

On visible satellite photos of thunderstorms, you can often see the shadow of the thunderstorm anvil that is cast on the lower clouds or on the ground. The anvils (marking the tops of thunderstorms) in the satellite image below are circled in red.

Figure ST.4 Satellite visible image of thunderstorms. Image downloaded from UCAR.

When the thunderstorm is close to you, the anvil might already be overhead, as shown below. Thunderstorms normally move from southwest to northeast in the Northern Hemisphere. The best place to view thunderstorms is southeast of the storm - that way, you stay out of its path, but you can have a good view. The photo below shows the view that you would have if the thunderstorm is west or northwest of you.

Figure ST.5 Anatomy of a thunderstorm. Photo from NOAA Photo Library.

Sometimes, mammatus clouds are visible on the underside of anvil clouds. While beautiful looking, particularly at sunset, these give NO clue as to the intensity of the thunderstorm, or whether tornadoes are present. Not all thunderstorm anvils have mammatus clouds, and sometimes these clouds form on the underside of non-thunderstorm clouds. In the cloud identification segment at the end of this unit, we will look at many different clouds types, and discover some more that are associated with thunderstorms.

Figure ST.6 Mammatus clouds. Photos from NOAA Photo Library.

TRANSITION FROM VIGNETTE TO SAGA: All of the energy that drives these violent motions comes from heat in the air, in the form of sensible heat and latent heat (stored in water vapour). To understand the way that latent heat helps to power the storm, we must first define what we mean by humidity. This is explained next, in the quantitative part of this lesson.

3. Cloud Identification

Some important concepts to look for in this image collection are:

  1. Visual differences between stratiform and cumuliform clouds

  2. Types of stratiform clouds

  3. Types of cumuliform clouds

  4. Identification of thunderstorm clouds, and types of other clouds that are sometimes found attached to thunderstorms.

Figure ST.10 The Earth's atmosphere, viewed from space. Photo courtesy of NASA.

3a. Visual Differences

i) Stratiform clouds. Stratiform clouds are layered clouds. They have very large horizontal extent (10s to 1000s of km), but are often relatively thin (0.01 to 1 km). They are formed by mostly smooth, horizontal winds, and are named by their altitude. Cloud base is NOT related to the Lifting Condensation Level (LCL, to be defined later) for these clouds.

  • cirrus, cirrostratus, cirrocumulus - high altitude (about 10 km), thin, made of ice crystals

  • altostratus, altocumulus - medium altitudes (about 5 km), made of water droplets

  • stratus, nimbostratus - low bases (0.1 to 2 km), thick, with widespread drizzle from the nimbostratus

Figure ST.11 Stratiform clouds are classified by their altitude. Drawing by R. Stull (2006).

ii) Cumuliform clouds. Cumuliform cloud tops look like popcorn, cotton balls, or cauliflower. They have significant vertical motion and turbulence, and are often formed from air parcels rising from near the ground under the cloud. They usually have flat bases, at altitudes very close to the theoretical LCL altitude. Often, their diameter is roughly equal to their thickness. Cumuliform clouds are named by their size:

  • cumulus humilis - fair-weather clouds, about 1 km in size

  • cumulus mediocris - medium size, about 4 km in size

  • cumulus congestus - towering cumulus, about 7 km in size

  • cumulonimbus - thunderstorms, about 11 km in size, with precipitation

Figure ST.12 Cumuliform clouds are classified by their thickness. Drawing by R. Stull (2006).

You can do a web search on "cloud identification", "cloud classification", or "cloud atlas", to find excellent photographs of clouds.

There are many other cloud types, some with amazing beauty. In this course, we will focus on the most violent clouds -- thunderstorms (cumulonimbus clouds).

3b. Types of Stratiform Clouds: High Level Clouds

i) Cirrus clouds (photos above)

ii) Cirrostratus clouds (photos above)

iii) Cirrocumulus clouds (photos above)
Figure ST.13 High level clouds. i) Cirrus clouds, photos by R. Stull (left) and by Simon Eugster WikiMedia Commons (right); ii) Cirrostratus clouds, both photos by R. Stull; iii) Cirrocumulus clouds, both photos from NASA.

3b. Types of Stratiform Clouds: Middle Level Clouds

i) Altostratus clouds (photos above)

ii) Altocumulus clouds (photos above)

Figure ST.14 Middle level clouds. i) Altostratus clouds, photo from WikiMedia Commons by PiccoloNameki (left). Photo on right (taken by Angie J. Venturato) was taken on a late morning in February 1996, temperature at 28 °F. Note how the altostratus covers most of the sky and gives the sun a "muddy" appearance. Courtesy of Windows to the Universe; ii) Altocumulus clouds, photos from WikiMedia Commons by Simon Eugster (left) and by Ralph F. Kresge, NOAA (right).

3b. Types of Stratiform Clouds: Low Level Clouds

i) Stratus clouds (photos above)

ii) Nimbostratus clouds (photos above)
Figure ST.15 Low level clouds. A. Stratus clouds photo by R. Stull (left) and by Ralph Kresge courtesy of NASA; B. Nimbostratus clouds; photo from R. Stull (left); photo taken from Guemas Island, toward Orcas Island, Washington by Steven Businger (right).

3c. Types of Cumuliform Clouds: Vertical Clouds

ii) Cumulus congestus clouds (photos left and above)

Figure ST.16 Vertical clouds, rise far above their bases and can form at many heights. A. Cumulonimbus clouds, photo on left from WikiMedia Commons by Simon Eugster; on right taken in Spain, courtesy of University of Washington Mesoscale Group; B. Cumulus congestus (towering cumulus) clouds, both photos from NOAA.
i) Cumulonimbus clouds (photos above)


3c. Types of Cumuliform Clouds: Low Level Clouds

i) Cumulus Mediocris clouds (photos above)

ii) Cumulus humilis clouds (photos above)

iii) Stratocumulus clouds (photos above)

Figure ST.17 Low level cumuliform clouds. A. Cumulus mediocris, photos from R. Stull (left) and from Earth Science Picture of the Day, Universities Space Research Association by Thierry Lombry (right); B. Cumulus humilis clouds, photo (left) from R. Stull and (right) taken from Platte, North Dakota courtesy of Arthur L. Rangno; C. Stratocumulus clouds, photos courtesy of University of Washington Mesoscale Group (left) and by Simon Eugster WikiMedia Commons (right).

3d. Thunderstorm Clouds

i) Overshooting top of thunderstorms (photos above)

ii) Mammatus clouds (photos above)

iii) Cumulonimbus base features: Beaver tail (left) and wall cloud (right) (photos above)
Figure ST.18 Thunderstorm clouds. A. Overshooting top of thunderstorms, photos (clockwise from top left) from downclimb.com, inflightphotos.com, and NASA; B. Mammatus clouds; photo on right, note mammatus clouds are under the anvil cloud. Photos from NOAA Photo Library; C. Cumulonimbus base features: photos from http://www.australiasevereweather.com/ by Jimmy Deguara.

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