Outline
1. Vignette: ThunderstormsThunderstorms 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. 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.
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). 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 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. 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. 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.
3. Cloud IdentificationSome important concepts to look for in this image collection are:
3a. Visual Differencesi) 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.
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:
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
3b. Types of Stratiform Clouds: Middle Level Clouds
3b. Types of Stratiform Clouds: Low Level Clouds
3c. Types of Cumuliform Clouds: Vertical Clouds
3c. Types of Cumuliform Clouds: Low Level Clouds
3d. Thunderstorm Clouds
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