Chapter 10: Thunderstorms Flashcards
A thunderstorm should be thought of as
a process which takes heat and moisture near the Earth’s surface and transports it to the upper levels of the atmosphere.
A thunderstorm should be thought of as a process which takes heat and moisture near the Earth’s surface and transports it to the upper levels of the atmosphere. The by-products of this process are
the clouds, precipitation, and wind that we associate with the thunderstorm
All thunderstorms, whether or not they become severe, must have three conditions present in order to form.
moisture
instability
lift
The first necessary condition is moisture in
the lower to mid-levels of the atmosphere.
The first necessary condition is moisture in the lower to mid-levels of the atmosphere. As air rises in a thunderstorm updraft, moisture
condenses into small water drops which form clouds (and eventually precipitation).
The first necessary condition is moisture in the lower to mid-levels of the atmosphere. As air rises in a thunderstorm updraft, moisture condenses into small water drops which form clouds (and eventually precipitation). When the moisture condenses
heat is released into the air, making it warmer and less dense than its surroundings. The added heat allows the air in the updraft to continue rising.
The second necessary condition is instability. If the air-mass is unstable
air which is pushed upward by some force will continue upward
The second necessary condition is instability. If the air-mass is unstable, air which is pushed upward by some force will continue upward. An unstable airmass usually contains
relatively warm (usually moist) air near the earth’s surface and relatively cold (usually dry) air in the mid and upper levels of the atmosphere.
. An unstable airmass usually contains relatively warm (usually moist) air near the earth’s surface and relatively cold (usually dry) air in the mid and upper levels of the atmosphere. The low-level air will
rise in an updraft because it is less dense than the surrounding air.
An unstable airmass usually contains relatively warm (usually moist) air near the earth’s surface and relatively cold (usually dry) air in the mid and upper levels of the atmosphere. The low-level air will rise in an updraft because it is less dense than the surrounding air. The air will
continue to move upward until it becomes colder and more dense than its surroundings.
The third necessary condition is a source of lift. Lift is a mechanism for
starting an updraft in a moist, unstable airmass
The third necessary condition is a source of lift. Lift is a mechanism for starting an updraft in a moist, unstable airmass. The lifting source can take on several forms. The most common source is called
differential heating (convection).
differential heating (convection)
). As the sun heats the earth’s surface, portions of the surface (and the air just above the surface) will warm more readily than nearby areas. These “warm pockets” (bubbles) are less dense than the surrounding air and will rise. If the air has sufficient moisture and is unstable, a thunderstorm may form
The source of lift can also be
mechanical in nature
……………………………. also act as triggers by lifting moist, low-level air to the point where the low-level air is warmer and less dense than its environment at which time thunderstorms can form.
Orographic lifting,
advancing cold fronts,
outflow boundaries,
drylines, and
sea breeze fronts
The Thunderstorm Life Cycle
cumulus stage
mature stage
dissipating (decaying) stage
Cumulus Stage
Warm, moist air rises in a series of convective updrafts. As this occurs the air begins to condense into a cumulus cloud. The condensation releases heat into the cloud, warming the air. This, in turn, causes it to rise adiabatically
Cumulus Stage
Warm, moist air rises in a series of convective updrafts. As this occurs the air begins to condense into a cumulus cloud. The condensation releases heat into the cloud, warming the air. This, in turn, causes it to rise adiabatically. The cloud edges during this stage are
sharp and distinct, indicating that the cloud is composed primarily of water droplets
Cumulus Stage
Warm, moist air rises in a series of convective updrafts. As this occurs the air begins to condense into a cumulus cloud. The condensation releases heat into the cloud, warming the air. This, in turn, causes it to rise adiabatically. The cloud edges during this stage are sharp and distinct, indicating that the cloud is composed primarily of water droplets. The process continues and works to form a
towering cumulus cloud
Cumulus Stage
Warm, moist air rises in a series of convective updrafts. As this occurs the air begins to condense into a cumulus cloud. The condensation releases heat into the cloud, warming the air. This, in turn, causes it to rise adiabatically. The cloud edges during this stage are sharp and distinct, indicating that the cloud is composed primarily of water droplets. The process continues and works to form a towering cumulus cloud. The convective cloud continues to
grow upward, eventually growing above the freezing level where supercooled water droplets and ice crystals coexist.
……………………………………………….. start the initiation of cool downdrafts, which leads to the second stage
The beginning of falling precipitation and cool air entrainment from the environment
Mature Stage
This stage is characterized by
the presence of both updrafts and downdrafts within the cloud.
This stage is characterized by the presence of both updrafts and downdrafts within the cloud. The downdrafts are initiated by
by the downward drag of falling precipitation
Mature Stage
This stage is characterized by the presence of both updrafts and downdrafts within the cloud. The downdrafts are initiated by the downward drag of falling precipitation. The downdraft is strengthened by
evaporative cooling, as the rain falling with the downdraft enters drier air below the cloud base and evaporates
Mature Stage
This stage is characterized by the presence of both updrafts and downdrafts within the cloud. The downdrafts are initiated by the downward drag of falling precipitation. The downdraft is strengthened by evaporative cooling, as the rain falling with the downdraft enters drier air below the cloud base and evaporates. This cold descending air in the downdraft will often
reach the ground before the precipitation
As the mature-stage thunderstorm develops, the cumulus cloud continues to
increase in size, height and width
Cloud to ground lightning usually begins when
the precipitation first falls from the cloud base
This stage is characterized by the presence of both updrafts and downdrafts within the cloud. The downdrafts are initiated by the downward drag of falling precipitation. The downdraft is strengthened by evaporative cooling, as the rain falling with the downdraft enters drier air below the cloud base and evaporates. This cold descending air in the downdraft will often reach the ground before the precipitation. As the mature-stage thunderstorm develops, the cumulus cloud continues to increase in size, height and width. Cloud to ground lightning usually begins when the precipitation first falls from the cloud base. During this phase of the life cycle
, the top of the resulting cumulonimbus cloud will start to flatten out, forming an anvil shape often at the top of the troposphere
Dissipating (Decaying) Stage
This stage is characterized by
downdrafts throughout the entire cloud.
Dissipating (Decaying) Stage
This stage is characterized by downdrafts throughout the entire cloud. Decay often begins when
when the supercooled cloud droplets freeze and the cloud becomes glaciated, which means that it contains ice crystals
This stage is characterized by downdrafts throughout the entire cloud. Decay often begins when the supercooled cloud droplets freeze and the cloud becomes glaciated, which means that it contains ice crystals. Glaciation typically first appears in
the anvil, which becomes more pronounced in this stage.
The glaciated cloud appears
filmy, or diffuse, with indistinct cloud edges. The cloud begins to collapse because no additional latent heat is released after the cloud droplets freeze, and because the shadow of the cloud and rain cooled downdrafts reduce the temperature below the cloud.
The decay of a thunderstorm can also be initiated when
the precipitation within the storm becomes too heavy for the updrafts to support, when the source of moisture is cut off, or when lifting ceases.
Storms are classified according to their
actual physical characteristics
There is actually a continuous spectrum of thunderstorm types, but there are four broad categories of storms:
single cell storms,
multicell cluster storms,
multicell line storms, and
supercell storms
- Single Cell Storm
Single cell thunderstorms have lifetimes of
20-30 minutes
- Single cell thunderstorms have lifetimes of 20-30 minutes.
They usually are not strong enough to
to produce severe weather
pulse storm
Strong single cell storms
A true single cell storm is
rare
Even with separate appearing storms in weak vertical wind shear, the gust front of one cell often
triggers the growth of another cell some distance away
single cell storm usually occurs in
- a more unstable environment with white cloud shapes
Although most single cell storms are non-severe, some single cell storms may produce brief severe weather events. These storms have
slightly stronger draft speeds and typically produce marginally severe hail and/or brief microbursts. Brief heavy rainfall and occasional weak tornadoes can also be expected (it should be remembered that any thunderstorm is theoretically capable of producing a tornado).
why is it is difficult to forecast exactly when and where severe weather will occur?
Because single cell storms are poorly organized, and because they seem to occur at random times and locations
The multicell cluster is the
most common type of thunderstorm.
The multicell cluster consists of
- a group of cells, moving along as one unit, with each cell in a different phase of the thunderstorm life cycle. As the cluster moves along, each cell takes its turn as the dominant cell in the cluster.
when do new cells tend to form at the upwind?
When multicell storms form in environments with winds which veer from southerly to westerly and increase with height, new cells tend to form at the upwind (usually western or southwestern) edge of the cluster.
(multicall cluster storm) Mature cells are usually found at
- the center of the cluster with dissipating cells at the downwind (usually eastern or northeastern) edge of the cluster.
Although each cell in a multicell cluster lasts only about
20 minutes
), the multicell cluster itself may persist for
several hours
Multicell clusters are usually more intense than
single cell storms
Multicell cluster storms can produce
heavy rainfall (especially if a number of cells mature over the same area), downbursts (with wind speeds up to about 80 miles per hour), moderate-sized hail (up to about golf-ball size), and occasional weak tornadoes
Severe weather will tend to occur where
updrafts and downdrafts are close to each other (i.e., near the updraft- downdraft interface (UDI) associated with mature cells).
The right (east) side of the complex is
dominated by anvil outflow, moving with the storm from left to right
In fact most flash floods probably occur with
multicell complexes. As with all thunderstorms, the threat to the aviation community is quite high.
- Multicell Line Storm (Squall line)
- Squall lines generally form along or ahead of cold fronts and drylines. Squall lines can extend to hundreds of miles in length. They also can travel quickly – at speeds up to 60 mph.
- Multicell Line Storm (Squall line)
They can produce
severe weather in the form of heavy rainfall, strong winds, large hail, weak tornadoes and frequent lightning. But they are best known as prolific downburst producers. As with multicell cluster storms, squall lines usually produce severe weather near the UDI
Occasionally, an extremely strong downburst will
- accelerate a portion of the squall line ahead of the rest of the line. This produces what is called a bow echo. Bow echoes are easily detected on radar but are difficult (or impossible) to observe visually.
Weak to strong tornadoes may occur with the
- comma head, while gustnadoes may form on the strong bow echo gust front.
Flash floods occasionally occur when
- the squall line decelerates or even becomes stationary, with thunderstorms moving parallel to the line and repeatedly across the same area.
- Multicell Line Storm (Squall line)
It consists of
a long line of storms with a continuous, well-developed gust front at the leading edge of the line. As the gust front moves forward, the cold outflow forces push unstable air into the updraft usually at the leading (eastern) edge of the storm, with the heaviest rain and largest hail just behind (to the west of) the updraft. Lighter rain, associated with older cells, often covers a large area behind the active leading edge of the squall line.
If tornadoes are associated with a squall line, they will usually develop in
cells that are just north of a break in the line or in the southern most cell line’s (sometimes called the “anchor cell”). Cells in these locations tend to behave more like supercells than typical squall line cells.
- Supercell Storm
The supercell is a
highly organized thunderstorm
Like the single cell storm, the supercell consists of
- one main updraft. However, the updraft in a supercell is extremely strong, reaching estimated speeds of 150-175 miles per hour.
The main characteristic which sets the supercell apart from the other thunderstorms is
- the element of rotation.
The rotating updraft of a supercell, called a
mesocyclone
The rotating updraft of a supercell, called a mesocyclone helps the supercell to
produce extreme severe weather events, such as giant hail (more than 2 inches in diameter), strong downbursts of 80 miles per hour or more, and strong to violent tornadoes
Although supercells are rare, they pose an inordinately high threat to life and property. The supercell is the most dangerous because of the extreme weather generated. This storm can produce
baseball hail
The supercell environment is characterized by
high instability, strong winds in the mid and upper atmosphere, and veering of the wind with height in the lowest mile or so.
The supercell environment is characterized by high instability, strong winds in the mid and upper atmosphere, and veering of the wind with height in the lowest mile or so. The veering winds with height assist
- the mesocyclone formation within the supercell.
Supercells are extremely dangerous, but excellent warnings are possible once
the storm has been properly identified
- Downbursts
Recall that a downburst is defined as a strong downdraft with an outrush of damaging winds on or near the ground
Downbursts are subdivided based on
their size
macroburst
. If the swath of damaging winds is 2.5 miles or greater in diameter, then it is termed a macroburst
microburst
. If the swath is less than 2.5 miles, it is called a microburst
In general, microbursts are
quick-hitting events and are extremely dangerous to aviation
Microbursts are sub-classified as
dry or wet microbursts, depending on how much (or little) rain accompanies the microburst when it reaches the ground
The formative stage of a microburst occurs as
the downdraft begins its descent from the cloud base. The microburst accelerates downward, reaching the ground a short time later. The highest wind speeds can be expected shortly after the microburst impacts the ground. As the cold air of the microburst moves away from the center of the impact point, a “curl” will develop. Winds in this “curl” will accelerate even more, resulting in even greater danger to aircraft in the area. After several minutes, the microburst dissipates, but other microbursts may follow a short while later
visual clues for identifying microbursts.
- Patches of virga mark potential microburst formation areas. Virga is defined as precipitation which evaporates before reaching the ground. As the precipitation evaporates, it cools the air and starts a downdraft. If atmospheric conditions are right, the downdraft may accelerate and reach the ground as a microburst.
- Localized areas or rings of blowing dust raised from the ground usually mark the impact point of dry microbursts.
- A small, intense, globular rain area may mark a wet microburst.
- As the microburst reaches the ground and moves away from its impact point, a plume of dust may be raised from the ground. This plume is called a dust foot and also marks a possible microburst.
A gust front is the
leading edge of cool air rushing down and out from a thunderstorm.
There are two main reasons why the air flows out of some thunderstorms so rapidly.
- The primary reason is the presence of relatively dry air in the lower atmosphere. This dry air causes some of the rain falling through it to evaporate, which cools the air. Since cool air sinks (just as warm air rises), this causes a down-rush of air that spreads out at the ground. The edge of this rapidly spreading cool pool of air is the gust front.
- The second reason is that the falling precipitation produces a drag on the air, forcing it downward. If the wind following the gust front is intense and damaging, the windstorm is known as a downburst
