ST L4: Downbursts, Gust Fronts and Hail/ How Thunderstorms Harness the Energy

Question 1

What are downbursts?

Answer 1

Regions of rapidly descending air. They are invisible but are hazardous to aircraft especially just after takeoff or just before landing.

Question 2

What causes downbursts?

Answer 2

Falling rain that drags some air down with it, and by the evaporation of some of the rain as it falls, which cools the air and causes it to sink.

Question 3

How fast are downdrafts? How about horizontal speed near ground?

Answer 3

20km/h to 90 km/h; horizontal speeds near ground can reach up to 250km/h.

Question 4

What are microbursts?

Answer 4

Downbursts that are small diameter (~1km)

Question 5

What are gust fronts? What causes them?

Answer 5

The leading edge of cold, spreading air of violent straight-line winds. Occurs when downburst hits the ground.

Question 6

What is a haboob?

Answer 6

A dust or sand storm produced by gust front blowing over dry dusty surfaces.

Question 7

What is an arc cloud?

Answer 7

A cloud created by advancing gust front in humid regions; the gust front can push the surrounding warm, humid air upward to create arc cloud.

Question 8

Gust front and arc cloud: where do they initially form? What happens after sometimes?

Answer 8

Initially they form immediately around the precipitation area, but sometimes they advance faster than the thunderstorm moves and end up in front of the storm. The winds can blow down large trees, destroy weak structures, and are also a hazard to aircraft during takeoff and landing.

Question 9

How to keep safe from hazards of downbursts and gust fronts?

Answer 9

Avoid standing around or hanging out near weak buildings and trees that could fall due to strong winds. Airports have sensors to send warnings too.

Question 10

What makes downbursts visible?

Answer 10

Ones that contain cloud and rain drops and ones that are in the process of producing a wet microburst.

Question 11

How is a wet microburst characterized?

Answer 11

By a well-defined foot shape on the left side of the rainshaft.

Question 12

How to know when a gust front is approaching?

Answer 12

You’ll feel the hot humid air in advance. Just before the gust front arrives, the air sometimes becomes still or calm. Then, the low dark arc cloud sweeps overhead and you feel the sudden gusts of cold wind on your face that smells cleaner than the previous. Sometimes it has the sharp smell of ozone caused by lightning discharges. It stays very windy after the gust front passes, as the precipitation region is advancing toward you. Finally the rains reach you and drench you with huge drops.

Question 13

Where do haboobs form, generally?

Answer 13

In desert regions which are so dry there is insufficient moisture to make an arc cloud.

Question 14

What is virga?

Answer 14

rain that evaporates before reaching ground.

Question 15

What is hail? Where does it fall down from?

Answer 15

Irregularly shaped balls of ice, falling out of the strongest thunderstorms. Most hail are the size of peas or marbles, but some bigger ones can be the size of baseballs and grapefruit. They can cause crop damage, break car windshields, dent the metalwork, and cause severe injuries. Should never be outdoors in hailstorm unless under solid roof. Extremely hazardous to aircraft and can cause them to crash.

Question 16

Where does normal precipitation form?

Answer 16

When small ice crystal near the TOP of the thunderstorm fall in the downdraft regions into warmer air, where they melt and continue as rain.

Question 17

What does the size of raindrops tell us?

Answer 17

The larger the drop, the deeper the cloud that it fell from. (thus we see large drops from deep thunderstorms.

Question 18

When does hail form?

Answer 18

When small ice crystals near the top of the thunderstorm re-circulate and fall down into the updraft region, instead of the downdraft region. This allows them to accumulate many layers of ice, growing like layers of an onion. Hailstones formation require strong updrafts. Hailstones are heavy, so they often fall very close to the updraft part of the storm. It rarely falls from the anvil of a thunderstorm.

Question 19

Where is hail most frequent in the USA? In Canada?

Answer 19

In Oklahoma and Texas and Kansas.
In Canada, it is most frequent just east of the Rockies, centered near the town of Red Deer (btwn Calgary and Edmonton). There are also hail regions in central BC, southern Sask, southern Manitoba.

Question 20

In Alberta, how do crop insurance companies try to reduce the hail? Does it scientifically work?

Answer 20

They seed the thunderstorms with silver iodide dispersed from the aircraft to try to reduce the hail. It does not work, but the companies do it for public relations.

Question 21

Which town in Canada has highest number of hail days per year?

Answer 21

Red Deer, Alberta

Question 22

What is a hail swath?

Answer 22

The damage path left by a moving hailstorm.

Question 23

How are the updrafts needed to create a hailstorm made possible?

Answer 23

A thunderstorm harnesses the initially-diffuse sensible and latent-heat energy, and concentrates it into a 10km diameter region (thunderstorm).

Question 24

What are some hazards from downpours (rain)? How to be safe during downpours?

Answer 24

flash floods, reduced driving visibility, rain-triggered landslides. Move to high ground, don’t drive through water of unknown depth.

Question 25

Which storms does hail most commonly come from?

Answer 25

Supercells (low precipitation storms).

Question 26

What are some hazards of hail? How to be safe during hailstorm?

Answer 26

Injury or death, dents in metal cars, broken windows, flattened crops, injured or killed livestock. To be safe, be indoors. If in car, make U-turn to leave the hail area, or park under a roof. If car exposed to strong hail, pull over to the side of the road and park; close eyes to keep out glass shards.

Question 27

An air parcel that rises will always move into lower/higher? pressure surroundings. The initially higher pressure inside the air parcel pushes outward with greater force than the surrounding environment pushes back. Net result: air parcel ____ as it rises.

Answer 27

lower; expands

Question 28

A law of thermodynamics: expanding gases become warmer/cooler?

Answer 28

cooler

Question 29

What is an adiabatic process? Does this apply to a rising air parcel?

Answer 29

A process for which there is no heat transfer. Yes, a rising air parcel will expand and cool adiabatically.

Question 30

What is a lapse rate?

Answer 30

The decrease of temperature with height.

Question 31

What is the special “process” lapse rate associated with rising air parcels? What are the two subdivided types?

Answer 31

The adiabatic lapse rate. Dry adiabatic lapse rate and most adiabatic lapse rate (a.k.a. saturated adiabatic lapse rate).

Question 32

How much does the temperature of a rising air parcel decrease per 1 km it rises?

Answer 32

9.8 degrees C. (so if it descends by 1 km, it will gain 9.8 degrees C)

Question 33

When an air parcel rises, what does it carry with it?

Answer 33

All of the molecules within it: nitrogen, oxygen, water vapour. A conceptual air parcel that doesn’t mix with its surroundings as it rises means that the relative abundance of these various molecules remain constant. So for water vapour molecules, the mixing ratio r is constant in rising and descending air parcel.

Question 34

The mixing ratio for water vapour molecules remains constant as the air parcel increases/decreases. Does the value of saturation mixing ratio change? Why?

Answer 34

Yes it changes, because cooler air parcel holds less water vapour at saturation that a warmer one. So the value of saturation mixing ratio decreases as it cools. But since the parcel cools as it rises, we infer that the value of the saturation mixing ratio decreases as an air parcel rises. So the max. amount of water that can be held in the air as vapour decreases in air parcel decreases as the parcel rises.

Question 35

What is the Lifting Condensation Level (LCL)?

Answer 35

The altitude at which cloud droplets first form in rising air parcels. This is the height of the cloud base for convective (cumulus clouds) such as thunderstorms.

Question 36

What are convective clouds?

Answer 36

Clouds that form from air rising more-or-less vertically from near the Earth’s surface.

Question 37

What are two processes that happen to the air parcel as it continues to rise above the LCL?

Answer 37

Adiabatic cooling and warming caused by the release of latent heat in the condensing water. However, the amount of warming is less than amount of adiabatic cooling, so rising air parcel continues to cool, but it cools slower with altitude compared to an unsaturated air parcel.

Question 38

What is the saturated adiabatic lapse rate (moist adiabatic lapse rate)?

Answer 38

the rate of temperature change with altitude for saturated air.

Question 39

The extra warmth that drives storm circulations come from what?

Answer 39

The saturated cloudy parcel does not cool as fast, and ends up much warmer than an unsaturated parcel.

Question 40

Why is it possible for there to have liquid (unfrozen) cloud and rain drops at temperatures below freezing?

Answer 40

In the atmosphere, the cloud droplets are gently floating in the air, and there are relatively few ice-nuclei dust particles to trigger the freezing. So they can stay liquid at temperatures as cold as -40 degrees C.

Question 41

What are conceptual models used for in meteorology?

Answer 41

to understand the dominant mechanisms

Question 42

How are thunderstorms like large engined?

Answer 42

They convert the fuel of warm humid air into motions.

Question 43

Describe the circulations that occur in a thunderstorm.

Answer 43

The rising air (A) cools and reaches saturation at cloud base LCL. As it continues to rise within the cloud (B), latent heat is released due to condensation of water vapour into cloud droplets. This heating helps to keep the rising air (in the tube) warmer than its surroundings (outside the tube), hence the air continues to rise.

As air rises up the tube, continuity effects cause it to draw air in horizontally near the base of the tube (C). Because this air is coming from all around the base of the tube, it is converging toward the center. This converging inflow air is coming from near the ground – a layer called the boundary layer. But sunlight acting on the ground has caused the ground to add heat and moisture to the boundary-layer air. Thus, this air has the high heat and humidity that serves as the fuel for the storm.

At the top of the storm, the updraft hits the base of the stratosphere. Because it can’t rise into the stratosphere, the air spreads out (diverges) horizontally instead (D). Why?

This is analogous to a cork released from under water. The cork quickly rises within the water up to the surface, because the cork is less dense (lighter) than water. It has positive buoyancy. However, the cork does not continue to rise completely out of the water into the air, because the cork is denser (heavier) than air. Thus, the cork floats at the interface between the dense water and the less-dense air.

Question 44

Why can’t the the updraft of the storm rise into the stratosphere?

Answer 44

The updraft is denser than the stratosphere, so the air just spreads out/ diverges. The stratosphere is relatively warm with lower density air. The troposphere is cold with higher density. So the air parcel from the thunderstorm rises up to the tropopause (separation between troposphere and stratosphere. The parcel will just bob there.

Question 45

How is the anvil or mushroom cloud of the thunderstorm formed?

Answer 45

So many air parcels will have accumulated that they push each other away horizontally.

Question 46

What does the diverging air aloft and the converging air near the ground require to maintain continuity.

Answer 46

A return circulation (downdraft), which is helped by precipitation of heavier raindrops formed in the cloud. The falling precipitation drags some of the air with it.

Question 47

Where does the thunderstorm extract latent heat?

Answer 47

from humid air (found in the boundary layer) in the rising updraft

Question 48

Where does the thunderstorm dump out the exhaust? What is considered the exhaust?

Answer 48

Rain and outflow air, at the ground and tropopause respectively.

Question 49

How can a daughter thunderstorm be spawned?

Answer 49

From the downburst/gust front from a thunderstorm.

Question 50

What is a favorable environment needed to create long-lasting storms? What is even more favorable (causes more violent and long-lived thunderstorms?)

Answer 50

Stored energy in the boundary layer.

Wind shear and stored energy for more violent and long-lived storms.

Question 51

What caps the boundary-layer air and prevents it from rising?

Answer 51

The temperature inversion or stable layer on top of the boundary layer.

Question 52

What kind of triggers do thunderstorms need to force or draw some of the boundary layer air upward past the capping stable layer?

Answer 52

Triggers include cold fronts, gust fronts, sea-breeze fronts, dry lines. They are the leading edges of cooler heavier masses of air that move along the ground and push the warmer air in front of them up and out of the way.

Question 53

How do mountains trigger thunderstorms?

Answer 53

When horizontal winds are forced upslope, they can push the boundary layer air up through the capping inversion. Thunderstorms triggered by mountains are called orographic thunderstorms.

Question 54

Are thunderstorms able to create their own favorable environment?

Answer 54

No

Question 55

What does the sun’s energy mainly go into?

Answer 55

modifying the temperature sounding in the bottom part of the troposphere (boundary layer)

Question 56

When thunderstorms feed on the boundary layer air, they consume all of the nearby warm air, then run out of fuel. They can die after a lifespan of roughly ___ minutes to ____.

Answer 56

15 minutes; half an hour

Question 57

What are two ways for a storm to continue? (as in, last for hours in form of supercell.

Answer 57

propagation of storms: the cool downburst from one mature or dissipating storm can spread out as a gust front, triggering a daughter storm nearby, where there is still fuel in the form of warm, humid boundary layer.

favorable wind shear