Static Stability and Cloud Development

Cloud Formation What is static stability? It describes what would happen to an air parcel if it was forced upwards. It could keep rising It could return back to it’s original/starting height How is air is cooled to the saturation point? If atmosphere is stable: If atmosphere is unstable: Generally through vertical lifting Flat-layered type clouds form (stratus) Cumulus-type clouds form (puffy clouds)

Stratus

Cumulus

Parcel rising and sinking in the atmosphere

How does air get forced upward? Topography Surface heating/convection Convergence of surface air Lifting associated with weather fronts

Convection

How to create an unstable atmosphere? Get lots of warm air underneath cold air Heating the surface considerably (convection) Wind/advection brings warm air in from another region Add moisture near the surface moisture makes air more buoyant and able to rise Bring in cold air to upper atmosphere troughs of low pressure do this

How to create a stable atmosphere Get cold air below warmer air. This is called a temeprature inversion, and this prohibits convection A cool ocean nearby can accomplish this by creating a marine layer Remove moisture near the surface “drying out” the air makes air less buoyant and less likely to rise Bring in warm air to upper atmosphere ridges of high pressure do this

Thunderstorms/ Lightning and Tornadoes

Thunderstorms Thunderstorms are generally classified into one of two groups: Air Mass Thunderstorms Mesoscale Convective Systems These are not-so-intense storms that are short-lived and localized. These systems lead to the generation of numerous thunderstorms.

Air Mass Thunderstorms These thunderstorms form within a single air mass and are not tied to fronts or mid-latitude cyclones. Daily solar heating is primarily responsible for rising air motion, producing rising cumulus clouds. These storms may go through growing, mature and dissipation stage In just an hour or two Often associated with summer storms.

Air Mass Thunderstorms Air mass thunderstorms are “self extinguishing” – their natural evolution forces their dissipation An updraft is required to release the latent heat that drives the thunderstorms. In the later stages, rainfall will lead to air cooling and a downdraft. This largely kills the updraft and thus the thunderstorm

Severe Thunderstorms As the name suggests, these thunderstorms are part of a mesoscale system. The horizontal scale of up to a few hundred kilometers. The actual structure of an these storms can vary considerably.

Severe Thunderstorms As the name suggests, these thunderstorms are part of a mesoscale system. The horizontal scale of up to a few hundred kilometers. The actual structure of an these storms can vary considerably. mesoscale convective complexes squall line thunderstorms supercell thunderstorms

Severe Thunderstorms

Severe Thunderstorms Change in wind with altitude important to formation: “Wind Shear”

Squall-line Thunderstorms

Severe Thunderstorms Characteristics Definition of Severe Thunderstorm: Capable of producing large hail Strong gusty surface winds Flash floods Tornadoes ¾ inch hail or Surface wind gusts of 50 knots

Supercell Thunderstorms

Average # of days thunderstorms observed

Average number of days hail observed

Lightning and Thunder Lightning is the discharge of electricity that occurs within a thunderstorm. The extreme heating associated with lightning causes air to expand rapidly, and produces sound waves we recognize as thunder. If you want to estimate the distance of an approaching thunderstorm: Count the number of seconds between a lightning strike and the sound of thunder For every five seconds, the storm is 1 mile away

Normal charge separation in a mature thunderstorm Charge separation not fully understood, but rapid convection (vertical motion) is certainly important

Separation of Charge in Clouds One theory suggests that the separation of charge is due to collisions between ice particles. After exchanging charge, the heavier ice crystals settle toward the cloud base. - - ++

Development of lightning stroke

Cloud to Ground Lightning Strike Positive charge is drawn up to the stepped leader. Usually through the highest conducting object. The return stroke can travel at 1 ´ 108 ms-1 (roughly 1/3 the speed of light.) This return stroke is visible to the human eye. There are commonly three or four strokes along a common path, which is why it is sometimes appears like the lightning flickers.

Why one shouldn’t shelter under a tree during a thunderstorm…

Lightning Facts It is estimated that globally 5000 people are killed by lightning annually. At any given moment there are ~ 1000 thunderstorms occurring over the globe. Two thirds of all lightning strikes occur within the tropics. Most lightning strikes are NOT cloud to ground strikes (20%) - rather cloud to cloud strikes are most common. Aircraft are usually not damaged by lightning strikes.

Tornadoes … … are also called twisters or cyclones. … are rapidly rotating winds that blow around a small area of intense low pressure. … come in many shapes, but mostly look like funnels or tubes. … often descend from large cumulonimbus clouds. A funnel cloud is a tornado that doesn’t hit the ground. A waterspout is a tornado-like storm that occurs over the ocean

Tornado Characteristics Majority of tornadoes rotate counter-clockwise (cyclonic) Most tornadoes only last a few minutes Most tornadoes are ~ 100 – 600 m (300-2000 ft) in diameter

Power of the Wind The force, or power associated with the wind is proportional to the cube of the wind speed. This means if the wind speed doubles, the force increases by the factor of eight. A wind speed of 100 mph is 8 times more powerful than a 50 mph wind

Enhanced Fujita Tornado Damage Scale EF0...... Light damage. Peels surface off some roofs; some damage to gutters or siding; branches broken off trees; shallow-rooted trees pushed over. Winds of 65-85 mph EFl...... Moderate damage. Roofs severely stripped; mobile homes overturned or badly damaged; loss of exterior doors; windows and other glass broken. Winds of 86-110 mph EF2......Considerable damage. Roofs torn off well-constructed houses; foundations of frame homes shifted; mobile homes completely destroyed; large trees snapped or uprooted; light-object missiles generated; cars lifted off ground. Winds of 111-135 mph

Enhanced Fujita Tornado Damage Scale EF3...... Severe damage. Entire stories of well-constructed houses destroyed; severe damage to large buildings such as shopping malls; trains overturned; trees debarked; heavy cars lifted off the ground and thrown; structures with weak foundations blown away some distance. Winds of 136-165 mph EF4...... Devastating damage. Well-constructed houses and whole frame houses completely leveled; cars thrown and small missiles generated. Winds of 166-200 mph EF5......Incredible damage. Strong frame houses leveled off foundations and swept away; automobile-sized missiles fly through the air in excess of 100 m (109 yd); steel reinforced concrete structure badly damaged; high-rise buildings have significant structural deformation; incredible phenomena will occur. Winds over 200 mph So far only one EF5 tornado has been recorded since the Enhanced Fujita Scale was introduced on February 1, 2007.

Tornado Formation Formed in association with severe thunderstorms Conditionally unstable atmosphere is important Multiple tornadoes can come from a single storm (like a supercell storm) Example: May 4-5, 2003, during a 24 hour period, there were over 80 reported tornadoes.

Recipe for a tornado Strong wind shear - Can enhance rotation - Caused mainly by jet stream Warm moist air below dry colder air - Large instability - Explosive growth due to latent heat release Supercell storms good candidate for tornadoes (they already have rotation).

Tornado occurrence? Tornadoes possible everywhere in the world, but most are in the U.S. (tornado alley Texas –Nebraska 3/4 of the tornadoes occur from March to July, with the maximum in _____. Jet stream is still a large influence Most often occur in the _______________ Least frequent ______________ May late afternoon (4-6pm) before sunrise

Tornado incidence by state 25 year total

Why is Tornado Alley the most likely place to get tornadoes? Perfect location for the mixing of air masses Warm, moist gulf air to the south Cold, dry to the north/northeast Rockies mountains to west/northwest Downslope flow is cool and dry Right latitude for the polar jet stream

On Radar, the presence of a hook echo indicates a mesocyclone; a region in a thunderstorm very likely to spawn a tornado