thunderstorms and hurricanes Flashcards
what is the trigger for rising air?
- Unequal heating at surface
- Random turbulent eddies
- Terrain effects that lift air locally or along mountain barriers
- Diverging upper level winds and converging surface winds and rising air
- Warm air rising along frontal zone
what is the cumulus stage of thunderstorms?
warm air rises and condenses into a cloud. The latent heat of condensation warms air so keeps rising. No rain as updrafts keep water and ice suspended. The adjacent drier air is then drawn in (entrainment), and then the drier air causes some rain drops to evaporate, latent heat of evaporation cools the air.
what is the nature stage of thunderstorms?
this is the most intense stage. The top of the clouds reaches stable zone. Eg tropopause spreads out into an anvil shape, like a bowl shape. The drier air causes some rain drops to evaporate, and the latent heat of evaporation cools the air. The air which is now cooler and heavier, descends as downdraft. As the air descends, ice particles melt and latent heat of fusion colds the air more and enhances downdraft. This forms a circulation cell. The intense turbulence causes light and thunder and heavy precipitation.
waht is the dissipating stage of thunderstorms?
this stage is 15-30 mins for a single cell. The gust front moves away and no longer enhances updraft. The downdrafts dominate through most of the cloud, and no more humid rising air, so clouds dissipate with light precipitation. Only cirrus anvil remains at the top. A single cell thunderstorms may go through all three stages in 1 hour or less.
what is a multicell thunderstorm?
Forms in zone of wind shear, near the frontal boundary. As pre-frontal squall line. Multicell thunderstorms forming as a squall line ahead of a cold front. Pre-frontal squall line thunderstorms may form as upper air-flow forms waves ahead of front.
Clouds do not penetrate far above tropopause as stratosphere is stably stratified.
what is a supercell thunderstorm?
- Downdraft never undercuts updraft. Wind shear strong resulting in rotation. When a single violently rotating updraft forms, a supercell results.
- Typical of spring over central US plains. Collision of contrasting air masses causes wind sheers.
what is the supercell thundestorm relation to upper air flow?
Relation to upper air flow: polar front Jet Stream swings over provides area of divergence hat enhances surface convergence and rising air.
with hurricanes, why are seasonal changes small?
• Daily heating, cumulus clouds develop, afternoon thunderstorms. Most not severe, but sometimes align to give: tropical squall line, may form along atmospheric topical waves.
what is a tropical storm like?
Tropical storm with winds >64 knots, typically 500 km across with Eyre with very low pressure. Surface winds spiral counter-clockwise to reach max. In Eye wall that contains a ring of intense thunderstorms.
The eye is the bit in the middle of the clouds which causes the spiral action of the hurricane;
how are atlantic hurricanes forced over africa?
Most Atlantic hurricanes may be forced by tropical waves initiated over Africa. This tends to be more and stronger when W. Africa is wettest.
what is the first stage in hurricane formation?
• The first stage is clustering of thunderstorms around a low pressure area. Latent heat releases into cloud. This warming of the air aloft creates areas of high pressure. Air aloft moves outwards from high, cools and sinks. Waring of air intensifies surface low. As surface winds rush into low, they collect more sensible heat and latent heat and moisture from ocean. Then the warm moist air is swept upwards in winds of the eyewall. The whole cycle is repeated and reinforced.
what are the heat engines for hurricanes?
• Analogous to heat engines: heat engine is heat taken in at high T, and is converted to work then ejected at low T. The hurricane takes heat in near warm ocean surface converted to kinetic energy winds, lost at top through radiational cooling.
what are the topical disturbances for hurricanes?
• Tropical disturbance: mass of thunderstorms. Tropical depression- winds 20-34 knots around developing low. Topical storms- 35-64 knots, hurricane when 64 knots or more.
how long do hurricanes last?
- Over warm water: may travel 1000’s km and last 24 days
- Over cold water: rapidly weaken as they lose heat source. If water below eyewall cools by 2.5 degrees celcius, storms dissipate. Also if warm water layer is shallow, since turbulence mixes.
- Over land- lose energy source: land friction causes winds to blow into storm raising pressure.
how do hurricanes move?
- Steered y Easterlies. Gradually swing poleward around subtropical highs, by mid-latitude speed may be up to 50 knots.
- Paths taken by tropical cyclones
- Why no cyclones and hurricanes near equator?
