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Thunderstorms. Three Conditions are needed:. 1 ) Abundant source of moisture in the lower atmosphere 2) Moisture condenses, releasing latent heat. Latent heat keeps the cloud warmer. then the air around it. Keeps the upward motion going to the inside of the cloud.
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Three Conditions are needed: • 1) Abundant source of moisture in the lower atmosphere • 2) Moisture condenses, releasing latent heat. Latent heat keeps the cloud warmer. • then the air around it. Keeps the upward motion going to the • inside of the cloud. • 3) Portions of the atmosphere needs to be unstable.
Limits to Growth Three conditions need to meet: • · Air keeps rising • · More moisture condensing and releases latent heat • · Continues until rising air reaches stable air and growth stops • *Most cumulonimbus clouds grow to the height of 18,000 meters. • *Most thunderstorms last 20 - 30 minutes with a diameter of 24 • kilometers
Air Mass Thunderstorm • Classification by the lift mechanisms. • Air lifted upward due to the unequal heating of the earth within one air mass. • Maximum heating occurs during mid-afternoon - optimum time for those storms.
Two common thunderstorms 1) Mountain thunderstorms - air mass rises over mountains (orographic lifting)
Sea breeze thunderstorm • - local air mass thunderstorm caused by the temperature - the difference between land and water
Frontal Thunderstorms • Produced by advancing cold front and they meet the warm fronts. Upward motion produces a line of thunderstorms. • Initial lift comes from the cold front, why? Air is not dependent on day time heating - occurs at night!
Warm Fronts • Can develop thunderstorms. When the warm front slides up and over a cold front within stable air - • mild thunderstorm.
Thunderstorm development • Three stages of development - Stages classified by the direction of air movement
Cumulus Stage • 1) a. Begins with thermals created by insulating heating in the g r ground or winds blowing over terrain • b. Winds force moisture upward • c. Moisture begins to cool and forms tiny droplets of H2O • d. Water droplets accumulate and forms cumulus clouds • e. Condensation process releases heat into surrounding air • f. Heat causes updrafts and creates low pressure under cloud
Mature Stage • a. Air cannot rise any farther • b. Cumulus clouds become cumulonimbus clouds. • c. H2O droplets increase in size. Air cools around it. • d. Rain falls quickly-creating downdrafts. • e. Updrafts and downdrafts create convection cells that turn into gusty winds.
Dissipating Stage • Downdrafts - thunderstorms undoing the convection cells that only exist with a supply of warm air. • Once the warm air supply is gone, downdrafts continue until it runs out of raindrops.
Severe Thunderstorm • Characteristics of a severe thunderstorm • 1) Hail • 2) Swirling tornadoes • 3) 160 mph winds • How do they form? • Cold air moves over warm land that provides a steady supply of moisture.
Super Cells • Super Cells • Thunderstorms with large temperature gradients that exist between the upper and lower atmosphere of the storm. • · Air becomes more unstable producing stronger updrafts and downdrafts. • · Cold fronts more with low pressure systems with pockets of cold air - creates super cell. • · Super cells have intense updrafts (rotating ) can reach speeds of 240 km/hr. Only 10% of 100,000 thunderstorm reach that level
Lightning • Electrical discharge caused by rapid movement of air with in a cumulonimbus cloud Friction between updrafts of air and of downdrafts cause the airs to lose e- forming Ions. These are (+) and (-) charged particles.
Creates and electrical imbalance which in turn creates an invisible channel of negative charged air between the cloud and the ground • The channel meets the (+) charge round in close areas - return strike is upward towards the cloud - lightning
Lightning bolts heat the air surrounding it to 30,000°C • Thunder - rapid expansion and contraction of air, caused by intense heat. • Know safety tips - Table 13.1 p 336
Downburst • Violent downdraft in small areas • · Areas that are 5 km wide are called macrobursts - last 30 • minutes wind speed - 200 mph • · Areas less than 3 km wide are called microbursts - 10 • minutes or less - wind speeds 250 mph - can cause • extensive damage but difficult to detect
Wind Shear • When wind changes speed, direction and magnitude quickly and/or violently
Hail - Precipitation that falls as ice balls - occurs interior section of the U.S. -spring • Two Conditions for hail: • · Droplets in upper atmosphere reach temperature below • freezing - become ice pellets • · Strong updrafts and downdrafts carry the pellets up and down with other super- cooled water droplets added on. The balls of ice fall to the ground because of the pull of gravity
Floods • Floods - slow moving storms unleash rain over a restricted area - ground saturated. No where for water to go.
Tornados • Violent whirling column of wind in the air - called funnel clouds -sparked by super cells. When winds’ speed and direction changes quickly with height. • There are an average of 800 tornados nationwide per year. • Tornadoes are made visible by debris.
