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- A cloud that is in contact with the ground.
- Hazardous when it obscures visibility for travel and when air pollutants are added to form smog or vog
Meaures the body's perception of air temp, taking into consideration humidity
- Considered to be prolonged periods of extreme heat that are both longer and hotter than normal.
- Criteria for establishing what constitutes longer and hotter than normal varies geographically w climate and living conditions.
prolonged per of freezing rain during which thick layers of ice accumulate on all cold surfaces, can be more or just as damaging as blizzards.
- a natural, high voltage electical discharge b/w a cloud and the ground, other clouds, or w/i clouds. The discharge takes a few tenths of a sec and emits a flash of light that is followed by thunder.
- Discharge of millions of joules of electricity. can heat air in path up to 30k degrees C (54kF) 5x hotter than sun
- Amt of moisture in the air.
- A measure of the amt of water vapor in the air compared with the amt that would saturate the air at a given temp and pressure; commonly a percentage.
A violently rotating column of air associated with extreme horizontal winds, causing tremendous damage and loss of life
Lowest layer of atmosphere [8-16km above surface]
Urban Heat Island Effect
A climatic condition resulting from various design and land use practices in a city, large town, This condition intensify heatwaves and cause temp in metro to be [up to 22*] warmer than the surrounding rural area.
Indicates that the area affected is in danger and ppl should take immediate action to protect themselves.
- Moving air rapidly cools exposed skin by evaporating moisture and removing warm air from next to the body.
- Reduces the time it takes for forstvite to form.
- Enhanced Fujita Scale-
- Graduated range of values from EF0 - EF5 describing tornado intensity based on max 3-sec wind velocity inferred from damage to buildings.
What is the role of water vapor in the air?
- Cloud formation and atmospheric circulation.
- Humidity describes the amt of water vapor in the air.
- Hydrologic cycle
Warm air holds more water vapor than cold air.
Humidity is largely a function of temp; natural changes in relative humidity occur each day, increase at night bc temp is cooler, decrease with daytime heating.
The lowest layer of the earth's atmosphere
The troposphere, defining characteristic: rapid upward decrease in temp, the most visible feature is abundant condesed water vapor in the form of clouds. Not even the highest mt's breach the upper boundary known as the tropopause.
Clouds and weather occur primarily in the troposphere
- most of the atm water vapor condenses in the troposphere along w most of the atm carbon dioxide and methane [important to global warming]
- Clouds are made of very small water droplets or ice crystals. Cumulus: puffy fair weather; Cumulonimbus thunderstorm, release lots of energy by condesation
High pressure vs Low pressure
The combined Effects of temp and air movement produce Low-pres centers and High-pres centers. At the surface, air flows from areas of high pres to areas of low. Less dense air rises in areas of low pres and diverges in the upper troposphere. Generally creates an area of High P aloft on top of the surface Low P center.
Unstable air moves while stable air stays in place - warm unstable air often rises to form clouds.
- An airmass is considered unstable if parcels w/i it are rising until they reach air of similar temp and density.
- Atm becomes unstable when lighter warm or moist air is overlain by denser cold or dry air.
- Severe weather, t-storms, tornados are associated.
Fronts, cold front vs warm front, stationary front
- The boundary b/w a cooler and warmer air mass is a Front.
- Cold front: when cold air is moving into a mass of warm air
- Warm front: the opposite
- Stationary: boundary b/w warm n cold shows little movement
- Occluded: rapidly moving cooler air overtakes another cooler air mass and warm air is wedged above the frontal boundary
Thunderstorms occur mostly in equatorial regions and require 3 basic atm conditions
- 1. Water vapor must be avail in the lower atm to feed clouds and precipitation as the storm forms
- 2. A steep vertical temp gradient must exist in the environ so the rising air becomes warmer than the air it's moving through. This gradient places colder air over warmer, moist air.
- 3. An Updraft must force moist air up to colder levels of the atm.
Most individual tstorms [air mass tstorms] last less than an hour.
- Developing/Cumulus Stage: Starts as moist air forced upwards, cools, n water vapor condenses to form cumulus cloud. If the moisture supply and updraft continue, relative humidity increases in air surrounding the cloud causing it to grow instead of evaporating; becomes cumulonimbus [califlower]
- Mature Stage: begins when the downdraft n falling precipitation leave the base of cloud. Stage has both up and down drafts, continues to grow until reaches top of unstable atm/tropopause. Clouds have anvil shape. Heavy rain, lightening and thunder, hail
- Dissipative stage: begins when upward supply of moist air is blocked by downdrafts at the lower levels of the clouds. Downdrafts incorporate cool dry air, causing evaporation and more cooling. Deprived of moisture the tstorm weakens and ends.
Types of severe tstorms
NWS classifies a tstorm as severe if windspeeds in excess of 93km[58mph]; or hailstones over 1.9cm; or a tonado happens
- Mesoscale Convective Systems [MCS]: roughly circular clusters of storm cells; most common
- Squall Lines: Linear belts of tstorms; drylines of SW US
- Supercells: Large cells w single updrafts; most damaging
generally funnel-like columns of rotating winds 65 - 450km[40-280mph] that extend downward from clound to ground.
Tornados form by wind shear in sever tstorms, esp supercells, n r most common in central US. Damage is rated on the Enhanced Fujita Scale. waterspouts are generally weaker tornados forming under fair-weather conditions along ocean coastlines and over lakes.
by the most intense damage that they have produced along their path
- Each assigned a value on EF-Scale based on poststorm damage survey of 27 types of buildings, towers n poles, hard and softwood trees
- Waterspouts: tornados that form over water
Geographic distribution of tornados [fig 8.21]
Most common in US. Mainly in Plain states, b/w rocky's and Appalacians, known as Tornado alley
Benefits of severe weather
- lightening produces forest fires and storms topple dead trees and freezes increase plant n animal diversity in forests.
- Some blizzards, snow storms, tstorms, tropical storms are in some areas the only source of water
Watches vs Warnings
- Watch does not guarantee that the event will occur
- Warning: when a tornado has been sighted or detected by weather radar, indicating that the area affected is in danger n ppl should take immediate action to protect themselves