THUNDERSTORMS AND AIR MASSES AND HURRICANES
Thunderstorms are some of the Earth's most spectacular events. Their lightning shows and their deafening thunder always get our attention.
ORDINARY (AIR MASS) THUNDERSTORMS – These thunderstorms have three stages to them. The Cumulus stage which is as humid air rises; it cools and condenses into a single cumulus cloud or a cluster of clouds. There is no lightning or thunder in this stage. As the cloud builds above the freezing level the cloud particles grow larger and heavier and they being to fall causing downdrafts. The beginning of downdrafts signifies the start of the mature thunderstorm. This is the most intense stage of the thunderstorm. Updrafts and downdrafts reach their greatest strength in the middle of the cloud, creating sever turbulence. In about 15 -30 minutes the dissipating stage occurs. Updrafts weaken and downdrafts ten to dominate throughout much of the cloud. Light precipitation usually falls during this period and then tends to leave behind an anvil as the only sign that there ever was a thunderstorm present.
SEVERE THUNDERSTORMS – The difference between an ordinary thunderstorm and a severe thunderstorm is that they are able to produce large hail, strong gusty surface winds, flash floods, and tornadoes. The National Weather Service defines a severe thunderstorm as having ¾ inch hail and/or surface winds gusts of 58 mi/hr. Updrafts are a major component of severe thunderstorms, they can cause the storm to tilt and allow updrafts to remain strong for an extended period of time or the updrafts could be so strong that precipitation does not have time to form. Updrafts also form the familiar “mushroom cloud”, also known as Cumulonimbus, to form well into the stable stratosphere. These violent updrafts also keep hailstones from falling enabling them to grow considerably in size.
SUPERCELL AND SQUALL-LINE THUDNERSTORMS – A supercell storm is a rotating thunderstorm whose updrafts and downdrafts are structured so that it is able to maintain itself as a singe entity for hours on end. Supercell storms can produce surface winds of over 100 mph; hail the size of grapefruit and large long lasting tornadoes. These storms are right movers; the rapidly rising air of updrafts interacts with horizontal winds that change direction with height enabling a new cell to form to its right. These storms have a vertical tower that looks like a corkscrew. Squall-line thunderstorms are simply thunderstorms that formed on the squall line or with 100 – 300 km in front of it. These storms may extend over 600 mi and cause damage all along their path.
Lightning and thunder are simply a discharge of electrical potential energy between two different parts of a cloud and the ground.
LIGHTNING - Lighting is not completely understood, but there are many theories. Lighting is simply a discharge of electricity which occurs in mature thunderstorms. As a side note, lightning also occurs in snowstorms, dust storms, and in rare occasion’s nimbostratus clouds and in the gas of a volcano. As an interesting fact the majority of lightning strikes are cloud to cloud while only 20 percent are cloud to ground strikes. Lightning is five times hotter that the sun at around 54,000 degrees Fahrenheit.
What causes a lightning stroke? As a visual, think of your favorite vinaigrette. Your oil, which is heavier, is negatively charged and is your surface atmosphere; your vinegar, lighter, is positively charged atmosphere. When you mix your two ingredients it creates and “explosion” of flavor. When you combine the negative and positive charged surfaces in a cumulonimbus cloud this effect creates some of our most beautiful and terrifying lightning on our earth’s surface.
Looking at the lightning stroke itself, it is broken down into three steps. When enough negatively charges air particles reach the bottom of a cloud a flow of electrons, otherwise know as a stepped leader, rushes toward the earth. At the same time a rush of positively charged air particles from the tops of trees and buildings charges up towards the stepped leader. Finally a strong electric current, a bright return stroke, carries the positive charge upward back into the cloud.
Another type of leader is called a dart leader. This is a bolt of lightning that follows the same path of the stepped leader but moves much more quickly because the resistance has been cut. A single lightning stroke may have up to three or four leaders and it will only last for a mere second. As a result during most mature thunderstorms you will have a distinct and beautiful show. Always different, and always bright.
THUNDER – Extreme heating of the air causes the air to expand explosively, therefore initiating a shock wave that becoming a booming sound wave that travels outwards in all directions from the flash. When the lighting stroke is very close, several hundred feet or less, the thunder sounds like a clap or a crack followed by a loud bang. When it is farther away it usually rumbles.
By definition an Air Mass is a large body of air that has similar horizontal temperature and moisture characteristics. Air masses are usually characterized by their temperatures and their humidity’s.
There are 5 basic names given to air masses:
Continental Artic (cA) - Forms primarily in the Artic and Antarctic regions and descend toward the equator bitterly cold and extremely dry in the winter, cool and dry during the summer.
Continental Polar (cP) - Form over dry lands. Cold and dry during he winter, mild and dry during the summer.
Continental Tropical (cT) - Forms over deserts and plains. In the U.S., a flow into the U.S. out of Mexico often sends a cT air mass northward. Typically hot and dry during summer and mild and dry during the winter.
Martitime Polar (mP) - Marine type humidity’s with cool and cold weather. Typically provide for miserable, damp, gray days. Mild to cold and humid with low stratus clouds and precipitation is often the rule with Maritime Polar air masses.
Maritime Tropical (mT) - Hot, humid, sticky weather. A good example of when mT air masses affect the U.S. s during the summer with the Bermuda High phenomena. Southerly flow of hot, humid, sticky weather is circulated northward into the U.S. Rarely will mT air masses affect the U.S. during the winter.
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