Chapter 11 Summary-Part 2 How a Thunderstorm Works
Air temperature drops upward from the ground surface through the troposphere at average rate of 6 °C/km. This is a stable rate of decrease in the troposphere. When lapse rate (the actual rate of cooling with height) is greater than 10°C/km, the atmosphere is unstable. Atmosphere instability increases as the temperature differences increase between warm air on the bottom and overlying cool air. Warm, low latitude air is less dense and it wants to rise upward. Air will rise as long as it is less dense than the surrounding air. If bottom air is warm and moist, the warm air may rise enough to pass through the lifting condensation level, allowing condensation of water vapour to begin. This marks the cloud base for a thunderstorm. Individual thunderstorms form on sunny days in late afternoon/early evening when temperature of ground surface and lower troposphere are the highest. Microbursts: An Airplanes Enemy
Microbursts: Sudden, violent downbursts of winds confined to a small area of tens of metres to4km in diameter. Commonly mistaken for a weak Tornado because winds often exceed 200km/h. Dangerous to airplanes because they are so close to the ground that an unexpected downdraft can push the plane to the ground. Doppler radar is commonly used to detect microbursts. Thunderstorms in Canada
Distribution of thunderstorms in Canada shows a non-uniform pattern. Southern Ontario has the fewest because it is humid. Has the warmest summers and it is away from the effect of the great lakes. A severe thunderstorm is issued when: 1. Wind gusts of 90km/h or more 2. Hail of 2cm in diameter or larger 3. Rainfall of 50mm or more within one hour or 75mm or more within 3 hours. Hail
Layered ice-balls dropped from severe thunderstorms that fall at speeds exceeding 150km/h. Requirements for hail are: 1. Large thunderstorms with buoyant hot air rising from heated ground. 2. Upper-level cold air creating maximum temperature contrasts. 3. Strong updrafts needed to keep hailstones suspended aloft while adding coatings of ice onto ever-growing cores. Destructive, large hail abounds in the colder prairies, especially central Alberta. Layering on hailstones indicated that it has travelled through parts of the thunderstorm cloud with greater and lesser amounts of super-cooled liquid content. July is the month of peak hailstorm activity (range from May- October)
Lightning
Thunderstorms generate lighting, and lightning causes thunder. Major cause of weather-related deaths in Canada in the summer. Kills people in ones and twos. Lightning flashes occur in Canada about three million times a year. Southern Ontario is the lightning “hot spot”. How Lightning Works
Involves the flow of electric current as areas with positive charges seek a balance with places having excess negative charges down low. During buildup of tall clouds, charged particles separate, creating an abundance of positive charges up to and an excess of negative charges down low. The charge imbalance comes about as the freezing and shattering of super cooled water drops initiates charge separations that are distributed by updrafts and downdrafts within thunderclouds. The charge separations occur during the cloud buildup of the early stage and then lightning bolts forth during the mature stage. Thunderclouds interact electrically with the ground. The abundance of negative charges in the basal part of the cloud creates a buildup of positive charges on the ground surface, because opposite charges attract. Lightning can move from cloud to earth, earth to cloud, or cloud to cloud. Lightning moves at speeds over 10,000km/s and includes several strokes occurring within 0.5 to 2 seconds. The sequence within a lightning flash includes; 1. Static electricity builds up within the lower thundercloud and induces opposite charges on the ground. 2. Discharge begins within the cloud and initiates a dimly visible, negatively charged stream of electrons propagating downward. 3. The conductive stream moves earthward in 50m jumps as a stepped leader. 4. As the stepped leader nears the ground, the electrical field at the surface increases greatly, attracting streamers of positive sparks upward and connecting with the stepped leader about 50m above the ground. 5. The connection closes the electrical circuit and initiates the return stroke, sending positive charges up to the cloud with a brilliant flash. 6. More lightning strokes occur as charges flow between the cloud and the earth. The electrical discharge of lightning can briefly create temperatures as high as 30,000°C. The heat from the lightning cause surrounding air to expand to produce sound waves we call thunder. The 30/30 Lightning Rule
Many deaths from lightning occur ahead of the storm because people try to wait to the last minute before seeking shelter. As the jingle goes: “if you can see it flee it; if you can hear it, clear it. Environment Canada recommends the 30/30 rule:
1. Thirty-second flash to bang rule: When lightning is recognized, count the seconds until the bang of its thunder. If the time lapse is 30 seconds or less you should take appropriate shelter immediately. 2. Thirty minute rule: Once lightning has been recognized, it is recommended to wait 30 minutes or more after the last flash of lightning is witnessed or thunder is heard before leaving the safe location. Any subsequent lightning or thunder after the beginning of the 30 minute count should reset the clock and another count should begin. Winds
Tornadoes derive their name from the Spanish verb tornar, which means “to turn”. The Spanish word derecho, meaning “straight ahead,” was applied to widespread, powerful, straight-line winds
Derechos
Can be as damaging as a small tornado. They commonly extend along a line at least 400km long with wind gusts of 100km/h; max recorded wind speeds are 240 km/h. In North America, derechos mostly occur in the middle and eastern states, and in Southern Ontario. Tornadoes
A tornado is a rapidly rotating column of air usually descending from a large thunderstorm. Tornadoes have the highest wind speeds of any weather phenomenon. The strongest tornadoes are more intense than the biggest hurricanes, but they affect smaller areas. 75% of the tornadoes on earth occur on the Great Plains region of Central North America. Only slightly more than 1% of tornadoes have wind speeds in excess of 320km/h but hey are responsible for over 70% of deaths. The core is usually less than 1km wide and acts like a gigantic vacuum cleaner, sucking up air and objects. Funnel clouds initially form hundreds of metres up in the atmosphere, and many never touch the ground. May touch ground only briefly, or may stay in contact for many kilometers. How a Tornado Works
In North America, the following conditions occur simultaneously: 1. A low altitude, northerly flow of warm moist air, often from the Gulf of Mexico, has temperatures at the ground in excess of 24°C. 2. A middle-altitude, cold dry air mass, often from Canada or the Rocky Mountains, at speeds in excess of 80km/h. 3. High altitude jet-stream winds racing east at speeds in excess of 250km/h. These 3 air masses, all moving in different directions, set up shearing conditions, imparting spin to a thunder cloud. The warm moist air lifts vertically, releasing its latent heat and forming a strong updraft that is sheared and spun at mid-levels by the fast-moving dry air and then twisted in another direction at it’s upper levels by jet-stream.
This motion is enhanced by vertical movements of air: warm air rising on the leading side, with cool air descending on the trailing side. In a single-cell thunderstorm, warm air rises by connection to build a nearly vertical cloud mass. In the upper air, water vapour condenses and rain falls back down through the thundercloud, causing a cool downdraft that blocks the upward flow of energy-carrying warm air. Sometimes wind shear tilts the thundercloud mass and it may grow into a super-cell thunderstorm. The tilt allows warm air to rise in the middle of the cloud, while most of the rain falls in the forward flank of the storm with the associated precipitation downdraft. On the trailing side, downdrafts of cool, drier air exist, and it is between the updraft and the near flank downdraft the tornadoes usually form. Rotation within a Tornado develops in the wide zone. Rotating wind speeds of a tornado are highest a hundred metres or so above the ground. Most likely due to the winds at ground level being slowed by the drag resistance of earth, trees, buildings, cars and such. Death and Destruction Caused by Tornadoes
Tornadoes cause destruction in several ways: 1. High-wind speeds blow away buildings and trees. 2. The furious winds debris that act like bullets or shrapnel, breaking windows and killing people. 3. When the fast winds lift and blow away a roof, he exposed walls are easily knocked over and the furniture removed. The Fujita Scale: Classifying Tornado Intensity
Scheme for classifying tornado strength according to the potential damage to homes and other structures. It is a six-point intensity ranking, designated F0 to F5, in order of increasing wind speed and storm damage. (See page334 for full scale). Tornadoes in Canada and the United States
The U.S. is the tornado capital of the world with a yearly tornado frequency between 1,000 and 1,200. Canada is a distant second with 80 to 100 tornadoes yearly. In Canada, 63% of tornadoes occur in June and July- when contrasting warm/moist and cool/dry air masses are more prevalent.
The 1912 Regina Cyclone
June 1912 Tornado swept through regina, creating a path of destruction six blocks wide and tearing down about 500 buildings. Cyclone was the worst killer tornado in Canadian history, leaving 28 dead, 300 injured, and 3,000 homeless.
The 1987 Black Friday, Edmonton Tornado
Deadliest in recent history Thunderstorms developed over the Rocky Mountain Foothills and moved toward the capital. Baseball sized hail and torrential rains of 40 to 50mm. Tornado cut a 40km swath up to 1km wide and was on the ground for a full hour. When it was over, 27 people were dead, hundreds of others injured and more than 750 left homeless. Property damage totaled $215 million Tornadoes and Cities
Urban concrete, asphalt, and stone absorb heat during the day and radiate heat at night. The arm air rising above a city creates its own low-pressure cell, a convecting plume of heat that can rise, cool, condense, and form thunderstorms. Volcanism and Weather
Large, explosive Plinian eruptions can blast fine ash and gas high enough to be above the normal zone of weather. Free from the cleansing effects of rainfall, the volcanic products can float about in the stratosphere for years and interfere with incoming sunlight. Fine volcanic ash (diameter of about 0.001mm) can stay suspended in the atmosphere for years. Much of the gases blown into the stratosphere disappear into space, but Sulphur Dioxide (SO2) picks up oxygen and water to form an aerosol of Sulphuric acid (H2SO4) that may stay aloft for years. The combined ash and sulphuric acid produce haze, reducing the amount of sunshine that reaches the troposphere and the ground surface; thus cooling results.
El Chichon, 1982 o o o o o
Located in the state of Chiapa in southern Mexico. Small volcano called El Chichon Four large Plinian eruptions on march 29th to April 4th 1982 blew out about 0.6. km^3 of material, leaving a 1km diameter crater and killing 2000 people. Large volume of SO2 gases were pumped into the stratosphere along with volcanic ash. The SO2 gases combined with O2 and water vapour, converting to sulphuric acid (H2SO4) aerosol.
Mount Pinatubo, 1991
Philippines island of Luzon in spring. 1,745m high summit was blasted to bits and replaced by a 2km wide caldera as up to 5km^3 of dense magma was blown out as pyroclasts debris. Over 300 people died in the pyroclastic flows and lahars and following the eruptions was a major storm that poured torrential rains on the loose pyroclastic debris, setting in motion numerous large volume lahars. 20 million tonnes of SO2 gas was put into the stratosphere (triple that of El Chichon). The H2SO4 aerosols reflected 2 to 4% of incoming short wavelength solar radiation back to space, causing a 20 to 30% decline in solar radiation directly reaching the ground. Mean global temperatures at the ground surface dropped 0.5 °C.
Tambora, 1815
Mount Tambora, on the island of Sumbawa in Indonesia, stood 4,000m tall. Eruption 10-11 April 1815. 150km^3 of rock and magma were blasted out during the eruption, producing 175km^3 of ashes and other pyroclastic debris. Killed about 10,000 people outright by pyroclastic flows and another 117,000 indirectly through famine and disease.
Volcanic Climate Effects 1. 2. 3.
4.
Gas rich eruptions can decrease the amount of incoming solar radiation and thus cause agricultural production to decline, which in turn can lead to famine, disease, and death. Plinian eruptions affect weather significantly for only a year or two. Rarer caldera eruptions may cause longer-lasting climate changes. The main variables that must come into play for volcanism to affect climate include The size and rate of eruptions. The heights of eruption columns. The types of gases and the atmosphere level they reach. Sulphur dioxide in the stratosphere reflects sunlight and cools the climate below. Carbon dioxide in the atmosphere creates a greenhouse effect. Low-latitude eruptions spread atmospheric debris across more of the world and have greater global effects than high-latitude eruptions.