Module 7 - Lecture notes 7 PDF

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Lecture 7 notes...

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GEO 103 – Earth Science Module 7           

Learning Goals Explain what an air mass is and how air masses differ from one another. Describe how air masses acquire their characteristics; and how they are modified as they move over Earth’s surfaces. Describe the chief forcing mechanisms that cause air to rise in Earth’s atmosphere. Describe the chief characteristics of the four types of fronts. Identify the criteria used to locate cold and warm fronts on weather maps. Explain how fronts and air masses are related. Describe the characteristics of a mid-latitude cyclone and explain how they form. Explain the connection between the upper-level westerlies and the formation and development of mid-latitude cyclones. Identify, describe, and characterize the atmospheric conditions that produce thunderstorms, tornadoes, and hurricanes. Describe the phenomena of lightening, thunder, and hail; and explain how they form. Compare and contrast mid-latitude and tropical cyclones.

Professor’s Notes (Lecture Set 11): Introduction and Overview of Module 7

Welcome to Module Seven! We have come to the point in our discussion of the Earth-Atmosphere system where we have the necessary information and background to understand weather systems and features that we commonly hear weathercasters on TV and radio talk about, and that we can see on (synoptic-scale) weather maps. I’m referring to such weather entities as fronts, mid-latitude cyclones, and hurricanes—the “big ticket” weather phenomena—so to speak. We also have the foundation to learn about one really nasty weather maker, that is too small (in space and time) to depict on a synoptic-scale weather map—but that can pack a particularly nasty punch— thunderstorms that is, which always generate lightning and thunder, and sometimes hail and tornadoes. As you read your notes and text, and work-through the ancillary exercises, please reflect on what you have already learned (e.g., how the earth-atmosphere system receives insolation and is heated, what controls vertical motions in the atmosphere, how water moves through and is dispersed in the atmosphere, and how the air moves

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in the horizontal), and think about how these items relate to the each of the weather systems being discussed this week. The material begins with a discussion of air masses—large blobs of air with temperature and moisture characteristics that are continuous in both the horizontal and vertical. Familiarize yourself with the air masses that affect North America, and pay particular attention to the air mass (mT air) that is responsible for North Carolina’s hot, humid summers. Next I discuss how we get air to rise (i.e., lifting mechanisms). Recall that when air rises it cools, and with sufficient rising (vertical motion), the air parcel may cool enough to form clouds, and maybe even produce precipitation. So, getting the air to rise is important! In the mid-latitudes an important lifting mechanism is frontal lifting. I describe each of the four types of fronts and even mention how we can identify them on a weather map. Make sure that you know why we tend to get severe weather ahead of a cold front—as opposed to ahead of a warm front (i.e., it’s because of the much steeper slope of the cold frontal surface—a warm front has a much more gentle slope). We then go on to talk about mid-latitude cyclones—what they are and how and why they form. (You might think about why we do not have mid-latitude cyclones in the tropics or Polar Regions of the Earth?) Next I discuss thunder storms (T’Storms) and phenomena that occur in association with them: lightning, hail, and tornadoes. Most of this discussion is rather descriptive in nature. Please understand the stages of an air mass T’Storm. Note that most severe weather (T’Storms and tornadoes) occur ahead of cold fronts, and the most severe T’Storms and Tornadoes occur in tornado alley. Note that FL has the greatest number of T’Storms but not the greatest number of tornadoes—you should think about why this is the case. The Fujita Scale is used to “rate” tornadoes. Note that most tornadoes are not violent, but it’s the most violent ones that cause the greatest loss of life and property damage. (BTW, you should know and understand what causes most of the damage and loss of life from tornadoes.) Module 7 concludes with a discussion of hurricanes. I discuss where and how hurricanes form, the role an easterly wave plays in their formation, their structure, and the damage they can cause. (I urge you to compare and contrast hurricanes and tornadoes—how they form and their respective characteristics.) You are going to love this module! Dr. J.

Begin Body of Content – Module 7

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I. Air Masses, Fronts, Midlatitude Cyclones, Tornadoes, & Hurricanes A. Air Masses air mass (Defn)- a huge volume of air (hundreds of square miles in horizontal size) uniform in its temperature and humidity characteristics. air mass advection (Defn) - movement of an air mass (and thus its properties of temperature & moisture) in the horizontal, from one location to another Air Masses Affecting North America     

continental arctic (cA) - very cold & dry continental polar (cP) - cold & dry continental tropical (cT) - hot & dry maritime polar (mP) - cool & moist maritime tropical (mT) - warm & moist

Air Masses

Air Mass Advection

B. Atmospheric Lifting Mechanisms 1) Orographic Lifting - air is lifted upslope as it is pushed against a mountain; the airs cools adiabatically as it is lifted 

the "windward" slope receives more precipitation than the "leeward" (downwind) slope of the mountain

Orographic Lifting

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2) Convection (convectional lifting) - a cooler (& moist) air mass moves over a warmer land area -heating from the land causes the air to rise - if the atmosphere is unstable it continues to rise (e.g., occurs at: tropical islands, Florida, ITCZ)

Average Annual # of T'Storm Days

3) Fronts - also provide a means of atmospheric lifting

C. Fronts Front (defn) - a transition zone between two air masses having different densities 

fronts typically separate air masses with different temperatures and dew points

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since air masses have both a vertical and horizontal extent, a front also extends upward in the vertical - the "frontal surface" (frontal zone)

Fronts, Air Masses, Pressure Systems

Mid-Latitude Cyclone, Clouds, & Fronts

Types of Fronts (1) Cold Front 

cold, dry, stable polar air is advancing into warm, moist, unstable tropical air

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a cold front is represented by a solid blue line with triangles pointing in the direction that the front is moving

Criteria Used to Locate Fronts: (a) sharp temperature changes in the horizontal (b) sharp moisture (Td) changes in the horizontal (c) wind direction shifts (d) clouds and precipitation near the front

Cold Front

(2) Warm Front 

advancing warm, moist, tropical air replaces colder air

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is depicted by a solid red line with half-circles pointing in the direction of movement (movement is toward the colder air)

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warm fronts move more slowly than cold fronts (~1/2 the speed of cold fronts)

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the rising of warmer, less-dense air over the colder more-dense air is referred to as "overrunning"

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overrunning produces clouds and precipitation well in advance of the front's surface boundary

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warm fronts have more inclined slopes than do cold fronts

Warm Front

(3) Stationary Fronts 5

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there is essentially no movement of the front

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is depicted by alternating red semicircles and blue triangles (the blue triangles point into the warmer air; while the red semicircles point into the colder air

(4) Occluded Fronts 

occur when a cold front catches up with and overtakes a warm front, and all the warm air is forced aloft

Formation of an occluded front

D. Mid-Latitude Cyclones & Cyclogenisis:

Satellite Image of a Mid-Latitude Cyclone

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the frontal systems that we've been examining are part of a larger storm system referred to as a "middle latitude (wave) cyclone"

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mid-latitude (wave) cyclones - are migrating centers of low pressure - the air flows inward and CCW in the N. Hemisphere (inward & CW in the S. Hemisphere) - there is rising motion in the center of the low/cyclone > clouds & precipitation.

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Norwegian meteorologists (just after W.W. I) developed a model explaining the life cycle of the mid-latitude cyclone called the "polar front theory"

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according to this theory/model the cyclone begins its development along the polar front

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cyclogenisis (defn) - the development or strengthening of a mid-latitude cyclone

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in addition to the polar front, there are other preferred areas of cyclogenisis: eastern slopes of the Rockies ("lee-side-lows"), the Gulf of Mexico, and the Atlantic Ocean east of the Carolinas.

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Lifecycle of a mid-latitude cyclone: 

"birth", "maturity", & "death"

Cyclogenisis

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why do some waves grow into big storms, while others never really develop?

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we need to look to the upper level winds for the answer !!

Example of Upper-Level Mid-Latitude Westerlies

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the air-flow aloft is comprised of a series of waves, both long and short

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longwaves (Rosby Waves): wavelengths of thousands of miles; 4-to-6 around the globe; move very slowly 7

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shortwaves: move more quickly through the longwaves; deepen/weaken when they approach a trough/ridge

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these deep troughs are important for the development of surface lows because they are associated with divergence and convergence

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convergence - the piling-up of air above a region - aloft it leads to the formation & maintenance of surface anticyclones (highs)

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divergence - the spreading-out of air above a region - aloft it leads to the formation & maintenance of surface cyclones

Divergence and Convergence in Upper-Level Gradient Winds

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for a surface low (cyclone) to be maintained/deepened, divergence aloft must exceed surface convergence.

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for a surface high (anticyclone) to be maintained/intensified, convergence aloft must exceed surface divergence.

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convergence aloft occurs on the west side of the trough; divergence aloft occurs on the east side of the trough.

Convergence, Divergence and Vertical Motions Associated With Surface Highs and Lows

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E. Thunderstorms 

defn: a storm containing lightning & thunder; it may produce gusty winds with heavy rain & hail; severe T'Storms may spawn tornadoes

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the storm itself may be a single cumulonimbus cloud, a cluster of them, or even a line of clouds stretching for more than 100 km (62 miles)

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T' Storms develop: within an air mass, at/ahead of fronts (particularly cold fronts), along the ITCZ, and at mountainous locations

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the birth of a T'Storm occurs when warm humid air rises in an unstable environment

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in North America most T’Storms occur in areas dominated by mT air masses

Life Cycle of an Air-Mass T'Storm

Lightning: flashes of light resulting from electrical discharges within the cloud (the majority of strikes), between the cloud and the ground (~20% of strikes), from one cloud to another, or from a cloud to the surrounding air 

lightning is created when there is a buildup of electrical energy between areas within the cloud, or between the cloud and the ground

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for lightning to occur, separate regions containing opposite electrical charges must exist in a cumulonimbus cloud

"Thunder" - results from the expansion of suddenly heated air, which sends out shock waves (the air is heated by the lightning - 15,000-30,000 degrees Celsius) "Hail" - forms within cumulonimbus clouds when raindrops are repeatedly carried above and below the freezing level - layers of ice are added until it becomes too heavy and can no longer be supported by the updrafts

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F. Tornadoes 

are rapidly rotating winds that blow around a small area of intense low pressure

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are spawned from severe T'Storms

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"funnel cloud" - the visible swirling circulation extending from the parent cumulonimbus cloud

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"tornado" - a funnel cloud that extends all the way to the ground

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tornado diameter: a few meters to a few hundred meters

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typical forward speed: ~20 to 40 knots

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most last only a few minutes & have an average path length of ~ 7 km (4 mi)

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horizontal wind speeds in tornadoes: can exceed 300 mph - most have lesser wind speeds

"Fujita Scale" - based on wind speed and property damage; F0-F5   

65% of tornadoes are weak (wind speed < 112 mph) 33% of tornadoes are strong (wind speed:113-206 mph) 2% of tornadoes are violent (wind speed >207mph)

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all 50 states have experienced tornadoes

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North America is by far the biggest recipient of tornadoes (avg. 800 annually)

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in particular, the greatest number occur in the "tornado belt" (alley) of the Central Plains - from Texas to Nebraska - because of its unique geographical location with regard to contrasting air masses

Average Annual Tornado Incidence (per 10,000 square miles)

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3/4 of tornadoes occur from March- July; May has greatest #; most frequent in late afternoon (4-6 pm)

Average # of Tornadoes and Tornado Days Each Month (27-year period)

Tornado Formation: 

In order for a T'Storm to spawn a tornado, the updraft (& the T'Storm itself) must rotate

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This rising, spinning column of air (5-10 km across) is called a "mesocyclone"

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Inside the mesocyclone a spinning vortex (tornado) may appear near the mid-level of the cloud and gradually extend downward to the cloud base; and then perhaps down to the ground

G. Hurricanes (Tropical Cyclones): 

Defn: an intense storm of tropical origin, with sustained winds exceeding 64 kts (74 mph)

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tropical cyclones have different names in different regions of the world

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the Saffir-Simpson Scale is used to categorize hurricanes on the basis of pressure, wind speed, storm surge, and damage

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form within the homogeneous air of the tropics (23.5S-23.5N)

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Form over the warm (790F) tropical waters where winds are light - from summerfall in the tropical N. Atlantic & N. Pacific oceans

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(Hurricane Season: June-November)

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For a hurricane to develop from an unorganized mass of T'storms, the surface winds must converge 11

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Surface convergence takes place on the eastern side of an "easterly wave" (a weak westerly moving area of low pressure within the trades)

Easterly Wave and Surface Convergence

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Hurricanes will not form on the equator - no Coriolis force for convergence

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It is the release of latent heat that drives & fuels the hurricane

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The heat released aloft warms the upper troposphere, creating higher pressure aloft and diverging air - surface pressures then fall

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As long as outflow aloft exceeds surface inflow, the hurricane will intensify

Hurricane

Mid-Lat Low (top) vs. Hurricane (bottom)

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When hurricanes move over land (or colder water) they lose their source of heat and fuel & die out

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Hurricane destruction results from storm surge (which is responsible for 90% of all hurricane-caused deaths, and is responsible for a large share of property loses), wind damage, and inland freshwater flooding. o

"storm surge" is a dome of water 65 to 80 km wide that sweeps across the coast near the point where the eye of the hurricane makes landfall; the storm surge is the height of the water (ignoring wave activity) above normal tide level - it commonly adds 2-3 meters to normal tide heights.

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End Body of Professor’s Notes – Module 7 Graded Assignments

MG Graded Assignment

See Module 7 Assignment in the Assignments area of the MG web site End Module 7

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