2FRONTSWeather is variable in the mid-latitudes. It can vary from mild and sunny, to cold and clear, to snowy and windy.What causes these changes?They are mostly caused b the movement of low-pressure systems and their associated frontal systems.
3What’s a Front? Air masses of different types don’t mix easily. FRONT – the boundary that separates opposing air massesFronts vary in size (width can range from 200m 200km, and can be as high as 5km and as long as 2000km)Fronts can affect weather patterns in very large areasFronts are most common at mid-latitudes where southward- moving polar air masses and northward-moving tropical air masses often meet.
4The air masses on either side of a front may differ in temperature, humidity or both At the front, the less dense air mass rises over the denser air massFronts usually bring precipitation – at the frontal surface, the less dense air rises high into the troposphere. The air cools as it rises, and if the air is humid enough, clouds and precipitation form
5Kinds of FrontsThe weather associated with a particular front depends on the types of masses involved and the speed at which the front is moving.Fronts can occasionally occur between air masses that have the same temperature but different humidity.Scientists usually define fronts according to the temperature of the advancing air mass.4 basic kinds of fronts:ColdWarmOccludedStationary
6Cold FrontsCold front – the boundary between an advancing cold air mass and the warmer air mass it is displacingSince cold air is denser than warm air, cold air slides underneath the warm air and forces it upward.The type of weather a cold front brings depends on the type of air mass it is displacing
7Cold FrontsEXAMPLE – if a cold air mass displaces warm, humid air in the summer, we often get thunderstormsIf cold air displaces hot dry air, very little changeIn winter, a cold front is often marked by rain or snow showersCold fronts have a steep slope, so the precipitation associated with the front covers a narrow band of groundCold fronts move quickly and precipitation usually ends soon after the front passes
8Warm FrontsWarm front – boundary formed between warm air displacing cold airThe advancing warm air rises above the denser cold air mass, which retreats slowlyThe slope of a warm front is more gradual than that of a cold front, and the weather changes associated with the warm front are less dramatic
9Signs of approaching warm front: High cirrus clouds, followed by cirrostratus and lower, thicker stratiform cloudsFollowing cirrus and cirrostratus clouds are altostratus, which almost screen out the sun and moonFinally, heavy nimbostratus clouds arrive, and steady rain or snow beginsThe rain or snow associated with warm fronts covers a lot of ground area, and lasts for a day or more (different than cold fronts!)Thunderstorms not typical of warm frontAfter front passes, weather warms
10Occluded FrontsCold fronts typically move twice as fast as warm fronts.If a cold front “catches up” to a warm front, the result is an occluded front.The warm air is caught between the two colder air masses rises. As this warm air rises, it cools and causes cloudiness and precipitation
11Stationary FrontsIf a front is not moving forward, it is called a stationary front.As with other fronts, the warmer air rises over the denser, colder air, and clouds and precipitation may resultIf the front remains stationary for too long, flooding can occur.
12Weather Associated with Pressure Systems When a low pressure system passes, the weather you experience depends on where the center of the low passes in relation to youFor example, for us, if a low passes north of us, warm front may move over us, followed by a cold frontIf the low passes to the south, no fronts will move over us, but we might experience steady snow or rain
13If a low pressure system heading East passes North of us…….. A warm front approaches from the west, cirrus clouds lead to cirrostratus, altostratus, nimbostratus and stratus. Steady snow or rain, followed by drizzle, marks the front’s approachWhen the warm front passes, the temperature warms, winds shift, and the skies may slowly clear. If the air is humid, showery precipitation may occur, particularly nearer the center of the low
14As the cold front approaches, it is preceded by scattered showers and possibly thunderstorms As the cold front passes, the temperature drops, winds shift again, and the sky clears
15High Pressure SystemsHigh pressure areas are associated with fair weather, clear skiesWinds blow outward from a highLittle or no wind in the center of the highThe still air there takes on the temperature and humidity of the aria, so highs are where air masses generally form.
16Thunderstorms!Thunderstorms and their cumulonimbus clouds form in warm, moist, unstable airStorm clouds may be as high as 20km, and they bring torrential rain, damaging wind, lightning, thunder, hail and tornadoesUsually occur in afternoon because surface warming throughout the day causes air to become unstable
17How do thunderstorms develop? A “trigger” forces part of the air to begin risingThe trigger can be a mountainside or a front standing in the way of the unstable air, or the air can collide with an opposing wind.As the rising air reaches the condensation level, the heat released during condensation makes this air warmer and less dense than the surrounding airA cumulonimbus cloud quickly grows and a thunderstorm begins!
18More about thunderstorms……. They consist of one more convection cellsThey often form along fronts because frontal boundaries force air to riseFrontal thunderstorms are associated with large-scale low pressure systems and may start and stop for daysAll thunderstorms produce lightning!
19Lightning!! Lightning is a discharge of electricity from a thundercloud to the ground, to another cloud, or to another spot within the cloud itselfLightning can also occur in the clouds of snowstorms, dust storms, or volcanic eruptions
20Tornadoes!A tornado is a violently rotating column of air that usually touches the groundIn order for a thunderstorm to produce a tornado, it must contain a rotating updraft called a mesocycloneThis updraft occurs when low-altitude winds are blowing at a different speed and in a different direction than winds higher up
21Before a tornado develops, the rotating clouds of the mesocyclone may become visible at the base of the storm and may lower to form a wall cloudAbout 10 to 20 min later, a tornado may descend from the wall cloudHowever, only a third of all mesocyclones produce tornadoes, and a tornado may form without a visible mesocyclone or wall cloud
22A tornado often appears as a vortex or a funnel-shaped cloud of flying debris Some tornadoes are rope shaped, some wedge shaped with more than one vortexA tornado’s funnel cloud results when the air pressure at its center is very low and air sucked into the funnel expands and coolsWater vapor in the air condenses, forming a funnel-shaped cloudIf the air is drier, or the pressure inside the tornado is higher, the tornado may consist of a cloud of dust and debris, creating a loud roaring sound
23A tornado usually appears at the back edge of its parent thunderstorm and travels with the thunderstormTornadoes can occur anywhere and at any time of the yearHowever – most common in “Tornado Alley” in spring and early summerTornado Alley – Texas S. Dakota
24So, why do some thunderstorms turn into tornadoes? Cumulonimbus clouds are produced when a mass of warm, moist air meets a mass of cold, dry air, generating a strong frontal systemIf there is a strong high-altitude jet stream operating above it with winds of 150 mph or more, then the three moving air masses interact to form a strong wind shear that gets the cloud spinning
25In tornado alley, there are the perfect conditions to make frequent tornadoes During spring and summer, masses of cold, dry air come down from CanadaThese meet warm moist air from the Gulf of MexicoAt the same time, the jet stream shifts southward and focuses its energy over this regionWarm dry air can move east from the Southwest desertThe Rocky Mountains block the spread of these air masses to the west, so they end up concentrated in this narrow zone
26Where do they occur? Every continent except Antarctica Most common in American MidwestDocumented in Africa, western Australia,New Zealand, South-central and eastern Asia, east-central South America and northwestern and southeast EuropeBangladesh – kill 200 people/year!
27Increased sunlight of spring and early summer warms the air near the ground, while the air higher up is still coolThe temperature difference makes the air unstable, resulting in severe thunderstormsWind conditions in Tornado Alley ideal for mesocyclones to form because wind direction changes with elevation
28Fujita Tornado Intensity Scale CategoryF-ScaleEstimated Wind SpeedEffectsWeakF065 – 118 km/hMinor damage, snaps small branches, breaks some windowsF1119 – 181 km/hDowns trees, shifts mobile homes off foundationsStrongF2182 – 253 km/hRips roofs off houses, destroys mobile homes, uproots large treesF3254 – 332 km/hPartially destroys buildings, lifts carsViolentF4333 – 419 km/hLevels sturdy buildings, tosses carsF5420 – 513 km/hLifts and transports sturdy buildings
29http://video. nationalgeographic Tornado video clip 2:45 – national geographicTornadoes 101
30Describe where and when it took place Was there any warning? For you to do:Research a tornadoDescribe where and when it took placeWas there any warning?What was its intensity on the Fujita Scale?Effects – damage, injury, lives lost, damages to infrastructure and homes, socioeconomic issuesRecovery/relief effortAt least 2 sourcesPictures and video clips