1. TORNADOES!

2. FRONTS
Weather 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.

3. What’s a Front? Air masses of different types don’t mix easily.
FRONT – the boundary that separates opposing air masses
Fronts 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 areas
Fronts are most common at mid-latitudes where southward- moving polar air masses and northward-moving tropical air masses often meet.

4. The 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 mass
Fronts 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

5. Kinds of Fronts
The 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:
Cold
Warm
Occluded
Stationary

6. Cold Fronts
Cold front – the boundary between an advancing cold air mass and the warmer air mass it is displacing
Since 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

7. Cold Fronts
EXAMPLE – if a cold air mass displaces warm, humid air in the summer, we often get thunderstorms
If cold air displaces hot dry air, very little change
In winter, a cold front is often marked by rain or snow showers
Cold fronts have a steep slope, so the precipitation associated with the front covers a narrow band of ground
Cold fronts move quickly and precipitation usually ends soon after the front passes

8. Warm Fronts
Warm front – boundary formed between warm air displacing cold air
The advancing warm air rises above the denser cold air mass, which retreats slowly
The 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

9. Signs of approaching warm front:
High cirrus clouds, followed by cirrostratus and lower, thicker stratiform clouds
Following cirrus and cirrostratus clouds are altostratus, which almost screen out the sun and moon
Finally, heavy nimbostratus clouds arrive, and steady rain or snow begins
The 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 front
After front passes, weather warms

10. Occluded Fronts
Cold 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

11. Stationary Fronts
If 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 result
If the front remains stationary for too long, flooding can occur.

12. Weather 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 you
For example, for us, if a low passes north of us, warm front may move over us, followed by a cold front
If the low passes to the south, no fronts will move over us, but we might experience steady snow or rain

13. If 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 approach
When 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

14. As 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

15. High Pressure Systems
High pressure areas are associated with fair weather, clear skies
Winds blow outward from a high
Little or no wind in the center of the high
The still air there takes on the temperature and humidity of the aria, so highs are where air masses generally form.

16. Thunderstorms!
Thunderstorms and their cumulonimbus clouds form in warm, moist, unstable air
Storm clouds may be as high as 20km, and they bring torrential rain, damaging wind, lightning, thunder, hail and tornadoes
Usually occur in afternoon because surface warming throughout the day causes air to become unstable

17. How do thunderstorms develop?
A “trigger” forces part of the air to begin rising
The 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 air
A cumulonimbus cloud quickly grows and a thunderstorm begins!

18. More about thunderstorms…….
They consist of one more convection cells
They often form along fronts because frontal boundaries force air to rise
Frontal thunderstorms are associated with large-scale low pressure systems and may start and stop for days
All thunderstorms produce lightning!

19. Lightning!! Lightning is a discharge of
electricity from a thundercloud to the ground, to another cloud, or to another spot within the cloud itself
Lightning can also occur in the clouds of snowstorms, dust storms, or volcanic eruptions

20. Tornadoes!
A tornado is a violently rotating column of air that usually touches the ground
In order for a thunderstorm to produce a tornado, it must contain a rotating updraft called a mesocyclone
This updraft occurs when low-altitude winds are blowing at a different speed and in a different direction than winds higher up

21. Before 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 cloud
About 10 to 20 min later, a tornado may descend from the wall cloud
However, only a third of all mesocyclones produce tornadoes, and a tornado may form without a visible mesocyclone or wall cloud

22. A 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 vortex
A tornado’s funnel cloud results when the air pressure at its center is very low and air sucked into the funnel expands and cools
Water vapor in the air condenses, forming a funnel-shaped cloud
If 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

23. A tornado usually appears at the back edge of its parent thunderstorm and travels with the thunderstorm
Tornadoes can occur anywhere and at any time of the year
However – most common in “Tornado Alley” in spring and early summer
Tornado Alley – Texas  S. Dakota

24. So, 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 system
If 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

25. In tornado alley, there are the perfect conditions to make frequent tornadoes
During spring and summer, masses of cold, dry air come down from Canada
These meet warm moist air from the Gulf of Mexico
At the same time, the jet stream shifts southward and focuses its energy over this region
Warm dry air can move east from the Southwest desert
The Rocky Mountains block the spread of these air masses to the west, so they end up concentrated in this narrow zone

26. Where do they occur? Every continent except Antarctica
Most common in American Midwest
Documented in Africa, western Australia,
New Zealand, South-central and eastern Asia, east-central South America and northwestern and southeast Europe
Bangladesh – kill 200 people/year!

27. Increased sunlight of spring and early summer warms the air near the ground, while the air higher up is still cool
The temperature difference makes the air unstable, resulting in severe thunderstorms
Wind conditions in Tornado Alley ideal for mesocyclones to form because wind direction changes with elevation

28. Fujita Tornado Intensity Scale
Category
F-Scale
Estimated Wind Speed
Effects
Weak F0
65 – 118 km/h
Minor damage, snaps small branches, breaks some windows F1
119 – 181 km/h
Downs trees, shifts mobile homes off foundations
Strong F2
182 – 253 km/h
Rips roofs off houses, destroys mobile homes, uproots large trees F3
254 – 332 km/h
Partially destroys buildings, lifts cars
Violent F4
333 – 419 km/h
Levels sturdy buildings, tosses cars F5
420 – 513 km/h
Lifts and transports sturdy buildings

29. http://video. nationalgeographic
Tornado video clip 2:45 – national geographic
Tornadoes 101

30. Describe where and when it took place Was there any warning?
For you to do:
Research a tornado
Describe where and when it took place
Was there any warning?
What was its intensity on the Fujita Scale?
Effects – damage, injury, lives lost, damages to infrastructure and homes, socioeconomic issues
Recovery/relief effort
At least 2 sources
Pictures and video clips