Severe Weather

Chapter 24 Sec. 2

High and Low Pressure Centers

• In a High Pressure center the air sinks. Because of the high pressure the air can not rise and condense so there are usually no clouds and dry air.

• In a Low Pressure center the air rises and cools. Eventually the water vapor condenses forming clouds and precipitation.

FrontsCan you label the different parts of this meteorological map?

Severe Weather!!!!!

• Ok so all these low pressure and high pressure centers mean what?

• Well if conditions are right you can have severe weather systems like thunderstorms, tornadoes, and Hurricanes!

• A system is categorized as severe when it brings strong winds and heavy precipitation..

Thunderstorms• Thunderstorms develop along cold fronts where air is forced rapidly

upward causing water droplets to form.• As these large droplets fall they created a down draft which is the

strong winds associated with thunderstorms. Dangerous hail can also form.

• Lightning and thunder are also related in thunderstorms. Where air uplifts rapidly, electric charges form. Lightning is the almost instantaneous energy flow that occurs between areas of opposite electrical charge.

Tornadoes• Tornadoes also form along fontal boundaries. If

conditions are just right, updrafts of rising air can start to spin into a rotating vortex. This crates a funnel cloud.

• If it reaches Earth’s surface it is called a tornado. They are violent and extremely destructive. The area in central United States is called Tornado alley because of the frequent recurrences of tornadoes.

http://www.weather.com/maps/maptype/currentweatherusnational/index_large.html

How do tornadoes form?

- Most tornadoes form from thunderstorms- You need warm, moist air from the Gulf of

Mexico and cold, dry air from Canada- When these two air masses meet – this

creates instability in the atmosphere

Continued

- A change in wind direction and…- An increase in wind speed (with increasing

height, create an invisible, horizontal spinning effect

- Then rising air within the updraft tilts the moving (rotating) air from horizontal to vertical

- This take up most of the thunderstorm

Tornado Condition Model

Funnel Cloud

- Most of us think that a funnel cloud is a tornado…NOT SO!

- A funnel cloud is a rotating cone-shaped column of air extending downward from the base of a thunderstorm, but not touching the ground.

- It’s only a tornado when it touches the ground!!!!

Tornado Model

Water Spouts!

• A waterspout is just a weak tornado that forms over water. They are most common along the Gulf Coast. Waterspouts can sometimes move inland, becoming tornadoes causing damage and injuries.

Tornado Alley!• The geography of the central part of the United States is

suited to bring all of the ingredients together to forms tornadoes.

• More than 500 tornadoes typically occur in this area every year and is why it is commonly known as "Tornado Alley".

Tornado Alley

Fujita Scale of Tornado Intensity• F0 40-72 mph Light damage: Branches broken off trees; minor roof damage

• F1 73-112 mph Moderate damage: Trees snapped; mobile home pushed off foundations; roofs damaged

• F2 113-157 mph Considerable damage: Mobile homes demolished; trees uprooted; strong built homes unroofed

• F3 158-206 mph Severe damage: Trains overturned; cars lifted off the ground; strong built homes have outside walls blown away

• F4 207-260 mph Devastating damage: Houses leveled leaving piles of debris; cars thrown 300 yards or more in the air

• F5 261-318 mph Incredible damage: Strongly built homes completely blown away; automobile-sized missiles generated EF5over 200 mph

Tornadoes in action

• Tornado

Hurricanes Review

• A Hurricane is a large storm that begins as an area of low pressure over tropical oceans.

• The Coriolis effect causes winds to rotate counter clockwise around the center of the storm.

• As the storm moves it pulls in moisture.

Hurricanes

• Hurricanes are measured using the Saffir-Simpson scale. It gives the hurricanes a rating of 1-5 depending on intensity.

• They measure the winds, wind gusts, and storm surge (size of waves)

Saffir-Simpson Hurricane Scale

Category Wind speed

Storm surge

mph(km/h)

ft(m)

5 ≥156(≥250)

>18(>5.5)

4 131–155(210–249)

13–18(4.0–5.5)

3 111–130(178–209)

9–12(2.7–3.7)

2 96–110(154–177)

6–8(1.8–2.4)

1 74–95(119–153)

4–5(1.2–1.5)

Additional classifications

Tropicalstorm

39–73(63–117)

0–3(0–0.9)

Tropicaldepression

0–38(0–62)

0(0

Floods!

- A flood results from many days of heavy rain, melting snows, or when rivers rise and go over their banks

Flash Floods

• Is sudden flooding that occurs when floodwaters rise rapidly, with no warning, after many hours of heavy rains

• Flash floods are the #1 weather-related killer in the U.S.

• 80% of flash floods deaths occur in cars• We can protect ourselves by knowing if we live in

a flood zone!• A car can float away in just 2 ft. of water!!!

Flood Lingo• FLOOD WATCH - means that an overflow of water from a river is possible for your

area.

• FLASH FLOOD WATCH - means that flash flooding is possible in or close to the watch area. Flash Flood Watches can be put into effect for as long as 12 hours, while heavy rains move into and across the area.

• FLOOD WARNING - means flooding conditions are actually occurring in the warning area.

• FLASH FLOOD WARNING - means that flash flooding is actually occurring in the warning area. A warning can also be issued as a result of torrential rains, a dam failure or snow thaw.

Flash Floods

Flash Flooding

Flash Flood in Action

Flood Safety• DURING A FLOOD: Move to a safe area quickly. Move to higher

ground, like the highest floor of your home. Avoid areas subject to sudden flooding like low spots and canyons. Avoid already flooded areas. If a flowing stream of water is above your ankles stop, turn around and go the other way. Do not attempt to drive through a flooded road. The depth of the water is not obvious and the road may be washed away. If your car stalls, leave it and seek higher ground. Rapidly rising water may engulf the car, pick it up and sweep it away. Kids should never play around high water, storm drains or viaducts. Be cautious at night, because its harder to see flood dangers. If told to evacuate, do so immediately.