1.2 Extreme weather in the US

• Aardrijkskunde
• Middelbare school
• havo, vwo
• Leerjaar 3

Chapter 1: U.S.

§1.2 Extreme weather

Learning objectives through these questions:

1. What is air pressure?
2. Which 4 types of air masses are prevalent in the US?
3. Which weather extremes can occur as a result of these air types?
4. How does a tornado form?

What is air pressure?

Air pressure: The weight of the air pressing down on the earth.

Measure with: 

-barometer

-unit: hectopascal (hPa) or millibar (Mb)

High pressure

High pressure = falling air

• This air is heavy
• Symbol on the map = H
• Pressure: around 1020 hPa

Which weather type belongs to H?

- no clouds

- dry 

- why?

High pressure

High pressure = falling air

• This air is heavy
• Symbol on the map = H
• Pressure: around 1020 hPa

Which weather type belongs to H?

- no clouds

- dry 

- falling air warms up near earth surface, clouds dissolve, clear skies

Low pressure

Low pressure = rising air

• the weight of that air is low
• Symbol on the map = L
  
• pressure: around 1000 hPa
  

Which weather type belongs to L?

- clouds

- precipitation

- why?

Low pressure

Low pressure = rising air

• the weight of that air is low
• Symbol on the map = L
  
• pressure: around 1000 hPa
  

Which weather type belongs to L?

- clouds

- precipitation

- (moist) air rises, air cools down, condensation takes place (cold air holds less moisture than warm air)

Air moves

Air moves due to a difference in air pressure, from high pressure areas to low pressure areas

Influence of air pressure on the weather

- California (Mediterranean climate): High pressure area in summer -> dry and sunny weather

In winter more western winds occur, bringing rain

 L

4 types of air masses

moist -> above sea

dry -> above land

cold -> in the north

warm -> in the south

where 2 different types of air masses collide: front

At fronts, low pressure areas (depressions) can occur, causing extreme weather situations.

Air types and their effect

(Collision of) air types have a major influence on the weather in the US.

Which 3 air types in the picture cause the following examples of extreme weather? Why only those 3?

• Northerns (cold wave)
  
• Southerns (heat wave)
  
• Tornadoes
• Hurricanes

Front

Front = grensgebied tussen twee verschillende luchtsoorten.

Op deze plek ontstaan depressies (lage druk) Vaak is het dan slecht weer.

Tornadoes

Tornado occurs where different types of air come together. Cold, dry air (Canada) collides with warm, moist air (Gulf of Mexico).

There are about a thousand tornadoes in the US every year, especially in spring spring (April, May and June).

Formation tornadoes

1. Collision of dry cold air from the north with warm moist air from the south
2. Warm air rises quickly, rotating
3.  Cold air descending, also rotating
4. A funnel cloud forms 
5. Suction of surrounding air (as a huge vaccum cleaner)

Forms during heavy thunderstorms with with large wind speed in the higher air layers

Tornado's

Characteristics tornadoes

• Tornadoes form in the spring; location unpredictable
• Tornadoes form usually on land
• Tornadoes are small in size (local phenomenon): < 1 km diameter
• Higher wind speeds than hurricanes: 100 - 400 km/hour
• There are more tornadoes than hurricanes per year in the US

Lesdoelencheck

1. Wat is luchtdruk?
2. Met welke 5 luchtsoorten heeft de VS te maken?
3. Wat ontstaat er, wanneer  deze luchtsoorten botsen?
4. Welke weerextremen kan er ontstaan door deze luchtsoorten?
5. Leg in stappen uit hoe een tornado ontstaat/werkt