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 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

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

In winter: more western winds, bringing rain

 L

4 basic 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 masses in the US:

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

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

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 (works as a huge vacuum cleaner)

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

Tornadoes

Characteristics tornadoes

• Tornadoes form in the spring
• Location unpredictable (as opposed to hurricanes)
• They form on land (as opposed to hurricanes)
• 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 (around 1000 per year)