1.2 Extreme weather in the US

Chapter 1: U.S.
§1.2 Extreme weather
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Chapter 1: U.S.
§1.2 Extreme weather

Slide 1 - Diapositive

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?

Slide 2 - Diapositive


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)

Slide 3 - Diapositive

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?

Slide 4 - Diapositive

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

Slide 5 - Diapositive

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?


Slide 6 - Diapositive

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)


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



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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

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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:

Slide 10 - Diapositive

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.

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


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Slide 15 - Vidéo

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


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Tornadoes

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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)

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Slide 21 - Vidéo