18 Fronts And Depressions Flashcards
Describe the formation of a Polar Front Depression
Stage 1:
- Where the mP and mT air masses meet there is a stationary front
- Due to PGF and CE the cold air moves South and to the West and the warm air moves North and to the East
Stage 2:
- The warm air slowly pushes up and over the cold air.
- The less dense air on top lowers the pressure at one point enough to form a depression
- Air begins to rotate around the Low (anti-clockwise direction)
Stage 3:
- Cold front and the warm front separate further forming a full Polar front dpression
Describe how a Polar Front Depression dissipates
- The cold air moves at the speed of the Geostrophic wind within the warm ‘wedge’ of air
- The warm front only moves at 3/4 of the speed of the Geostrophic wind
- The cold front slowly catches up with the warm front until they meet
- This then forms an Occluded front
Describe the following as you approach and cross the Warm front of a Polar Front Depression:
- Clouds
- What happens to the cloud base
- What happens to the visibility
- What happens to the temperature and dew point
- What happens to the Pressure
- Any other Weather
- Wind direction and speed
- Clouds: Ci, Cs, As, Ns, St
- Cloud base lowers
- Visibility reduces (drizzle)
- Temperature increases and becomes closer to dew point
- Pressure decreases (slow at first then rapidly reaching the front)
- Frontal Fog, Drizzle, Shows, Verga
- Wind Veers and slows down
Describe the following as you travel across the Warm section of a Polar Front Depression:
- Clouds
- What happens to the cloud base
- What happens to the visibility
- What happens to the temperature and dew point
- What happens to the Pressure
- Any other Weather
- Wind direction and speed
- Clouds: in summer Fair weather Cumulus, in winter Stratus
- Cloud base remains constant
- Visibility is poor in winter due to St and good in summer
- Temperature remains constant
- Pressure remains constant
- Radiation Fog (winter), Drizzle
Describe the following as you approach and cross the Cold front of a Polar Front Depression:
- Clouds
- What happens to the cloud base
- What happens to the visibility
- What happens to the temperature and dew point
- What happens to the Pressure
- Any other Weather
- Wind direction and speed
- Clouds: Ns, Cb, St, As, Cs, Ci
- Cloud base rises
- Visibility improves
- Temperature and dew point drop
- Pressure initially drops due to Cb and TS updrafts then increases
- Wind Speeds up and Veers
What happens to the wind direction if a Polar Front Depression moves across south of you?
The wind backs
What happens to the wind direction if you climb up through the warm front to the Jet stream?
The wind Veers
What happens to the wind direction if you climb up through the cold front to the Jet stream?
The wind backs
Describe the following regarding secondary lows:
- Where they form
- How they travel in relation to the main depression
- What kind of weather they create
- Form behind the cold front of a Polar Front Depression
- They travel anti-clockwise around the main depression (NH)
- The weather is more intense and severe than the main depression
How do you determine:
- What direction a Polar Front Depression is moving
- What direction the individual fronts on PFD are moving
- The PFD direction is determined by the direction of the Geostrophic winds in the warm air
- The individual fronts spread out in a direction 90 degrees to their surface
Describe the different types of PFD Occluded fronts and the differences between them
Warm Occluded Front:
- Occluded line joins smoothly with the warm front
- Cold air behind and coldest air in front
- Cold front is pushed up and over the warm front
- Winter
- Precipitation in front of the occluded front only
Cold Occluded Front:
- Occluded line joins smoothly with the cold front
- Coldest air is behind and cold air in front
- Warm front is pushed up and over the cold front
- Summer
- Precipitation both sides of the occluded front
