Depressions Flashcards
Define a depression
A closed off area of relatively low atmospheric pressure. Also called a ‘low’ or ‘cyclone’.
General characteristics of Depressions
- Convergence at low levels (due to friction near SFC, causing wind to blow slightly across the isobars towards lower pressure)
- Divergence at upper levels due air moving through an area of decreasing absolute vorticity.
- Upward motion due low level CONV and upper level DIV.
- Thick cloud formation/precip due upward motion (concentrated around the front)
How does DIV and CONV create a low pressure system?
What enhances this?
- DIV aloft results in vertical motion
- If upward motion generated at the top of the troposphere exceeds upward motion generated by frictional CONV at the SFC more air is removed from air at top of column that is entering it at the bottom.
- Pressure at SFC falls and low will develop.
- Enhanced/deepened when there is a considerable amount more DIV aloft than CONV at SFC.
Trough (ccccc) to Ridge (cc) flow
- DIV of the parcel of air (expansion) aloft, because of DECREASE in absolute vorticity. Low pressure formed at SFC
Ridge (cc) to Trough (ccccc) flow
-CONV of the parcel of air (compression) because of increase in absolute vorticity. High pressure formed at SFC.
What happens if DIV or CONV are intensified at the tropopause?
Their relative low/high pressure systems will also intensify.
Factors enhancing intensity of upward motion (4)
- Sharpness of the curvature around the upper trough/ridge. (increase in sharpness = increase in vorticity and vice versa)
- An increase in shear vorticity (from passage of jet stream over area)
- Increased diffluence (spreading out of isobars)
- Cloud development (releases latent heat, destablises air which accelerates up drafting air).
What is a Lee depression/how does it occur?
Is formed in strong wind conditions in the Lee of a range or mountain, (doesn’t behave the same as a normal low pressure system. Occurs when;
- An airmass is forced over/around a mountain barrier
- Build up of air on windward side = ridge of high pressure
- Corresponding drop in pressure on Lee side = low pressure trough
Important aspects of Lee Depressions (3)
- Usually associated with strong winds = mechanical turb and lee wave activity in lee of ranges.
- Significant cloud and vis problems due precip on windward side. On lee side, there should be nil cloud/precip.
- Combined effects of windward ridge and Lee trough can result in large QNH changes over short distances
What is a thermal or heat Low
A shallow Low pressure system caused by the heating of the SFC and air near it.
- Air near SFC is heated, becomes less dense and bouyant.
- Convection (rising) occurs.
- Divergence between 2 - 5000ft occurs, SFC pressure is lowered and Low pressure system created.
Important aspects of Thermal Lows (3)
- Convective turbulence as a result of thermals
- Possible afternoon Cbs with showers, TS and hail
- Warm temps and low pressures increases the density ALT and decreases take-off performance.
Features of a Tropical Cyclone (TC) (7)
- Moist, unstable, warm core = lots of energy
- Generally smaller, but more intense, winds can exceed 150kts at GL.
- Have several bands of heavy rain
- Are given names
- Only form over oceans warmer than 27 degrees C
- Therefore tend to form between 5 - 15 Deg N or S.
- Needs some pre existing disturbance/movement in the atmosphere to form, as well as weak vertical windshear and strong DIV aloft.
