CLIMATE AND
WEATHER
,REVISION FROM GRADE 11:
KEY TERMS
Atmospheric pressure: Pressure exerted by the weight of the atmosphere on the earth’s surface.
Isobars: lines on a weather map joining places with the same air pressure
Hectopascals (hPa): unit of measurement for air pressure. Normal sea level pressure is 1013 hPa
Low pressure (LP): an area where the atmospheric pressure is lower than that of the surrounding area.
Lows are associated with warm air and atmospheric lifting. Because of this, lows normally produce clouds,
precipitation and unstable conditions.
A low pressure cell is called a cyclone or depression. Air moves in a clockwise direction around a cyclone in
the Southern Hemisphere.
High Pressure (HP): an area where the atmospheric pressure is higher than that of the surrounding area.
Highs are associated with cooler, subsiding air. Because of this, highs normally produce clear (cloudless),
dry and stable conditions.
A high pressure cell is called an anticyclone. Air moves in an anti-clockwise direction around an anti-
cyclone in the Southern Hemisphere.
Warm font: where a warm air mass rises gradually up over a cold air mass.
,CONVENTION CELLS:
Air takes on characteristics of surface on which it lies.
Air is heated, it expands and becomes less dense (lighter) and is forced to rise à causes LOW PRESSURE
CELL à creates UNSTABLE AIR CONDITIONS (wet, cold, cloudy weather)
- As it rises, it cools down at rate of 1oC/100m (DALR – dry adiabatic lapse rate)
- At dew point air condenses from water vapor (gas) to water (liquid) à this creates clouds,
therefore, unstable
Air cools as it rises, it contracts and becomes dense and is forced to descend. It then diverges at the
surface à causes HIGH PRESSURE CELL à creates STABLE AIR CONDITIONS
- As air descends it heats up at 1oC/100m (DALR)
- Associated with warm, clear, pleasant (stable) conditions
LP CELLS => CYCLONE
HP CELLS => ANTI-CYCLONES
, PRESSURE GRADIENT FORCE:
Pressure gradient = difference in atmospheric pressure across the earths surface. PGF causes air to move
from a HP cell to a LP cell.
HP CONVENTION CELL LP CONVENTION CELL
• Winds diverge out HP on surface • Winds converge into LP on surface
• Highest pressure at center • Lowest pressure at center
• Pressure lower than 100hPa usually
considered LP
à stable sunny conditions à unstable windy conditions
à air diverges à air converges
• Steep pressure gradient = strong-winds = close isobars. Gentle winds = further apart isobars
• Speed of air movement depends on isobar
CORIOLIS FORCE:
CF means that wind is deflected to the LEFT in the SOUTHERN HEMISPHERE and to the right in the
northern hemisphere. Exists because of the earth’s rotation. 0 at the equator
HP CONVENTION CELL LP CONVENTION CELL
• Deflected to the left in SH • Deflected to left in SH
• Causes anti clockwise rotation in SH • Causes clockwise rotation in SH
à anti-clockwise in SH à clockwise in SH
à clockwise in NH à anticlockwise in NH
WEATHER
,REVISION FROM GRADE 11:
KEY TERMS
Atmospheric pressure: Pressure exerted by the weight of the atmosphere on the earth’s surface.
Isobars: lines on a weather map joining places with the same air pressure
Hectopascals (hPa): unit of measurement for air pressure. Normal sea level pressure is 1013 hPa
Low pressure (LP): an area where the atmospheric pressure is lower than that of the surrounding area.
Lows are associated with warm air and atmospheric lifting. Because of this, lows normally produce clouds,
precipitation and unstable conditions.
A low pressure cell is called a cyclone or depression. Air moves in a clockwise direction around a cyclone in
the Southern Hemisphere.
High Pressure (HP): an area where the atmospheric pressure is higher than that of the surrounding area.
Highs are associated with cooler, subsiding air. Because of this, highs normally produce clear (cloudless),
dry and stable conditions.
A high pressure cell is called an anticyclone. Air moves in an anti-clockwise direction around an anti-
cyclone in the Southern Hemisphere.
Warm font: where a warm air mass rises gradually up over a cold air mass.
,CONVENTION CELLS:
Air takes on characteristics of surface on which it lies.
Air is heated, it expands and becomes less dense (lighter) and is forced to rise à causes LOW PRESSURE
CELL à creates UNSTABLE AIR CONDITIONS (wet, cold, cloudy weather)
- As it rises, it cools down at rate of 1oC/100m (DALR – dry adiabatic lapse rate)
- At dew point air condenses from water vapor (gas) to water (liquid) à this creates clouds,
therefore, unstable
Air cools as it rises, it contracts and becomes dense and is forced to descend. It then diverges at the
surface à causes HIGH PRESSURE CELL à creates STABLE AIR CONDITIONS
- As air descends it heats up at 1oC/100m (DALR)
- Associated with warm, clear, pleasant (stable) conditions
LP CELLS => CYCLONE
HP CELLS => ANTI-CYCLONES
, PRESSURE GRADIENT FORCE:
Pressure gradient = difference in atmospheric pressure across the earths surface. PGF causes air to move
from a HP cell to a LP cell.
HP CONVENTION CELL LP CONVENTION CELL
• Winds diverge out HP on surface • Winds converge into LP on surface
• Highest pressure at center • Lowest pressure at center
• Pressure lower than 100hPa usually
considered LP
à stable sunny conditions à unstable windy conditions
à air diverges à air converges
• Steep pressure gradient = strong-winds = close isobars. Gentle winds = further apart isobars
• Speed of air movement depends on isobar
CORIOLIS FORCE:
CF means that wind is deflected to the LEFT in the SOUTHERN HEMISPHERE and to the right in the
northern hemisphere. Exists because of the earth’s rotation. 0 at the equator
HP CONVENTION CELL LP CONVENTION CELL
• Deflected to the left in SH • Deflected to left in SH
• Causes anti clockwise rotation in SH • Causes clockwise rotation in SH
à anti-clockwise in SH à clockwise in SH
à clockwise in NH à anticlockwise in NH
