Cyclones and Anticyclones, Ridges and Troughs
Schematic surface map showing a cyclone / anticyclone pair
● Isobars on surface maps often have a closed appearance (as illustrated)
● Areas of low pressure are called cyclones, while areas with high pressure are called anticyclones
● Rule of thumb:
E
N
H
L
1014
998
Example surface plot from last spring, showing closed contours around highs and lows
Surface isobars and cloud cover (yellow) for the same case, showing cloudy conditions over the low pressure area
Radar image showing areas of rain (green) and snow (pinkish) associated with the low pressure system
heights of the 500 mb surface (km)
● On upper-level charts, height contours often have a wave-like appearance
- The part of the wave with higher heights is called a ridge, while the part with lower heights is called a trough
E
N
5.55.45.3
5.6
Height contours for the 200 mb pressure surface for the same case as before, showing a wave pattern over the US
Cyclones and anticyclones on a surface map
Ridges and troughs on an upper-level chart
Temperatures and Pressure-Surface Heights
Why?
warmcold
column of air expands
column of air contracts
● If the air is warm, then the mass of air expands. And if the air is cold, then it contracts.
● This expansion of the column where it's warm means that the pressure surfaces at upper-levels will have higher heights. And vice-versa where it's cold.
500 mb surface
warmcold
column of air expands
column of air contracts
● If the air is warm, then the mass of air expands. And if the air is cold, then it contracts.
● This expansion of the column where it's warm means that the pressure surfaces at upper-levels will have higher heights. And vice-versa where it's cold.
● One consequence is that in general, pressure surfaces slope downwards from equator to pole
500 mb surface
Same 200 mb surface as before, showing heights decreasing towards the pole
11.2 km contour
12.2 km contour
High to low, look out below! Planes flying with pressure altimeters have to be careful when flying into cold air.
