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

E

N

H

L

low pressure area / cyclone: cloudy or rainy weatherhigh pressure area / anticyclone: fair weather

● Rule of thumb:

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

E

N

L

trough ridge

5.55.45.3

5.6

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

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

As a rule, warmer temperatures in the lower troposphere imply higher pressure surface heights at upper levels (e.g., at 500 mb). Why?

Temperatures and Pressure-Surface Heights

As a rule, warmer temperatures in the lower troposphere imply higher pressure surface heights at upper levels (e.g., at 500 mb). Why?

● Well, for the sake of argument, suppose the pressure at the ground is nearly uniform, with value 1000 mb (roughly true)

Temperatures and Pressure-Surface Heights

As a rule, warmer temperatures in the lower troposphere imply higher pressure surface heights at upper levels (e.g., at 500 mb). Why?

● Well, for the sake of argument, suppose the pressure at the ground is nearly uniform, with value 1000 mb (roughly true)

● Now recall that between any two given pressure levels, the mass of air between the two levels is always the same

- To be concrete, we'll pick 1000 mb and 500 mb

Temperatures and Pressure-Surface Heights

As a rule, warmer temperatures in the lower troposphere imply higher pressure surface heights at upper levels (e.g., at 500 mb). Why?

● Well, for the sake of argument, suppose the pressure at the ground is nearly uniform, with value 1000 mb (roughly true)

● Now recall that between any two given pressure levels, the mass of air between the two levels is always the same

- To be concrete, we'll pick 1000 mb and 500 mb

● If the air is warm, then this mass of air expands, taking up more space. And if it's cold, it contracts, taking up less space.

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.