Cyclones and Anticyclones, Ridges and Troughs

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Candace Sutton
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Cyclones and Anticyclones, Ridges and Troughs Isobars on surface maps often have a closed appearance (as illustrated) Areas of low pressure are called cyclones, while areas with high pressure are

1 Cyclones and Anticyclones, Ridges and Troughs 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: 1014 H 998 Schematic surface map showing a cyclone / anticyclone pair L N E low pressure area / cyclone: cloudy or rainy weather high pressure area / anticyclone: fair weather

2 Example surface plot from last spring, showing closed contours around highs and lows

3 Surface isobars and cloud cover (yellow) for the same case, showing cloudy conditions over the low pressure area

4 Radar image showing areas of rain (green) and snow (pinkish) associated with the low pressure system

5 N E 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

6 N E L 5.6 trough ridge 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

7 Height contours for the 200 mb pressure surface for the same case as before, showing a wave pattern over the US

8 Cyclones and anticyclones on a surface map Ridges and troughs on an upper-level chart

9 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?

10 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)

11 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

12 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.

13 500 mb surface If the air is warm, then the mass of air expands. And if the air is cold, then it contracts. warm cold column of air expands column of air 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.

14 500 mb surface If the air is warm, then the mass of air expands. And if the air is cold, then it contracts. warm cold column of air expands column of air 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

16 11.2 km contour 12.2 km contour Same 200 mb surface as before, showing heights decreasing towards the pole

17 High to low, look out below! Planes flying with pressure altimeters have to be careful when flying into cold air.

ERTH 465 Fall Lab 5. Absolute Geostrophic Vorticity. 200 points.

ERTH 465 Fall Lab 5. Absolute Geostrophic Vorticity. 200 points. Name Date ERTH 465 Fall 2015 Lab 5 Absolute Geostrophic Vorticity 200 points. 1. All labs are to be kept in a three hole binder. Turn in the binder when you have finished the Lab. 2. Show all work in mathematical