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1

These long waves also called the

Rossby waves generally found in mid‐ latitude (23.5°N/S‐66.5°N/S) westerlies

2

Rossby waves form primarily due to

the earth's geography

3

Rossby waves form primarily due to the earth's geography, which results in:

a) Differential heating due to uneven earth’s surface and
b) Vertical motion forced by orography

4

The length of long waves

 vary from around 6,000 to 8,000 km or more

5

There are usually between ..................... longwaves that encircle the NHS at any given time.

2 and 7

6

long waves generally move 

very slowly from west to east. But occasionally they will become stationary or retrograde (move east to west)

7

The height of a pressure surface at any level (e.g., the 500 mb surface) is related to 

the (density averaged) temperature of the atmosphere below that level

8

The height of a pressure surface at any level (e.g., the 500 mb surface) is related to the (density averaged) temperature of the atmosphere below that level:

the higher the temerature, the higher the height of the pressure level

9

Where the height lines bow northward (....................)

(a ridge) warm air has moved north

 

10

where the height lines bow southward (...............) 

(a trough), cold air has moved south

11

warmer than average temperatures can be expected underneath

ridges

12

colder than average temperatures can be expected underneath

troughs

13

digging 

Cold air moving south results in strengthening or ‘digging’ southward of an upper level trough

14

building

Warm air moving north results in strengthening or ‘building’ northward of an
upper level trough

15

compare zonal and meridional patterns

A ‘zonal’ (more flat) 500 mb pattern exhibits little wave structure, while a ‘meridional’ (or amplified) 500 mb pattern contains large northward and southward excursions.

16

Surface level storm systems, and associated cold fronts, move

 very fast from west to east in zonal flows as they have very little north to south (or south to north) movement.

17

The 500mb pattern can also be used to

locate where surface storms and precipitation are most likely to be occurring

18

The 500mb pattern can also be used to locate where surface storms and precipitation are most likely to be occurring Surface storms and precipitation are most often found over

areas downstream of 500 mb trough.

19

The 500mb pattern can also be used to locate where surface storms and precipitation are most likely to be occurring Surface storms and precipitation are most often found over areas downstream of 500 mb trough. this is because

 the convergence downstream of the trough forces the surface air to move upward towards the tropopause (dynamical lifting of air).

20

Conversely, sinking air motion is forced over areas

downstream of ridges, where clouds do not develop. Underneath these areas fair weather is most likely

21

Thus, by looking at a 500 mb map, one should be able to distinguish where

precipitation is most likely and where fair weather is most likely

22

The shape and strength of a 500 mb trough often indicates its 

dynamical strength, i.e., its potential to force strong rising motion in the atmosphere and hence strong areas of precipitation.

23

The factors (related to 500 mb pattern), that act to increase divergence and hence
rising motion and precipitation are:

  1. Stronger winds increase divergence. Therefore, the more closely spaced the height lines, the stronger the divergence downwind of troughs.
  2. The more amplified the pattern (amplitude of the ridge/trough pattern), the stronger the divergence downwind of troughs
  3. The sharper the curvature of a trough, the stronger the divergence downwind of the trough
  4. The orientation of the trough axis with respect to a north‐south line.

24

(GRAPH) Stronger winds increase divergence. Therefore, the more closely spaced the
height lines, the stronger the divergence downwind of troughs.

25

(GRAPH) The more amplified the pattern (amplitude of the ridge/trough pattern), the
stronger the divergence downwind of troughs.

26

(GRAPH) The sharper the curvature of a trough, the stronger the divergence downwind of the trough.

27

negative tilt

Troughs that are oriented along a northwest to southeast line

28

positive tilt

troughs oriented along a northeast to southwest axis

29

 a negatively tilted trough indicates a

stronger weather system

30

(GRAPH) Troughs that are oriented along a northwest to southeast line are said to a have a "negative tilt", while troughs oriented along a northeast to southwest axis are said to have a "positive tilt."