1. Constant Pressure Maps
ATMO 4300
Spring 2008

2. Two Related Variables Height and Pressure
On a surface weather map, height is constant and pressure varies
What height is used and why?

3. Surface Weather Map Figure from www.rap.ucar.edu/weather/model
Slide11.mp3
The surface weather map, as far as depicting horizontal pressure gradients is concerned, is really a constant level map since all the station pressures have been converted to sea level. To represent horizontal pressure gradients in the upper atmosphere we could use constant level maps such as a 5 km map or an 8 km map. However that is not what is done.

4. Question
Where does the information come from that is plotted on a surface weather map?
Mostly from ASOS (Automated Surface Observation System)

5. ASOS

6. Upper Air Maps Also called constant pressure maps
On these maps, pressure is constant and height varies

7. 500 mb Map
Slide34.mp3
This is an example of a 500 mb constant pressure map. The solid dark lines connect equal height values and are called contours. They depict the variation in height of the 500 mb pressure surface. Everywhere on that surface the pressure is 500 mb. The idea of a constant pressure chart may be somewhat confusing, but we’ll return to this in the lesson on Air in Motion and discuss constant pressure charts in greater detail.

8. Question
Where does the information come from that is plotted on a constant pressure map?

9. Upper Air Observations
Radiosonde
Measures temperature, pressure, humidity
Rawinsonde - wind
Figure from
Slide30.mp3
Therefore it is important to observe temperature, wind and other weather variables above the surface all the way up through the troposphere and into the stratosphere. These upper air observations are made using an instrument package called a radiosonde. It contains instruments to measure temperature, pressure, and humidity. Sometimes this device is called a rawinsonde because it also allows us to obtain wind speed and direction.

10. Launching a Radiosonde Photo from www. srh. noaa. gov/mob/balloon
Launching a Radiosonde Photo from map from
Slide31.mp3
The radiosonde is a relatively small instrument package that is attached to a balloon and launched twice a day at selected locations that you see on the map at the right. As the balloon carries the radiosonde up through the atmosphere the radiosonde measures temperature, pressure, humidity and wind at many levels and radios that information back to the ground station where the data are collected. The radiosonde usually ascends to an altitude of about 30 km or close to 19 miles. Once the balloon bursts, a parachute opens that allows the radiosonde to descend back to earth.

11. Depicting Upper Air Data
1. Constant Pressure Maps
2. Temperature/height diagram:
Vertical profile of
temperature, dew point and wind
Slide39.mp3
A second way of depicting observations of the upper atmosphere is by plotting data from a single radiosonde site on a temperature-height diagram, also known as a thermodynamic diagram. This method shows how temperature, dew point and wind change in the vertical.

12. Thermodynamic Diagram
Slide40.mp3
This is an example of a thermodynamic diagram. We can use pressure values as shown on the left side of the diagram to represent height since pressure decrease with height and pressure is what is measured by the radiosonde. The thicker red line shows how air temperature changes with height as measured by the radiosonde. The green line shows how dew point temperature changes with height. Over on the right side of the diagram you can see the wind direction and speed at various altitudes. Weather data plotted in this fashion is called a sounding.

13. Depicting Upper Air Data
1. Constant Pressure Maps
Slide33.mp3
There are basically two ways of depicting observations of the upper atmosphere. One is to plot observations made by radiosondes, satellites and aircraft on a map in the form of a station model. This is similar to the way surface observations are plotted on a map. Maps on which upper air observations are plotted and then analyzed are called constant pressure maps.

14. Standard Upper Air Maps
Pressure Height (ft) Height (m)
200 mb ,000 ft ,000
300 mb ,000 ft
500 mb ,000 ft
700 mb ,000 ft
850 mb ,000 ft
Slide37.mp3
The standard pressure values used to label upper air charts are 850, 700, 500, 300 and 200 mb. The approximate altitude above sea level in both feet and meters at which these pressure values are found in the atmosphere are shown here. By plotting the height of one of these pressure values at all the locations that launch a radiosonde we have a constant pressure map.

15. Constant Pressure Maps
Pressure decreases with height
Rate of decrease varies
Height of a given pressure varies
500
500
Slide36.mp3
Let’s use a pressure of 500 mb for this example. The pressure at each little horizontal line is 500 mb. You can see that 500 mb occurs at a different height above each location and it’s that height that is plotted on a constant pressure chart. So on a constant pressure map we keep the pressure the same and plot the different heights of where that pressure is measured above each location. If you can imagine connecting all the points in the atmosphere where the pressure is the same, it would define an imaginary constant pressure surface.
500 up
500

16. 500 mb Map
Slide38.mp3
This is an example of a 500 mb constant pressure map. The solid dark lines connect equal height values and are called contours. They depict the variation in height of the 500 mb pressure surface. Everywhere on that surface the pressure is 500 mb. The idea of a constant pressure chart may be somewhat confusing, but we’ll return to this in the lesson on Air in Motion and discuss constant pressure charts in greater detail.

17. Question
Why does the height of a constant pressure surface vary from place to place?

18. Standard Atmosphere Figure from ww2010.atmos.uiuc.edu
Slide13.mp3
To explain constant pressure maps let’s look at the influence of temperature on pressure using these figures. On the left are two columns of air, A and B, both having the same average temperature. You could think of column A as being above Amarillo and column B being above Lubbock or use any 2 locations you prefer.

19. Non Standard Temperatures Figure from ww2010.atmos.uiuc.edu
Slide13.mp3
To explain constant pressure maps let’s look at the influence of temperature on pressure using these figures. On the left are two columns of air, A and B, both having the same average temperature. You could think of column A as being above Amarillo and column B being above Lubbock or use any 2 locations you prefer.

20. Temperature & Density Which column has the greater density?
Pressure decreases at a greater rate with height in the cold air
Figure from apollo.lsc.vsc.edu/classes/met130
Slide14.mp3
Consider two columns of air. The average temperature in the column of air on the left is colder than in the column of air on the right. If the same amount of air occupies both columns, in other words both columns of air contain the same mass, which has the greater density?

21. Construction of a 500 mb Map upper left map from www. srh. noaa
Construction of a 500 mb Map upper left map from 1 3 2
500 4
Slide21.mp3
Let’s see how a constant map would be constructed. We’ll choose a 500 mb map. Twice a day at locations over the U.S. (as seen in fig. 1) a radiosonde is launched that measures pressure, temperature, humidity and wind at various altitudes. At all these locations, the altitude at which the radiosonde measures a pressure of 500 mb is determined (fig 2). That altitude can vary from place to place because of horizontal differences in temperature.
500
500
500

22. Use of Const. Press. Maps 850 mb map
- Low level winds – Low Level Jet (LLJ)
- Temperature advection – vertical motion
- Moisture advection
- Frontal zones

23. Use of Const. Press. Maps 700 mb map
- Temperature advection – vertical motion
- Winds
- Relative Humidity
T-Td = 5 or less: possible clouds/
upward motion
- Quick look at possible cap: T > 12-14C

24. Use of Const. Press. Maps 500 mb map
- Identify Long-waves & Short-waves
- Vorticity – related to vertical motion
- Wind flow patterns (see following slides)

26. Upper-Level Flow Zonal Flow – dominant W to E flow
Meridional Flow – large north/south component to the flow

27. Meridional Flow Image from www.rap.ucar.edu/weather

28. Split Flow Diagram from www.theweathrprediction.com/blocking
Note the northern branch and southern branch of the westerlies

29. Cut-Off Low Diagram from www.theweathrprediction.com/blocking

30. Omega Block Diagram from www.theweathrprediction.com/blocking

31. Rex Block Diagram from www.theweatherprediction.com/blocking

32. Use of Const. Press. Maps 300/250/200 mb maps
- Locate Polar & Subtropical Jet Streams
- Locate Jet Streaks (Jet Max)

33. Polar Jet