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ATS 351 Lab 4

September 28, 2009

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2

3

G23

72

9

45

998

-16

.09FNL

Temperature (oF)

Visibility (miles)

Current Weather

Dewpoint (oF) Sky

CoverStation

ID

3 hour precipitation

3 hour pressure change

Station Pressure

Wind speed, direction, and peak gust

4

G23

72

9

45

998

-16

.09FNL

Station Pressure

Sea-level pressure is plotted in tenths of millibars (mb), with the leading 10 or 9 omitted. For reference, 1013 mb is equivalent to 29.92 inches of mercury. Below are some sample conversions between plotted and complete sea-level pressure values:

410: 1041.0 mb103: 1010.3 mb987: 998.7 mb872: 987.2 mb

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5

6

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7

1 kt = 0.514 ms-1 = 1.15 mph

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� A line that connects equal values of some quantity. (iso =‘same’ in Greek)

� Examples• Isotherm: line of constant temperature (5 or 10°F

intervals)• Isodrosotherm: line of constant dew point (5 or 10°F)• Isobar: line of constant pressure (4 mb)• Isotach: line of constant wind speed(5 knots)

� Contouring • Drawing isopleths on weather maps• Lines closer together mean the value is changing more

rapidly over a short distance (i.e. cold front)

1) Isopleths should never cross.

2) Isopleths should not branch or fork.

3) Do not draw isopleths where there is no data.

4) Label at both ends, and color code the isopleths.

5) Isopleths can form closed loops.

6) Eliminate what appears to be incorrect data.

7) Start with locating highest and lowest values.

8) Interpolate between given values.

9) Lines should be smooth.

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• Recall that the height of a column of air can be changed based on the mass in the column and the temperature (density) of the column.

• Thus, the height of a given pressure above the surface can change from place to place depending on the mass and temperature of the column overhead.

500 mb

500 mb

1000 mb 1000 mb

5 km

6 km

The height of a given pressure can be contour plotted in a manner similar to a topographical map.

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• Similarly to surface station plots, upper air maps abbreviate geopotential heights by plotting in decameters (10’s of geopotential meters)

• For example, 540 on the previous map would refer to 5400 gpm (geopotential meters)

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• Troughs are associated with cooler temperatures, low pressure and precipitation

• Ridges are associated with warmer temperatures, high pressure and stability