Altitude and vertical thermal structure of the atmosphere
Exponential decay of Pressure with Height
Approximate relationship:
P=P0 exp(-z/8100)
Where P0=1013 mb and
8100 m = e-folding distance or “scale height”
Common Unit Conversions and Conventions in Meteorology
•Temperature:
•Pressure:
•Winds:
•Lengths
•Areas
•Volumes
•Energy
•Power
5 ( F 32)
9
F 9 C 32
5
K C 273.15
C
Surface pressure is usually measured in mb or in. Hg and adjusted mathematically to the
pressure that would be read if the station were at sea level.
1013 mb = 101300 Pa = 1013 hPa = 1 atm = 760 mm Hg = 29.92 in Hg
Compass rose with wind blowing FROM 0 degrees denoting a North Wind, 90 an East Wind,
180 a south wind and 270 a West wind
1 kt = 1.15 mph ; 1 mph = 1.61 km/hr
Large lengths usually in km or mi; small dimensions almost always in meter fractions
m = 3.28 ft; 1 mi=5280 ft; 1 mi=1.61 km; 1 mm= 1x10-3m; µm= 1x10-6m
1 sq. mi = 2.6 km2=2.6x106m2; 1 acre=4087 m2
1m3 = 1x109 mm3; 1 ml=1cm3=1x10-6 m3
The energy required to heat 1 ml of water 1 degree C is a calorie – a food Calorie (large C)
is 1000 calories (small c)
1 cal = 4.184 J
Power expresses the rate of energy
W=1 J/s
The Station Model for surface observations
PRESENT WEATHER:
Indicates presently occuring precipitation. Other
unusual phenomena may also be indicated in
text report. Visibility is often indicated to the left
of this symbol.
WIND
Wind is plotted in increments of 5 knots (kts), with the outer end of the symbol pointing toward the
direction from which the wind is blowing. The wind speed is determined by adding up the total of
flags, lines, and half-lines, each of which have the following individual values:
Flag: 50 kts
Line: 10 kts
Half-Line: 5 kts
If there is only a circle depicted over the station with no wind symbol present, the wind is calm.
Below are some sample wind symbols:
PRESSURES and TRENDS
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:
PRESENT SKY COVER:
Usually, but not always in quarters of cloud
cover. Traditionally observed in eighths.
410: 1041.0 mb
103: 1010.3 mb
987: 998.7 mb
872: 987.2 mb
The pressure trend has two components, a number and symbol, to indicate how
the sea-level pressure has changed during the past three hours.
Trends are often omitted
The number provides the 3-hour change in tenths of millibars, while the symbol
provides a graphic illustration of how this change occurred. Below are the
meanings of the pressure trend symbols: