AOS 101
Weather Observation
Jan 29 (302), Jan 31 (304)
Background
• Observations allow meteorologists to
assess the current state of the
atmosphere
• Synchronized: taken at same time at
every location
• Standardized: all measurements taken
the same way
• Two main types:
– SURFACE and UPPER AIR
Surface Observations
• Taken hourly at thousands
of sites around the world
• Originally was manual,
but now mostly
automated
• Mostly at airports
• Stations identified by four
letter code
– KMSN = Madison
– KORD = Chicago O’Hare
ASOS station
• Spaced very close together, 48 stations in
Wisconsin alone = Ave. spacing of 60 km.
Upper Air Observations
• Radiosondes (weather
balloons) measure the
atmosphere aloft.
• Released twice daily at
the same time globally.
– 6 AM and 6 PM CST
• Can reach 90000 feet
• 900 stations globally
– 72 in the continental U.S.
Radiosonde
• Measures temperature, moisture and
wind direction/speed to attain vertical
profile of atmosphere
Other data types
• Ships
• Commercial
Aircraft (ACARS)
• Satellite
• All this data goes into forecast models.
7 important measurements
• 1. Sky Cover
– How much sky do clouds cover?
– Partly cloudy, mostly cloudy, etc.
• 2. Current Weather
– Is there precipitation falling? What type?
– Is it foggy? Thunder?
3. Wind
• Speed
– 1 mph = 0.869 kts = 0.447 m/s
– Anemometer
• Direction
Cup anemometer
– Measured from a direction
– Either cardinal direction or degrees
• Examples: 0o = N, 225o=SW
– Wind vane
• 4. Temperature
– oF = (9/5 * oC) + 32; oC = (oF - 32) * 5/9
– Thermometer
• 5. Air pressure
– Units: hectopascals (hPa),
millibars (mb) or inches of
mercury (“ Hg)
– hPa = mb
– 1000 hPa = 29.53” Hg
– Falling pressure = stormy
weather is on the way
(usually).
– Barometer
Barometer
• 6. Dewpoint
– Related to amount of moisture (water vapor)
in the air except in temperature units.
– The temperature to which the air must be
cooled for condensation to take place
• Example: Morning Dew
– If dewpoint is close to temperature (within
3o), expect fog, haze or precipitation.
• 7. Visibility
– How far one can see horizontally.
– Clear day visibility more than 10 miles
– Fog or heavy snow can cause visibilities of less
than one mile
Station Model
26
181
+14
3
23
• Combines all seven measurements into a
readable figure
S
D
TT
vv
ww
dd
N
PPP
app
• N = Sky Cover
– Quarters of sky
that are cloud
covered
S
D
TT
vv
ww
dd
N
RAIN
PPP
app
• ww = current
weather
– Symbols representing
certain weather
conditions
– Omitted if no current
weather
SNOW
DRIZZLE
FRZ. DZ.
SLEET
T’STORM
FOG
HAZE
S
D
TT
vv
ww
dd
N
PPP
app
• D = Wind direction
– Line (wind barb)
drawn in direction
wind is from.
S
D
TT
vv
ww
dd
N
PPP
app
• S = Wind speed (in knots)
–
–
–
–
–
Lines drawn at end of barb
Full line = 10 kts
Half line = 5 kts
Flag = 50 kts
Calm = circle around
station
S
D
TT
vv
ww
dd
N
PPP
app
• TT = Temperature
– In Fahrenheit
S
D
TT
vv
ww
dd
N
PPP
app
• dd = Dewpoint
– In Fahrenheit
S
D
TT
vv
ww
dd
N
PPP
app
• PPP = Pressure (in hPa)
– If PPP>500, place a 9 in front of PPP and divide by 10,
example 876 = 987.6 hPa
– If PPP<500, place a 10 in front of PPP and divide by 10,
example 181 = 1018.1 hPa
• app = Pressure Tendency (in hPa)
– Change in pressure over last 3 hours in tenths of hPa
(ALWAYS with a + or -), also a symbol describing how it
has changed (see handout).
S
D
TT
vv
ww
dd
N
PPP
app
• vv = Visibility (in
miles)
– can have a fraction
• Several station models can be plotted on
a map and analyzed to find fronts,
high/low pressure systems, cold/warm
areas, and areas of cloud cover…