AOS 101

Jan 29 (302), Jan 31 (304)

Weather Observation

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– Measured from a direction– Either cardinal direction or degrees

• Examples: 0o = N, 225o=SW

– Wind vane

Cup anemometer

•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

• Combines all seven measurements into a readable figure

26

233

181+14

TT

ddvv ww

PPPappN

D

S

TT

ddvv ww

PPPappN

D

S• N = Sky Cover

– Quarters of sky that are cloud covered

TT

ddvv ww

PPPappN

D

S

TT

ddvv ww

PPPappN

D

S

• ww = current weather– Symbols

representing certain weather conditions

– Omitted if no current weather

RAIN

SNOW

DRIZZLE FRZ. DZ. SLEET

T’STORM FOG HAZE

TT

ddvv ww

PPPappN

D

S

TT

ddvv ww

PPPappN

D

S• D = Wind

direction – Line (wind barb)

drawn in direction wind is from.

TT

ddvv ww

PPPappN

D

S

TT

ddvv ww

PPPappN

D

S

• 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

TT

ddvv ww

PPPappN

D

S

TT

ddvv ww

PPPappN

D

S

• TT = Temperature– In Fahrenheit

TT

ddvv ww

PPPappN

D

S

TT

ddvv ww

PPPappN

D

S

• dd = Dewpoint– In Fahrenheit

TT

ddvv ww

PPPappN

D

S

TT

ddvv ww

PPPappN

D

S

• 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).

TT

ddvv ww

PPPappN

D

S

TT

ddvv ww

PPPappN

D

S • 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…