Weather Maps, Observations and Tools:
I Satellite Imagery
- Big boost when this came around!!!
- Allows one to see what’s coming
- Can’t see precipitation falling from space, but can see cloud types that would
indicate precipitation, along w/ fronts, storms etc.
- Two main types: visible and infrared
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Time Convention:
In order to avoid confusion, weather maps often have the times listed in
Greenwich Mean Time (GMT) a.k.a. Coordinated Universal Time (UTC)
a.k.a. Zulu time
Why do this? Because local time to someone if London is different than local
time to someone in NYC.
How to read a map in terms of local time?
o Right now…we are on Daylight Savings (PDT), so local time is GMT
time – 7 hrs
A Visible Imagery
- What we see, so only good during the day!
- Color of cloud depends on its albedo!!!
- Albedo: reflectivity of an object
- Albedo of clouds depends on how thick they are, so
o Thick clouds: high albedo: bright white
o Thin clouds: lower albedo: darker
- Also picks up surface features such as snow/ice caps
B Infrared imagery
- aka IR—a type of radiation
- Works by seeing temperature differences: amount of radiation emitted (by
clouds or surface) depends on its temp.
o It’s why cops use it to catch crooks in dark
- Calibrated so that cold clouds are whiter, warm clouds are darker
- Works 24 hours/day
- Works well when using alongside visible
- Review questions:
o What property of a cloud or surface determines how bright it will
appear on an visible satellite image?
o What property of a cloud or surface determines how bright it will
appear on an infrared satellite image?
o How is it possible to tell daytime from nighttime by looking at IR
imagery alone?
o How is it possible to make out the Grand Canyon using IR imagery
alone?
II Isopleths
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Iso = same.
Lines or contours showing the same thing
o Isobars, lines of constant pressure
o Isotherms, lines of constant temperature
o Isotachs, lines of constant wind speed
III Surface Observations
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Give temperature, dew point temperature, air pressure, cloud cover, wind
speed and direction and weather conditions
o http://www.hpc.ncep.noaa.gov/html/stationplot.shtml
IV Measuring and reporting precipitation
* Need guidelines for consistency so that an inch of rain here is an inch of rain everywhere
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Rain/snow: implies consistent
Showers: rain off and on
o size of droplets depends on whether it is called drizzle, light rain,
heavy rain
Snow:
o Flurries = light snow
o Blizzard: strong winds and snow. Whiteout conditions
- We measure precipitation in inches, so…
o Snow is melted down and calculated as liquid.
o 1” of rain is ~ 10” snow, but is not constant
 It depends on temperature of atmosphere and density of snow
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Measured using rain gauges
o Several types, but all have a standard size and diameter for consistency
 20 cm in diameter; 50 cm tall
 Faults: rain/snow must fall in cylinder
o Tipping bucket: When 1/100” of rain is reached, it tips and records.
 Faults: heavy rains cannot handle heavy rates and may
underestimate rain totals
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Doppler radar
o RAdio Detection And Ranging
o A type of EM wave that has a longer wavelength than IR and Visible
o Measures drop size and intensity, but not amounts
 Shows where rain is falling and how hard
 Shows movement of rain bands
 Indicates tornado formation w/ hook echoes