Weather
Observations
and Time
Zones
Val Bennington
AOS 101
September 2008
Types of Observations
• Temperature
• Dewpoint
• Barometric Pressure
• Wind Speed and Direction
• Cloud Cover
• Present Weather
• Cloud Type
• Etc…
• Observations are taken by
automated sensors as well as by
individuals
Temperature and
Dewpoint
• Both are measured with a thermometer
that is shielded from direct sunlight
(hygrothermometer)
• Temperature around the world recorded
as degrees Celsius
• Scientists use degrees Kelvin
• U.S. still uses degrees Fahrenheit
Tk  Tc  273.15
Tf  9 5  (Tc  32)
Tc  (Tf  32) * 5 9
Dewpoint (Td)
• Air around us contains water vapor
• Warmer air can hold much more water vapor
than cold air
• Dewpoint tells us the temperature at which
the current amount of water vapor would
saturate the cooler air and start to condense
(form dew)
• If we only have a little moisture in the air,
have to cool the air more to get it to condense
• Think of hot chocolate --- the sugar stays
mixed when it is hot, but as it cools, the sugar
falls out. If you make weak hot chocolate, it
has to cool more before the sugar falls out
• Why do you think this is reported????
Barometric Pressure
• Measured with
barometer
• Units of hectopascals
(hPa), millibars (mb)
or inches of mercury
(in Hg)
• Reported values are
adjusted to sea level
-- otherwise
pressure maps would
simply reflect
topography
Wind Speed and Direction
• Anemometer
measures speed
• Speed measured in
knots:
• 1 knot = 0.514 m/s
• 1 knot = 1.151 mph
http://www.spl.org/images/branch/
BAL_art/anemometer.jpg
• Weak hurricane has
winds over 65 knots
(75mph)!
• Weather vane tells
us wind direction
• Why do we care
about direction?
Clouds
• Cloud amounts and height
• Cloud type done only by sight
www.free-online-private-pilot-ground-school.com
Upper Air Observations
• Balloons with
radiosondes released
twice a day at 72 US
stations (~900
worldwide)
• Measures P,T,Td,
wind speed and
direction as it rises
• Create atmospheric
“soundings” from
these observations
to understand the
vertical profiles of
T,P,wind,Td
Australian Sounding
• How does temperature change with height? Dewpoint? Wind
speed?
• What might the solid black lines be?
• How is dewpoint related to temperature?
• What is surprising about dewpoint at 6000 feet?
Station Models
Lots of
data on
one
map!
Station Map Layout
http://weather.cod.edu/notes/stnmodel.html
Interpretation
Temperature - Two digits for temp (C,K, or F)
Dewpoint - Two digits for dewpoint (same units as T)
Interpretation Continued
• Cloud Coverage How much of the
sky is covered with
clouds? What
fraction?
Interpretation Continued
Wind Direction - Line gives direction FROM WHICH wind is coming
Wind Speed - Barbs used to tell us wind speed
Short barb = 5 knots, Long barb = 10 knots, Triangle = 50 knots
Wind Barbs
Interpretation Continued
Surface Pressure (mb) - Adjusted to sea level. First digit is dropped! If
number starts 0,1,2,3,4 -- put 10 in front of it! Last digit is after decimal
point. 236 = 1023.6mb
If number starts with 6,7,8,9 -- put 9 in front of it! 869 = 986.9
Pressure Tendency – Change in pressure over last three hours. Change
in pressure is represented by a value and line indicating how the
pressure was changing.
Pressure Tendency
Interpretation Continued
Visibility – How far we can see
from the observing point,
expressed in units of miles.
Present weather conditions –
Symbols are used to convey
this information (rain, snow,
ice, etc.).
Examples
Universal Time = Greenwich Mean Time
Expressed as Z or UTC:
6:00 am in London = 6Z