September 2008September 2008

Weather Observations

and Time Zones

Weather Observations

and Time Zones

Val BenningtonAOS 101

Val BenningtonAOS 101

Types of ObservationsTypes of Observations

• Temperature• Dewpoint

• Barometric Pressure• Wind Speed and Direction

• Cloud Cover• Present Weather

• Cloud Type• Etc…

• 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

• Observations are taken by automated sensors as well as by individuals

Temperature and Dewpoint

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

• 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

€

Tf = 95∗(Tc + 32)

Tc = (Tf − 32)* 59

€

Tk = Tc + 273.15

Dewpoint (Td)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????

• 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 PressureBarometric 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

• 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 DirectionWind Speed and Direction• Anemometer measures

speed• Speed measured in

knots:• 1 knot = 0.514 m/s• 1 knot = 1.151 mph

• Weak hurricane has winds over 65 knots (75mph)!

• Weather vane tells us wind direction

• Why do we care about direction?

• Anemometer measures speed

• Speed measured in knots:

• 1 knot = 0.514 m/s• 1 knot = 1.151 mph

• Weak hurricane has winds over 65 knots (75mph)!

• Weather vane tells us wind direction

• Why do we care about direction?

http://www.spl.org/images/branch/BAL_art/anemometer.jpg

CloudsClouds

• Cloud amounts and height• Cloud type done only by sight

• Cloud amounts and height• Cloud type done only by sight

www.free-online-private-pilot-ground-school.com

Upper Air ObservationsUpper 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

• 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 SoundingAustralian 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?

• 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 ModelsStation Models

Lots of data on one map!

Station Map LayoutStation Map Layout

http://weather.cod.edu/notes/stnmodel.html

InterpretationInterpretation

Temperature - Two digits for temp (C,K, or F)Dewpoint - Two digits for dewpoint (same units as T)

Interpretation ContinuedInterpretation Continued

• Cloud Coverage - How much of the sky is covered with clouds? What fraction?

• Cloud Coverage - How much of the sky is covered with clouds? What fraction?

Interpretation ContinuedInterpretation Continued

Wind Direction - Line gives direction FROM WHICH wind is coming Wind Speed - Barbs used to tell us wind speedShort barb = 5 knots, Long barb = 10 knots, Triangle = 50 knots

Wind BarbsWind Barbs

Interpretation ContinuedInterpretation 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 TendencyPressure Tendency

Interpretation ContinuedInterpretation 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.).

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.).

ExamplesExamples

Universal Time = Greenwich Mean TimeExpressed as Z or UTC:6:00 am in London = 6Z

Universal Time = Greenwich Mean TimeExpressed as Z or UTC:6:00 am in London = 6Z