Atmospheric Measurements

Nick Bassill

January 28th 2009

What types of atmospheric variables are measured?

What types of atmospheric variables are measured?

Some examples:

• Temperature

• Moisture content (often in terms of relative humidity or dewpoint)

• Atmospheric pressure

• Wind (both speed and direction)

• Visibility

• Cloud cover

• Types of weather (rain, snow, fog, etc.)

Measuring Temperature

• Measuring instrument: the thermometer

• Commonly measured in units of Fahrenheit (F) or Celsius (C)

• For ˚F, 32˚ represents the freezing point, while 212˚ is the boiling point

• For ˚C, 0˚ represents the freezing point and 100˚ represents the boiling point

• For converting between the two,• ˚C=˚F*5/9-32˚, or ˚F=˚C*9/5+32˚

• Records (http://en.wikipedia.org/wiki/Temperature):

Lowest temperature recorded on Earth: -128.2˚F

Highest temperature recorded on Earth: 136.4˚F

http://en.wikipedia.org/wiki/File:Galileo_Thermometer_closeup.jpg

Measuring Moisture

• Measuring instrument: the hygrometer• Often given by the “dewpoint” or the “relative humidity”• Relative humidity is defined as the ratio of the amount

of water vapor currently in the atmosphere to the maximum amount possible (this is most often given as a percentage)

• The dewpoint is the temperature at which the atmosphere would achieve 100% relative humidity

• When the atmosphere is warmer, it can hold more water vapor

Measuring Atmospheric Pressure

• Measuring instrument: the barometer• Commonly given in inches of mercury, hectoPascals

(hPa or mb)• Stations above sea level normally report lower

pressures than those at sea level (due to their height)• Therefore, normally pressures are converted to “sea-

level” pressures, to make comparisons from location to location easier

• Conventionally, “low” pressures are associated with active weather, while “high” pressures are associated with sunny weather

The Stevenson Screen

• The Stevenson screen is a commonly used enclosure for these measuring devices

• It is a box with slits in the side to allow for air movement

• It is frequently situated roughly 6 feet (2 meters) above the ground

http://en.wikipedia.org/wiki/Stevenson_screen

Measuring the Wind

• Measuring instrument: the anemometer

• Wind has both velocity (speed) and direction components

• Wind speeds is often recorded in miles/hours (mph) or meters/second

• Aside from compass directions, wind direction can be measured in degrees (where 0˚ is North, 90˚ is East, etc.)

http://en.wikipedia.org/wiki/File:Prop_vane_anemometer.jpg

Measurements Above The Surface

• In order to measure these quantities above the Earth’s surface, a “radiosonde” is used

• These weather balloons are conventionally released twice a day in the United States

• These observations help us understand what is occurring in the atmosphere

http://personalpages.manchester.ac.uk/staff/geraint.Vaughan/UFAM/radiosonde_launch.jpg

Station Models

These display virtually all the relevant current weather observations for a given weather station

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

Meanings

Wind Speeds

An Example From

Tuesday Morning:

http://www.ral.ucar.edu/weather/surface/

Observations about Observations

• Conventionally, only temperature, dewpoint, wind speed and direction, cloud cover, pressure, current weather, and visibility (if less than 10 miles) are shown

• However, much of the planet goes unobserved• Large differences in temperature, dewpoint, etc.

can exist from location to location• This is why we must do contour analysis in order

to “fill in” the missing data