1. Atmospheric Measurements Nick Bassill January 28 th 2009

2. What types of atmospheric variables are measured?

3. 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.)

4. 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

5. 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

6. 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

7. 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/S tevenson_screen

8. 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

9. 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.manchest er.ac.uk/staff/geraint.Vaughan/ UFAM/radiosonde_launch.jpg

10. Station Models These display virtually all the relevant current weather observations for a given weather station http://weather.cod.edu/notes/stnmodel.html

11. Meanings Wind Speeds

12. An Example From Tuesday Morning: http://www.ral.ucar.e du/weather/surface/

13. 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