Observing the Atmosphere. Assignment 8. Due October 30

Discussion: (1) Pressure. Where portability is not an issue, the most accurate and widely used instrument is the mercurial barometer. The mercury barometer relies on the accurate measurement of the height of a mercury column supported by the pressure of the earth's atmosphere. However, the height of the mercury column is not solely a function of atmospheric pressure; temperature variations as well as variations in the local acceleration due to gravity affect the length of the mercury column. For automated reporting of pressure, the pressure transducer is an accurate and rapidly responding instrument. The transducer is a ceramic capsule that deforms in proportion to applied pressure. As the pressure increases, electrodes on the inner surface of the capsule are moved closer together and increase the capacitance for a charge imposed by the sensor.

(2) Temperature. Mercury in glass thermometers remain one of the most convenient ways to measure temperature as a result of the expansion and contraction of the liquid as a function of temperature. The freezing/boiling point of liquid mercury determine the range of this instrument: -39 to +357 degrees Celsius. A platinum resistance thermometer, such as the CS500 temperature probe, is a positive resistance device in which the resistance to a current is related directly to temperature. The thermistor is a hard ceramic-like elctronic semiconductor that measures temperature in terms of the resistance to an applied electric current.

(3) Humidity. The amount of water vapor in the atmosphere can be determined manually using the sling psychrometer by comparing the `dry bulb' temperature (the air temperature) to the `wet bulb' temperature (the temperature of the air that is reached at constant pressure when water is evaporated into it). The hygristor is an electronic humidity sensor that consists of a hygroscopic (water attracting) film over a mirrored surface. As the amount of water vapor in the air changes, the electronic properties of the film changes, which can then be converted into a voltage.

Measuring pressure.

• Follow the directions on the accompanying sheet and record the pressure from the mercury barometer at nearly the same time on three successive days in both inches and mm Hg.

• Use the accompanying tables and correct the pressure to compensate for temperature and gravity effects.

• Convert the values of your readings to mb (.75006mm/mb;.02953in/mb).

• Log into a workstation and record the 5 minute averaged pressure from the pressure transducer on the top of WBB at roughly the same time as the manual pressure readings are obtained.

• Compare your reported pressures from the two sensors at each time. What factor(s) might explain any differences?

• Relate any variations in pressure over the 2 day period to weather changes.

Measuring temperature and relative humidity.

• Using the information in the Weather Station Installation manual, go outside as a group and determine if there is any location that would meet the general requirements for determining a site for temperature and relative humidity.

• Even if you can't find a suitable site, take out the sling psychrometer and Kestrel unit.

• Record temperature and relative humidity from the Kestrel and temperature and wet bulb temperature from the psychrometer.

• Repeat in a clearly non-optimal locations for temperature and relative humidity measurements.

• From the WWW server, record the 5-minute average temperature and relative humidity observations on the top of the WBB at the time of your readings outside.

• Tabulate your results neatly. Compare your reported temperatures and humidities at all surface locations to those at the top of the WBB. What factor(s) might explain any differences?