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This page provides information
concerning the Department of the Earth Sciences Rooftop Weather
Station. The station, located on the roof of Lennon Hall, is pictured
at left. The weather station was purchased from Climatronics Corporation of
Bohemia, New York. Wind speed and direction, temperature, relative
humidity, and solar irradiance are measured by instrumentation on the
metal tripod. A tipping bucket raingauge is located nearby; the
heated precipitation collector also measures snowfall water
equivalent. These data are collected by a datalogger, which also
contains a barometer to measure atmospheric pressure. The data are
then automatically transferred to a personal computer, where data
collection software runs continuously. The data are written to a disk
that is also mounted by our local UNIX computer network. On the UNIX
system there are a series of programs that perform additional
computations, write the data into html format for display on the web,
and store the data in GEMPAK format. In addition to the current
weather display, a brief
climate summary for the previous day is now available. Starting
in August 1997, an archive of climate summaries for past
days is available.
| Temperature: 64.7 F | Wind: 220 degrees at 25.3 knots |
| Relative Humidity: 39.4% | Sea Level Pressure: 1007.5 mb |
| Dewpoint Temperature: 39.8 F | Solar Radiation: 501.6 Watts/m2 |
The instrument used for measurement of incoming solar radiation is
called a pyranometer. The particular instrument used by our
weather station is a model Mk 1-G pyranometer, built by Matrix,
Inc. of Mesa Arizona. Pyranomemeters operate by measuring the
temperature difference between two types of absorbing materials, one
that is a nearly-perfect absorber of solar wavelengths (black
material) and another that reflects nearly all solar radiation (white
material). This temperature difference is related to the
intensity of incoming solar radiation; when there is no solar
radiation (i.e., at night) the temperature difference between the two
materials is negligible. Perhaps the
most familiar units for solar radiation are Watts per square meter
(W/m2).
The thermometer and relative humidity sensors are located in the white plastic shelter pictured at left. The relative humidity sensor, a Rotronic model MP601A, provides a 0 to 1 volt output signal that linearly corresponds to 0-100% relative humidity. The enclosure for these sensors is white in order to ensure that the temperature reading is not biased by absorption of incoming solar radiation. The location of the temperature sensor on top of Lennon Hall (approx. 60 feet above ground level) may not be representative of conditions near the ground on clear, calm nights. In these instances, cool air will generally be confined to a shallow layer near the surface. Therefore the minimum temperature readings shown here will occasionally be several degrees warmer than readings from the airport or other nearby weather stations where the thermometer readings are taken closer to ground level.
Precipitation measurements are taken with a model TRP-525 tipping bucket raingauge built by Texas Electronics. The collection cone is heated for wintertime precipitation measurement. There is some problem with exposure at this buildingtop site because increased winds above the surface could reduce the amount of precipitation "captured" by our gauge. This will particularly be true in high-wind situations. However, we have the luxury of intercomparison with our weighing gauge located at a nearby site. The weighing gauge provides more reliable measurements during winter because tipping-bucket gauges are not designed to measure snowfall accurately.
Pictured at right is a closeup of the inside of the tipping-bucket
gauge. The black object in the center of the gauge is a two-sided
bucket mounted on a fulcrum halfway between the two buckets.
As precipitation collects in one side of the bucket, it eventually
tips the bucket, activating a contact switch in the process. (A
magnet swings close to an electric switch.) Then,
the other side of the gauge fills, and the bucket tips back, and
so forth. Each switch activation is recorded by the datalogger
and stored on the departmental UNIX system. Every bucket tip is
equivalent to 0.01" of rainfall.
Spent water drains out the bottom of the gauge.
The Wind Mark IIIa wind sensor measures the wind speed and direction
with separate sensors. Wind speed is measured with the three-cup
anemometer pictured above. The cups rotate and the frequency of
rotation is measured by a "light chopper". The frequency is
proportional to wind speed.
The wind vane is coupled to a potentiometer; the voltage on the
potentiometer is then related to the wind direction.
We have thus far only undertaken rough calibration of the wind
direction, so there may be an error of up to 20 degrees in our
currently reported values.
More careful calibration will be made shortly.