Falmouth Monthly Climate Reports

Falmouth Monthly Climate Reports

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Definition of parameters on monthly climatological data reports

Column Units Description

DAY Day of the month

DAYNO Year day number

SORADM watt-hours/m² Daily total of observed vertical solar energy flux

SORADC watt-hours/m² Daily total of observed vertical solar energy flux with no atmosphere (calculated)

AT Dimensionless Atmospheric transmittance = SORADM/SORADC

NOONALT Degrees Altitude of sun above horizon at solar noon

TMAX Deg F Maximum air temperature during 24 hour period

TMIN Deg F Minimum air temperature during 24 hour period

DD65 Deg F Heating degree day total for house maintained at 70 deg F
DD65 = (TMAX + TMIN)/2 - 65

DD60 Deg F Heating degree day total for house maintained at 65 deg F
DD60 = (TMAX + TMIN)/2 - 60

PRECIP Inches Precipitation in 24 hour period

Comments on measurements

Solar radiation measurements are made with an Eppley PSP pyranometer mounted on the roof of the Clark Building, Quissett Campus, Woods Hole Oceanographic Institution. The output signal of the sensor is sampled at 10 second intervals and averaged over an hour by a Campbell data logger. The hourly totals have been archived since 1991. Daily totals are computed from the hourly totals and have been archived since 1975.

TMAX, TMIN, and PRECIP are recorded by Falmouth Water Department employees at the town pumping station located on the shore of Long Pond, the town reservoir. Beacuse this is an inland site, about 1.5 miles from the shore of Buzzards Bay, air temperatures may differ considerably from temperatures at shore sites. The thermometers have not been calibrated since they were purchased in 1960 and their absolute accuracy cannot be vouched for. TMAX and TMIN have been recorded since 1960, PRECIP since 1966. All data were archived by Dick Payne.

Atmospheric Transmittance

Solar radiation received at the earth's surface can vary because of clouds, latitude, time of year, and time of day. If we consider only daily totals we eliminate the time of day. Because of the dependence on latitude and date, daily totals cannot be compared directly from one place and time to another but the dependence on latitude and date can be compensated for mathematically. Knowing a few astronomical parameters we can compute the amount of solar radiation which would fall on the earth's surface at a given location and date. If we compute it for Woods Hole and divide our measured daily totals by these computed daily totals, we get an index for each day which has a value between 0 and about 0.75. This index is called the atmospheric transmittance (AT). A clear day yields an index of about 0.75; a very cloudy day might have an index of 0.1. This index changes primarily with the effects of the atmosphere (clouds, water vapor) but has the effects of latitude and date removed. This index can then be used to compare atmospheric effects at one location with those at another and one time of year with another.