Observations by column
EXPLANATION OF OBSERVATIONS BY REFERENCE NUMBER
Observational records have been kept continuously since August 31, 1979.
Station moved on Feb. 8, 1986 to -LATITUDE 38° 50' 26" LONGITUDE 77° 09' 56".
This is about 2.7 miles north and 1/3 of a mile west of the earlier location.
THIS NEW LOCATION IS ABOUT 1.8 MILES ENE OF ANNANDALE, VA,
INSIDE THE BELTWAY AND IS JUST NE OF BELVEDERE ELEMENTARY
SCHOOL (FAIRFAX CO. PUBLIC SCHOOL). THE ALTITUDE IS 360 FEET
ABOVE MEAN SEA LEVEL.
1. DATE: MONTH - DAY - YEAR
TEMPERATURE - #2 AND #4
The maximum and minimum temperatures are read from a 1996
-upgraded Nimbus digital thermometer and recorded on a daily observation
sheet to the nearest 1/10 of a degree but are rounded off when entered on the
monthly observation sheet until 2007. Now Davis Vantage Pro II is used since 2007.
Example: 50.5°F would be recorded 51°F.
2. TEMP. °F MAX. - MAXIMUM TEMPERATURE DEGREES F FOR DATE
Maximum temperature in degrees F for a 24 hour period from midnight until
midnight at exactly 5 feet above ground level.
3. HR. OF MAX. - TIME THE MAXIMUM TEMPERATURE OCCURS
The time of the maximum is read from a Davis Vantage Pro II since 2007.
4. TEMP. °F MIN. - MINIMUM TEMPERATURE DEGREES F FOR DATE
Minimum temperature in degrees F for a 24 hour period from midnight until midnight
at exactly 5 feet above ground level.
5. HR. OF MIN. - TIME THE MINIMUM TEMPERATURE OCCURS
The time of the minimum is read from a Davis Vantage Pro II since 2007.
6. TEMP. RANGE - DAILY TEMPERATURE RANGE
Temperature range in degrees F obtained by subtracting the minimum temp. from
the maximum temp. for the day.
7. TEMP. MEAN - MEAN TEMPERATURE IN DEGREES F FOR DATE
Mean temperature equals the maximum temperature plus the minimum temperature
and divide the sum by two.
8. MEAN CHANGE - MEAN TEMPERATURE CHANGE IN DEGREES F FROM YESTERDAY
Change in today's mean compared to yesterday's mean temp. or yesterday's mean temp.
minus today's mean.
9. GROUND TEMP.- GROUND TEMPERATURE IN DEGREES F
The ground temp. is taken remotely with a Davis Vantage Pro II stainless steel thermometer
probe at a depth of one foot.
SOLAR RADIATION #10 AND #11
The NW sky camera is used to determine sky cover. If the sky is covered with thin cirrus
and significant solar radiation is passing through, a remark will be made to the effect that
sunlight is passing through high thin clouds.
10. SKY A.M. - AMOUNT OF AVERAGE CLOUD SKY COVERAGE SEEN FROM SUNRISE
TO NOON
Cloud cover in A.M. is taken from sunrise until noon. 0 = clear (0 to 33% sky coverage),
1 = partly cloudy (34% to 66% sky coverage), 2 = cloudy (67% to 100% sky coverage).
11. SKY P.M. - AMOUNT OF AVERAGE CLOUD SKY COVERAGE SEEN FROM NOON TO
SUNSET See #10 above as explanation is the same except for time.
12. PRECIP. MELTED - melted precipitation measured in hundredths of an inch
Starting Jan. 1, 1983 the melted precipitation for 24-hour period will be recorded from
midnight to midnight measured in .01 of an inch. T means Trace, less than .01 inches
of precipitation. From Aug. 31, 1979 to Dec. 31, 1982 precipitation was recorded
sunset to sunset. In the event a measurement could not be made at midnight then it
will be noted under remarks. Precipitation is checked with a nine-inch diameter gauge,
a four-inch diameter gauge and a 8 inch National Weather Service gauge.
13. MAX. 1 HR. PRECIP. - MAXIMUM PRECIPITATION OCCURRING IN A ONE HOUR
PERIOD FOR DATE
This information is observed from a NIMBUS remote precipitation monitor, which is battery
operated and thus will record data during power outages that occur during thunderstorms.
Instrument is reset manually. If the precip. pd. goes through midnight and it can't be
determined if the max. pd. of precip. occurred before or after MT a note will be made
under remarks and the observation will be recorded on the date it is thought to have
occurred or will be omitted.
14. MAX. 5 MIN. PRECIP. - maximum precipitation occurring in a five minute period for date
This information is recorded from a NIMBUS remote precipitation monitor, which is
battery operated and thus will record data during power outages that occur during
thunderstorms. Instrument is reset manually. ALSO NOTE THE ADDED REMARKS
OF #13 AS THIS ALSO APPLIES TO #14.
15. FROZEN PRECIP. - frozen precipitation measured in tenths of an inch for date
Amount of new snow or frozen precipitation recorded for a 24-hour period from
midnight until midnight starting Jan. 1, 1983, from 1979 to Dec. 1982 it was sunset
to sunset. Measurement will be taken when the snowfall has just stopped before
melting caused by sunlight, etc. In deep snows the frozen precip. #15 and snow on
the ground measurements #16 will differ because of packing of new snow by the
weight of overlaying snow. If the precipitation is other than snow it should be noted
under remarks #39. The amount is measured in tenths of an inch. This represents
the max. frozen precipitation depth on the ground at any one time. If a measurement
cannot be made or determined at midnight a measurement will be made as near to
midnight as possible and recorded as to time under remarks. Starting 1996 on deep
snows (over five inches) a separate reading will be taken on snowfall. The snow board
is cleared each hour or before snow depth reaches five inches this is called the five inch
running total method (RTM) and will be recorded under remarks as an additional observation.
16. SNOW ON GROUND- SNOW TOTAL MEASURED IN TENTHS OF AN INCH AT SUNSET
Snow total is a measure of the average amount of snow or frozen precipitation on the
ground at sunset measured in inches. Example: if 3 inches of snow is recorded in
shadows and one inch in sunny areas an average of 2 inches would be recorded.
When T or trace is used it means there are only some patches of snow left in shadowed
or colder areas. Trace or T will be recorded until all snow patches have melted in the
area where the station is located. Numbers 15 and 16 should be the same if it is a very
cold day and if no previous snow was on the ground. Numbers 15 and 16 could be
different if some of the new snow melted before the sunset observations. Also the
decrease in snow on the ground #16 may decrease on a day when the maximum
temperature is less than 32° due to melting or settling due to the heat of the sun in
the open areas. In very deep snows 15 and 16 could be different because of
settling or packing with temperatures even much below freezing.
17. START TIME - FIRST STARTING TIME OF PRECIPITATION
The time precipitation started in 24-hour time. Time of 1 means midnight or continued
precipitation through time midnight. Time may not be given if less than .01 precip. fell.
Time listed would be when enough precipitation had fallen to wet the pavement or
approximately .005 inches. Snowfall time is recorded when the first snow flakes
were observed during daylight hours and not later than .01 hundreds snowfall when
melted when falling during late hours of night or early hours of morning when readings
are recorded on a precipitation recorder not directly observed. Since November 2003
precipitation periods are measured with a precipitation sensor that records the slightest trace.
*** The following only applies to observations between Aug. 1979 to Dec. 1982:
If a time is listed for the date but no precipitation is recorded then the precipitation
started between sunset and midnight and the amount would be recorded in the following day.
18. END TIME - FIRST ENDING TIME OF PRECIPITATION
Time precipitation ended - see #17 for additional information. End of precipitation
is most often determined from the precipitation recorder when the last precipitation
dot has been recorded. One dot equals about .005 of an inch rain. Since November 2003
precipitation periods are measured with a precipitation sensor that records the slightest trace.
19. DATE: MONTH - DAY - YEAR
20. START TIME - SECOND STARTING TIME OF PRECIPITATION
If more than one time is given for a date then two distinct periods (interval of >0.5
hours separating periods of precipitation) of precipitation occurred. If more than two
periods of precipitation occurred a note should be found under remarks #39, example:
period of off and on showers from 1400-2000 hours. Since November 2003
precipitation periods are measured with a precipitation sensor that records the slightest trace.
21. END TIME - SECOND ENDING TIME OF PRECIPITATION
SEE #20 ABOVE FOR MORE INFORMATION.
22. WIND DOT AVE. NOW WIND RUN is USED - UNITS TO REPRESENT THE AVE. WIND SPEED
The number of wind dots from midnight until midnight. One dot represents about
618 turns of the wind cups.
THIS IS TWICE THE NUMBER OF TURNS OR 1/2 THE NUMBER OF DOTS
BEFORE NOV. 8,1987. Since May 2002 wind run has replaced wind dots as a method
of measuring average wind speed and is now recorded at a height of 30 feet.
WIND VELOCITY #23 AND #24
The Anemometer for the average velocity is about eight feet above the ground
and was moved to a more exposed area about forty feet up a hill on July 1, 1998
thus there may be some increase in the average wind velocity due to the change
in location. Since 2003 is the maximum wind gust is recorded for the 24 hours
period from midnight to midnight.
The other anemometer used for max. wind velocity is about five feet above the
apex of the house roof at 32 feet above the ground but is still protected from the
wind some by tall trees to the north and south but is well exposed to east
or west winds.
23. MAXIMUM WIND GUST SINCE 2003
Since 2003 is the maximum wind gust is recorded for the 24 hours period from
midnight to midnight.
EARLIER--- MAX. SUNRISE. - MAXIMUM WIND SUNSET YESTERDAY TO SUNRISE TODAY
The maximum wind recorded from sunset to sunrise as recorded by a maximum
wind gust recorder. Recorded by the Vigilant wind gust cups and a Nimbus wind
monitor recorder. Since Nov. 19, 1998 Davis Monitor II has been used to obtain
the velocity and time of the maximum gust which will be recorded under remarks
when strong winds occur of generally greater than 30 mph. ht.
24. TIME of MAXIMUM WIND GUST SINCE 2003
Since 2003 is the maximum wind gust time is recorded for the 24 hours period from
midnight to midnight.
EARLIER--- MAX. SUNSET. - MAXIMUM WIND SUNRISE TODAY TO SUNSET TODAY
The maximum wind recorded for the 12-hour period from sunrise to sunset as
recorded by a maximum wind gust recorder. If a morning observation is not
recorded for #23 then the maximum wind gust for 24 represents the maximum
wind gust to occur in a 24-hour period from sunset to sunset. Recorded by the
Nimbus wind monitor. Since Nov. 19, 1998 Davis Monitor II has been used to
obtain the velocity and time of the maximum gust, which will be recorded under
remarks when strong winds occur of generally greater than 30 mph.
25. MAX. R.H. - MAXIMUM HUMIDITY FOR 24 HR. PERIOD MT TO MT
SINCE 2003 the relative humidy is taken from the Davis Vantage Pro II.
Starting on Jan. 1st 1991 the maximum relative humidity reading will be taken
from a Nimbus digital remote humidity sensor. The Nimbus instrument has an
accuracy of 2% from (20%-100%) at 70° F and 5% from (20%-100%) at 32° F.
Since November 15, 1998 the Davis Monitor II has been used as it is more accurate
(2% for the full range of scale) for R.H. When the observer is absent for a few days
the maximum relative humidity readings will be taken from a hydrothermograph for
the 24 hour period of midnight until midnight or from a computer data by way of
interfacing with the Nimbus and Davis weather instruments. (Also read #26)
26. MIN. R.H. - MINIMUM HUMIDITY FOR 24 HR. PERIOD MT TO MT
SINCE 2003 the relative humidy is taken from the Davis Vantage Pro II.
Starting on Nov. 15, 1998 the min. relative humidity reading was taken from a
Davis Monitor II digital remote sensor with an accuracy of 2% for the full range
of scale. The Nimbus instrument was replaced as it gave too low a reading before
its repair on July 15,1997 and too high after the repair until its replacement with the
Davis instrument on November 15, 1998. Thus relative humidity readings for 1997
and 1998 may be in error by ten percent. READINGS OF LESS THAN 16% ARE
GENERALLY CHECKED WITH A WET AND DRY BULB THERMOMETER.
(Also read #25)
27. MAX. BAROMETER - MAXIMUM BAROMETER READING FOR DATE
SINCE 2003 the barometer reading is taken from the Davis Vantage Pro II.
The maximum barometer reading is taken from a barograph for the 24-hour period
of midnight until midnight. The barometer reading will also be checked against a
NIMBUS digital barometer sensor and Davis Monitor II sensor.
28. MIN. BAROMETER - MINIMUM BAROMETER READING FOR DATE
SINCE 2003 the barometer reading is taken from the Davis Vantage Pro II.
The minimum barometer reading is taken from a barograph for the 24-hour period
of midnight until midnight. The barometer reading will also be checked against a
NIMBUS digital barometer sensor and Davis Monitor II sensor.
29. AVE. BAROMETER - AVERAGE BAROMETER READING FOR THE DATE
Mean barometer equals the maximum barometer plus the minimum barometer
and divide the sum by two.
30. BAR. CHANGE - AVE. CHANGE MEAN BAROMETER READING FOR DATE
COMPARED TO YESTERDAY'S
Change in today's mean barometer compared to yesterday's mean barometer reading.
31. BAR. & PRECIP. - BAROMETER READING AT THE ONSET OF PRECIPITATION
The atmospheric pressure when precipitation started and if there is more than one
period of precipitation then the pressure is taken at the time of the first precipitation.
32. MAP FEATURE - MAP FEATURE INFLUENCING TODAY'S WEATHER
Code for map features through the use of the following numbers. The feature listed
is the one most effecting the day's weather. If more than one is recorded then they
both were responsible for the day's weather and in the order listed.
8 Low pressure 11 Cold fronts
9 High Pressure 21 Warm fronts
9R. Elongated high Pressure 24 Stationary front
10 Air Mass ( mT- maritime tropical) 23 Upper level disturbance or a trough line
26 Occluded front
33. FEATURE LOCATION - location of the influencing map feature given
by state using the post office's state abbreviations. If C is placed after a
state abbreviation it means the feature is located off the coast of that state
or G for off the gulf coast, A for off the Atlantic coast. The feature location is
given for a time between 6:00 and 8:00 A.M. local time.
34. HP. / LP. TRACK - HIGH OR LOW PRESSURE TRACK DIRECTION
Only given for High or Low Pressures as to the direction they track
(often NE at our latitude)
35. FRONT DIRECT. - DIRECTION OF FRONTAL APPROACH
Indicates the direction of frontal approach, if there is more than one front
then the direction of nearest front will be given.
36. FRONT INTEN. - FRONTAL INTENSITY THE STRENGTH OF THE FRONT
INFLUENCING THE AREA'S WEATHER FOR THE DATE.
S = STRONG FRONT [greater than 10 degrees temp. difference on
either side of the front]
AVE. = AVERAGE FRONT [approximately 5 to 10° of temp. change on
opposite sides of the front]
W = WEAK FRONT [less than 5° temp. change across front]
37. TIME PASS. - TIME OF FRONTAL PASSAGE
If there are two fronts in 24 hours then two times will be given and the
second front will be recorded under remarks # 39 and 24 hour time will
be employed.
38. DATE: MONTH - DAY - YEAR
39. REMARKS
Remarks will include other observations of interest.
Example: SNOW to WATER ratio, coldest since Dec. 23,1989
Symbols commonly used in remarks column are as follows:
LP Low Pressure TH Thunder but no storm observed
LPT L. Pressure Trough L Lightning but no storm observed
HP High Pressure SH Shower
HPR H. Pressure Ridge SHS Showers
CF Cold Front TS Thunderstorm
WF Warm Front TSH Thundershower
OF Occluded Front DCF Double Cold Front or 2 Fronts in 24 hours
SF Stationary
TP Triple Point - The point where the occluded front separates into a WF. And a CF.