Date Explanation for |
Jan Null, CCM
Golden Gate Weather Services
Source: "Comparative Climatic Data", National Climatic Data Center, NOAA, 2001
The following gives a full explanation of all symbols and caveats used to explain the data values in the above data tables.
I.Observed Data (Monthly and Annual) - These values are the
means and extremes for the Period of Record (number of
years) indicated. Period of record are documented in the
Local Climatological Data annual publications.
A. Temperature - Highest of Record, Degrees
Fahrenheit (F)
B. Temperature - Lowest of Record, Degrees
Fahrenheit (F)
The annual temperature entry for each station
represents the highest or lowest observed
temperature in any month.
C. Mean Number of Days with Minimum Temperature
32 Degrees F or less.
The mean number of days with a minimum
temperature of 32 degrees F or lower indicates
the frequency of occurrence of days with
freezing temperatures.
D. Mean number of days with Maxium Temperature 90 degrees F or More
(70 Degrees F or More for Alaska Stations)
E. Mean number of Days with Precipitation 0.01
Inches or More
This table shows mean number of days per month
with at least 0.01 inch of precipitation.
This is the smallest amount of precipitation
numerically recorded, and includes the liquid
water equivalent of frozen precipitation. The
frequency of days with precipitation should
not be considered as frequency of cloudy days.
F. Snowfall (including ICE PELLETS and SLEET) -
Average Total in Inches.
NOTE: Beginning in April 1988 HAIL is
also included under Snowfall.
Therefore, some stations may
show snowfall during the warm
months.
G. Wind - Average Speed (MPH)
The average wind speed is based on the speed
of the wind regardless of direction.
H. Wind - Maximum Speed (MPH)
This table expresses both a maximum wind speed
for the stations and, where available, the
direction (referenced to true North) from
which it blew. Short gusts are listed only
for stations denoted with a (G).
If the direction is expressed as one of the 16
compass points (N, NNE, NE, etc.) the maximum
speed is calculated from the minimum time
during which one mile of wind passed the
station. Note that some stations that report
short gusts (G) may also use the 16 compass
point reference for wind direction.
If the direction is expressed numerically, the
maximum speed is the highest one minute
average value recorded by the observer.
Direction is given in tens of degrees
clockwise from true North.
I. Sunshine - Average Percentage of Possible
The total time that sunshine reaches the
surface of the earth is expressed as the
percentage of the maximum amount possible from
sunrise to sunset with clear sky conditions.
J. Cloudiness - Mean Number of Days - Clear (CL),
Partly Cloudy (PC), Cloudy (CD)
This table shows the mean number of days per
category of cloudiness. The categories are
determined for daylight hours only. Clear
denotes zero to 3/10 average sky cover.
Partly cloudy denotes 4/10 to 7/10 average sky
cover. Cloudy denotes 8/10 to 10/10 average
sky cover.
K. Average Relative Humidity (percent) - Morning
(M) and Afternoon (A)
The relative humidity is expressed as a
percentage measure of the amount of moisture
in the air compared to the maximum amount of
moisture the air can hold at the same
temperature and pressure. Average humidity
values are given for selected morning and
afternoon observations. Maximum relative
humidity values usually occur during morning
hours. In this publication, the Local
Standard Time (LST) of morning and afternoon
humidities are shown below.
Atlantic, Alaskan (M morning 8 A.M.) (Afternoon 2 P.M.)
Eastern, Bering, 165W Meridian (M morning 7 A.M.) (Afternoon 1 P.M.)
Central, 180E Meridian (M morning 6 A.M.) (Afternoon NOON)
Mountain, 165E Meridian (M morning 5 A.M.) (Afternoon 5 P.M.)
Pacific, 150E Meridian (M morning 4 A.M.) (Afternoon 4 P.M.)
135E Meridian (M morning 9 A.M.) (Afternoon 3 P.M.)
II. Climatological Normals (Monthly and Annual) - these
are the 30-year average values computed from the data
recorded during the period 1961-1990. Normals are
updated decennially, for the most recent 30-year
period. If an instruments exposure was changed,
mathematical adjustments are made to make the data
representative of the current location. The values are
statistically determined and cannot be recreated solely
from the original record.
A. Normal Daily Maximum Temperature, Degrees F
B. Normal Daily Minimum Temperature, Degrees F.
C. Normal Daily Mean (Average) Temperature,
Degrees F.
In A, B, and C above, the temperature data are
the normal daily values for each month. They
have been adjusted, as necessary.
D. Normal Heating Degree Days (July-June)
E. Normal Cooling Degree Days (January-December)
Degree day data are used to estimate amounts
of energy required to maintain comfortable
indoor temperature levels. Daily values are
computed from each days mean temperature (max
+ min/2). Each degree that a day's mean
temperature is below or above 65 degrees
Fahrenheit is counted as one heating or
cooling degree day.
F. Normal Precipitation, Inches
The normal precipitation is the arithmetic
mean for each month over the 30-year period,
adjusted as necessary, and includes the liquid
water equivalent of snowfall.
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