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Climate services
Daily weather of the Netherlands: Explanation
General information
"Daily data of the
weather in the Netherlands" gives a survey of the observations as taken
by the KNMI at six locations throughout the years. The names and coordinates
are stated in the tabel mentioned below (map).
The names in parenthesises refer to the present name of the observing station.
The weather on the
day indicated is based on the measurements in a natural day (0-24 hrs).
Time is expressed in UT=Universal Time, where 12 hrs UT corresponds with
13 hrs CET and 14 hrs CEST. The normals are the mean values covering the
period 1971-2000. These values are derived from measurements in ten-days
periods ("decades"). Every month has three decades: 1st-10th,
11th-20th and 21st - end of the month. *Altitude of site of temperature
and precipitation measurements.
The most recent data may not yet have been
unvalidated. As a result, some values may change within a few working
days. Warning: because of station
displacements and changes in observation methodologies these time series
of daily values are not homogeneous!
Temperature
Temperature is a measure of the warmth
condition of the air, and is recorded at a measuring height of 1,50 meters.
The mean stated is the average of
24 observations in a natural day. The maximum and the minimum are respectively
the highest and lowest of the measured values. Temperatures are expressed
in degrees Centigrade.
Sun,cloudiness and
visibility
At present sunshine duration is calculated
from 10-minutes radiation values with an algorithm developed by the KNMI.
Formerly (before 1991) an instrument was used allowing the sun to burn
a track on a special registration strip via a glass ball. The relative
sunshine duration is the recorded duration expressed as percentage of
the longest possible duration on that specific day. This takes account
of the seasonal varations in day lengths.
The covering of the sky by clouds is determined every hour and is recorded
in octas. The values given are the means of 24 hourly observations in
a natural day. 0, 1 and 2 represent blue sky, 3, 4 and 5 partly cloudy,
6, 7 and 8 cloudy and 9 sky invisible.
Formerly the minimal
visibility was observed every hour by recognizing landmarks, whose distance
to the observr was known, in the surroundings of a measuring station. At
present visibility is measured electronically.
Air pressure
The air pressure
or atmospheric pressure is the force per surface unit resulting from the
weight of the atmosphere above the measuring point. Air pressure is represented
in hecto Pascal 1 (hPa = 1 mbar). The mean air pressure stated is the average
of 24 hourly observations in a natural day. Nowadays every hourly value
is based on a continuous measurement during one minute. To enable comparisons
with other stations the measured values have been reduced to mean sea level.
Precipitation
Precipitation comprises rain, drizzle, supercooled rain, snow, sleet, hail, frozen rain,
spicules of ice and precipitation resulting from fog and dew etc. The precipitation refers
to the volume reaching the ground per square meter. The duration concerns the cumulative
time (possibly the total of separate precipitation duration times) during which is question
of measurable precipitation.
Wind
The measured wind refers to the horizontal movement of air on a level of 10 meters
above surface. The mean wind speed is the average of the 24 hourly measured
values in a natural day. The highest of these 24 hourly means is mentioned
separately under: highest mean hourly speed. The measured highest peak
of the wind velocity any time during the day is called the highest gust.
To determine the wind direction the hourly wind directions are resolved
in east-west and north-south components, from which a resultant vector,
representing the total air movement during the natural day, is calculated.
A compass card (pdf)
can be used to convert the wind direction in degrees to wind direction
in quarters. Our
Beaufort scale (pfd) converter can be used to convert the wind speed
in meters per seconds to wind speed in Beaufort. Note that wind speed
on the Beaufort scale refers to a period of at least five minutes; so
the highest gust cannot be expressed in Beaufort. |
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