Guidance to open data observations

How to interpret open data observations? Here we have the summarized some of the most used observations and what should be taken into account when using them.

Observations are available in the original UTC-time or in local time. In local time the current daylight saving time (dst) rules are applied to all past decades whether dst was in use or not at the time.

Weather observations

Parameter name	Temporal resolution	Interpretation
Cloud cover	Instant	Total cloudiness given in numbers out of eight: 0 = clear skies, 8 = overcast, 9 = cloud coverage could not be determined. Takes into account all cloud layers.
Precipitation amount	Instant	Precipitation amount in previous 10 minutes. Calculated from precipitation intensity for easier usage. Observations only done every 10 minutes. Value 0 = No precipitation. Value - = No observation carried out
Wind speed	Instant	The average wind speed during the previous 10 minutes.
Gust speed	Instant	The highest 3-second average wind speed during the previous 10 minutes.
Wind direction	Instant	In degrees. 360/360 means that the wind is blowing from the north towards the south. 0/360 means calm conditions.
Air pressure	Instant	The air pressure reduced to sea level.
Relative humidity	Instant
Snow depth	Instant	Snow depth from automated weather stations. Accuracy +- 2 cm.
Horizontal visibility	Instant	Presently automated, previously also manually estimated visibility.
Weather description	Instant	Estimated weather conditions from automated sensors.
Dew-point temperature	Instant
	Hourly observations	Based on the previous hour.
Mean temperature	Daily	The average temperature based usually on 4 or 8 observations per day.
Maximum temperature	Daily	The highest temperature during two 12 hours period, i.e. between 8 p.m. previous evening and 8 p.m. this evening (9 p.m. - 9 p.m during summertime).
Minimum temperature	Daily	The lowest temperature during two 12 hours period, i.e. between 8 p.m. previous evening and 8 p.m. this evening (9 p.m. - 9 p.m during summertime).
Minimum ground temperature	Daily	The lowest temperature near ground between 8 p.m previous evening and 8 a.m (9 p.m. - 9 a.m. during summertime).
Precipitation amount	Daily	The daily precipitation is measured between 8 a.m. (9 a.m. in summertime) and 8 a.m. (9 a.m. in summertime) local time the following day and is given in millimetres (snowfall=water equivalent).
Snow depth	Daily	Snow depth is measured at 8 a.m. local time (9 a.m. during summertime).
Precipitation	Monthly	The monthly precipitation sum is the sum of daily precipitation sums.
Mean temperature	Monthly	The monthly mean temperature is the average of daily mean temperatures.

Sea observations

Parameter name	Interpretation
Sea level	Sea water level referred to the theoretical mean water (= theoretical mean sea level) or N2000-system (=BSCD2000) in millimeters.
Significant wave height	Wave height is the vertical difference between the wave through and the wave crest. The wave field consist of waves of different heights. The significant wave height is approximately the average of the highest third of all waves and corresponds to how one visually estimates the wave height. It is calculated exactly from the energy spectrum.
Wave direction (from)	In practice the direction from where most of the waves come from. Exactly it is the direction from which most of the wave energy comes from. Given as degrees and for example 90° mean that the waves come from east.
Wave directions spreading	An irregular wave field always consists of waves traveling in different directions. Their distribution with respect to the mean direction is described by spreading.
Modal period	The wave period is the time difference between two wave crests. The modal period describes the dominant wave period.
Water temperature	The water temperature is measured, depending on the buoy, from 35–45 cm deep.

Air quality observations

Parameter name	Interpretation
All	Mean values of the previous hour
Air Quality Index	1=good, 2=satisfactory, 3=fair, 4=poor, 5=very poor

Radiation observations

Parameter name	Interpretation
Diffuse radiation	Global radiation, with direct solar radiation shaded out. Measured on a horizontal black surface (W/m²).
Direct solar radiation	Shortwave radiation from the sun measured on a perpendicular black surface (W/m²).
Global radiation	Shortwave radiation from the sun and atmosphere measured on a horizontal black surface (W/m²).
Long wave solar radiation	Longwave outgoing radiation reflected from the atmosphere, measured on a horizontal surface and corrected for temperature (W/m²).
Long wave outgoing solar radiation	Longwave radiation from the Earth's surface to space, measured on a horizontal surface and corrected for temperature (W/m²).
Radiation balance	The difference between incoming and outgoing radiation at the Earth's surface. Instantaneous value (W/m²).
Reflected radiation	The portion of global radiation reflected from the Earth's surface to the atmosphere, measured on a horizontal black surface 2 meters above cut grass (W/m²).
Sunshine duration	Unit: seconds per minute. Sunshine duration is accumulated if the intensity (power) of direct radiation is greater than 120 W/m².
UV index	The UV Index (UVI) is a dimensionless index that measures the flux density of UVB radiation weighted by skin erythema (sunburn sensitivity). The UVI corresponds to the flux density of erythema-weighted UVB radiation (in units of W/m²) multiplied by a factor of 40 m²/W.