Metar Reports and Renewable Energy

Metar Reports are intended for use in aviation, but they offer a useful package of data for the renewable energy sector.  The performance of many forms of renewable energy systems are dependent on the weather.  Wind turbines don't turn on calm days, solar panels of all forms work better on clear summer days than they do below a solid mass of cloud in the bleak mid-winter.

Scope

Metar reports are typically compiled once an hour during the operational day of an airfield, if conditions require it, reports can be more frequent.  They are available from many sources, but the NOAA website is the most accessible one.  All reports for the preceding 24 hours are available in cycle files.  For modelling purposes, this is the most convenient form, however, processing these files which contain tens of thousands of records can be a computer intensive task.  The geographic scope is worldwide, however, it is suggested that a potential user of the data evaluates what is available.  For example, we monitor reports from an airfield seven km away, this only operates during daylight hours, as our interest is primarily related to solar energy this is not a problem, but does mean that there is no overnight wind or temperature data.

Timebase

The time base of a report is UTC.  Programming languages such as the .net family of inbuilt functions to convert from UTC to a local time zone.  The report itself contains the day, hour and minute of the report, but not the month and year.  These latter values are are available as part of the download from NOAA, or they can be inferred from time the report was harvested.

Units

There are regional variations in the units used in Metar reports, this is generally clear from the data, for example, in North America the atmospheric pressure is supplied in inches of mercury, whilst in Europe it is given in mB.

Items relevant to Renewable Energy

Within a given report the most interest blocks of data for renewable energy are:

Wind	Direction and speed
Cloud cover	The height and extent.  The report can contain information on up to three layers.  For automatic stations, information is limited to 12,000 feet above ground, thus no information is available on high level cloud.
Pressure	The atmospheric pressure is the value to set an aircraft's altimeter to show the elevation of the airfield.  To get the station pressure it is necessary to adjust this figure for the elevation of the airfield.
Weather	The two main groups relate to precipitation (rain, snow etc.) and obscuration (fog, mist etc.)
Temperature	The report contains the surface temperature and the dew point.  There are many applications of this, for solar devices, it gives the ambient temperature prevailing with a given amount of cloud cover.  Also it facilitates the calculation of the Lifted Condensation Level.

A sample report is shown below, the reader is urged to consult Wikipedia for a fuller description and links to decoding resources.

EGKA 081550Z 04016G26KT 4500 SN SCT007 BKN012 02/00 Q1008

The decoding is shown below:

The first item is the four character ICAO code of the airfield, EGKA is the code for Shoreham.

The next item is the time of the report, this one was harvested during February 2010, the date time of the report is 2010-Feb-08 15:50, the Z indicates that the time base is UTC.

Next comes the wind speed and direction, the first three characters are the direction, the next two characters are the speed in knots (the unit is given in the final characters of the group).  In this case the wind is gusting at up to 26 knots.

The 4500 indicates that the visibility is 4500 metres.

SN is part of a weather group and indicates that it was snowing.  There can be additional qualifiers and descriptors which can indicate the intensity and provide additional information, e.g. fog can be freezing fog.

The next two groups describe the cloud cover.  The lower level is scattered cloud at 700 feet.  Scattered means that there are between 3 and 4 octas of cloud.  The height is given in multiples of 100 feet.  The next layer up is broken cloud at  with the base at 1,200 feet.  Broken indicates that there are between 5 and 7 octas of cover.

Next comes temperature, the first two digits represent the air temperature in deg. C and the second two the dew point.  If the temperatures are below freezing, the digits are preceded by M for minus.  Some stations repeat the temperature information at a higher precision giving the temperatures in increments of 0.1 deg.

Finally comes the altimeter setting, the prefix Q indicates that the units are in mB, an A would mean that they are in inches of mercury.

These are the main items which are present in most reports they can be followed by remarks and more detailed information. When parsing the report it is important to differentiate what is in the main report and what is part of a comment.

Application

The graph below is being used as part of a model of solar panel performance, it shows cloud height and cover, the lifting condensation level and precipitation/obscuration. This image has been reduced to fit on the page.

This graphic was produced from metar reports harvested at an interval of one hour, whilst it gives a good impression of the day's weather, it is possible that some reports have been omitted.

Page Updated: 03-Feb-10