We recently re-added realtime METAR (Aviation Routine Weather Report) rosettes to the Scan B radar displays. The attached image shows the rosettes for Kithener-Waterloo and near Fergus, Ontario. These reporting stations are usually sited at airports and may or may not be automated. Their currency may lag by up to 2 hours, although they usually more current than that.

The rosette EWR displays consists of a centre circle, to which a wind vane is attached, and is surrounded by three or four numbers.

The centre dot may contain one of the following codes: M – no sky cover report (ie “missing”; a full white dot (clear sky), a full black dot (fully cloudy), or a partial black/white dot that is quartered in black and white in proportion to the relative cloud cover. In the image, the KW cloud cover is “50%”. Additionally, our METAR dots will also show a red down-arrow (sharply decreasing pressure) or a green up-arrow (sharply rising pressure). These latter two items report in relation to storm cells typically. The outer rim of a storm cell will be strongly high pressure due to the storm cell downdrafts and outflow winds, while the storm centre or core will show a strong low pressure due to rising air in the cell. These rapid changes are indicative of a strong convection cell, and should be viewed as indicative of potentially severe weather imminent.

The wind vane radiating from the centre dot indicates direction the wind is coming from; the “feathers”, the strength of the wind, in knots. There are two scale lengths to the feathers, long and short. A long feather is 10 knots, a short feather, 5 knots. The vane itself represents a 5 knot breeze.

The top left number in white is the reported temperature in degrees Fahrenheit. The lower green number is the dew point. The upper right number is the last 3 digits of the atmospheric pressure in kilopascals, ie, for Fergus, “237” is 1023.7 kpl.

Other chart symbols may show from time to time (we have no control over what comes down the METAR feed) – this chart link will show the many symbols which may turn up, depending on what’s on the feed at the time.