The image below shows the results of observations over a day. Coloured arrows that indicate horizontal wind direction are plotted against altitude or height in kilometres (up the page) and time (along the page). An upward pointing arrow indicates wind blowing toward the north, one pointing to the right indicates a wind blowing toward the east (as on a compass dial). The lack of an arrow indicates that no data was obtained for that height and time. The colour of the arrow indicates the wind speed as shown in the scale bar on the right hand side.
Wind speeds of 50 m/s (180 km/h) are not uncommon at these heights! The east-west winds (around the planet) are usually (not always) greater than the north-south winds (pole to pole). Although the radar measures returns from 40 km to 108 km, the analysis does not always yield an answer that passes various quality checks. The range over which we are currently able to measure winds is indicated by the presence of arrows in this image. The base height of observations usually moves downwards in the southern winter months.
A few days
A few days of the Zonal (east-west, eastward positive) and Meridional (north-south, north positive) wind velocities at a height of 86 km are presented below. The horizontal axis is time, while the vertical axis is velocity in m/s. These values have been averaged into hourly blocks. Solar heating of the ozone in the stratosphere (around 25–55 km) and water vapour in the troposphere (up to approximately 12 km) brings about variations in the wind in the mesosphere (86 km) that are of 1-day period and are subharmonics of a one day period. These are known as atmospheric tides. Tidal variations of the wind (with periods of 24 hours and 12 hours) can be seen in the velocity time series. There are also variations of shorter periods due to atmospheric gravity waves.
Over the seasons, the circulation of this part of the atmosphere changes significantly. As shown in the image below, the wind at 80 km blows toward the east during the southern winter (positive zonal velocity) and toward the west (negative zonal velocity) during the summer. The north-south winds are weaker but are more variable during the winter than through the summer. This is because wavelike motions that flow around the planet are only able to reach this height during the winter.
MFSA radar signal to noise ratio
The signal-to-noise ratio is an important parameter in deciding if a wind determination is possible. During the summer, when the sun is illuminating the sampled height range throughout the day, there is little variation in the signal-to-noise ratio. This is less true in winter and during periods of high solar activity.