Description of the Polar Frequency tool

The Openair Polar Frequency tool has been designed to visualise the distribution of wind speeds and directions at a site, in a way similar to a traditional wind rose, but is enhanced to also present the variation of pollutant variables. The tool bins pollutant data by wind direction and wind speed which form the angle and radius, the colour of each bin then represents the chosen statistic for the pollutant data falling within that bin. A wide range of statistics can be considered.

The pollutants measured at each site along with modelled wind speed are shown in the variables box once a site has been selected from the drop-down list of site names at the top of the page. Polar frequency plots can only be produced for one site and one pollutant (or modelled wind speed from the UK air quality forecast) at a time on this website.

Data can be plotted over user defined date ranges by clicking on the drop-down lists containing the day, month and year and using these to change the start and end dates accordingly. Data can be chosen on an annual basis by highlighting the 'Specific Years' option and using the year drop-down menu to alter the start and end year. The reliance of the tool on having wind direction and wind speed data means that the time range for plotting is restricted to that where modelled wind direction and wind speed from the UK air quality forecast are available.

The Type menu allows the user to split the data in several ways. The default option will produce a single plot using the entire data over the selected time range. Other types include 'Hour' (splitting the output into plots by hour of the day), 'Weekday' (for plots by day of the week), 'Month' (for plots by month of the year), and 'Season' (for plots split into normal seasons).

The Statistic drop-down menu allows the choice of the statistic that is applied to each wind speed and direction bin. Choices of 'Frequency' (the default option), 'Mean', 'Median', 'Maximum', 'Standard Deviation' and 'Weighted Mean' are available. The output in each wind speed/direction bin provides does provide the absolute value for the statistic for each bin segment; this is in contrast to the polar plot tool where smoothing is applied. The 'Weighted Mean' option is generated from the concentration multiplied by the frequency at which it occurs and can be useful to highlight which wind speed and directions contribute most strongly to the mean level.

The Colours drop-down menu allows the user more control over the colour scheme used to plot which can be useful to highlight the trends in the chosen statistic. A variety of colour palettes are available.