Description of the Polar Plot tool

The Polar Plot tool produces polar plots of pollutant concentrations by wind speed and wind direction. To do this on this website, the tool includes modelled wind speed and wind direction from the UK air quality forecast for each monitoring site by default. The variation in concentration is plotted as a continuous smooth surface, modelled using a statistical smoothing approach called a Generalised Additive Model (GAM), for further detail see the Openair manual. Polar plots are useful to gain a quick graphical representation of the relationship between pollutant concentrations and the meteorological conditions. This can be useful in identifying potential sources of pollution affecting the location, for example particle suspension is increased at higher wind speeds.

The pollutants measured at each site 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 plots can only be produced for one site and one pollutant 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 both wind direction and wind speed data means that the time range for plotting is restricted to that where modelled wind speed and direction from the UK air quality forecast is available.

Type 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' (for hour of the day), 'Month' (for months of the year), and 'Season' (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 'Mean' (the default option), 'Median', 'Maximum', 'Frequency', 'Standard Deviation' and 'Weighted Mean' are available; because of the smoothing applied the output provides an indication the concentration trends but not the absolute concentration levels. 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 concentration trends. A variety of colour palettes are available.