Description of the Time Variation tool

The Time Variation tool produces four separate panes combined into a single plot: The plotted output shows the average variation by day of the week and hour of the day combined (the top-most pane), hour of the day (diurnal variation, shown in the lower left pane), month of the year (seasonal variation in the lower middle pane) and day of week (lower right pane) of one or more variables or at one or multiple sites over a user selected time range. The variation of a pollutant by time of day and day of week can reveal useful information concerning the likely sources at a particular site. Likewise, it allows temporal trends for particular pollutants to be examined across a range of sites.

Monitoring sites are chosen from the drop-down name list. Multiple sites can be selected/de-selected by pressing and holding the CTRL key whilst highlighting the desired sites using the left mouse button. If there are no common measurements at the sites selected, 'Sorry, no pollutants found for the data options selected' will be displayed. As multiple sites are selected the number of pollutants displayed in the variables list may decrease to reflect the extent of corresponding pollutant measurements undertaken at different sites within the network. To confirm which sites a pollutant is measured at, first choose the pollutant from the drop-down menu below. This will reduce the list of sites to only those at which the pollutant is measured. Then select the sites to be used.

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. The number of pollutants measured at each site varies across the network. Multiple pollutants can be selected/de-selected by pressing and holding the CTRL key whilst highlighting the desired pollutants using the left mouse button. Modelled wind speed and wind direction from the UK air quality forecast are available in the variables list along with the pollutants and can be selected in the same way.

The period of dates over which data is available is site-dependent and shown in the greyed-out block below the date range. This is set as the default the date range. For multiple pollutants or modelled meteorological variable selections the date range shown is, for example, the date of when the first pollutant measurement was taken to the last measurement taken at the site. Therefore the date range may not correspond to the start or end of a specific pollutant measurement at the site or the start or end of modelled meteorological data.

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. 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.

Selecting normalise allows the pollutant concentrations to be divided by their mean values. This helps to compare the shape of the temporal trends for variables on very different scales.

Selecting confidence intervals produces a shaded-line plot. The shading shows the 95% confidence intervals of the mean and the line the mean. If multiple variables are chosen over-lapping confidence intervals can render plots unreadable. For this reason the maximum number of variables that can be selected has been set at 10.