polarFreq: Function to plot wind speed/direction frequencies and other...

In openair: Tools for the Analysis of Air Pollution Data

Function to plot wind speed/direction frequencies and other statistics

Description

polarFreq primarily plots wind speed-direction frequencies in ‘bins’. Each bin is colour-coded depending on the frequency of measurements. Bins can also be used to show the concentration of pollutants using a range of commonly used statistics.

Usage

polarFreq(
  mydata,
  pollutant = NULL,
  statistic = "frequency",
  ws.int = 1,
  wd.nint = 36,
  grid.line = 5,
  breaks = NULL,
  labels = NULL,
  cols = "default",
  trans = TRUE,
  type = "default",
  min.bin = 1,
  ws.upper = NA,
  offset = 10,
  border.col = "transparent",
  key.title = paste(statistic, pollutant, sep = " "),
  key.position = "right",
  strip.position = "top",
  auto.text = TRUE,
  plot = TRUE,
  key = NULL,
  ...
)

Arguments

mydata	A data frame minimally containing ws, wd and date.
pollutant	Mandatory. A pollutant name corresponding to a variable in a data frame should be supplied e.g. pollutant = "nox"
statistic	The statistic that should be applied to each wind speed/direction bin. Can be one of: "frequency": the simplest and plots the frequency of wind speed/direction in different bins. The scale therefore shows the counts in each bin. "mean", "median", "max" (maximum), "stdev" (standard deviation): Plots the relevant summary statistic of a pollutant in wind speed/direction bins. "weighted.mean" will plot the concentration of a pollutant weighted by wind speed/direction. Each segment therefore provides the percentage overall contribution to the total concentration. Note that for options other than "frequency", it is necessary to also provide the name of a pollutant.
ws.int	Wind speed interval assumed. In some cases e.g. a low met mast, an interval of 0.5 may be more appropriate.
wd.nint	Number of intervals of wind direction.
grid.line	Radial spacing of grid lines.
breaks, labels	If a categorical colour scale is required then breaks should be specified. These should be provided as a numeric vector, e.g., breaks = c(0, 50, 100, 1000). Users should set the maximum value of breaks to exceed the maximum data value to ensure it is within the maximum final range, e.g., 100–1000 in this case. Labels will automatically be generated, but can be customised by passing a character vector to labels, e.g., labels = c("good", "bad", "very bad"). In this example, 0 - 50 will be "good" and so on. Note there is one less label than break.
cols	Colours to use for plotting. Can be a pre-set palette (e.g., "turbo", "viridis", "tol", "Dark2", etc.) or a user-defined vector of R colours (e.g., c("yellow", "green", "blue", "black") - see colours() for a full list) or hex-codes (e.g., c("#30123B", "#9CF649", "#7A0403")). See openColours() for more details.
trans	Should a transformation be applied? Sometimes when producing plots of this kind they can be dominated by a few high points. The default therefore is TRUE and a square-root transform is applied. This results in a non-linear scale and (usually) a better representation of the distribution. If set to FALSE a linear scale is used.
type	Character string(s) defining how data should be split/conditioned before plotting. "default" produces a single panel using the entire dataset. Any other options will split the plot into different panels - a roughly square grid of panels if one type is given, or a 2D matrix of panels if two types are given. type is always passed to cutData(), and can therefore be any of: A built-in type defined in cutData() (e.g., "season", "year", "weekday", etc.). For example, type = "season" will split the plot into four panels, one for each season. The name of a numeric column in mydata, which will be split into n.levels quantiles (defaulting to 4). The name of a character or factor column in mydata, which will be used as-is. Commonly this could be a variable like "site" to ensure data from different monitoring sites are handled and presented separately. It could equally be any arbitrary column created by the user (e.g., whether a nearby possible pollutant source is active or not). Most openair plotting functions can take two type arguments. If two are given, the first is used for the columns and the second for the rows.
min.bin	The minimum number of points allowed in a wind speed/wind direction bin. The default is 1. A value of two requires at least 2 valid records in each bin an so on; bins with less than 2 valid records are set to NA. Care should be taken when using a value > 1 because of the risk of removing real data points. It is recommended to consider your data with care. Also, the polarFreq function can be of use in such circumstances.
ws.upper	A user-defined upper wind speed to use. This is useful for ensuring a consistent scale between different plots. For example, to always ensure that wind speeds are displayed between 1-10, set ws.int = 10.
offset	offset controls the size of the 'hole' in the middle and is expressed on a scale of 0 to 100, where 0 is no hole and 100 is a hole that takes up the entire plotting area.
border.col	The colour of the boundary of each wind speed/direction bin. The default is transparent. Another useful choice sometimes is "white".
key.title	Used to set the title of the legend. The legend title is passed to quickText() if auto.text = TRUE.
key.position	Location where the legend is to be placed. Allowed arguments include "top", "right", "bottom", "left" and "none", the last of which removes the legend entirely.
strip.position	Location where the facet 'strips' are located when using type. When one type is provided, can be one of "left", "right", "bottom" or "top". When two types are provided, this argument defines whether the strips are "switched" and can take either "x", "y", or "both". For example, "x" will switch the 'top' strip locations to the bottom of the plot.
auto.text	Either TRUE (default) or FALSE. If TRUE titles and axis labels will automatically try and format pollutant names and units properly, e.g., by subscripting the "2" in "NO2". Passed to quickText().
plot	When openair plots are created they are automatically printed to the active graphics device. plot = FALSE deactivates this behaviour. This may be useful when the plot data is of more interest, or the plot is required to appear later (e.g., later in a Quarto document, or to be saved to a file).
key	Deprecated; please use key.position. If FALSE, sets key.position to "none".
...	Addition options are passed on to cutData() for type handling. Some additional arguments are also available: xlab, ylab and main override the x-axis label, y-axis label, and plot title. layout sets the layout of facets - e.g., layout(2, 5) will have 2 columns and 5 rows. fontsize overrides the overall font size of the plot. annotate = FALSE will not plot the N/E/S/W labels. limits sets the colour bar limits, if breaks is not used.

Details

polarFreq is its default use provides details of wind speed and direction frequencies. In this respect it is similar to windRose(), but considers wind direction intervals of 10 degrees and a user-specified wind speed interval. The frequency of wind speeds/directions formed by these ‘bins’ is represented on a colour scale.

The polarFreq function is more flexible than either windRose() or polarPlot(). It can, for example, also consider pollutant concentrations (see examples below). Instead of the number of data points in each bin, the concentration can be shown. Further, a range of statistics can be used to describe each bin - see statistic above. Plotting mean concentrations is useful for source identification and is the same as polarPlot() but without smoothing, which may be preferable for some data. Plotting with statistic = "weighted.mean" is particularly useful for understanding the relative importance of different source contributions. For example, high mean concentrations may be observed for high wind speed conditions, but the weighted mean concentration may well show that the contribution to overall concentrations is very low.

polarFreq also offers great flexibility with the scale used and the user has fine control over both the range, interval and colour.

Value

an openair object

Author(s)

David Carslaw

See Also

Other polar directional analysis functions: percentileRose(), polarAnnulus(), polarCluster(), polarDiff(), polarPlot(), pollutionRose(), windRose()

Examples

# basic wind frequency plot
polarFreq(mydata)

# wind frequencies by year
## Not run: 
polarFreq(mydata, type = "year")

## End(Not run)


# mean SO2 by year, showing only bins with at least 2 points
## Not run: 
polarFreq(mydata, pollutant = "so2", type = "year", statistic = "mean", min.bin = 2)

## End(Not run)

# weighted mean SO2 by year, showing only bins with at least 2 points
## Not run: 
polarFreq(mydata,
  pollutant = "so2", type = "year", statistic = "weighted.mean",
  min.bin = 2
)

## End(Not run)

# windRose for just 2000 and 2003 with different colours
## Not run: 
polarFreq(subset(mydata, format(date, "%Y") %in% c(2000, 2003)),
  type = "year", cols = "turbo"
)

## End(Not run)

# user defined breaks from 0-700 in intervals of 100 (note linear scale)
## Not run: 
polarFreq(mydata, breaks = seq(0, 700, 100))

## End(Not run)

# more complicated user-defined breaks - useful for highlighting bins
# with a certain number of data points
## Not run: 
polarFreq(mydata, breaks = c(0, 10, 50, 100, 250, 500, 700))

## End(Not run)

# source contribution plot and use of offset option
## Not run: 
polarFreq(mydata,
  pollutant = "pm25",
  statistic = "weighted.mean", offset = 50, ws.int = 25, trans = FALSE
)

## End(Not run)

openair documentation built on April 2, 2026, 9:07 a.m.