Here comes the rain again…

By: Natalie Harvey

British people are well renowned for their obsession for talking about the weather. This is partly because it is a “safe” topic for conversation and partly because it is really fascinating! This is especially true in the UK where our weather is so varied.

Since my son started school in 2017, I have found myself having numerous conversations about the weather and the accuracy of weather forecasts at drop off and pick-up times. These conversations are often critical of the forecasts, which I do my best to dispel in the short time I have. The UK Met Office four-day forecasts are now as accurate as our one-day forecast was 30 years ago, with over 90% of forecasts of next day temperature accurate to within 2 °C [1]. Bauer et al. (2015) [2] give a detailed description of the revolution in numerical weather prediction over the last 40 years.

Another question I hear a lot is “why does it always rain at pick up time?” Now, I live a fair way from my son’s school, so my walk is longer than most and I don’t really feel like it rains that much. However, last Wednesday, at 3pm, it did! I got very soggy. It made me think about the question again as it is something that we can test using rainfall data from the Atmospheric Observatory based on the University’s Whiteknights campus [3] which is a short walk from my son’s school.

Figure 1 – Screenshot from the Met Office weather forecasting app showing rain forecast at 3pm.

In Figure 2a the bars show the fraction of time it rains for rain aggregated over two-hour windows from the last 5 years (2015-2019). Each two-hour window is split into five-minute chunks and each five-minute chunk is counted if at least 0.2mm of rain is recorded. The black bars indicate all days, teal bars indicate summer days (June, July, August) and light blue bars indicate winter days (December, January, February).

Overall, as I thought, it doesn’t rain much per 2-hour window, with each 5-minute chunk being classified as raining just once a month on average. In general, the fraction of time it rains does increase throughout the day, with the signal strongest in the summer months. This is most likely related to the diurnal cycle of convection over land in the summer which brings heavy rain showers. The effect is not very large though, with rainfall during the afternoon around 20% more common than between 8 and 10am, school drop off time.

Figure 2 – (a) Fraction of time rain occurs throughout the day for 2015-2019 in two-hour windows. Black bars indicate all days, teal bars indicate summer days and light blue bars indicate winter days. (b) Mean rainfall per 5-minute window throughout the day when rain is recorded.

Figure 2b shows the mean rainfall (bars) for the times that it is raining in the same 2-hour windows. The grey bars indicate the 5th and 95th percentile of the observed rain at these times. These are included to give an indication of the variability of rainfall and show the highly skewed distribution of rainfall rates. The mean rainfall is reasonably uniform throughout the day with a higher value between 2 and 4pm in the summer months, again this is likely to be related to convective events. During that time window there are some large rainfall events, so it is possible that my fellow parents at the school gate memories are skewed by a few events when they (and their children) got very soggy or maybe it is the rain affecting their mood [4]. This is only a very small-scale study using a small amount of rainfall observations, but it seems to me that it doesn’t always rain at school pick up time but maybe it is best to have your umbrella ready just in case, especially in the summer.

[1] Met Office, 2020 Global accuracy at a local level. Accessed 4 December 2020, https://www.metoffice.gov.uk/about-us/what/accuracy-and-trust/how-accurate-are-our-public-forecasts

[2] Bauer, P., Thorpe, A. & Brunet, G., 2015: The quiet revolution of numerical weather prediction. Nature 525, 47–55. https://doi.org/10.1038/nature14956

[3]Reading University Atmospheric Observatory, 2018: Atmospheric Observatory. Accessed 4 December 2020, https://research.reading.ac.uk/meteorology/atmospheric-observatory/

[4] Flook,J 2019: Can’t stand the rain? How wet weather affects human behaviour. Accessed 4 December 2020, https://www.theguardian.com/science/blog/2019/mar/06/cant-stand-the-rain-how-wet-weather-affects-human-behaviour

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