What happened
June 2016 was wet and unsettled across much of southern England, with day after day of showers and thunderstorms. At this station 99.2 mm of rain fell, on 23 of the 30 days. June 2016’s own page shows how it compares with other Junes.
A downpour, and a day of steady rain
Two of the wettest days had their rain in very different ways. On 12 June, 24.4 mm fell, 17.8 mm of it in the single hour to Sun 12 Jun, 18:45: a thunderstorm’s downpour. On 20 June, 16.6 mm fell through the morning, the most in any hour 8.3 mm, ahead of a likely warm front at Mon 20 Jun, 13:30: a warm front’s wide band of cloud brings its rain before the front itself arrives.
They’re the two ways rain forms. Showers and thunderstorms grow when warm, moist air near the ground rises on its own, like bubbles in a pan: it builds towering clouds that dump a lot of rain in a short time, on a small patch of ground. Steady rain comes from wide sheets of cloud where air is lifted gently over a large area, usually along a front: lighter, but it lasts for hours. More on rain.
Why downpours are so local
A thunderstorm’s heaviest rain falls from a cloud only a few kilometres across, so one street can flood while the next stays dry. That shows on this site: its rain history comes from the Environment Agency’s gauge 3.6 km away (the station’s own is sheltered), and on a thundery day a downpour can hit one gauge and miss the other. The steady rain of a front falls much more evenly.
Muggy air: the storms’ fuel
Thunderstorms feed on humid air. As moist air rises and cools, its water vapour condenses into cloud and gives up heat, which drives the air up faster still. The dew point measures how much moisture the air holds: above about 16°C it feels muggy. Here it reached 19.3°C at Thu 23 Jun, 14:05, and the warmest night, Thu 23 Jun, never fell below 16.8°C. That was 23 June, the day of the EU referendum, when thunderstorms flooded parts of London and the south-east; 10.2 mm fell here. More on dew point.
What to look for in the charts
- Rain: tall, narrow bars for the downpours, against longer runs of small ones for steady rain.
- Temperature and dew point: the dew point high and close to the temperature on the muggiest days.
- Pressure: no deep lows: showers and thunderstorms don’t need them.
- Wind: mostly light.