Read this: Summers are generally warm and sometimes hot. London's average July high is 24 °C (75.2 °F). On average London will see 31 days above 25 °C (77.0 °F) each year, and 4.2 days above 30.0 °C (86.0 °F) every year. During the 2003 European heat wave there were 14 consecutive days above 30 °C (86.0 °F) and 2 consecutive days where temperatures reached 38 °C (100.4 °F), leading to hundreds of heat related deaths. Winters are generally cool and damp with little temperature variation. Snowfall does occur from time to time, and can cause travel disruption when this happens. Spring and autumn are mixed seasons and can be pleasant. As a large city, London has a considerable urban heat island effect, making the centre of London at times 5 °C (9 °F) warmer than the suburbs and outskirts. The effect of this can be seen below when comparing London Heathrow, 15 miles west of London, with the London Weather Centre, in the city centre.

What occasional weather event causes problems for drivers in London?
What is the answer? (If it cannot be answered, return "unanswerable")
Snowfall