Highway Code Rule 237
Keep your vehicle well ventilated to avoid drowsiness. Be aware that the road surface may become soft or if it rains after a dry spell it may become slippery. These conditions could affect your steering and braking. If you are dazzled by bright sunlight, slow down and if necessary, stop.
Section: Driving in adverse weather conditions (rules 226 to 237) Subsection: Hot weather (rule 237)