Extreme heat grips Northern Hemisphere on summer solstice

A woman watches the sun rise at the stones of the Stonehenge monument on the summer solstice near Amesbury, Britain June 21, 2017. REUTERS/Neil Hall

SINGAPORE (Reuters) - Extreme heat across large tracts of the Northern Hemisphere raised fears for crops in China, fuelled forest fires in Portugal and Russia's Far East, forced flight cancellations in the Southwest U.S., and melted tarmac on roads in Britain.

As Wednesday marked the summer solstice - the longest day of the year - forecasters said temperatures in Paris were expected to hit 37 Celsius (100 Fahrenheit), Madrid could see 38C, and London was set for 34C with warnings of thunderstorms.