A new study has suggested hot weather is a factor in how students perform in exams.

The Harvard Kennedy School in the US examined the impact of cumulative heat exposure on cognitive skill development.

It found that students scored lower when they had just experienced a hot school year than when they had just experience a cool school year.

It said low-income and minority students were impacted by heat more than others - while air conditioning in schools all but completely eliminated the impact of heat.

The authors write: "We show that only school-day exposure to higher temperatures affects test scores; hot summers and weekends have little impact on achievement and controlling for such exposure does not shrink the magnitude of impact of hot school days.

"This suggests that heat’s disruption of instruction or homework time is responsible for the observed drop in test scores."

The drop in performance is significant, the research showed.

On average, student achievement fell by the equivalent of 1% of a year's worth of learning for each additional degree Fahrenheit in temperature during the year preceding the exam.

But having air conditioning in classrooms changes everything, the research showed, offsetting nearly all of the damaging impacts.

"School air conditioning penetration reported in 2016 mitigates the adverse effect of hot temperatures substantially, such that moving from a school with no air-conditioned classrooms to a school with all air-conditioned classrooms reduces the impact by approximately 78%."

The study was based on data from 10 million American students from the school classes of 2001 to 2014 who took the PSAT exam multiple times.