Fire crews were called out to 157 grass fires in a week following the heatwave and rising temperatures - with a number started 'on purpose'.
Merseyside Fire & Rescue Service says on average, it has dealt with more than 20 grass fires a day since 14 July.
The emergency service believes a number of the fires were started on purpose.
It said: "Deliberate fire setting not only affects people's ability to enjoy our parks & woodlands, but it can also have an effect on wildlife."
In a tweet, they encouraged parents to speak to their children about the consequences of deliberate fire setting.
It said: "It might seem like a bit of harmless fun or 'a laugh' but it's arson - a serious offence that can lead to a criminal record".
The country experienced its hottest ever day on record on Tuesday after Britain was issued the first ever red extreme heat warnings.
Merseyside Fire & Rescue Service has reminded people even though temperatures have dropped, the ground is still extremely dry.
It is advising people to leave their barbecues at home, and warning then not to light fires in woodlands and to dispose of cigarettes properly.