It was the length of time it went on for, in particular the drought. Scorched grass everywhere.
I remember it well as it was the year of my final exams at Uni.
Do you remember the ladybirds?
Bearing in mind I can't take the heat and while it was hot I don't remember it feeling stupidly hot

As I recall it possibly wasn't too humid and it did often go cooler overnight so you got some respite from the heat, I do remember getting up to go to school and it actually feeling a bit chilly early on until the sun really got going
I also remember my Dad kept going on about the Azores High that was influencing our weather, apparently if you get high pressure over the Azores it often results in the UK getting a spell of good weather and this particular high pressure system was only small but just stayed put for ages.
The weather forecasters would come on and say there was a low pressure system coming in to challenge it and my dad who was always prone to being negative would start his doom and gloom "Well that's it now, there's no way it'll stay good" but somehow it kept batting the big lows away

Didn't even know what Air conditioning was back then, remember as a kid having a day out at the beach and then having to get into a car that had been shut up and sitting all day in the blazing sun.
Black plastic / vinyl seats and sitting on them with bare sunburnt legs

We just got on with it, these days kids would be taking their parents to court for child abuse
