Temperatures on Bristol's buses have got close to 100ºF during the hot spell.

Without air conditioning, First buses have been a sticky place for passengers and drivers this week and yesterday was no exception.

The Bristol Evening Post took a trip on a bus in the afternoon heat and, with the help of a digital thermometer, found temperatures inside hitting a sweltering high point of 37.3ºC (99.1ºF).

The average temperature during a 20-minute journey from Hengrove to the city centre was about 35ºC, well above the outdoor reading of about 27ºC.

Even after the late afternoon rain yesterday, the temperature reached 33.1ºC onboard a packed number 51 bus from Temple Meads to Knowle just before 5pm.

Forty minutes later on the return journey to Temple Meads, with fewer passengers on board, the temperature had dropped slightly to 30.8ºC.

The Evening Post's thermometer recorded the outside temperature before both bus journeys yesterday afternoon at 28ºC.

But with a fall in temperatures and heavy showers predicted for the next four days, passengers can expect to cool off soon.

Drivers have faced an even more sweltering prospect. Ordinarily their seating booth is the only part of a First bus that does have air conditioning.

But thanks to an equipment fault in certain models, the air conditioning on 69 buses in Bristol has not been operating.

First says the fault lies with "cooling packs" on 57/58 plate Volvo double deck vehicles, a problem that operators across the country are facing with the same models.

The packs provide some climate control for drivers who sit in the cab behind the assault screen.

One driver, who did not wish to be named, said: "We're not very happy, it's quite hot in the bus and uncomfortable with the heat.

"You are constantly sweating, you come off dripping sweat."

First denied claims the air conditioning had been deliberately switched off to save fuel and said it was working with Volvo around the clock to resolve these problems, fitting modified drive shafts to all the vehicles affected after they have stopped running for the day.

Alan Stewart, Unite Branch Secretary for Bristol, said: "While we recognise that there is an issue with the cooling packs in the drivers' cabs in the 57/58 plate Volvo double deck vehicles, meaning that drivers get hot, we understand that First is doing everything it can do to obtain the necessary parts from Volvo, the manufacturer, so the matter can be resolved as quickly as possible."

The company has issued advice for passengers struggling with hot weather. It says customers travelling during the hot weather should sit out of direct sunlight wherever possible; wear loose-fitting clothing; take water with them and ensure that all windows near them are open.

Should a customer begin to feel unwell they should notify the driver who will be able to summon help if necessary.

First spokeswoman Karen Baxter said: "We would like to offer our apologies to any customers struggling to cope with the hot weather.

"A few days of intense sunshine always prompt calls for buses to be air conditioned but air conditioning units are heavy and you also use more fuel, and thus generate more carbon emissions.

"We have also taken steps in the past week to ensure drivers turn off all the interior lights in the daytime to avoid adding to the residual heat in the vehicles."