How Hot Is Too Hot?

Most outdoor instructors have a pretty good feel for when it's too cold to run an activity.

Strong winds, driving rain, freezing temperatures and winter conditions are familiar territory. We've spent years learning how to manage those risks, and most of us have a clear idea of when a plan needs to change.

For many instructors in the UK, regularly dealing with temperatures in the high twenties and thirties is a relatively new phenomenon. As summers become warmer, a question that I have been asking myself more often is: how hot is too hot?

Climbing in the shade and a sun hoody to keep from burning.

The HSE describes heat stress as what happens when the body is unable to regulate its core temperature effectively while working in hot conditions, leading to strain on the heart and circulation and an increased risk of heat-related illness. (hse.gov.uk)

The UK's Health and Safety Executive doesn't set a maximum temperature for working outdoors because heat stress depends on far more than the number on the weather forecast. Humidity, direct sun, wind, the amount of effort you're putting in and even the colour of the rock can all make a difference.

Anyone who has climbed on a south-facing crag in North Wales on a still summer day will know this already. A forecast of 25°C on a humid day can feel surprisingly hard work when you're standing on a ledge with no shade, no breeze and the sun reflecting back off the rock.

One reason for this is that your body relies on sweat evaporating to keep cool. When the air is humid, sweat doesn't evaporate as efficiently, so your body has to work much harder to regulate its temperature.

This is where wet-bulb temperature comes in. Wet-bulb temperature combines heat and humidity into a single figure that gives a better indication of how difficult it is for the body to cool itself. Two days with the same air temperature can feel very different depending on how humid the air is. A hot, dry day often feels more manageable than a slightly cooler but very humid one because sweat can evaporate more easily.

Interestingly, climbing isn't the only outdoor activity grappling with hotter conditions. Wimbledon uses a formal heat rule based on Wet Bulb Globe Temperature (WBGT), which takes into account humidity and solar radiation as well as air temperature. Players are allowed an extended break when WBGT exceeds 30.1°C. Football also uses WBGT guidance, with cooling breaks and match modifications introduced as conditions become more demanding. The common theme is that decisions are increasingly being based on WBGT and how that contributes to heat stress rather than air temperature alone.

Heat exhaustion is the body's warning sign that it is struggling to keep cool. Symptoms can include heavy sweating, dizziness, headache, muscle cramps, nausea, unusual fatigue and difficulty concentrating. Someone with heat exhaustion will often look unwell but will usually remain alert and able to hold a conversation.

Heat stroke is serious and is a medical emergency. The body's temperature regulation starts to fail and the person may become confused, disorientated, unsteady on their feet or behave unusually. They may stop sweating. In severe cases they may become unconscious. At this point immediate cooling and emergency medical assistance are required.

In the UK, health alerts are often issued when temperatures move into the high twenties and low thirties. That doesn't mean climbing becomes unsafe at 30°C, but it is the sort of weather where heat-related illness becomes a realistic consideration, particularly for people who are carrying equipment, spending long periods in direct sun or are not used to exercise in hot conditions.

For climbing days, I tend to pay more attention to the conditions at the crag than the headline temperature. A breezy North facing mountain route in the shade can feel comfortable on a 30°C day, while a sheltered single-pitch venue can become quickly unpleasant.

If the forecast is hot, a few simple adjustments go a long way. Start earlier, choose a venue with shade, carry more water than you think you'll need and take sensible breaks. I also find it comfortable to wear light weight, light coloured, trousers and a synthetic hooded base layer sun proof layer. rather than the classic shorts and cotton t shirt. Don’t forget the sun cream and most importantly, don't be afraid to change plans.