It's August as I type this article. Here in the UK it's high Summer and we're in the middle of a heatwave. The recent run of hot weather has also been accompanied with high humidity levels and frequent electrical storms. 

Several runners I've spoken to have mentioned how much harder they feel it is to run in the humid weather. Some have questioned whether they're losing fitness - because they simply can't maintain the sort of pace that they would usually.

I've had to explain that they're not alone in feeling the way they do. The reason it's felt harder is because it has been harder - for everyone.

In this article I'm going to spell out why running in high heat and humidity feels so much harder. 

High humidity can be nasty to run in. Because it can sap your energy, makes it difficult to breath and leaves you feeling as if you're drowning in your own sweat. 

I've written about running in the heat before. But my previous blog article only concentrated on the heat aspect. Humidity can make things a whole lot worse than just heat alone. 

Reading my previous article along with this one on electrolytes and this one about mosquitos - will provide you everything you need to know about running in the heat this Summer. 

Running when it's Hot & Humid

Sweating is our primary method of cooling our bodies. When humidity is high, it means that the air is already quite saturated with moisture. So it doesn't readily absorb any more. That means our sweat will not evaporate as quickly. Which means we don't cool down as effectively. As a result our core body temperature can rise.

This puts additional energy loads on our bodies, so that things we can do easily in dryer/less humid conditions require more energy when humidity rises. Because of this it can feel a lot hotter than the reading on the thermometer suggests, as the heat index table below illustrates.

Heat Index Chart

Apparent Temperature

As noted in the table above apparent temperature is how hot it feels when you take air temperature and relative humidity into account. Wind speed will also come into play as wind will help evaporate sweat. Where as if there is no breeze it will feel even more uncomfortable.

Generally relative humidity needs to get above 40% before it starts to feel hotter than the reading on a simple thermometer says it is. But when humidity levels get above 40%, it's quite possible that your running performance will suffer.

As an example last week there were highs of 32°C (90°F) with relative humidity of 80%. That would make it feel like 45°C (113°F). That's the sort of temperature you get in deserts. It also sits firmly in the orange zone, making running fast a bad idea. But if the relative humidity was only at 20% putting a lot of effort into a good interval session would be manageable.

Yellow Zone - when it feels like it's 32°C (90°F) or above it's possible to get heat cramps whilst running.

Orange Zone -  When it feels like 41°C (105°F) getting heat cramps and suffering from heat exhaustion whilst running are likely and developing heat stroke is a possibility.

Red Zone - as the colour suggests this is the DANGER zone. When it feels like 54°C (130°F) running is almost a dead cert to result in heat stroke.

Heart Rate

Another tactic our bodies use to attempt to dissipate body heat is sending blood to our skin for 'cooling' by the air. This is why we 'flush' or become red and blotchy. Flushing along with the demands of exercise result an increase in heart rate. Another factor at play with increased heart rate is dehydration. Because the more we sweat fluids out - the less fluid we have in our bodies. So there is a reduction in blood plasma volume. Making our blood thicker and harder to pump around. This makes it the heart work much harder increasing the heart rate further.

Relative humidity levels above 60% can increase your heart rate by 10 beats a minute or more. So if you train using a heart rate monitor, you'll need to take that into account and appreciate the effects that heat and humidity has on you. Or just ditch the heart rate monitor and train by feel - using RPE or Rate of Perceived Effort instead.

Sweat Rate

As previously mentioned humid air doesn't readily accept more moisture. So air drying your clothes will take longer on humid days. This also means that our sweat rates increase too, as our bodies attempt to keep us cool.

Increased sweat rates, increase the chances of getting dehydrated or developing electrolyte imbalance. Because electrolytes are lost through our sweat. So we need to make sure that we stay hydrated, but that we're also replenishing our electrolytes too. If we fail to do this, especially whilst running, we risk heat-illness.

Let's take a look at what that might entail...

Heat Cramps

Cramps

Heat cramps are a form of exercise associated muscle cramping. They can be sudden and very painful. Runners most often experience them in the major leg muscles (quads, hamstrings, calfs). Though they can also affect the arms, back and sometimes the stomach and digestive tract too.

Whilst the exact causes of heat cramps are unknown. It is understood that fatigue, dehydration and electrolyte losses play a major role in creating them. So understanding your sweat rate, drinking to replace lost fluids/electrolytes during exercise, performing regular strength and conditioning exercises and heat acclimation should all help avoid them in the first place.

Though they can ruin a run or a race, heat cramps are not serious. You can quickly recover from them with rest (in a cool or air-conditioned room if possible), stretching of the muscle, and replacement of lost fluids and electrolytes.

Heat Exhaustion

Stepping up a notch from heat cramps, heat exhaustion can come about if you're core temperature rises and you stop being able to regulate your body temperature. Like with heat cramps, dehydration and electrolyte imbalance are also factors.

The symptoms of heat exhaustion include;

  • headache
  • dizziness and confusion
  • nausea and loss of appetite
  • pale, clammy skin & sweating profusely
  • cramps in the arms, legs and stomach
  • fast breathing or pulse
  • raging thirst

If not treated promptly heat exhaustion can develop into heatstroke. So if you start developing any of the above symptoms whilst running. It's best to stop immediately, get yourself to a cooler spot, drink plenty of cold sports drinks containing electrolytes. If you don't improve within half an hour or so, or start to get worse - seek medical attention.

Heat-Exhaustion

Heststroke

The next level of heat-illness up from heat exhaustion is heatstroke, which can be a life-threatening condition.

Symptoms of heatstroke include;

  • feeling unwell after half an hour of resting in a cool place and drinking plenty of water/electrolytes
  • not sweating even though you feel too hot
  • shortness of breath or rapid breathing
  • confusion
  • a fit (seizure)
  • loss of consciousness

Heatstroke can be very serious if not treated quickly. Someone suffering from heatstroke will most likely need urgent medical attention and/or a trip to hospital.

Top Tips for running in hot humid conditions:

  • Change Your Timing - adjust the time of day that you run. First thing in the morning can be much cooler and make running a more pleasant experience.
  • Pre-cool - plan ahead and put some drinks in the fridge, or add ice to your drinks bottle or hydration backpack/bladder. You can also do the same with bits of kit. Like caps or headbands.
  • Hydrate - make sure you start the run well hydrated and take enough liquids with you to cover up to 80% of the sweat loss during your run + a bit extra to account for higher humidity/higher sweat rate.
  • Slow Down - You will be working harder. So to avoid over-cooking/over-training slow down. Also be prepared to stop if need be.
  • Run by Feel not pace or heart rate - use RPE (Rate of Perceived Effort)
  • Be Flexible - just because your training plan calls for a hard interval session. It doesn't mean that it's written in stone. If you can swap some of your workouts around during the week and do the hard workouts on cooler/less humid days. Do it and keep your efforts easy when the temperature and relative humidity levels rise.
  • Wear Technical Wicking Clothing. It may be tempting to wear as little as you can get away with and not get arrested for public indecency. 

But as counter-intuitive as it may seem you'd be better off wearing some fabrics with decent wicking properties. You'll feel more comfortable as they will move the sweat away from your skin quicker and keep you dryer. Plus they'll protect you from the sun. So you won't need to apply as much sun-screen. Which can block your pores and make it difficult for you to sweat freely.

Caffeine and Heat

There have been several studies (conducted on cyclists) that have demonstrated that caffeinated sports drinks give a performance boost in hot weather.

The main reason for this I suspect is that caffeine helps to lower the level of perceived exertion. In other words it can make hard exercise seem easier.

But I would caution runners against using caffeinated drinks in hot and humid conditions. Because there's a chance they could lead you to 'over-exert' . Plus the quantity of caffeine in some of the drinks available on the market is quite high. And if you're not used to drinking very strong coffee or are quite sensitive to caffeine. Glugging a caffeinated sports drink in the latter stages of a long hard training session could make you feel quite unwell.

It's likely to make your heart beat a lot faster too. Which could be rather disconcerting.

It's very easy to overtrain if you push yourself too hard during hot and humid spells. Make sure you don't fall into that trap. Make a note of the humidity levels on the weather forecast and adjust your training accordingly.

Learn to Run by Feel. Use RPE and don't bother with pace or heart rate.

The risk of being struck by lightning is higher too, as electrical storms are far more likely to happen when the weather is hot and humid - Just saying...

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About the author 

Coach D

Hi, I'm Dave. I'm a UK Athletics qualified and licensed Coach in Running Fitness (CiRF), Endurance Event Group Coach and Certified Running Technique Coach. I coach groups and individuals of all abilities both online and in person.I particularly enjoy coaching beginner and improver runners in the 40+ age range.I'm also a regular recreational runner and I've been competing in races from 5k to marathon distance for over 30 years.

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The Able Runner


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