5 Tips to Help You Run Your Best Marathon on a Hot, Humid Race Day

Heat and humidity are performance killers when it comes to long distance running. This holds especially true with an event like a marathon.

So what do you do when race day gets close and the weather looks terrible?

Do you cry and just stay in bed? No. Do you defer to another race? Possibly, if you really care about hitting a specific time.

But if you approach the race the right way, you can salvage a decent performance on a hot, humid day. On the other hand, if you approach the race the wrong way, you’ll be miserable. And quite possibly a failure.

So here are a few quick tips to make sure you make the right choices – and come out the other side with a performance you can be proud of.

Why is Heat and Humidity So Bad for Running?

First, let’s think about why heat and humidity are so bad for your performance as a runner.

When you run, your body generates a lot of heat. It’s normal, and your body is pretty good at dissipating that heat.

There are three main things that help you lose excess heat – the difference in temperatures between your body and the air, the evaporation of your sweat, and the wind flowing across your skin.

If it’s cold outside, you’ll dissipate heat quickly and easily. If it’s 30 degrees outside, you’ll feel warm while you’re running. But as soon as you stop, you’ll feel cold. The air temperature itself is enough to let the heat radiate away into the world.

But when the air temperature rises and approaches your body temperature, this just doesn’t work. Temps in the 80’s or 90’s are so close to your ambient body temperature that your extra heat has nowhere to go.

Sweating can help. Your body’s primary way to rid itself of excess heat is to perspire and then let that perspiration evaporate. Through the magic of science, evaporating water also has a cooling effect. If it’s dry out, this is great. It’s why people say a “dry heat” isn’t so bad.

And it’s true. When your sweat can evaporate easily, you can endure hotter temperatures. But when the humidity nears 100%, you’ll quickly get miserable. Instead of evaporating, the sweat clings to your body, soaks your clothes, and leaves you hot and wet. Don’t believe me? Here’s a description of what it’s like to run in Florida.

Wind can also help. It can aid in the evaporation of sweat and the movement of the air itself also helps in heat dissipation. So even if you’re stuck running in the heat and humidity, pray for some wind.

At the end of the day, all of this – heat, humidity, and/or lack of wind – contribute to making it harder for your body to dissipate heat. When that happens, your body starts to overheat more easily. You get dehydrated. Your rate of perceived exertion goes up.

Ultimately, you’ll find it impossible to perform in the same way that you could on a nicer day.

So How Do You Beat the Heat?

First, acknowledge that you can’t win. You must adjust your approach and run a different race that accounts for the heat. With that in mind, using the following tips, you can have a decent day:

  • Run Slower
  • Watch Your Heart Rate
  • Hydrate – But Expect Dehydration
  • Wear Less Clothes
  • Cool Yourself Artificially

Adjust Your Plan and Run Slower

First and foremost, you need to re-evaluate your pacing plan and pick a slower goal. There’s really no way around this. Heat and humidity will slow you down, and if you don’t slow down first … they’ll slow you down later.

So how fast should you slow down?

That depends. You can find many calculators on the internet to adjust efforts by temperature and humidity. This research study into race performance has a complex flow chart to help you identify a specific reduction.

But generally, you’re going to be looking at at least a few minutes for moderate temperature – and 5 or 10 (or more) minutes in worse conditions. Everyone reacts a little differently to heat and humidity, so there’s really no single answer. But over time, you need to learn how to make adjustments for yourself.

The key thing to remember is that the heat and humidity increase the effort your body requires to move at a certain pace. In a perfectly paced marathon, you’ve dialed into the precise effort you can sustain for 26.2 miles. If the weather shifts that even 1% or 2%, you’re going to be in for pain, suffering, and failure. So you need to dial in a modified goal.

When I ran a marathon in Jersey City, in moderate but humid temps, I didn’t do that. I stuck with my initial, aggressive pacing plan. I crashed and burned.

When I ran my next marathon at Erie, it was a little warmer and just as humid. This time, I modified my expectations. Instead of aiming for a goal of 3:05 – which should have been perfectly attainable in good conditions – I dialed it back to a more conservative 3:10. This turned out to be a perfect plan. Despite fading slightly after 20 miles, I came in under my goal time.

Watch Your Heart Rate More than Your Pace

Although it helps to modify your race expectations, you should also pay particular attention to your heart rate.

Early on, your rate of perceived exertion may be deceiving. You’ll feel good. It takes a while for the heat to catch up to you. But your heart rate doesn’t lie. Know how you perform in a marathon intensity run, and target that same heart rate.

I looked over my past marathons, all of which involved some slowing in the second half, and I realized that 160bpm seemed to be a threshold for me. Below that, a pace was sustainable. Above that, and I eventually started to fade. I paid attention to my marathon paced runs in training, too, and I noticed that my heart rate for them was typically in the high 150’s.

So as I prepared for my latest race, I also aimed to keep my heart rate below 160 for the first half. Although I felt great, and I could have pushed the pace, I kept things reigned in. That’s because I looked at my watch and saw my heart rate sitting around 158 – right in the zone, but with little room to spare.

In the second half of the race, the heat, humidity, and dehydration will eventually catch up to you. When that happens, after an hour or two, expect your heart rate to rise. That’s ok, because you can sustain that for a while. You just can’t sustain it for the entire race.

If you start off with a modified pace and your heart rate gets into the danger zone in the first few miles, you need to dial things back sooner than later. Because once you’ve run an hour at that intensity, it’s too late to reign it in.

So keep an eye on your heart rate, and use it to keep you honest about what pace is really sustainable in your race conditions.

It helps to have a good running watch. Check out this advice on how to pick the right Garmin, if you don’t already have one.

Hydrate Enough – But Expect to Get Dehydrated

Hydration strategy is an important part of running in hot, humid conditions. But that strategy doesn’t simply mean drink as much water as you can.

There is such a thing as too much water. It can be counterproductive, and in the extreme it can be downright dangerous. Drinking too much water can dilute the amount of sodium and other electrolytes in your blood, leading to a dangerous condition known as hyponatremia. In severe cases, it can be fatal.

The key is to consistently take in a moderate amount of fluids while also taking in electrolytes. It’s particularly important to take in electrolytes on humid days, because the excess sweating causes you to lose excess salt. If you only drink water, you’ll replace the liquids but not the salt.

You should already have a strategy for nutrition, and that probably includes gels. These should have electrolytes in them, so it makes sense to wash them down with water. But at the other aid stations, I’d suggest taking Gatorade (or whatever sports drink they offer). How often you stop is up to you, but every two miles is probably good for warm days and every mile (if there are that many aid stations) on hot days.

While you should be taking on liquids, you should expect to still get slightly dehydrated. It’s inevitable.

There’s a common piece of outdated advice that you should aim to drink enough water to replace your water loss during a run. If you weigh yourself before and after a run, you should weigh the same. This is bad advice, and it will lead to you consuming too much water.

Instead, you should expect to lose a few pounds – probably at least 3 to 5 over the course of a marathon. Maybe a little more. On a hot, humid day, you will lose fluids. You will end the race dehydrated. But as long as this is mild, you’ll be fine.

Because what really matters isn’t the amount of fluids in your body – it’s the balance of electrolytes in your blood. When you drink too much water (or even sports drink) in an effort to counteract dehydration, you throw off that balance.

Things will get back to normal after the race. Just give it time.

Wear As Little as Possible

Clothing can exacerbate the effects of heat in a few ways.

First, anything you wear can get wet. On a humid day, it will get soaked before your race is over. That means more weight that you have to carry.

A commenter on Reddit recently asked if he should get new shorts for a hot, humid marathon he had coming up. I advised him to absolutely do so. He had been running in longish, 8 inch shorts. That’s fine, if it’s cold out. But when they get wet, they will be impossible. I learned this the hard way early on in my running career when I used to wear long shorts and I got caught out in a rainstorm. Miserable.

Now, I prefer 3-inch split shorts. It took some time to get used to them, but I rarely notice during a run when they get wet. On humid days, I often end a long run with them being soaked through. But they’re so small that I hardly notice.

Clothing also covers up your body and prevents air from getting in contact with your skin. This prevents heat dissipation through the air and through wind.

This is where your top becomes important. Wear a tank top, a sports bra, or no shirt if it’s possible. Avoid anything with sleeves that will cover up your shoulders or upper arms. Expose whatever skin you can.

Finally, the color of your clothing also matters if it’s going to be sunny out. Dark colors absorb sunlight and heat. That’s … bad. White or reflective colors do a better job of reflecting heat back into the air instead of absorbing it.

If it’s going to be overcast, this doesn’t matter much. But if the sun is going to be out, definitely avoid dark colors and opt for something light or reflective.

Figure Out How to Cool Yourself Artificially

Finally, you should figure out some way to cool yourself down artificially. Your body’s usual tactics aren’t going to cut it on a hot, humid day. So you need to help out.

How do you do this? Most solutions are going to involve some combination of water and/or ice.

A simple tactic is to pull into an aid station and dump a cup of water on your head. Water conducts heat better than air, so even if the water is warm it’ll be cooler than your body. It will feel good, and it will help cool you off a little bit.

On particularly hot days, a race might offer a sponge in an ice bath. This helps – because cold water is obviously much better at cooling you off than hot water. If the race isn’t going to provide this, you might be able to get some assistance from a family member who is strategically placed along the course. Have them ready with some ice cold water.

Another take on this is to dunk a hat in ice water and then put it on. I personally can’t stand running with a hat, but this should be effective. It keeps the cold water in contact with your body longer, rather than letting it run off. This is more common on looped courses, when you can have someone strategically placed to provide the ice water.

If you’re lucky, there might be some mist along the course. This is probably more common on local 5k’s, when families break out the garden hose. But big city marathons have been known to put mist attachments on fire hydrants. If you can find a cloud of mist to run through … do it.

Finally, you may be able to do some amount of pre-cooling. Try one of the above tactics before the race. Prevention is, of course, the best medicine. You’ll still heat up, but if you keep yourself cool in the minutes leading up to the race you can fend off the inevitable for at least a little while.

How Do You Deal With Running a Race On a Hot Day?

If I had a choice, I would always opt for the cool day. When I ran Philly, it was 30 degrees and windy. I’ll take that any day over warm and humid. I’d rather shiver than sweat.

But sometimes, you don’t have a choice. If you have a goal race – maybe a Major like Boston or Chicago – there’s always a chance that the weather will not cooperate. And switching to another race may just not be an option.

If you follow these tips – especially modifying your expectations and hydrating appropriately – you can at least make the best of a bad situation. And hopefully you can avoid being one of the casualties on the side of the road, walking the last few miles or being taken to the medical tent.

What are your favorite tips with dealing with the heat on race day? I’d love to hear them in the comments.

If you’re still training for your marathon, check out these tips for dealing with heat during marathon training. It’s a little different than dealing with it on race day.

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