You’ve probably noticed that your water bottle sweats on hot summer days, especially when it’s full of ice cold water.
Obviously it’s not water leaking out of the bottle, so what exactly is causing all those little droplets of water to form on your bottle?
Well, what you’re seeing is condensation.
Condensation forms on water bottles when the surface of the bottle is cold enough to cool the air around it past its dew point. This causes water vapor in the surrounding air to convert back to liquid water, which in turn clings to the surface of your bottle as droplets.
Below I’ll explain what exactly causes condensation to form on water bottles, why your bottles sweat much more sometimes than other times, and how you can prevent it from happening altogether.
What Exactly Is Condensation?
Simply put, condensation is the opposite process of evaporation. Instead of water turning from liquid to gas, gaseous water vapor is converted back into liquid water.
There are two reasons condensation happens:
- Air is cooled below its dew point — which is the temperature in which air becomes saturated with water.
- Or air becomes so saturated with water vapor that it’s unable to hold any more.
The former is the main reason condensation forms on water bottles. So let’s take a closer look as to exactly why.
What Causes Condensation On A Water Bottle?
Even though you can’t see it, the air around you always contains some amount of water vapor. The measurement of the amount of water vapor in the air is what’s known as humidity.
When you introduce something cold to humid air, like a nice cold bottle of ice water, it cools down the air surrounding it quickly and drops it below its dew point.
Because colder air is able to hold less water vapor than warmer air, this causes some vapor to be converted back to liquid water, which in turn clings to the surface of your bottle in the form of water droplets.
You may have heard this referred to as ‘sweating’ — which certain bottles (looking at you Nalgene) are notorious for.
Why Is There More Condensation On My Bottle On Hotter Days?
The amount of condensation that will happen depends on the amount of water vapor in the air (humidity) and (less directly) the temperature of the atmospheric air compared to the temperature of your bottle.
The hotter and more humid it is outside and the colder your bottle, the more vapor will turn back to liquid water and cling to your bottle.
This effect is exaggerated in higher temperatures for two reasons.
For starters, water molecules convert to water vapor at a much faster rate in the heat, so the air is normally more humid.
And second, as the amount of water vapor in the air increases (an increase in humidity), the higher the air’s dew point becomes.
So on hot, humid days, an ice cold bottle is able to drop the surrounding air below its (already higher) dew point much faster. And as a result, you’ll notice more condensation on your bottle.
How To Prevent Condensation On Your Water Bottle
Condensation on a water bottle can be pretty annoying.
If you’re sick of leaving rings of water on your desk or having it drip down your arm every time you take a sip, there are several ways you can stop condensation from forming on your water bottle — or at least slow it down considerably:
- Room temperature water in a water bottle won’t cause condensation to build up. Because the exterior of your bottle remains about the same temperature as the air surrounding it, it won’t cause water vapor to convert back to liquid water.
- An insulated bottle won’t sweat because the exterior of the bottle won’t cool down, no matter how cold the water is inside. Vacuum-insulated bottles in particular are great at preventing condensation because a vacuum seal insulates so well.
- Using an insulating sleeve won’t work nearly as well as an insulated bottle, but will still help slow condensation a good bit. Plus, any condensation that still happens will be absorbed by the sleeve.
For more info and a few extra tips, check out my complete guide on how to prevent condensation on a water bottle.
What About Condensation Inside A Water Bottle?
While we normally think of condensation happening on the outside of our water bottles, it can also happen on the inside — the conditions needed are just less common and, when it does happen, it’s not as inconvenient so we’re much less likely to notice.
Condensation on the inside of a bottle happens when the temperature of the air inside the bottle reaches a higher temperature than the air outside it.
This causes more of the water in the bottle to convert to vapor, eventually saturating and depositing on the interior walls of your bottle.
Normally this happens with a bottle full of warm water outside in colder temperatures, when the temperature outside the bottle changes and the temperature inside lags behind, or when the bottle sits in direct sunlight.
If you want to learn more, check out my deep dive on condensation inside of water bottles.
Is It Safe To Drink From A Water Bottle With Condensation?
You may have heard that water bottles left in high temperatures are unsafe to drink from. But this is less a problem to do with condensation than a problem with certain plastics.
Condensation on its own is perfectly safe — whether in or on your water bottle. After all, it’s nothing more than pure water that’s been converted from vapor back to liquid.
The question of health comes from the bottle itself.
Certain plastics contain potentially harmful chemicals, BPA being the most well known offender, that may leach from your bottle when it’s heated.
If you’re concerned, avoid drinking from opened disposable water bottles that have been left in the heat and consider swapping to a BPA-free reusable plastic bottle or stainless steel bottle to be sure.
