I keep a motorcycle outdoors under a waterproof cover, but noticed that with morning dew the bike is still noticeablely wet on the inside of the cover.
Meanwhile a buddy has his bike in a plywood shed that is by no means air tight but has 4 walls and a roof, but no insulation or air handling fans/AC and he says dew is never an issue..what’s the difference?
In: Planetary Science
It’s about the temperature…. Your bike under the cover gets to a low enough temperature to hit the dew point. Your buddy’s bike is staying a little warmer due to the wood enclosure.
Your buddy’s bike gets just a little wet from dew inside the shed. Dew hundreds of feet above the shed fall on the shed roof and then rolls off instead of hitting your buddies bike.
Bikes outside get wetted by the full hundreds of feet thick column of air worth of dew.
Your buddies bike gets a tiny bit wet from dew that forms inside the shed, but there is so little water that you don’t notice. The roof of the shed diverts all the rest.
[removed]
The air in the enclosed shed acts like insulation to slow the change in temperature. Depending on the region things that you keep in a shed might still rust but not as fast
Dew is not rain, and it doesn’t ‘fall’ from the sky. It’s condensation of water vapor that’s in the air.
For your bike, there’s air (that contains water vapor) underneath your cover when you put it on. At night, the temperature of that air drops, and the air can no longer hold as much moisture. The water vapor then condenses onto your bike. I’d bet that you also get dew on the outside of the cover, since the same thing is happening to the ambient air (that’s why dew appears on grass and other surfaces at night).
For your buddy’s bike, the air temperature inside the shed is simply not changing enough for moisture to condensate. Even though it’s not ‘airtight’ or insulated, a relatively closed space makes a huge difference in restricting air movement and slowing down how quickly the air temperature inside changes. In extreme cases (for example, a very hot and humid day followed by a very cold night), I actually would expect some condensation inside the shed, but it’s probably rare.
The amount of water air can carry is dependent on temperature.
At 32 degrees Fahrenheit, the air can hold at maximum 4.8 grams of water per cubic meter of air.
At 100 degrees Fahrenheit, the air can hold approximately 10x more water in the same volume of air.
“Relative” humidity is the percentage of max capacity, 4.8 grams of water per M3 on a cold winter day is a clammy 100% relative humidity. 4.8 grams of water per M3 is today’s bone dry forecast for Death Valley.
Back to the motorcycle, your friend’s shed is still a lot more insulating than your motorcycle cover. The shed is going to warm up from the sun all day, and the air inside will stay warmer overnight compared to the air outside. Warm air is drier air, it’s not cooling down enough to squeeze out the moisture onto his motorcycle, if anything the windows would be the coldest part of the shed and where water condenses first.
Dew forms because the ground (or other objects) radiate heat into space on clear nights, and thus get cooler than the air near the ground.
If the air has a lot of water vapor, then the vapor pressure of liquid water at the temperature of the object is lower than the partial pressure of the vapor in the air. It’s exactly the reverse of evaporation, which happens when the vapor pressure of water at the temperature of objects on the ground (or, say, the surface of a body of water) is higher than the partial pressure of water vapor in the air.
At night, on clear nights (when clouds aren’t reflecting infrared radiation back to the ground), objects on the ground cool down via radiation, and that sets up conditions for dew.
Something being covered — not insulated, just covered — can prevent dew formation. For example, when frost forms (which is just solid dew), typically you will see less of it under evergreen trees, or deciduous trees in the autumn before the leaves fall. That’s because the ground there is warmer, insulated from radiative cooling by the boughs of the tree above.
The plywood is doing a sufficient job of preventing radiative cooling; it’s working as a cloud. The waterproof covering is not.
Thermal mass, heat conductivity and blocking air flow.
Dew happens when an object gets cold enough to be below the dew point.
The bigger an object, the more thermal energy it has and the longer it will take to cool down. It takes a while for an object like a motorcycle to get cold. A plastic cover does not add much to that thermal mass. A shed has a bigger thermal mass and takes a while to cool down.
Heat conducts through some materials faster than others. Metal is a good heat conductor and will heat up or cool down quickly, wood is a poor heat conductor (called an insulator) – one side can be hot and the other cool for hours.
Finally is airflow. Air moves a lot of heat into and out of an area. While not perfectly air-tight, blocking most airflow can slow heat transfer a lot.
Water from the air condenses onto things that are cold, and things get cold at night because they radiate their heat away to the blackness of space, which gives nothing back. Dew tends to form on clear nights, because on cloudy nights, the clouds radiate energy back to the ground. If your friend’s shed is near trees or a building, those would block the view of the night sky, which would reduce cooling.
Cooling by radiating heat away is not very fast, compared to cooling by conduction (touching something else that’s colder). So big heavy things with a lot of thermal mass don’t cool enough overnight to form dew, while thin, lightweight things like grass, or your motorcycle cover, could cool enough to condense water.
Another reason that your motorcycle gets wet might be that it’s touching the tarp directly. Your friend’s shed might get dew on ceiling, but the bike is a few feet below the ceiling so the water doesn’t transfer.
Morning dew prefers the great outdoors, just like we do. It’s all about exposure – your bike gets more of it under that cover versus your buddy’s shed-protected bike.
Latest Answers