Why is the top of a ceiling fan blade dirtier than the bottom?

Pretty much, just like it says…
The times that I’ve cleaned off the blades of a ceiling fan, the top is usually much dirtier than the bottom. (Said fan blowing the air down, the majority (<75%) of the time.) Why does that happen?
I understand that the blades tend to attract dust, due to static electrical “cling”, but I’m just wondering why the top of the blade seems to collect a significantly greater amount, than the bottom. :confused:
I appeal to the greater wisdom of the SDMB, to enlighten me!:smiley:

Gravity. The blades aren’t spinning all the time, and some of what falls on the top of the blades sticks.

Perhaps the dirt that hits it isn’t getting ‘blown’ off since it’s angled away from the direction the fan it turning. You could try running the fan in the other direction for a few months and see if that changes anything.

My other idea would be simply because that’s where the dust settles. Maybe the dust doesn’t stick to it while it’s moving but after you turn it off. Just like any other piece of furniture, you don’t find nearly as much dust on the bottom surfaces as you do on the top.

Dust settles by gravity on the top even if the blades are continuously spinning. I have fans I only turn off to dust and they always have dust on the tops of the blades as well as the leading edges.

spark240 got it in one word. Same reason nobody dusts the underside of a shelf.

Gravity? :dubious:
For some reason, that just doesn’t sound quite right. For one thing, I’ve noticed the phenomenon occurs with ceiling fans that are almost never turned off.
The same thing also happens to “floor” fans, that are blowing air, horizontally. :slight_smile:

Well then, if gravity is the cause… How does that explain the dust on the underside of a shelf? :stuck_out_tongue:

There’s a thin layer of air around the fan that doesn’t move (called “the boundary layer”) - so dust doesn’t get blown off the top because for small enough dust particles, there is no wind to do the blowing

Gravity (IMO) is not an entirely sufficient explanation as there will typically be more dust on the front surface of a box fan blade (standing vertically) than the rear surface.

Some thoughts by other people

Two reasons: Gravity as mentioned for when the fan is not blowing. When the fan is blowing, there’s gravity, plus the air is flowing from the top to the bottom (when the fan is blowing down as you say it is 75% of the time). That means any dust in the air that impacts the fan blades due to the air flow is going to impact the top side 75% of the time.

Right. A fan blade, horizontal or vertical, creates a breeze and is in the path of that breeze. The air in said breeze has dust in it. The dust is being thrust onto the fan blades. Of course it’s going to accumulate on the side it hits, and not on the side it doesn’t hit.

dust will collect on the low pressure side of a moving fan blade and motor and supports that are in the airstream on the low pressure side of the unit. dust will collect on the top of nonmoving things as well.

The dust that settles on the top part of the fan blade is actually how they kill you in your sleep.

Maybe fan blades need spoilers :stuck_out_tongue: