density

920

Ice in bird bath
We are expecting a hard freeze tonight. The water in our bird baths will probably have a layer of ice, if they don't freeze solid. If cold water sinks, why does the ice form at the top?

925

Sandstone cliff
As winter approaches here in Johnson Canyon, I will be paying close attention to katabatic winds. Without looking it up, what are katabatic winds, and why would they be important to us in the canyon?

945

plastic bag on scale
I weighed this plastic, storage bag while it was empty. Then I opened it to fill it with air. Sealing the bag again, I put it back on the scale. Was its weight the same, more, or less? Why?

946

Balloon on scales
A plastic, storage bag would not weigh more when it when it was filled with air, but when I blew up this balloon, its weight did increase. Why?

951

Purple Gallinule
We talk about something being as light as a feather. For the average bird, what percent of its weight is due to its feathers?

Stirring Sand, part 2

In part one, I left you with a challenge. We put a little sand into a glass of water and stirred it. Instead of moving to the outside edge, as we might expect, the sand gathered in the center of the glass. I left you with the challenge of telling me why.

Congratulations! Several of you got the right answer, and several more got it at least partially right.

719

Death Valley
While many parts of Death Valley are below sea level, there are places where the ground is exactly at sea level. If you compared a cubic foot of air at sea level in Death Valley with a cubic foot of air at sea level in Florida, would they weigh the same? If not, which would be denser, and why?

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