I know there are conditions where H20 will change phase at temps below 32 deg. F. Can there be warm ice, however?
Yes, just recently published, AFAIK…
http://www.dailytimes.com.pk/default.asp?page=2010\08\02\story_2-8-2010_pg17_8
My google fu is weak today, but back in the Pleistocene (well, ok, college in the late 80s), we were taught about how there are several types of ice which form under different conditions and have different chrystallographic forms. You can obtain “normal” ice at temperatures higher than 32F by manipulating the pressure, IIRC it was unusual in that you had to lower it where for most substances you’d have to raise it (seeding methods like that one linked by Jersey Frank were theoretical at the time).
ETA: Eureka! Not recommended for the weak of Physics.
Yep, melting point is variable based on pressure. The melting points you see in charts are the melting point at a standard pressure. Interestingly, if temperature and pressure are at the “triple point” you can have water in all three forms. A similar phenomenon you sometimes see on the ocean in cold climates is an iceberg sitting in water with water vapor rising off of it, which makes for a pretty picture… Although, for the phase diagram of water, regular ice doesn’t form at higher temperatures, but other types of ice do.
I’ve scraped ice off my windshield on many mornings when the air temp is above 32 degrees.
But most likely your windshield was below 32 degrees.
During the night, your windshield radiates heat away into the night sky, and if the air is fairly calm, radiates enough that it can drop below the temp of the air. Thats why clear nights feel a bit cooler that cloudy ones.
And its easy to see evidence of this effect. Look at houses that have frost formed on them. You’ll often see frost parts of the roof that are well exposed to the sky. Parts that are well covered by trees will often not have frost on them (assuming this was a marginal night and not a really cold one).
The phase diagram for water is actually pretty interesting. Most substances have a line/curve with a positive slope for the solid/liquid boundary meaning as pressure goes up it takes higher temperatures to form the solid. Water has a negative slope meaning their is a fairly narrow range (between about 1 atm and .005 atm) where you can form water at higher temperatures than 0 degrees C. Even at a pressure that allows you to max out temperature though it would still only form at 0.01 degrees C.
Yes. If you’re driving along, and all the banks say the temperature is 42F, and it’s windy and drizzling, and then you drive over a long bridge–expect ice. (The thing I never understood about wind chill–they say it makes it “feel like” a lower temperature. But apparently, it can also make it actually a lower temperature, hence if the temperature drops to a daily low of 42F, the bridge can still be cold enough that the drizzle will freeze.)