Why does hot air not move cold water??

Here is a phenomenon that exists on the Great Lakes during springtime. Last week we had some very, very strong winds on a Low, almost hurricane force in places blow from the south. The isobars were nearly as tightly woven as a hurricane. In this case, the water temp was 37-39F, and the winds had temps of near 80F. The water barely moved and we had reports of 1-2 ft mush at the North End, despite NOAA predictions at buoys to 9-12 ft. At the same time, with wind speeds of the about the same velocities, with water temps in the low 30s and air temps about 37F, the same system generated waves to 8 feet up in Lake Superior.

Today, with winds nearly the same speeds and strong enough to make a wall collapse at a construction site in Chicago (gusting to 60 mph), SW winds howled all day. At mid Lake, the waves were reported to 6 ft, with buoy readings to 9.5 ft. The difference: water temps and air temps were far closer, with the spread being only 10 degrees or so.

I've seen this happen repeatedly during springtimes. As water then warms slowly but approaches air temps more closely and we get Low pressure storm systems on cold fronts, the water returns to stirring up as quickly as it does in the fall and wintertime, with less of a temp spread.

Is there an obvious reason for this??

Cold water is more dense? Like me?

Puzzling. Waves start from an initial instability/perturbation of the water surface and build upon that. Perhaps the combination of hot air and cold water is somehow damping the formation of the smallest ripples.

I know that our hottest winds, offshore Santa Anas, are very good at building waves, but over 55-65 F water though.

It might have something to do with the fact that hot air rises. On the West Coast I have rarely seen Santa Ana winds be anything other than gusty and 'Swiss Cheesy'...localized geography has a major effect on micrometerological conditions. Heat bubbles form, causing wind to go up, over and around.

Maybe building waves moving away from us towards the front side of our islands...not surfable ones ,mind you...as anyone who has been on a boat at the Channel Islands when the Santa Anas have come up will attest... It's been my experience that more times than not, during Santa Anas, the surf gets blown nearly flat as the Santa Anas are caused by vigorous high pressure systems parking themselves over the Four Corners Region of the country after moving in off of the Pacific...we get most of our swells from lows. Occasionally, swell will be combined with weaker Santa Anas making for near perfect surfing conditions at our beach breaks to the South. Colder air, generally always makes for better wind swell producing conditions.

WARDOG

Craig,

5 PM reading from 45002 was 10.5 ft. w/ an 8 sec period.

Yes, exactly my point! ck

Uhmm....

I had a crazy idea about that.... and I followed it up. Have a look at http://www.ndbc.noaa.gov/Maps/WestGL.shtml

Uh huh. It's not the water temperature so much as it is the fetch: how far across the water it blows. Nor just how strong the wind is and the air temperature. The different densities of cold and hot air and of cool and warm water really ain't enough to be a factor. It's what way the wind is blowing .

Take a southwest wind. Blow as it might, it's not gonna blow across a whole lot of water in Lake Michigan and thus it's not gonna make much in the way of waves. Lake Michigan is lined up pretty much north-south. Get a strong wind out of the north and you have something.

Compare it to Lake Superior - which is pretty much southwest-northeast, so that a southwest wind is gonna blow straight from the Wisconsin-Minnesota line straight to the 45001 buoy, with (at a very rough guess ) ten times the fetch that very same wind would have across water from Wisconsin to the 45002 buoy out in Lake Michigan. Quite a difference, as your observations above show.

So, what you might want to do is this -

See what the wind directions are..... and look at the buoys mentioned on the page above. And the map...... and Lud's bit on wave forecasting. Then you might be onto something.

Yes! You live on a damn lake !!!

Ah, no. As written above, the same phenomena is observed on the oceans. And, we do surf here, despite (as you say) we live on a 'damn Lake'.

Come and visit and observe. Then surf with me sometime in Chicago. As long as I'm around (and I am working on getting to the oceans and I fly outta here and surf the oceans often now) you are very welcome ro use my boards/wetsuits if you come around.

Open thy mind. Yes, we surf here and we have fun doing it. Maybe you would even be interested to find out that an offshore or clean chest high wave with a 10 second period is not uncommon here.... ck http://www.lakesurf.com

An interesting observation and worth watching over time. The fetch from the SW wind to 45007 isn't all that far, but enough to look for water you are getting at.

Pilots know that cold air is denser. Cold water certainly feels denser to me when breaking the surface when platform diving. While warm water feels like a much softer entry.

Finally, it may be that Lake Michigan is more of in a position for us to see such a contrast between air and water for wave generation. Certainly, most coastal surfers won't have any local experience with these sorts of temperature contrasts, nearby wave generation, etc.

Well no, the fetch across the channel is short, but the waves do come up, on Catalina for example, about as well as a theory that doesn't take into account air temperature would predict.

Others have commented on colder water being more dense. True, but not by much, only a couple percent, so that will not have much effect. On the other hand, the density of air responds much more over the same temperature range, so that could partly explain the effect Craig is observing.