I posted it in mid atlantic forum even though i surf southern nc and obx...Anyway I was wondering if you could help me out. It seems in the spring (aka now today was 75) that we get lows coming off of the se coast. Powerful systems with a lot of wind (hence all the tornados in Alabama and Georgia) in a short amount of time. This always seems to equal SW winds (which i understand), but why does it ALWAYS turn offshore right before dark? I'm talking systems here, not your normal glass off. Constantly the winds shift offshore an hour or two after dark. I think these systems have it out for us because I just can't figure it out. Is it our area? I live in Carolina Beach btw. Any feedback is appreciated.
Believe it or not, the systems don't always go offshore right after dark, but it is an interesting question... And, one that has crossed my mind before. A plausible hypothesis is that the diurnal heating/cooling influences the timing of the frontal systems.
Typically, land and water have different heat retention characteristics. Just remember that barring other influences, warm air rises, and cool air falls. Land cools and heats faster than water, while water retains and expels heat much slower. The heat differential from land to water, and from day to night creates air movement from cool to warm as the warm air rises, and cool air moves in to replace it. This, and so much more.
yea MDSurfer is pretty much got it. Its the sea breeze and land breezes that make the switch from onshore to offshore during the dusk/night. When the temps change, so does the pressure. When the pressure changes, the wind changes because wind comes from the change in pressure from high to low. Lower pressures, higher winds etc. So when the temps change as fast as they can on and the water is 40 degrees colder than the air, you can have significant switches like that.
it add to mdsurfer. a few examples. during the summer, in the afternoon when the temp's reach the highest of the day during a heat wave, the wind is the strongest, creating a southernly flow, then when the sun starts to set you experience a "glass off". If you ever noticed evening glass off are much more common when it's warm out either late spring, summer, or fall....at least to my experience. It also correlates to the ocean temperature, so really it just depends on the difference between the air and sea temperatures. Just pay attention to the weather when you surf and you'll naturally figure it out
Yeah I would have to agree with Micah on this one, theres probably not really any pattern. IF there is, I would guess its from diurnal cooling at night strengthening the cold air mass pushing the cold front, which could cause the front to speed up slightly just after dark (until it gets offshore and hits the "warm" water). The land-sea breezes that y'all are talking about play very little role when there is large scale flow as strong as with a cold front like the one you got last night (the low pressure actually passed over the NE and the SE just got the associated cold front i think...) but are super important when there is not a strong large scale flow pattern like in the summer when the bermuda high just sits a couple hundred miles offshore.
Last edited by surfing is neat-o; Feb 26, 2012 at 08:49 AM.
as stated above ^, the land/sea breezes are microscale and are very localized right along the coastline.
Land/Sea breezes are noticeable in the absence of macroscale systems.
The question raised, was about large scale frontal systems moving off the coast right after dark. The timing of the frontal systems are definitely more random, but I suggested that perhaps the diurnal heating/cooling along the coastline plays a role in the eastward speed of the low pressure systems.
If there is a correlation, it would be very dependent upon track of each system. You have to consider the whole journey of the front/low pressure center, as it moves from the Pacific on to the Mainland, or down from Canada, or up front the Gulf. Its an interesting question none the less, and I've thought about it before, but it would be a heavy research project to come up with some answers.