Wind & Tide ?

[Salin Sal]

Along tidal sea shores I noticed the wind often changes direction or comes up after a period of calm when the tide is nearing change in height either high or low. Why is this ?

Thanks

[Wild Wassa]

"T'is an ill wind that turns the tide ma' young hearty," isn't just for sailors to know about ... ask any stock broker.

On the south east coast of Oz, for instance, where there is steady fine weather with a N.E. wind blowing in from the sea, if the time of slack water after flood tide occurs about sunset, the sea breeze dies, and a gentle air from the W. then sets in. When the time of slack water is later, the N.E. wind blows far into the evening. I do not venture to say that the changes of wind and tide always keep time with one another.

It is caused by temperature differentials Mate ... and I might even guess that when the tide is at it's lowest and at it's highest points during the day, the air pressure changes too, possibly causing the wind to go through a slower period of transition more than just when the off shore or on shore changes caused by the temperature changes, happen.

It will only be a couple of days then I'll be spending time on the coast ... and I'll carry out some observations and check the timings and post what I find, young Grasshopper.

Warren.

[Ian McColgin]

I agree with the temperature differentials which actually happened a bit before the noticable wind shift.

The most rapid change in water level and the strongest tidal currents are around mid-tide. Fully one half of the total tidal change happens in the third and fourth hours. But it takes a little while for the effect, usually cooler water reducing the amount of warmer land, to stabilize into (most often) an on-shore zepher.

Like local tides, micro-weather has many particular causes and the general physics is only a start at understanding what's happening in a particular place. Local knowledge is a most wonderful thing.

G'luck

[Wild Wassa]

"I'll carry out some observations and check the timings and post what I find, ..."

Since this question was posted, I've talked to guys from the NSW Coastal Patrol and picked their brains, asked my sailing friends before they went on holidays to the coast to be aware of this possibly happening. I've also asked CYC sailors who have been racing in regattas (down) on the coast (it is that time of year Skippers) what have they noticed or could they make some observations while they were away and I asked the question to local Narooma fishermen and to a few Mystery bay locals walking on beaches, while we were holidaying. The short time that I spent on the NSW South Coast was not adequate to see a pattern for wind and tide but has shown me that the summer time breeze is far from straightforward (that I originally thought it was) on the coast and no matter how consistent and regular, it springs surprises or two. On the NSW South Coast (also known as the Sapphire Coast), I like how as the day progresses and heated up, the sea breeze deflects to the left, and it often ran parallel to the coast later in the day, as a Northerly, while over land (close by) it sill remained a strong Easterly. Mystery Bay is not a bay, it is open to the sea. The winds there don't deflect around headlands like they can in closed bays.

Back to the issue. Nothing emerged concerning the changes of tide creating a calm zone, from my questions, except ...

... when a shore breeze is a gradient wind with a softening gradient flow (as it leaves the land), if the sea breeze (from offshore) is a weak sea breeze, a calm zone will be created where they meet.

The timing of this phenomenu Sailin Sal, just may have coincided with the changes of tide that you noticed.

I've been paying particular attention to this summer's sea breeze since you asked this question. Despite being 90k's inland from the coast, each day I look forward to it cooling me down, if only slightly. When the sea breeze arrives, it isn't as refreshing as it is on the coast, it has had a chance to warm-up somewhat. The increase in warmth of the breeze over being cooler on the coast, hasn't stopped it from being windy when it arrives. It has made for some wild overpowered late afternoon rides. The sea breeze appears at about 2.30 - 3.00 and by 5.00, it's blowing the spots off Dalmatians. Twilight racing has been excellent here because of it ... the lake late in the day is 'Capsize Central'.

Warren.

[Tom Hunter]

I can only speak for my home waters (New England) but around here a period of calm is reuqired for certain wind shifts, for example if the wind is coming from the North we usually have a period of calm before it comes from the South or South West.

I have not noticed a connection to the tide, and I do pay some attention to this. But I will have to pay more attention, maybe your on to something.

[Thorne]

Haven't noticed the winds out here being linked to the tides, but I'm sure there is an impact of some sort.

Here in SF Bay the primary driving factor is the Central Valley, which during the summer heats up and creates a bit of a vaccum -- hence pulling the cool air in from way offshore.

If you aren't ready for 30+ knot winds and fog, don't go out on the Bay after 1pm on a summer afternoon. This pattern continues all along the Sacramento and San Joaquin rivers, plus various wind gaps in the coastal range.

During the winter you'd better have a full fuel tank or oars ready, as the water can be like glass -- I've seen folks water-ski underneath the Golden Gate in January.