Weather Question

[double_ender]

Can anyone tell me why the wind so often drops off at dusk.

I know it would on warm sunny days when the sea breeze goes. And in spring easterly weather it often blows day and night.
But so often I find that in moderate cloudy westerly weather the 10 -20 knts in the day falls away at dusk. Mind you very useful to make a quick dash at dawn to get home avoiding a strong head wind!.

 

[EagerV]

Cant answer your question, but is your boat a Marina 75.

If so its one of the nicest small yachts I have seen.
A retired fisherman friend of mine has bought one and I have been out a few times. Sails remarkably well and appears to take comfortably what ever weather is thrown at it.

Cheers,
Nigel.

 

[Skyva_2]

I'll give it a try and wait for better explanations:

Surface wind is basically due to the wind stream at altitude - known as gradient wind. During the day, the ground is heated by the sun and vertical convection is set up, mixing ground air with higher air; the gradient wind is therefore transferred to ground level by this mixing.

At dusk the ground cools and convection stops, so there is no mixing of high and low air, so surface wind speed will fall.

 

[double_ender]

[ QUOTE ]
Cant answer your question, but is your boat a Marina 75.

If so its one of the nicest small yachts I have seen.
A retired fisherman friend of mine has bought one and I have been out a few times. Sails remarkably well and appears to take comfortably what ever weather is thrown at it.

Cheers,
Nigel.

[/ QUOTE ]

Hi Nigel
About our boat, it is a Marina 75, there are very few about here. We have had Feshie for 10 years, typically away over 100 nights , >1000 miles around the islands of the West Coast. For 2 its absolutly ideal, with deck house etc. Will pm some photos, Keen to hear from your pal, I'll pm email address etc. For the summer we sail out of Ardfern, at present she's ashore on the Gareloch on the Clyde

 

[EagerV]

Nice pic Malcolm,

A real beauty of a Yacht,

My pal who has vast experience of the sea considers the Marina 75 has all he needs for all weather cruising. Cruised from Isle of man to Dingle in rotten summer, he claims he was always warm and comfortable. Also is very manouverable for a long keel.

Will pass on your details to Jackie, unfortunately he is not computer literate.
However he is quite likely to arrive in your area at any time! "Zuiderzee" is afloat at the present at Kilmore Quay and is used almost daily.

Was your boat the one featured in Yachting Monthly a few years ago?

Regards,

Nigel.

 

[tonybarebones]

The diurnal wind variability, manifest by daytime winds stronger and gustier than nighttime winds, has long been understood by folk weather observers, although not necessarily with an understanding of the science behind it. They expressed their knowledge in the simple rhyming couplet:

Winds of the day do wrestle and fight,
Longer and stronger than those of the night.
The "wrestle and fight" is the manifestation of the turbulence level observed in the wind as micro-changes in the heating and pressure fields of the lower atmosphere across a region cause air currents to vary in response. Throughout the day, the solar heating process continues, often altered by the movement cumulus humilis clouds born on rising air bubbles across the sky, causing shadows over the landscape. The wrestling winds gust up, change direction and then lull for a moment or two. The fight proceeds irregularly until the hours prior to sunset.

Eventually, the setting sun's rays are again spread too thin to heat the ground above the surrounding air temperature, and the rise and fall of air parcels diminishes ceases altogether. Unless perturbed by larger scale weather patterns, the surface wind speed drops, soon becoming part of the calmness of evening. With continued clear skies and light winds, the earth's surface will lose heat and cool. In many locations, the cooling will produce cold air drainage winds that will fill low terrain spots with cold, dense air that further dampen the turbulence in the breeze until it is finally becalmed.

The sun has set, and the day I had awoken too and watched so intently for so many hours has now ended. Hardly a leaf now rustles in the tree around me, even the avian activity has ceased for the night. The calmness of the evening ushers in a time for rest, a time for peace, as the waning June moon rises from the horizon.

 

[doca]

Yes, this is the time of the day that I really appreciate. We stay out all day and only head home to the marina when dusk is falling and the wind is almost gone. This makes it an easy task to glide the boat almost to a standstill next to the pontoon so that SWMBO can alight, tie up, and I say to myself "What a wonderfull world !"

 

[EagerV]

Hi malcolm,

Have sent you a pm,

Regards, Nigel.

 

[SnaxMuppet]

This is mostly right except that it isn't really true that ALL surface wind is due to convection but convection does have a significant effect particularly when the gradient is slight.

Much of the surface wind is due to the gradient but there is friction with the ground and ground disturbance that causes the wind to be more gusty then at altitude and also to back 20-30 degs.