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Sea breeze
- 1. Thermal breeze ENVSN Unit 3: Thermal breezes (sea/land breezes) - How do they form? - What are the encouraging parameters? - How do they behave?
- 2. Thermal breeze ENVSN The cause of thermal breezes is the temperature difference between land and sea. They mainly form in summer, in nice weather, when land becomes warmer at daytime and cooler at night, while sea temperature is quite stable. Wind goes from cold towards hot areas (cf module 1). ๏ The day thermal breeze comes from the sea. We call it ยซ sea breeze ยป. It begins at midday (when land warms up) and blows to evening. It can reach 15-20kts and extend to 20-30 nautical miles offshore. ๏ The night thermal breeze comes from land. We call it โland breezeโ. It begins in the middle of the night (when land is cooler) and blows to early morning. It is usually less important than the sea breeze. LAND SEA Sea breeze LAND SEA Night breeze
- 4. Thermal breeze ENVSN The sea breeze 1. In the morning, the sun warms the earth, which in turn radiates heat and warms the overlying air. 2. The warm air over land expands, which forces the high air towards the sea, and creates a down-flowing air from the land to the sea. SEA LAND 1.Airexpansion 2. Down flow 3. Offshore, the air falls back on the sea. 4. On land, warm air rises up (creating cumulus cloud on the coastline, cf module 1) 5. Sea breeze establishes SEA LAND Down flow Sea breeze 3 4 5 Thermal breeze is a fragile loop. It depends a lot on local conditions. Several ingredients are needed.
- 5. Thermal breeze ENVSN The land breeze 1. On the contrary at night, land cools. Cold air over land contracts. 2. This generates in high altitude a down flow from the sea to the land. 3. Over the sea, the warmer air rises up. 4. On land, the colder and heavier air goes down to the sea. 5. Land breeze establishes from the land towards the sea. Land breeze may be stronger along mountainy costs, as Corsica, where cold air goes down the slopes which cool quickly at night. SEA LAND land breeze SEA LAND 1 2 3 45
- 7. Thermal breeze ENVSN 1. A synoptic wind < 16- 18kts, otherwise the loop canโt work 2. A temperature difference between land and sea > 2ยฐ. The more the difference is important, the stronger and the sooner will be the breeze. ๏ Know the sea Tยฐ ๏ Read the forecasted temperatures at land for the day, especially just before the breeze may begin (ie at the end of the morning for the sea breeze) ๏ Observe the change of cloudiness over land the morning for the sea breeze. 3. Instable air. The vertical mixing helps a loop establishment, while a too stable air (for instance a big high pressure) acts as a cover. ๏ Look at the general synopsis ๏ A good visibility often reflects instability ๏ Cumulo-form clouds reflect instability as strato-form clouds reflect stability 4. A light synoptic wind from land to prime the loop of the sea breeze The establishment parameters Breezes
- 9. Thermal breeze ENVSN Thermal breezes veer (turn right) by the Coriolis effect in northern hemisphere ๏ Generally, sea breeze starts blowing from the sea to the land, perpendicular to the land. Then it veers (turns right) to blow almost parallel to the land. It reinforce with the heat in middle/end of the afternoon, then decrease in the evening. ๏ Generally, land breeze starts blowing from land to sea, perpendicular to the land. Then it veers (turns right) to blow almost parallel to the land. It reinforce with the cold in middle/end of the night, then decrease in the early- morning. 14H 19H 15H- 18H 13H 2H 6H 8H Breezes in Baie de Quiberon
- 11. Thermal breeze ENVSN 1. The breeze changes strength and direction all the day long. That is to say: If the outward is upwind, the return will not be necessarily downwindโฆ If we start with 10kts, we may return with more then 10ktsโฆ If we anchor sheltered from the wind in the evening, it probably will not be still the case in nightโฆ 2. Breeze is a local phenomenon, difficult to forecast. Therefore we have to understand the way it works and to locally observe in order to forecast. 3. At last, breeze is a very frequent phenomenon during our summer sailing. Becauseโฆ Breezes
- 12. Thermal breeze ENVSN Estimate the breeze probability for this afternoon. OR Remember a breeze case and note the day conditions (general synopsis, sunshine, sea and land temperature, synoptic wind, breeze behavior)