What is the opposite of an on-shore breeze?

Question

I know it occurs in the day, and that it's when the breezes roll on-shore to replace air heated by the sun. But what is it called at night, and where's the cold air going then?

Answer 1

It is called a land breeze. In many coastal areas where there are hills and mountains not far from it a katabatic wind is generated. A katabatic is a denser and relatively cold wind that flows down the slope of an elevated area. This usually happens overnight and early morning.

Answer 2

Along the Southern California coast, off shore winds have a few different names. The most often used is "Santa Ana's" This occurs in Southern California when high pressure forms over the great basin or the four corners area of the United States. The rotation out of this high (a high is a dome of cool sinking air rotating clockwise) flows across So Cal and out to sea. Another type of offshore is called a "sundowner". This occurs along the coast at Santa Barbara. This not related to a normal Santa Ana. The coastal area at Santa Barbara is narrow and the coastal range is about 5000' just behind Santa Barbara. The normal sea breeze happens during the day but when the sun goes down the mountains start to send cool air back down the coastal slopes across the coastal plain and out to sea. It only lasts for a few hours. The cool air over the ocean just moves further offshore.

Answer 3

off shore breeze!

Answer 4

Errr...off shore breeze...?

Answer 5

Probably an off-shore calm, I would think.

Answer 6

not sure but I was just going to say sea breeze -- didn't know there were so many different types ... http://en.wikipedia.org/wiki/Sea_breeze

Answer 7

Pure sea breeze

This "textbook" sea breeze requires little or no synoptic gradient, or an offshore gradient that is roughly perpendicular to shore — NNW to NNE on our hypothetical shoreline. A pure sea breeze is always preceded by a period of calm; if there is an offshore breeze, it must first die. As this happens, cumulus clouds will often "sprout" over land. Then the onshore sea breeze fills, typically as a line of new wind approaching from offshore.

A pure sea breeze flows onshore, rises with the thermal updrafts over land, then flows offshore and sinks back to the sea, only to return to shore again. This enclosed circulation expands with the day, both offshore and inshore. By sunset it can extend more than 50 miles, from its offshore edges to its inland reaches, in our latitudes.

The greater the air-to-water temperature differential, and the weaker the synoptic gradient, the earlier the sea breeze will fill. Hence, an incoming tide that brings cold ocean water close to shore will speed the onset of a sea breeze. Pure sea breezes forming from a NNW gradient are stronger than those forming from a NNE gradient, for the same reason that a corkscrew sea breeze is stronger than a backdoor sea breeze (more on that later).

As long as that offshore synoptic gradient isn't too strong, a circulating sea breeze can form with as little as a 4-degree air-to-water temperature differential. The smaller the differential, the weaker and later the sea breeze will fill. When the synoptic gradient is strong, it often blows a wedge of hot air offshore as the morning progresses. This wedge forms an "artificial shoreline" which can extend a mile or more offshore (Chicago, for example, in a gradient southwester). The sea breeze will form seaward of this artificial coastline, and then slowly push it toward shore, with a zone of calm in between. Sometimes the two will fight, and the sea breeze might not make landfall until late afternoon.

A pure sea breeze will try to orient itself perpendicular to shore as it fills, or due south in our example. (If the shoreline is irregular, try to take the average axis over a 10- to 20-mile span to determine perpendicular.) Once the sea breeze is "established," it then builds in velocity. This is when it will begin to veer from the Coriolis effect. The veer is typically 10 degrees per hour in locations in the northern U.S., and 5 degrees in the southern U.S. The shift continues until the wind is angled approximately 45 degrees to shore. At day's end, a pure sea breeze dies with little further change in direction.

Corkscrew sea breeze

This is the strongest type of sea breeze. It forms when there is a sideshore, or slightly offshore synoptic gradient — between the W and NW on our hypothetical coastline. In this scenario, the wind rarely goes calm before the arrival of the sea breeze. Instead, it slowly backs (shifts left) into a sea breeze, with a period of lighter winds as the breeze makes its initial move onshore. A corkscrew sea breeze circulates, as does a pure sea breeze. But instead of the circulation being roughly enclosed, it spirals down the coast in a helix pattern.

In the morning, the sideshore synoptic gradient creates an area of divergence, or slightly weaker velocity, just off the coast. This makes it easy for a corkscrew sea breeze to form, as air from aloft can sink into the divergent zone and initiate circulation. Hence, this type of sea breeze forms earlier, with less of an air-to-water temperature differential, and can overpower stronger synoptic gradients more easily than other types of sea breezes. It can even form on days when the synoptic gradient wind tops 20 knots, if the temperature differential is 10 degrees or more.

The onset of this sea breeze is signaled by the clearing of haze and low clouds offshore. (This is caused by air sinking into the zone of divergence; when air sinks, it compresses, warms and cloud moisture evaporates.) As the clouds clear, the morning wind will begin backing toward onshore. This backing trend will continue until the pressure gradient stabilizes. This will usually be early afternoon in the late spring, when the water is cold and the land is hot. But in late summer, when the air-to-water temperature differential is smaller, the sea breeze will not be firmly "established" until mid-afternoon.

Once the sea breeze is established, the velocity will build steadily. Only at that point will the breeze begin to veer from the Coriolis effect. From then on the veer will be continual, until the wind has returned to the original synoptic direction, usually by midnight.

Backdoor sea breeze

While the corkscrew sea breeze forms easily — let's say it comes through the "front door" — when the synoptic gradient is from the opposite direction, a sea breeze has a more difficult time developing. And when it forms, it is more variable than other sea breezes. For lack of a better term, we'll call this scenario a backdoor sea breeze.

When the synoptic gradient is sideshore or slightly offshore, between NE and E on our hypothetical shoreline, convergence creates an area of stronger velocity just offshore. The convergence also inhibits the sinking of air from aloft and disrupts the formation of sea breeze circulation. Because the sea breeze has trouble forming close to shore, it often forms farther offshore and then tries to fight its way inshore, across the zone of convergence. For a backdoor sea breeze to form, it takes a greater air-to-water temperature differential, and a weaker synoptic gradient, then it does with a pure or corkscrew sea breeze.

In a backdoor scenario, there will be more wind on the left early in the day, as the left is closest to the zone of convergence. Then, in early afternoon and given favorable conditions, a backdoor sea breeze can form offshore, as a SE breeze on our sample shoreline. In between the sea breeze and the convergent wind there is usually a zone of light and shifty air. Slowly, this zone will move toward shore as the sea breeze wins the fight, but it can take much of the afternoon for the sea breeze to make landfall. If the race is started during the fight, boats on both corners of the racecourse will often come out ahead of the those in the middle.

After the sea breeze has moved across the course and inshore, it will begin to veer. This shift will often come in "pulses" of 10 to 15 degrees, rather than a slow, steady, shift to the right. Each pulse is a response to increased heat on shore, the wind taking a "shot" right to relieve the pressure drop on shore. Once the pressure is relieved, the wind steadies out and oscillates as the pressure differential builds again. Then the process is repeated.

Given enough time before evening, this sea breeze can eventually veer to nearly perpendicular to shore. However, this type of sea breeze is not as well organized as a pure or corkscrew sea breeze. So it tends to fall apart early in the evening, and as it does, the wind will begin to back toward the synoptic gradient — toward the morning wind.


Synoptic sea breeze

This is the scenario whenever the synoptic gradient is onshore, as is typically the case on the West Coast of the U.S. By definition, an onshore gradient flows day and night, but an inversion sometimes leaves it calm at the surface in the morning. This is the case with the "marine layer" common on the West Coast, where the inversion is marked by low clouds offshore.

On our hypothetical coastline, a synoptic sea breeze would cover gradient winds between the SW and SE. But there are marked differences in how the two extremes of wind will behave. If the synoptic gradient is SW, and you took a look at a weather map of our coastline, you'd notice that there was lower pressure at the upper left, inland. High pressure would be at the lower right, offshore. As the day heats up, the effect is to further lower the pressure on land which intensifies and twists the onshore breeze to the right. But as you sail several miles offshore, away from the thermal effect, the wind will soften and back toward the synoptic gradient. So, as you approach the weather mark, the wind often softens and backs.