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Soaring the Sea Breeze

WHISTLE WHETTING

Imagine a warm summer/s flying day like no other flying day you may have ever flown. The wind aloft is light and variable under the influence of HIGH pressure. The lift you encounter is not only reliable, smooth, and seemingly ubiquitous, but it extends for miles and miles under the only cloud street that is visible for as far as the eye can see. As you head down the street, you pull in on the control bar and cruise for hours in hammer lane on the inter-coastal highway in the sky. Today, you are soaring the sea breeze and you wonder where it/s been hiding all this time.

Manquin and Highland aerotow flight parks are within striking distance of the sea breeze, making it possible for pilots to make long X-C flights parallel to the east coast. Highland flyers may be in the best position to catch the sea breeze, but they run out of room at the 115 mile (185 km) marker on the southern end of the Delmarva. On the other hand, Manquin pilots have a clean shot at making it deep into North Carolina if they can manage to successfully navigate the first 20-30 miles (32 - 48 km) over the flats to the SE before intercepting the sea breeze front.

SEA BREEZE MORPHOLOGY

The sea breeze is a diurnally driven meteorological phenomenon initiated by the same process that drives all of Earth/s weather - the Sun/s differential heating of the Earth/s surface. If all points on the planet were heated equally, there would be no weather, or at least not the weather to which we are accustomed. The simple fact that all surfaces not heated equally, and that the land heats faster than the water, gives rise to the sea breeze circulations.

A given area of land will gain and lose heat more quickly than a body of water because it has a much lower heat capacity. This is why it takes water in the ocean or a backyard pool so long to warm up in the summer, in spite of days with broiling afternoon temperatures. As anyone who has ever spent time at the beach during the summer can attest, it/s pleasantly cooler by the water. The differing heat capacities between land and water initiates the sea breeze and the sea breeze makes it all together possible for hang pilots to soar across long distances.

For the sake of the following discussion, the body of water will lie to the east and the land area will lie to the west. This simulates the location of major features that produce sea breeze circulations along the Middle Atlantic coast. In places such as the Florida or upper Delmarva Peninsulas, where the land area has significant bodies of water on either side, each coast might experience a sea breeze or in some cases, the two sea breezes may migrate and converge in the middle of the land area.

HOW IT ALL STARTS

As the rising Sun warms the ground throughout the morning, the air/s density and pressure near the Earth/s surface are lowered. This creates HIGH pressure aloft and relative LOW pressure aloft over the water, and LOW pressure over the land and relative HIGH pressure over the water. In response to this lower air pressure, cool stable and therefore more dense air from over the water moves west to replace the warm buoyant, less dense air being evacuated over the land. This inland advance of maritime air works in a similar fashion as an approaching cold front, where the cold air undercuts and lifts the warmer air downstream. This undercutting and lifting process is known as a direct thermal circulation, i.e., warm (cold) air rises (sinks). This process is depicted in the figures below.
 

(Figures from URL: ww2010.atmos.uiuc.edu/(Gh)/guides/mtr/fw/sea/flt.rxml)

The zone where the cooler maritime air meets and undercuts the warm continental air is known as the sea breeze front (SBF) or sea breeze convergence zone. The zone is usually marked by a linear field of robust cumulus clouds that are oriented more or less parallel to the nearby coastline. Cloudbase is relatively low because the shallow maritime air mass is usually ~1km deep. Behind the SBF, the sky is mainly clear, the temperature drops sharply, the humidity rises, and the wind freshens. If the atmosphere is moist enough and sufficient convective instability is present, showers and thunderstorms can develop along the SBF.

[Image]

This figure is a schematic depiction of the sea breeze front. At the front itself the inflowing dense current of air meets the land air, is lifted, and passes back above the trailing dense air, where strong turbulent mixing occurs in the form of unstable breaking waves that entrain warm air into the colder flow.

(Figure and caption from URL: www.phy.umist.ac.uk/Research/research_NL/NewsLetter1/wood.shtml)

DETECTING THE SBF

In general, the SBF becomes established between noon and 2 p.m. when the temperature difference between the water and land reaches ~15°F (8°C). A SBF will usually not form if the synoptic scale pressure gradient (i.e., too many isobars on the map) is too strong because a wind of moderate strength will easily overpower the weaker onshore flow. This is why the SBF is most likely on days when a HIGH pressure ridge axis is centered over, or just west of the flying area.

Initiation of the SBF can be detected by reports of onshore wind gusts in surface observations taken along the coast. The strongest surface winds are found on the cool side of the boundary, sometimes reaching speeds of 25 knots (29 mph) , with 8 - 15 (9 - 17 mph) knots being more common. The SBF can also be observed in satellite imagery once the cumulus develop and their bases have expanded to a width of 1 km or more.

As the afternoon progresses, the wind produced by the thermally induced pressure gradient force begins to adjust to the Coriolis Force (CF). The CF is a consequence of the Earth/s rotation, which in the Northern Hemisphere, causes trajectories to experience a deflection to the right. The CF adjustment causes the wind to veer such that on the east side of the SBF, the wind will shift from its initial easterly direction, into the SE, then become S and parallel to the SBF by late in the day. The same adjustment occurs on the warm side of the SBF, where the initial westerly wind veers into the NW, then becomes N and parallel to the SBF late in the day. With this in mind, an X-C pilot would wisely chose to exploit either the quartering NW tail wind while flying south, the quartering SE tail wind while flying north, or both if flying an out-and-return.

WHAT TO EXPECT FROM YOUR LOCAL SBF - RESEARCH RESULTS v. PILOT REPORTS

Atmospheric researchers have observed, studied, and modeled the SBF over the course of many years, however their findings only have limited applicability to hang flight. Among other things, the results of their studies show that the upward vertical motion in the vicinity of the SBF is on the order of 40 cm/s. This converts to a mere 80'/", which is enough to sustain a turkey vulture, but not a hang glider. What follows are published flying reports from a few hang pilots who have encountered the SBF.

"Hay Bluff - Wales 1988"

Excerpt from Bill Scotts/s HG Page @ www.billscott.co.uk/index.htm

The valley again ended with a Northerly ridge, this one over Crosskeys. I arrived over the top with about 500'. I ridge soared for about 10 minutes (supp open again), until I was rewarded with another 8 up. This one took me to cloud base at 2800' ato. The coast was now within reach, so I pulled on the VB and headed off. Problem - I was still going up. Despite pulling speed, I was now 1000' into the cloud and still in a 4 up. I was worried that if I continued in this direction, I would pop out of the cloud over the Bristol channel, so I turned due North, still going up. At 4300' ato, 1500' into the cloud, I saw a flash of blue and turned towards it. I was in a big blue hole in the cloud and pleased to see that there was still solid ground below. I circled down the hole and when I got down to cloud base, I realised I was directly over the M4, mid-way between Newport and Cardiff. I glanced at the coast and noticed some smoke drifting NE. Obviously the lift I had just escaped from was a sea breeze front.

"An Evening Sled Ride"

Excerpt from from ds.dial.pipex.com/angus.pinkerton/sled.htm

The ground starts to shrink, and I switch my vario to average, it says 680 feet pr minute. I let my glider circle smoothly, over 5000ft now and the lift has evened out. This lift is clearly the effect of the sea breeze, and less than half a mile away some of the straggles of the curtains are now below me. It starts to feel cold at 6000ft and I now realise that how lucky it was that I couldn't find my summer gloves before leaving home today. 7000ft and the lift has slowed. It REALLY feels cold. I have been considering my options during the last few thousand feet and as I gaze off towards the east coast 30 miles away, it feels almost in reach. As does almost everywhere else in the beautiful evening panorama. The high summits of the Cairngorms only 20 miles away, with snow still gathered in their folds, look most inviting. My parents house is only 40 miles to the south west, but that would be out of reach with no sea breeze remnants that way. I imagine being in a sailplane, its 60-1 glide taking me over the horizon... but it wouldn't have been able to get me here. Or of a Swift, or the Exxstasy, and what a 20-1 glide could do. As I reach cloudbase at 7600ft I decide to try out the Kiss' 12-1. Upwind I go, across the top of the Cairnwell, and past Loch Vrotachan high near its summit. Further north past Carn Aosta now, and everywhere I go, I seem to be able to find more gentle lift. But now I have been above 7000ft for half an hour, I'm getting cold and tired, and I would prefer a witness to my landing, just in case those skills are really rusty too.

"Thermal Conditions at Woodbury"

Excerpt from www.gfa.org.au/scot/

When good thermal conditions occur, there will usually be a sea breeze by mid to late afternoon. However, this does not always spell the end of soaring for the day. The Woodbury gliding site is not too far from the centre of where sea breezes from north/west and south/east meet up which leads to a powerful and often long lasting "sea breeze front". This can extend in a line for over 50 Kms on some occasions and last for two or three hours providing a reliable and constant source of lift at around 400 to 600 feet per minute. It's great fun to soar this. On a couple of occasions I have (in a Blanik) gone 30 or 40 Kms out and then back at a speed from 70 to 90 knots without circling. A higher performance machine could get along at top speed.

"East Hill "

Excerpt from www.eclipse.co.uk/freeflyer/records.htm

Regarded by Simon as the best site in the UK, but he is biased as having the record distance from any Condors site with an excellent flight of 80 miles. This was done in 1989, probably the most competitive year in the Condors' History. Other memorable flights were made by Jeff Hoer, who got sucked into a sea breeze front cloud and 3,000ft and spat out at 8,500ft, the record height gain int he South West. He then went on to complete one of four 50+ miles flights that he has made from this site.

From these anecdotal reports, hang pilots might expect to find lift rates of 4-700'/" and cloud suck beneath the SBF.

SBF FORECASTING

Ordinarily, a day marked by a big blue H directly overhead doesn/t bode well for hang flight. The surface temperature might get to be quite warm during the afternoon, but in this synoptic weather situation, warm air aloft is also beginning to flow into the area, which effectively stabilizes the soaring atmosphere. It/s at this point we begin to consider the possibility of soaring the SBF.

The ideal conditions for the SBF to develop would be a bright, sunny spring or summer day with a light offshore flow. Spring and summer are prime time for SBF development because the temperature contrast between the land and water is greater than during the fall and winter. The temperature differential between land and water should be 15°F (8°C) of higher. A light offshore flow contributes to a stronger SBF because it opposes, yet does not overwhelm, the onshore flow initiated by the thermally induced pressure gradient. If the flow is onshore, then a strong SBF is not as likely to develop although some enhancement to the onshore flow will likely be observed. If too many middle or upper level clouds are present or expected, then the temperature gradient between land and sea may not be large enough to produce the necessary pressure gradient for a SBF to develop.

SBF Forecast Checklist

Will the sky be sunny?

Is the ocean or bay water temperature at least 15°F (8°C) cooler than the forecast high temperature?

Is HIGH pressure with a weak pressure gradient over or west of the area?

Is the surface wind forecast westerly at 6 knots (7 mph) or less?

Did a SBF form yesterday?

A FEW CLOSING WORDS OF CAUTION

The SBF is a dynamic meteorological phenomenon that has the capacity to spawn strong thunderstorms when an SBF forms in a moist, sub-tropical air mass. The presence of a sub-tropical air mass can easily be identified by the presence of surface dew points greater than or equal to ~68°F (20°C).

Cloud suck will ingest the glider and pilot into the cumulus. Flying in white out conditions is inherently dangerous and should be avoided at all costs.