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This info is considered Public Domain, so use it as you wish, but not for commercial purposes.  It was created to increase the joy and enjoyment of flying gliders/ soaring. WARNING: All info is unsubstantiated, so use it at your own risk.

Basic discussion of atmospheric weather Soaring Conditions.
(In the Northern Hemisphere)

In the area surrounding a low pressure system, the air moves in a
counter-clockwise direction - veering slightly towards the center of the low.

Since the air in a low is converging, it is ascending. (It can't all go to the center of the Low without going up).

In the area surrounding a high pressure system, the air moves in a clockwise
direction - veering slightly away from the center of the high. (It is a descending air mass, and when it gets near to the ground, it has to spread out).

Since the air in a high is diverging, the area of the high is being filled from above with descending air.

The wind near the surface is generally parallel to the pressure lines on a weather chart - but it does diverge away from the center of the High, and converge towards the center of the Low.

The closer the isobars are together, the stronger the winds will be. Isobars close together means a strong pressure gradient - the pressure is changing more rapidly in a certain distance vs. isobars far apart and little change in pressure over a fixed distance. If there is little pressure change, there will be little wind!

The Jet Stream occurs on the South side of a Low and on the North side of a High.

When standing with your face into the wind, the low will be on your right.

To predict the height of the thermals, you need to have a good guess at what the maximum surface temperature will be, and what the temps aloft will be.

Best guess on the max surface temp usually comes from the local TV, radio, or newspapers.

Temps aloft may be copied via phone from FSS at 1-800 WX BRIEF.

FSS does not provide temps below 6,000 ft., or winds below 3,000 ft..
Winds are in degrees True, not Magnetic, and temps aloft are in Celsius, not Farenheit.

Plot temps on an adiabat chart, following the dry adiabat lines, and you will
see the max thermal tops for that period.

Dry adiabat lines are also called Dry Adiabat Lapse Rates, and are abbreviated as DALR. On the sounding chart / skew-t / adiabat chart / or whatever you want to call it, the DALR lines are presented as lines from the base of the chart / lower right side and they are drawn diagonally upward to the left.

Strength of thermals are easily computed by taking into account the height
of the thermal and the temperature difference between the top of the thermal
and the temp at some nominal altitude above the surface. We then take those
two values, and use a multiplier on it, yielding the strength of the thermals
in feet per minute or knots (1 knot is 6,000 ft per hr, which is 6,000 ft per
60 minutes, which is 100 ft per minute).

Example calculation for strength of thermals:
Thermals to 8,400 ft. where the temp is + 11,
with the temp at 3,000 ft of + 26 C. Difference is 15,
so multiply by 10 and get 150. (10 just works out, so use it).
Divide thermal height of 8,400 by 100 and get 84.

Add the two values and you get 234 and multiply it by a
constant. Here at Williams we typically use a constant of 4, but
at Minden or Air Sailing, use 8 instead of 4 as your constant.

Thermals in this example will be 900 to 1,000 ft per minute (234 x 4 = 936).

Thermals were Not as good as Forecast because  .....
Either the surface temp did not achieve the forecast maximum or the temperature of air mass was hotter than expected.

Reasons the surface temp was not hot enough ....
The surface temp did not achieve the forecast maximum because:
The clouds filtered or blocked the sunlight, or the surface wind cooled the
surface, or it rained, or the air was descending from the mountains - causing
katabatic cooling of surface air, or a combo of these.

Reasons the temp aloft was hotter than expected ....
The temperature of air mass was hotter than expected because:
The temp forecast was incorrect because an atmospheric
pressure change occurred sooner or later than expected, or the temp forecast
was unknown at lower levels, and you guessed that it would be cooler than
it actually was! That is, it was hotter than you guessed it would be at the
lower levels.

Surface wind was may not be conducive to thermal development because:
There is no wind at all, and thermals just bubbled off the hot spots without
coalescing, or the wind was parallel to the ridge lines, causing
the thermals to be weak along the ridge tops and isolated at the downwind
end of each valley because the hot surface air was channeled downwind
along the valley floors - rather than being allowed to gently climb the
lower slopes, and then up the sides of the ridges from the valley floor.

Clouds sometimes form in each thermal before you can climb to the
forecast thermal altitude.
If both the dry bulb temps and the wet bulb temps are parallel, the relative
humidity, or moisture content of the air is about the same all the way up,
but this is rarely the case. The level of moisture in the air usually varies with
altitude. If there is no cloud at the top of a thermal, then the air was not
moist enough to form a cloud. A cloud forms if the invisible
moisture becomes visible. This happens whenever the air is cooled sufficiently.
If there is too much moisture in the air below the top of your thermals,
you will have clouds forming before you reach the top of your thermals.

Sometimes thermals exist, but can not be used for climbing because of the
wind. If you are climbing, but are drifting downwind of the field you wish
to land at, the thermals are useless to you.

Thermals in a strong wind may be:
strong on the leeward side and weaker on the
windward side, very narrow, intermittent, not as high as forecast - wind
caused early mixing of the thermal with the surrounding air.

The location and frequency of thermals may be unusual because:
thermals may not very high, thus they will be very close together. This is
normal for low thermals, and you just have to be content with weak lift,
and take baby steps in the direction you want to fly. Conversely, thermals
may be very high, but will naturally be spaced quite far apart. This is
normal for high thermals.

Thermal spacing old rule of thumb:
thermals are spaced apart a distance that is equal to 2 1/2 to 3 times the
height of the thermals.

For example, if you have thermals that are 6,000 ft above the surface,
you can expect the thermals to be 2.5 to 3 miles apart.

Reasons the surface temp was hotter than forecasted ...
The surface temperature may be hotter than forecast because:
The TV/ radio weather man thought there would be high cirrus clouds, but
they didn't form, thus the sunlight was unfiltered.

Reasons the temps aloft were cooler than forecasted ...
The air mass temperatures may be cooler than expected because:
the atmospheric pressure changed sooner or later than expected.

The surface wind may be conducive to thermal development if:
the wind is perpendicular to the ridge lines, causing better thermals to form
at the top of each windward facing canyon - but thermals may be weaker
along the valley floor. Also, it there is no surface wind, this may cause the
hot air to centralize in the large dry lake bed areas of the valley floors,
resulting in larger and higher thermals in the valleys but weaker ones along
the ridge lines.

Thermals may not always be in the location we predict, or they may not
always be in the shape of cylindrical columns.

Mountain wave, which formed from a ridgeline up-wind of your location
may be forcing cool air down onto the ridge line you are working, thus
killing all thermals.

A break in the ridge line may be channeling the surface wind at high speeds,
thus preventing thermals at that location.

Thermals may be forming behind the foot of a ridge line due to the eddy of
the air mass - the swirling air mass circles behind the foot, gathering in all
of the hot air, and thermals rise near the center of the swirl.

Thermals may form downwind of a lake. This can happen if that area of the
air mass is stratified. There may be hot surface air ascending into the cooler
air coming off the lake, well above the surface, forming a thermal.

Thermals may form as the air above the surface is cooled due
to a sea breeze. This is common place anywhere a sea breeze comes in
during the late afternoon. As the sea breeze moves inland, so does the line
of lift.

Thermal lift, mountain wave lift, rotor lift, ridge lift, shear line lift, air mass convergence lift. Each type is discussed below.

Thermal Lift

Thermals are currents of rising air.

Convection is vertical mixing of the air.

Surface thermals are generated from a heat source on the ground.

Convection can begin or end at any altitude.

Thermals stop rising when the temperature of the thermal air matches the
temp of the surrounding air.

You can for soar hours, and travel hundreds of miles beneath an overcast
sky.

Thermals will normally not occur immediately downwind of a ridge line,
because the descending air is expanding and therefore cooling.

Cloud streets and thermals will occur parallel to and downwind of a ridge
line if the air is unstable, because the ridge line is blocking the wind, and
the air has a chance to heat up and rise thermally before being blown again
by the wind.

Thermals normally will not occur further downwind of a ridge line during
strong winds, because the descending air is creating an area of stable air -
cool below, and warm above, thus eliminating convection, as well as cooling
off the thermal sources.

If the sun is strong and the ridge line faces south, but the wind above the
ridge is from the north, the wind on the surface on the south side of the
ridge will be from the south, and thermals will form on the downwind side
of the ridge.

Thermals will not drift at the same rate as the wind is blowing,

When thermals stop rising, the cloud at the top will often be blown
downwind before it dissipates.

Mountain Wave Lift

Mountain wave begins slightly downwind of the obstruction that is forcing
the wind to wave.

Mountain wave is the result of both mechanical action and thermodynamics.
The wind is forced over the obstruction, as it descends on the downwind
side, the stable air which exists downwind is forced to rise to a higher level
momentarily, but then it descends again to attempt to achieve thermal
stability.

Distance between first and second and third mountain wave is a function of
wind speed. The stonger the wind, the further apart the waves will be.

Wave occurs parallel to the ridge line, not perpendicular to the wind.

For a given wind, the more stable the air is downwind of the ridge line, the
stronger the mountain wave will be.

Mountain wave begins slightly below the altitude of the top of the ridge
line.

Mountain wave is likely if the wind is within 30 degrees of being
perpendicular to the ridge line.

The more the wind increases in velocity as you go aloft, the stronger the mountain wave will be.

As you climb in a mountain wave, the area of strongest lift will move forward, towards the top of the ridge, the higher you climb.

Rotor Lift

Rotor lift will occur immediately below the lowest level of the mountain
wave.

Since mountain wave may begin and end at end at various altitudes due to temperature inversions occuring at various levels, the rotor may also be found at various levels.

There was a very informative article in SOARING magazine in mid-1999 about inversions and mountain wave lift.

Pilots only fly in rotor lift in order to ascend into the smooth mountain wave lift.

Ridge Lift

Ridge lift is the mechanical rising of the air. The surface obstruction forces
the wind up and over it.

Ridge lift occurs on the upwind side, at a distance which is equal to
one-third of the height of the ridge.

Shear Line Lift

Air in the valley gets hot from the sun.  The air rises.  Other air must replace the rising air.  If you live near the ocean, the air is drawn from the ocean to the valley.  This is a sea breeze.  This is also called the marine air. Also called the Marine Layer.  Where this cooler marine air meets the warmer valley air this line is called a shear line.

Air in this valley (which is oriented north - south) often flows south to north.  Air from the west is drawn in during the late afternoon by the low pressure created by the rising air in the valley.  As the westerly air meets the air that was flowing south to north, a shearline is formed - right across the middle of the valley.  At Maxwell- Williams area it happens regularly.  I've also seen in just north of Mount Shasta in Siskiyou County.  It also happens just south of Mono Lake at the north end of the Owens Valley as the air flows east ward over the sierras.

Please send me some things you know about this.

Air Mass Convergence Lift

Unlike the sea breeze/ marine air, this is just different air masses.  Often you will see that on the east side of the ridge the thermals are higher than those on the west side- or vice versa.  Or maybe you have seen the clouds coming up from the Owens Valley ending near mount Patterson, or maybe you have seen the line that froms north east of Quincy.  These are indications of two different airmasses, and where these two air masses come together, there is often a line of lift, because the temp of each air mass at various levels will undoubtedly be different, and mixing wll have to occur.  Where the warm air gets bouyed up there is lift - that's Air Mass Convergence Lift!

Please send me some things you know about this.
 

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