I am not a meteorologist by any means, but a couple of basic things can be done in the morning before flying to insure that you avoid one of those "sucky" days.  A lot of this stuff you can find in greater detail on the Internet or in Dennis Pagen's book, Understanding the Sky.  

First thing I will try to address is preventive measures the morning of a flight.  Couple of things you can look at that will make a huge difference on where the thermals are going to be topping out that day.  First off, cloud suck is most likely to occur in low pressure weather and in humid conditions.  We get a lot of it in the PI, but it is mild, compared to suck I have flown through in converging Elsinore.  I don't personally fly with a vario, but on days when cloud suck might become an issue it is probably a good idea, because based on the lift a thermal generates, it is possible, to guess at the height of the thermal.  

A few meteorologists have discovered that the strength of thermals is directly related to the height they reach, meaning the stronger the thermal the higher it reaches and vice versa. The strongest velocity in thermals occur when the lapse rate is the most unstable.  Dennis Pagen offers a table in his book that suggests a relation between thermal strength and thermal height: 

Dry Thermals

Moist Thermals

This information was probably calculated using Southern and Northern California conditions.  So checking the lapse rate is a good idea before the flight.  It can also be suggested that dry thermals are often heavier than moist thermals, because when you add water to the air density, the water decreases the density of the air, making it more buoyant...  h2o is lighter than o2.  So at the same temperatures; moist air is lighter than warm air, and more likely to rise. But this can't be said for desert conditions or very arid environments.  

Secondly, it is harder to change the temperature of water than the temperature of air, which means a thermal is likely to remain warmer longer in wet conditions than in dry conditions.  For figuring out thermal heights in high pressure or dry environments it is important to understand the concept of dry adiabatic lapse rate layers.  Brief explanation...  couple of different lapse rates that we must understand.  Air is heated from the ground up - as you go up in the atmosphere it becomes less dense.  This means warmer air is at the surface and it cools as you go up in the atmosphere (density altitude).  Ideal conditions in a normal or the average atmosphere around the earth is known as the Standard Lapse Rate reflect a drop of 3.6F per 100' or 2C per 300m.  Now this is not the normal lapse rate but rather the mean lapse rate on a global perspective.  

Now we need to understand the concept of the dry adiabatic lapse rate. In stable air... as air rises it expands due to a decrease in density.  This air will decrease in density and cool at a fairly standard rate in the lower 10,000ft.  This rate is 5.5F/1000' or 3C/1000' or 1C/100m.  This is in a perfect environment where no mixing of the air, or addition or subtraction of heat was initiated.  But we know our sky is not a perfect environment.  

Now if the lapse rate is less than 5.5F/1000' the rising air will cool faster than the surrounding air, and eventually it would reach the same temperature as the air around it - reaching a state of equilibrium.  This is stability.  Instability is just the opposite, if the lapse rate is more than 5.5F/1000' the air moving up will not cool as much as the surrounding air and will be out of balance, this air is unstable.  In a vertical dimension the air below the rising air will be warmer, so the air may want to sink, while the air above it is cooler and will want to rise.  According to Dennis Pagen, this is why on unstable days, wide spread sink can be found, or more buoyancy at ridge soaring sites.  Signs of stable conditions; layered clouds, general haze, steady winds, etc. Signs of unstable conditions; cumulus clouds, erratic winds, dust devils, clear air, etc.

Back to the above preventive measures knowing the height of cloud formations.  Remember as air is moving up through the atmosphere, in moist conditions, it also will expand, specifically the moisture in the air will expand so the relative humidity will increase.  Assuming this process continues, the relative humidity increases, it will  eventually reach a saturation point at 100% humidity...  this is when the air is said to be saturated and condensation occurs.  This is also know as the dew point, and at this level the base of the clouds begin to appear. 

In areas other than the tropics this cloud base, will remain somewhat consistent for a given air mass.  Because the air is now condensing it gives off latent heat to the air around it, so the air is no longer moving at the dry adiabatic lapse rate.  This lapse rate is known as the moist adiabatic lapse rate and the temperature now changes 2-5F/1000', but averages about 3F/1000' or 0.5C/100m.  If the lapse rate falls between the dry and the moist adiabatic lapse rates, it is said to be conditionally unstable, which means that it will be stable unless moisture is present in the air and the air is saturated.  At that point condensation will occur and clouds begin to form.  Good for flying right?  

Another factor which must be taken into account is inversion.  Inversion is a level at which the temperature either increases at as the altitude increases, or the temperature doesn't decrease as the altitude increases, but remains the same.  Inversion layers, can be a good thing at high altitudes, because one can rest assure that it will be difficult for thermals to pass that layer of warmer air.  And they usually stop there... at a so called ceiling.  This can also be seen by checking the lapse rate as a curve to the right on the lapse rate chart (remember to start at the bottom).

So these are a couple of the basics that must be taken into account before a flight... inversion layer, cloud base, dry adiabatic lapse rate, and lapse rate.  Now I'm sure there are more, but these are the things I usually check for.  So what happens if we get up high and realize the cloud is developing faster than we had intended.  What are some of the signs of impending cloud suck.  Often times a haze or grey area can be seen growing below a strong thermal cloud.  This is a sign of strong climbing tendencies below the cloud base, in which time the air in the thermal is rapidly cooled by its quick ascent and some of the pollution particles take on water long before the dew point is reached.  Sometimes an area of light lift or no lift occurs below a large cloud, because the cloud has become self sustaining and is no longer relying on a ground fed thermal to sustain its growth, but is rather drawing in the air around it, which still gives off heat.  Sometimes the sink associated with a thermal warms as it compresses and produces a layer of stable air just below the cloud.  This will often shatter our thermal and create turbulence. 

Other factors to consider that may lead to excessive cloud build-up and suck... rate of cloud build-up, extent of cloud vertical development, height of the base above ground, cloud size - horizontal dimensions, cloud darkness, shape, position, and movement, humidity of the surrounding air, presence of lightning and precipitation.  

Other things to remember:  Lift will be strongest towards the darkest part of the cloud (moist moisture), lift will be stronger upwind of a cloud, and sink may be present downwind, concaved upwards shaped base, area of highest cloud development, sharp outlines of a cloud and flat bases, other newly forming clouds. 

I hope this presents some help.  I might have missed a few things, but the point I am making is that cloud suck is a calculated risk, and that it can be avoided.  I talked a lot about lapse rate, because it is a very important idea that a lot of pilots don't take into account.  I also mentioned a few guidelines and things to look for to find potentially good thermalling clouds, or dangerous ones.  I might have missed a few points, but this has been a long account, so take it as you will.  Let me know if you have other questions or comments.

Gabe

Gabriel Jebb
Director / Flight Instructor
Philippines Sky Sports, Inc.
http://www.skysports.ph 
gabe@skysports.ph 
+63 (2) 825-7756