My other passion is thermal-duration sailplanes and a glider guider lives and breathes for lift. When conditions are right, a model sailplane can remain aloft almost indefinately until the source of lift has dissapated. There are many ways this lift is generated. One major way is called orographic lift. This occurs when moving air travels over terrain that goes from low to high, forcing the air up along with the rising ground. If you see soaring gulls out on a windy day they are using this orographic lift and it sure looks like fun! Hang gliders love this type of lift and Torry Pines bluffs in California are some of the most famous in the world for orographic lift. Another major way is by thermal activity. A thermal is when the ground is heated by usually the sun and a column of heated air will start rising. This is the same principal as hot air balloons use. As the air rises it creates a very localised low pressure area which causes the thermal to generate. The strength or weakness of the thermal will usually be determined by what is known as the lapse rate. The lapse rate is the rate at which the air naturally cools as we go up in altitude. The normal average lapse rate is 2 degrees Celsius per thousand feet. If the lapse rate is higher, the thermal activity will be strong, that is, if we now have a lapse rate of 3 or 4 degrees C per thousand the air will rise that much faster. This high lapse rate air mass is generally considered unstable. Those big puffy cumulus clouds in the sky are a dead giveaway that this condition exists. Conversly if the lapse rate is low, that is 1 degree C per thousand there will be a lot less thermal activity generated. This air mass is consided as being stable. If the air mass has the same temperature over a few thousand feet then the air mass is considered as being stagnant. This is usually caused by warmer air being aloft and cooler air on the surface. This is called an inversion. Picture Los Angeles or even Vancouver, BC for that matter when the smog is bad. Full scale glider pilots love it when the air is considered unstable and can once again stay up for hours and travel hundreds of miles without any power other than thermal/orographic generated lift.
In the northern hemisphere when thermal activity occurs, the rising air is in a thermal flows counter clockwise or is called anti-cylonic. It looks just like your toilet bowl when you flush with the exception that the air is going up instead of the water going down. This rotational movement is due to the Coriolis Effect of the earth. I will not go into great detail but this is the effect on the air that is influenced by the rotation of the earth. Google it for more info. At any rate the thermal will look like a funnel as it rises. Picture what a tornado looks like. This is an extreme version of a thermal. We've all seen a dust devil and this is essentially a mini tornado. There are other forms of lift such as warm and cold frontal lift but this really does not concern us for our ops other than making it rain or snow!
So we have talked about how lift is generated. This is a macro side of this which full scale pilots are concerned with but there is also a micro side to it. We as pattern pilots are dealing with the micro part. So here is the tie-in and the point of this whole diatribe.
Usually in any given cubic kilometer of airspace there will be ascending and descending air. Ascending air will produce lift and descending air will produce sink. As a glider pilot the name of the game is to seek out and stay in lift and avoid sink at all costs. As a pattern pilot this moving air produces some very unwanted deviation of the model flying in that air. As a thermal has an anti-cyclonic movement to it when it passes through our aerobatic box it can effect a blow in on one side of the box and a blow out on the other side!! Perceived prevailing downdwind and upwind segments can be totally reversed when strong thermal conditions exist. A fickle situation indeed! I'm sure all of you have experienced this in some degree at one time or another. Cursing at the bad trim job on the airframe is all for naught. I always blame my poor scores on the conditions! It would be wise that if you are aiming to trim an airframe and these conditions exist, the time might be better spent working on that tan by the pool or out on the golf course. Save the cursing for the golf game!
So how can one conteract this demon? There are many indicators that this activity is happening. One of the main ones is your windsock. If it indicates a small breeze and then balloons out with a gust and/or a change of direction for a fairly long period, you can be pretty sure that a thermal is moving through. If you suddenly feel a warm or cool breeze on your face, you can bet there is thermal activity. If there are trees or high crops, just look and if on one side of the field there are trees/crops moving and the other side indicates a calm wind, there is a good chance there are thermals about which may cause some grief if you are not prepared. Unfortunately there is no magic cure for this phenomenon other than the knowledge that you may have to do some rather fancy wind correcting that may not be consistently into the prevailing wind and be prepared and ready to do so.
So to recap, if you suspect that the conditions are ripe for thermals, have a look at the surrounding indicators for confirmation that these conditions exist just prior to flying your round. Looking at the previous pilot's flight will sometimes give you a false reading on the moving air. Man, there is a lot to think about when you step up to that flying station!!