full text of the classic FAA guide
Warm air forced upward over cold air above a frontal surface can provide lift for soaring. However, good frontal lift is transitory, and it accounts for a very small portion of powerless flight. Seldom will you find a front parallel to your desired cross-country route, and seldom will it stay in position long enough to complete a flight. A slowly moving front provides only weak lift. A fast moving front often plagues the soaring pilot with cloudiness and turbulence.
A front can on occasion provide excellent lift for a short period. You may on a cross-country be riding wave or ridge lift and need to move over a flat area to take advantage of thermals. A front may offer lift during your transition.
Fronts often are marked by a change in cloud type or amount. However, the very presence of clouds may deter you from the front. Spotting a dry front is difficult. Knowing that a front is in the vicinity and studying your aircraft reaction can tell you when you are in the frontal lift. Staying in the lift is another problem. Observing ground indicators of surface wind helps.
An approaching front may enhance thermal or hill soaring. An approaching front or a frontal passage most likely will disrupt a sea breeze or mountain wave. Post frontal thermals in cold air were discussed earlier.