Insects are not flying about in strong winds, cold temperatures and in persistent rain. When adverse weather conditions exist for more than a day or two, offering food is imperative. Without intervention, in the form of supplemental feedings, the purple martins will starve to death.

Supplemental feeding adult purple martins will increase survival rates when extended rain, cold or wind makes it difficult or impossible for adults to catch insects. Teaching your purple martins to accept feedings is a rewarding experience that is a lot easier than one might think.

What to feed:
Dead crickets available in most petstores or online are the food recommended by the PMCA.
Bought live and then frozen to death, the crickets are allowed to thaw completely before offering as food. Another alternative food often offered in Scrambled eggs cooked WITHOUT using any kind of butter or oil.
Mealworms are also often used in a feeding platform.

How to feed:
Food is offered by being flung into the air in the direction of the martins but never at them directly. Use a plastic spoon to slingshot the food into the air.
The motion will eventually entice a hungry martin to try the new food source. If your birds have never been fed in this manner, it may take 15 minutes before the food is accepted. Hunger and the insects motion, as it is flung into the air, is a strong stimuli and most report that within 15 minutes the martins are eagerly accepting both meal worms and crickets. Scrambled eggs make a very cost effective and convenient alternative to insects, but is usually less successful to birds unfamiliar to being fed.

Eventually, once the martins learn to accept crickets or eggs in flight as a food source, they will eventually transition to accept feedings off elevated platform feeders.
The PMCA (Purple Martin Conservation Association) has some information to help your purple martin colony survive these extended periods of poor weather conditions.