"A screaming comes across the sky. It has happened before, but there is nothing to compare it to now"

This quote from Thomas Pynchon's unique Gravity's Rainbow is an allegory for the concern and unpredictability of the future. The "howling" is a depiction of the unknown, as well as the fact that it has "occurred prior to" implies that it is a recurring phenomenon. The phrase "there is nothing to contrast it to currently" recommends that the future is uncertain and that no person can be sure of what will happen. This quote talks to the idea that the future is uncertain and that no person can anticipate what will certainly occur. It is a suggestion that we have to be planned for anything and that we must agree to adapt to whatever comes our method. It is a warning that we should be vigilant which we should be ready to encounter whatever comes our way.