Higher frequency of the allele for hemoglobin S (Sickle Cell Anemia)?

Consider two isolated population of people who have their homelands for untold generations. One population occupies a dry, windswept plain. The other population inhabits a lowland, tropical area with many swamps and streams. In which population would you expect to find the higher frequency of the allele for hemoglobin S (Sickle Cell Anemia)? Explain your reasoning.