As with solar, hydro, and biomass resource data in HOMER, you can specify ambient temperature data either by importing time series data from a data file, or by specifying monthly averages and having HOMER synthesize the time series data from those averages.  We use a clever algorithm (of someone else’s making) to synthesize hourly solar resource data from monthly averages, but we don’t yet have an analogous algorithm for synthesizing hourly ambient temperature data from monthly averages.  So when you enter monthly averages, HOMER simply assumes constant temperature within each month.  If that’s not good enough, you can import hourly (or even sub-hourly) time series data.