If you understand sql you can even try data.table package which is the fastest in entire R programming.
library(data.table)
climate.data %>% setDT()
climate.data[,.(temp_min=min(temp_min),
temp_max=max(temp_max)),
by=.(County,State)][
(temp_min > -15) & (temp_max <40),]
FYI
data.table has a syntax like sql something like this
from[where, select, group by]