> month_sum <- airquality %>% 
      group_by(Month) %>% 
      summarise(mean_temp = mean(Temp),
                sd_temp = sd(Temp))
> month_sum
Source: local data frame [5 x 3]

  Month mean_temp  sd_temp
  (int)     (dbl)    (dbl)
1     5  65.54839 6.854870
2     6  79.10000 6.598589
3     7  83.90323 4.315513
4     8  83.96774 6.585256
5     9  76.90000 8.355671