It's hardly a secret that apartment rents are on the rise. To cite just one statistic: average monthly rents increased 21% nationwide over the past year and 7% in a single month, according to a recent report from Redfin. These are the highest annual and monthly growth rates in at least two years—as far back as Redfin's rental data goes. Redfin also found that the national median monthly mortgage payment for homebuyers climbed at about the same annual rate—20%—but rose just 1% from October. Rents, in short, are climbing more quickly than mortgage costs, making it more affordable to buy a home than rent one in some markets. Much depends on the market in question, of course, as some cities are posting double digit growth in rental rates. In the slideshow above you'll find the cities with the highest year over year growth in rental rates, according to Redfin.