The exhaust port is behind the keyword, at the screen hinge. The fan runs at or above a minimum speed while the Mac Book Air is running.
On my late-2010 Mac Book Air 11 inch, the minimum speed is 2000 revolutions per minute (rpm).
If you have constant fan noise, and your Mac Book Air is not doing heavier activities, your Mac’s may be confused. The hottest temperature on the bottom of the case I’ve seen quoted in the press is 105F / 41C (Anand Tech) and in the wild is 109F / 43C (thanks for commenting Corbin).
(If you have a higher Mac Book Air 2012 / 2011 bottom case temperature, please take a screenshot and leave a comment below.) If your Mac Book Air is getting too hot, there are some possible fixes below.
The fan noise and heat issues are affecting the i5 Mac Book Air as well.
To help cool down your laptop, quit background apps or pause them using the 3rd party software App Tamer.
Note that it’s not just apps running in the background that consumes power.