Wi-Fi on MacBook Air only works within 10 feet of router
You published the hardware information which is useful, but hardly to change with upgrade to YOSEMITE.

First lets check the current condition:

Hold the Alt key and click on WiFi icon.

You get something like this:



RSSI is (Signal strength including the noise (disturbance)) this is confusing but the lower the number the better. 

Transmit Rate- the higher the better (minimum 10)

MCS Index- the higher the better (minimum 5)

There are some Apple provided analytical tools to take look at your WiFi.

If you hold the alt key and click on the WiFi symbol, at the bottom of that list there is a "Open Wireless Diagnostics". Enter your log in password and continue. Let it run till it ask you what to do next. Use the "Monitor my WiFi connection". Let it run and move the MacBook around closer and further from the router. Stop it if nothing happened. 

I get the "Crowded WiFi channel" on mine, to many of my devices and neighbors on same channel. 

On Yosemite hold the "alt" and click on WiFi, then select the "Open Wireless Diagnostics". In menubar Window tab and select the scan.

Once you have the result sort it by Channel to see how crowded it is.

Here is what I get:



As you can see aside of my self been on channel 11, there are to many others on it as well. Go to your router setup and change the channel to less crowded one.If you have 5g WiFi then switch to it.