A sensor stuck right on the window won’t give you accurate temperatures, it will always be too hot. It will get heat from the apartment in the winter and it will get additional reflective sun heat in the summer. It could easily be off by as much as 15°.
I would suggest instead that you just get the weather from a local service that is using a weather station near you. It’s quite easy to feed this information into SmartThings and have it look like it’s just another device, and it will be much more accurate than something placed on your own window.
There are two very quick ways to do this.
One) Ifttt weather channel. The free IFTTT service has a weather channel that you can use as triggers to then trigger SmartThings events
- SmartThings Weather tile
Or SmartThings itself has a device type which will allow you to set up a virtual weather device. Instructions in the following thread.
It puts a weather device in the things list in your mobile app which looks like this:
I would choose either of those over a sensor stuck to the window 23 floors up. They will be more accurate, they won’t cost you anything, and there’s no danger of the device falling off. Oh, and you never have to change batteries. 
JMO