How to reduce the Root mean square error
The problem is that you are using a classifier. You should use RandomForestRegressor or other type regressors. 

Those three airports seems like leverage points. Check this wiki topic. You can log transform your target values or simply remove those 3 airports from your training set.

But still you have very less observations. You may want to add new data to your set. After train test split 35 observations for train 15 for test seems not so satisfactory.