If miserable weather on the night of the concert requires you to leave much earlier for the concert hall and greatly extends the time it takes to get home afterward, the opportunity cost of attending the concert

The opportunity cost is simply anything you would have used the time of attending the concert for.

Explanation: An opportunity cost is simply alternative forgone. Something you could have done had it been you chose not to do what you did or something you could have bought if you chose not to buy what you bought.

More information are required in the question - information relating to the alternative activities you would have done if you had chosen not to attend the concert. If for instance, the concert took approximately 5 hours of your time. The 5 hours could have been used for other visits or adventures, study, sleep, cook, play some games, etc. If you chose any of these as the alternative use of your time, that would simply be your opportunity cost.  

increases

Explanation: due to the time spent in hurrying to the concert when you could have stayed home engaged in something else, and the time spent waiting for the weather to clear up so you can go home, the opportunity cost of attending the concert increases. The cost of an opportunity forgone (and the loss of the benefits that could be received from that opportunity) is defined as the opportunity cost. It is the most valuable forgone alternative. The opportunity cost of attending a concert could be whatever is the next best alternative you consider for you if you had not be hastened up or kept back by the miserable weather.

what

explanation: