Read Temperature/Humidity

Opening the serial port resets the Arduino. Closing the serial port resets the Arduino. Is there some reason for doing that every time you want the temperature and humidity?

            SReaderT1 = SerialPort1.ReadLine()

SReaderT1 is a String.

        TemperatureHumidity = SReaderT1

TemperatureHumidity is an Integer. You can't assign a String to an Integer.
Even if you could, which float value in the string do you want TemperatureHumidity to contain? The temperature OR the humidity.

This is REALLY a dumb name for a variable. As well as being the wrong type.

If you are just adding the serial data to the list box, why not just add the String?