Your car is overheating. You might need a new radiator, or have it flushed. Also, you may need a new water pump. That's dangerous, as you could blow your engine. Call a few mechanics and get some quotes. Many of them will look at your car for no charge, then once you know what it is, call around for detailed estimates. It might help to run your heater if your car starts over-heating regularly, before you can get it fixed. If the temp is rising when the vehicle is at hwy speed then above is probably correct, if this happens in stop and go traffic it is normal as long as the temp gauge is not in the red.