temp problem

The temperature depends on how CPU/GPU-intense the apps are that you are running. An idling MBA stays fairly cool, obviously.

Activity Monitor (under Applications -> Utilities) can tell you what draws which resources and how much.

Again, it's predominantly the CPU/GPU-load which causes the temperature to be lower or higher.

Of course, if you run multiple apps or apps that require huge amounts of RAM but you're low on RAM, the CPU will be busy with swapping data back and forth in between RAM and SSD, too.

If it stays hot in what you think is idling, maybe it isn't idling but running processes "silently" in the background. Again: Activity Monitor, best the CPU tab, and the table sorted by % CPU descending.

Before you ask: No, you cannot upgrade the RAM in an MBA since it's firmly soldered onto the logic board.

No one can help you if you don't read their posts. I repeat:

If it stays hot in what you think is idling, maybe it isn't idling but running processes "silently" in the background. Again: Activity Monitor, best the CPU tab, and the table sorted by % CPU descending.

You can quit processes there, but that way you can stall your machine, eventually. It's smarter to find out if the active, resources-hungry processes are necessary, and if not, what started them and go to the cause to avoid them getting started in the future.

Before you ask: In case of doubt, googling a process' name may help.