The minivan gives the outside temperature.
The GPS unit gives the altitude.
saturated adiabatic lapse rate.
He: That's because we are driving into a storm front.
She: Isn't that what I just said?
Bystander in the backseat: Huh?
Background: If a parcel of dry air is lifted adiabatically (without exchanging heat with its surroundings), then the temperature cools off 9.8 degrees Celsius per 1000 meters altitude. If the air is saturated, it cools off more slowly, about 5 degrees C per 1000 meters.
Traveling between Durango and Silverton, along the Million Dollar Highway, our elevation went up by over 3000 feet or about a 1000 meters, but the temperature only changed about 3 degrees C.
That very low lapse rate implied that we had crossed the boundary between two distinct air parcels--what is commonly called a front. It might have also implied that we got caught in the upslope of a diurnal mountain-valley circulation, but it was the opposite of the type of calm and sunny day that sets up that type of wind pattern. It was cloudy and cold. When two air masses collide, a violent storm can erupt.
Sure enough, it started to rain as we crested the pass, gradually becoming heavier and there were even a small amount of hail. What a great day for a picnic! ;-)
On that day, it did NOT look like this picture from Wikipedia commons, but it was beautiful nonetheless.