What are the aircraft in the sky that look like skywriters but don't write anything?

As I drive east on the highway in Oklahoma I notice aircraft that fly vertically. And I mean Vertically. Straight up. They also go east to west. West to east. Every direction. They look as though they are skywriters but do nothing. A long stream of smoke follows this little dot (aircraft). What in the world is that?
Also, there might be 5 or 6 at a time (in the sky) flying in different directions.
By the way, I know they are aircraft – It seems odd to see them flying in the manner that they fly and with so many in the sky at one time. I used to date a pilot and I also worked at a aircraft maintenance facility.
It just looks so odd driving down the highway. I just wonder what purpose they serve.

Thanks to everybody who have replied for the purpose of educating me.