When you're making photos out in the landscape, do you stop for a moment, and watch? In particular, do you pay particular attention to the speed of moving clouds? I do.
Sometimes participants on workshops ask me 'how long should I make the exposure for?' when they want to get blur in their photos. I think the answer can be found without asking me. You just need to look at the clouds and watch them as they drift across the sky, and while you're doing that, count the seconds it takes for them to move. It's really as simple as that. Only a lot of us aren't looking. We're not watching. We just fire the camera and wait to see what pops up on the screen.
But I love to anticipate. To study. To get to know the movement of clouds, waves, even the vibration of the trees due to a light wind. I'm a studier of movement in the landscape.
Particularly where long exposures are concerned. If it's a windy day, then I'm all excited as I know 20 or 30 seconds is an eternity and I'll get long streaks like the ones you see in my Harris photo above. If it's a calm day, then I know there's almost little to no movement and most probably - no point in using a long exposure.
But I still stand and watch, and wait, and watch some more. Just to make sure.