Assuming even light, if possible, you want to take your meter reading off of a middle grey - the pavement looks like a good choice here.
In most cases that will determine your exposure.
You certainly want to avoid having the sky determine your exposure, unless the sky is what you are most interested in.
In rare cases you need to go further. Sometimes there are subjects of interest in the frame that are relatively much lighter or much darker than average, where a normal and otherwise correct exposure won't result in those subjects appearing the way you want them. In those unusual circumstances you may make the decision to change the exposure in order to favour those subjects, at the expense of the rest of the frame. An example would be strongly backlit subjects, where the foreground is emphasized at the expense of the blown out background.
Generally speaking you don't adjust your EI with circumstances - you determine your EI when you calibrate your system, and stay with that EI. Subsequently you may make decisions to deliberately under-expose or over-expose certain subjects, based primarily on issues of contrast.
Thank you very much. I shall use your advice this weekend and see how it goes.