Right. If the ambient air goes over the coolest water first its going to get slightly warmer (1.5C or less) which will be removing heat from that water....it will then go over the much warmer water--removing the bulk of the heat. Its about temp difference. The air going over the coolest water will remove some more heat (energy), but not enough to increase the air temp v. the water temp in the hottest rad. So, the air should still have a significant temp difference compared to the water in the hottest rad....and remove the bulk of the heat as it passes through.....

The reverse would just be reheating cooled water.....