Relentless is a good word for describing something that's harsh, unforgiving, and persistent, like the hot sun in the desert, or a cold that keeps you in bed for days with a nose like a strawberry.

When you're relentless about something, you mean business. You're not stopping until you get what you want, and you're not taking "no" for an answer. People might try to steer you from your goal, but you have eyes only for the prize and they're just going to have to get out of your way. If you're a relentless student, for instance, you might stay up all night studying your vocabulary, just in case there might be a quiz in the morning.

Definitions of relentless

adj never-ceasing

“the relentless beat of the drums”
persistent, unrelenting
occurring without interruption; chiefly restricted to what recurs regularly or frequently in a prolonged and closely spaced series

adj not to be placated or appeased or moved by entreaty

relentless persecution”
grim, inexorable, stern, unappeasable, unforgiving, unrelenting
incapable of being placated