Hey all.

My sims wear their outerwear in any season when they go outside. is this the way its meant to be, or what?

Sims wear outerwear when it's cold enough to be wearing it, regardless of the actual season. Though winter is usually coldest and they'll almost always wear outerwear when outside in winter, it can get cold any time during the year (just like real life) so you can have it cold and even snowing in autumn or spring and even, rarely, in summer.