Learn Finnish: Kesällä sää on usein aurinkoinen ja lämmin. - In summer the weather is often sunny and warm.

In Finnish, seasons use the -llä / -llä ending (the adessive case) to express when something happens, rather than the "in" ending (-ssa / -ssä). So, kesä (summer) becomes kesällä (in summer).

Is aurinkoinen related to the word for "sun"?

Yes! Aurinko means "sun". By adding the common adjective ending -nen (which becomes -inen here), it turns into the adjective aurinkoinen, meaning "sunny".

Why are aurinkoinen and lämmin in their basic dictionary forms?

In Finnish, adjectives must match the noun they describe. Because the noun they are describing, sää (weather), is in its basic form (the nominative case) as the subject of the sentence, the adjectives stay in their basic form too.

I've seen the word ilma used for weather. What is the difference between sää and ilma?

Sää specifically refers to the "weather". Ilma literally means "air", but Finns often use it in everyday speech to mean weather too. In this sentence, you could replace sää with ilma and the meaning would be exactly the same.