Learn Hardcore Russian: В тени прохладнее, чем на солнце. - It is cooler in the shade than in the sun.

In Russian, the present‐tense form of быть (“to be”) is typically omitted.
So В тени (есть) прохладнее literally means “(It is) cooler in the shade…,” but Russians drop the “есть.”

What part of speech is прохладнее, and how is it formed?

прохладнее is the comparative form of the adjective прохладный (“cool”). Russian comparatives often add the suffix -ее or -ей:

сильный → сильнее (“stronger”)
прохладный → прохладнее (“cooler”)

Why is there a comma before чем?

In Russian, you place a comma before чем (“than”) when introducing a comparative clause or predicate. Here прохладнее is the main predicate, so the comma signals the comparison:
…прохладнее, чем на солнце.

Can I say более прохладно instead of прохладнее?

Yes. более прохладно (“more cool”) is grammatically correct, but Russian prefers synthetic comparatives like прохладнее for brevity. более + adjective is used more with longer adjectives or for emphasis.

Why do we use в тени but на солнце? Why not в солнце?

These are fixed, idiomatic preposition–noun combinations for location:

в тени = “in the shade” (a covered/inner space)
на солнце = “on the sun” (an open surface)
You also hear под солнцем (“under the sun”), but never в солнце.

Why aren’t there articles like the or a before тени and солнце?

Russian has no articles. Definiteness or indefiniteness is understood from context, so you simply say в тени or на солнце without the or a.

Can I invert the comparison and say На солнце жарче, чем в тени?

Absolutely. На солнце жарче, чем в тени means “It’s hotter in the sun than in the shade.” It’s a perfectly natural way to express the opposite comparison.