How hot it is today!
'Caldo' — CAL-do. Stress on the first syllable. 'Fa' — short, unstressed. The exclamation mark matters — this is expressive.
Use on hot days, especially in summer. In Italy, complaining about heat is a social ritual — particularly in the south and in humid Po Valley cities like Milan and Bologna.
In Italian weather expressions, 'fare' (to do/make) is the main verb: 'fa caldo' (it's hot), 'fa freddo' (it's cold), 'fa bello' (it's nice). You don't say 'è caldo' for weather — that would mean the object itself is hot. 'Fa' is impersonal, referring to the weather condition.
Si muore di caldo!
You'd die of the heat!
Very dramatic — very Italian. 'Morire di caldo' is standard hyperbole for hot weather
Questo caldo è insopportabile.
This heat is unbearable.
More measured — expresses genuine suffering rather than playful exaggeration
L'afa è terribile oggi.
The humidity is terrible today.
'Afa' = muggy heat — a very specific and dreaded Italian summer condition
Italian summers, especially in the south and the Po Valley, can reach dangerous heat levels. The 'colpo di calore' (heat stroke) is a genuine medical concern. Italians take heat very seriously and routinely cancel outdoor activities when it exceeds 35°C.