High tempreature and Low Humidity == Increased pressure from discharge of Powder…This is due to Two factors, one the Boyle’s Law, PV over T is a constant (ie, Pressure and Volume vary inversely as the temperature; if Volume is constant and the Temperature rises, the Pressure increases…
The second factor: The Humidity factor is that all Powder contains molecular Water (not all is “dried out” during either Sulfuric Acid washing in Nitration, or by the Alcohol-Acetone method;) High temperature also drives out the water, and this causes the Nitrocellulose to become more sensitive and produce a Higher pressure (and temperature).
Thus “tropical” ammo is loaded especially for the High temperatures (lower Powder charge)…a fact noted by the Australian manufacturer during the First Gulf War, when Normal 7,62 and 5,56 made for Australian Climatic conditions performed badly in Iraq. A similar reverse problem occurs with ammo which is meant to be used in Arctic areas. (higher Powder charge for same results as in temperate ziones.)
Tropicalised ammo (German) was also better sealed, to prevent humidity being absorbed during the cold nights in the desert ( condensation in cold nights after extremely hot days…)