What did darius expect alexander to do?

He ordered his troops to be well fed and then to rest. Darius, on the other hand, mindful of Alexander's unpredictable manoeuvres, ordered his army to stand to throughout the night in readiness for a surprise attack. ... By overrunning the Persian Empire, Alexander helped extend Macedonian and Roman rule.

b. Stage a night attack

the causes of the dust bowls were severe droughts,   lands became dry and hard, temperature was higher than usual, hot and dry conditions killed crops, high winds. because of those factors, the winds kicked up dry soil and created clouds of dust.