Cairo has a desert climate with hot, dry summers and mild winters. May-September temperatures are 90-105 F/32-40 C. January and February are generally the mildest months, with average temperatures peaking around 65 F/18 C and dropping to about 45 F/7 C. There is virtually no rainfall, so the city's drainage system is nonexistent.
Spring and fall are the most pleasant times of the year. Summer is blazingly hot in Luxor and Aswan but much nicer on the northern coast and the Red Sea and Sinai areas. Winter is cooler than most visitors expect between Cairo and Alexandria.
The windy khamasin (sandstorm) season runs late March-early May. The sandstorms are not heavy enough to bury you, but they do raise the dust and turn the air a hazy, gray-beige color.