WEATHER is an important element in any setting, providing sensory texture and contributing to the mood the writer wishes to create in a scene. With a deft touch, weather can enhance the character's emotional response to a specific location, it can add conflict, and it can also (lightly) foreshadow coming events.
However, caution must accompany this entry: the weather should not be used as a window into a character's soul. The weather can add invisible pressure for the character, it can layer the SCENE with symbolism, it can carefully hint at the internal landscape, but it must never OVERTLY TELL emotion. Such a heavy-handed approach results in weather cliches and melodrama (a storm raging above a bloody battle, a broken-hearted girl crying in the rain).
Sight: panting dogs, people sweating through their clothes, animals and people more lethargic than usual, a pale cloudless sky, a general decrease of animals and people outside for long periods of time (at playgrounds, exercising, at the park, etc.), increased acts of violence, brushfires and wildfires, shimmery heat waves rising off the concrete
Smell: hot air, asphalt, sweat
Touch: clothes sticking to the body, sweat slicking the skin almost immediately upon stepping foot outside, hot air rolling over you as if you were standing in front of an oven, sunburn, the feel of heavy air weighing you down
Sound: the crunch of dry grass under your feet, air conditioners and oscillating fans blowing full-blast, weather reports issuing from TVs and radios, people complaining/talking about the heat
Mood: Heat has an escalating effect on emotions. Long-term heat makes people crankier, impatient, and short-tempered. Violent acts often increase during a prolonged heat wave. Being uncomfortable without knowing when it will end and being powerless to do anything about it can evoke feelings of desperation. The lethargy that accompanies prolonged heat can slow mental processes and effect production at work and school.
Symbolism: oppression, sin/punishment,
Possible Cliches: the heat wave that builds throughout the story, only to break at the climax or point of revelation; the heat wave as a backdrop for the steamy love scene; the heat wave that is broken by a welcome rain just before the crops go under
OTHER: Technical definitions vary, but a heat wave is widely accepted as being two or more days of excessively hot weather (which varies according to region). Power outages are common, as are burst water lines, wildfires, heat stroke, and increased mortality among infants, the elderly, and the chronically ill.
Don't be afraid to use the weather to add contrast. Unusual pairings, especially when drawing attention to the character's emotions, is a powerful trigger for tension. Consider how the bleak mood of a character is even more noticeable as morning sunlight dances across the crystals of fresh snow on the walk to work. Or how the feeling of betrayal is so much more poignant on a hot summer day. Likewise, success or joy can be hampered by a cutting wind or drizzling sleet, foreshadowing conflict to come.