SWELTERING DAYS CREATE HOTHEADS.

Byline: David R. Baker Daily News Staff Writer

When his customers stagger in from the kiln-like parking lot in Woodland Hills, Ruby's Diner manager Gary Edelstein can see what this week's heat wave is doing to folks.

Are they thirstier? Yes. Crabbier? You bet.

``They become different people,'' he said. ``They're just irritable, sometimes quieter, sometimes less patient.''

It is an ugly fact of summer psychology: Stuck in high heat, even the mellowest souls can turn surly, irritated by things they normally wouldn't notice, psychologists say.

``We start sending out signals that say, Back off,'' psychologist Richard Landis said.

And tempers certainly were flaring Tuesday, as the mercury hit 100 in Woodland Hills and 107 in Chatsworth - with temperatures expected to drop by only 4 or 5 degrees today.

Experts say the crabbiness is a natural reaction, tied to how the body responds to soaring temperatures. The best way to deal with it, they say, is water, water and more water - plus a determination to keep trivial problems in perspective.

``Just being aware that it's a physical thing - not something someone else is doing - can help,'' Landis said.

So does trying to keep cool, literally.

Heather Marts finally broke down and bought an air conditioner this week after four sweltering summers in her Glendale apartment. The heat, she said, definitely affects her mood.

``It's called being bitchy,'' she said.

Profuse sweating aside, unusual heat does some unpleasant things to the human body. Many of these effects, psychologists say, conspire to make people sluggish, which in turn can lead to crankiness.

First, like physical exercise or hard work, heat can raise one's alertness level, eventually tiring the body out, said Albert Mehrabian, professor emeritus of environmental psychology at the University of California, Los Angeles. Prolonged high temperatures also can force the body to slow down - to avoid generating more heat, Landis said.

Furthermore, a heat wave will play hell with sleep schedules, leading, again, to drowsiness.

But since American culture lacks the tradition of a mid-day nap common in many warmer climes, people must push their tired bodies to work. That causes crankiness, Landis said.

``People don't want to move,'' he said. ``Unfortunately, most people have to work to eat.''

There also is a simpler explanation for why folks get testy in a heat wave: high temperatures are just plain unpleasant. And like other unpleasant environmental conditions - such as filth or overcrowding - they get on your nerves.

``Any of those things will tend to result in people being less friendly, less helpful, less respectful,'' Mehrabian said. ``When you're in a place that's very beautiful and pleasant, people are very nice to each other. Then you go to a place like New York, it's ugly and congested, and people are rude.''

No one need tell a waitress that scalding temperatures drive customers nuts.

``The heat's definitely making them crankier,'' said Kristin Greenfield, a server at the Wolfgang Puck restaurant in The Promenade at Woodland Hills.

Not like they give her any lip, or tip any less. But they can get pretty insistent, especially when it comes to demanding drinks.

``Most people are like, JUST GIVE ME WATER,'' she says, clenching her fists over the bar for emphasis. ``They drink a lot more: lemonade, iced tea, lots of water.''

In this, at least, her customers know what they're doing. Experts recommend drinking up to eight glasses per day to replace the moisture rushing from your pores. Avoid excessive caffeine, which can actually dehydrate the body, and don't drink alcohol after dinner, since it can interfere with sleep, Landis said.

Edelstein said that with a few liquids, and a few minutes in a cool, comfortable booth, most customers return to their more human selves.

``Nice, cool air-conditioning, an iced tea, they'll calm down,'' he said.

Daily News Staff Writer Lee Condon contributed to this story.