The first raindrops after weeks of dry weather could make your drive much more dangerous than you think.

When rain finally hits a road that's been baking in the sun, it can turn into a slippery mess almost instantly.

That's because oil, dirt and grime build up on pavement during dry stretches. When rain arrives, those substances mix with water, creating a slick surface that can make it harder for your tires to grip the road.

And the longer it's been dry, the greater the danger.

Behind the science

A University of California, Berkeley, study analyzing more than 1 million fatal crashes across 48 states found that less than a half inch of rain increased the risk of a deadly crash by 3.7% if it had only been two days since the last rainfall.

But after 21 days or more without rain, that same amount of precipitation increased the risk by 9.2%.

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(MORE: What every driver should know about driving in rain)

There's another factor at play, too: Drivers may simply be out of practice.

After weeks of sunshine, people aren't necessarily prepared for slippery roads, reduced visibility or the extra stopping distance that comes with rain.

This is especially important in drought-prone parts of the country, including the Southwest, Southern Plains and parts of California, where long stretches without rain can make the first showers particularly hazardous for drivers.

(MORE: How to navigate the dangers of sun glare while driving)

How to stay safe

If you're behind the wheel when those first drops finally fall, don't wait for a downpour to take precautions.

Slow down, leave extra space between your vehicle and the one ahead, avoid sudden braking or sharp turns, and give yourself more time to reach your destination.