Hot weather and poor air quality will smother the Santa Clarita Valley today.

The forecast calls for temperatures in the 90s, with light winds and above-average humidity today, said John Makevich, College of the Canyons weather specialist. The hot temperatures and high humidity is forecast to last through Friday, he said.

The culprit is a high pressure system above Southern California.

“High pressure does two things,” Makevich said. “It presses down on the air heating it up and it pulls moisture from the southeast into the area.”

The relative humidity is forecast to remain above 30 percent through Friday. Humidity that high is uncommon in California when the temperature rises above 90 degrees.

“Typically in Southern California, the hotter it gets the dryer it gets,” Makevich said.  

With the hot weather and high humidity will come poor air quality. According to the South Coast Air Quality Management District, today’s air quality will be unhealthy for sensitive individuals in the Santa Clarita Valley.

“The above-average humidity causes a strong inversion layer,” Makevich said.

The inversion layer is cold air held aloft by the warm air beneath it.

The layer acts like a trap holding in the hot air, Makevich said.

The inversion layer also holds in the air pollution, he added.

With poor air quality forecast for today, Jonathan E. Fielding, Los Angeles County health officer and public health director, advises people living or working in the Santa Clarita Valley with heart disease, asthma or other respiratory diseases to minimize outdoor activities.

Schools that have children with sensitive conditions, including heart disease, asthma, and other chronic respiratory diseases, should not participate in outdoor physical activity and should stay indoors as much as possible.  

The warm weather with temperatures in the 90s will continue through the Independence Day weekend, but the air quality could improve as the humidity diminishes, Makevich said.