U. S. National Weather Service - http://www.srh.noaa.gov/jetstream/global/hi.htm

How they occur —High pressure up traps heat near the ground, forming a heat wave.

Summer doesn’t officially begin until Sunday, but the Southland was basking today in some summer-like conditions, and authorities urged the public to take some simple common-sense steps to avoid heat-related ailments.

Temperatures are expected to be 15 to 25 degrees above normal across Southern California through the end of the weekend, according to the National Weather Service.

During the week, temperatures had already reached 99 in areas throughout the San Fernando Valley.

Forecasters said the heat wave is expected to continue into early next week, with the  highest temperatures expected over the weekend.

State workplace-safety officials called on employers with outdoor workers to protect them from the intense temperatures.

“During times of sustained high heat, it is especially important that employers take the necessary steps to prevent heat illness for their outdoor workers,” said Christine Baker, director of the state Department of Industrial Relations.

California’s 10-year-old regulations for heat-illness prevention were amended effective May 1. The changes clarify requirements related to the provision of water rest and shade protections for all outdoor workers, which are detailed in Cal/OSHA’s guidance on the new requirements.

Employer requirements under the heat regulation include:

◆ training for all employees and supervisors about heat-illness prevention;

◆ provision of cool, fresh water as close as practicable to the work area at no cost to workers;

◆ shade provided whenever the temperature rises above 80 degrees Fahrenheit and enough shade to accommodate the number of workers taking a break;

◆ encouragement of employees to take cool-down rests and monitoring for

symptoms of heat illness; and

◆ acclimatization to ensure that workers, especially new hires, safely adapt to increased temperatures during a heat wave.

Special high-heat procedures are also required when temperatures reach 95 degrees or higher and workers are at greater risk. At these times, supervisors must take the following extra precautions:

◆ observe workers for signs and symptoms of heat illness;

◆ hold pre-shift meetings on safety and remind workers to drink water frequently;

◆ require that workers take a cool-down rest every two hours; and

◆ ensure effective communication systems are in place so that emergency assistance can be summoned immediately if necessary.

Cal/OSHA inspects work sites in outdoor industries such as agriculture, construction, landscaping, and others throughout the heat season. Through partnerships with various employer and worker organizations in different industries, the agency also provides consultation, outreach and training on heat illness prevention.