Daily News Updates

  The National Weather Service issued another excessive-heat warning on Tuesday for much of Washington state and Oregon that will remain in effect until Sunday. There are also heat advisories in the Northeast, from Philadelphia to Boston. Here is what you need to know about these heat waves.  What causes a heat wave? In most parts of the country, temperatures must be above the historical average in an area for two or more days before the label “heat wave” is applied to a hot spell, according to the National Weather Service. But the definition can vary by region; in the Northeast, it is defined as three straight days in the 90s or above. Heat waves begin when high pressure in the atmosphere moves in and pushes warm air toward the ground. That air warms up further as it is compressed, and we begin to feel a lot hotter. What is a heat dome? As the ground warms, it loses moisture, which makes it easier to heat even more. And in the drought-ridden West, there is plenty of heat for ...