What is a 'thermal trough'?



    The thermal trough is responsible for almost all of our heat waves in the summertime. A lot of times, we associate low pressure with stormy, rainy weather. But intense heat can also cause low pressure.

    In the hot desert, you have a lot of warm air rising into the upper atmosphere. That creates lower pressure near the ground as you now have less air around.

    This is a near-daily occurrence in the summer in the Desert Southwest and California. However, if strong high pressure moves south out of Canada and into the Rocky Mountain states, that can actually push the thermal trough north into the Pacific Northwest.

    At this point, the placement of the trough is key to the temperatures around the area. In some sense, it might be best to think of the trough as a valley, and air will want to pour into the valley to fill it.
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    A few times a year, the trough will actually come north either along the Pacific Coastline or even out over the ocean. When that happens and the trough is to our west, you have air now rushing from the east out toward the offshore trough (See

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