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CHICO, Calif. - There are two big factors that drive extreme heat in California. The first is the placement of the jet stream and the second are the winds and placement of the surface high pressure system.
Before we get into the nitty gritty details, you are probably wondering, what is the jet stream. The jet stream is a river of air high up in the atmosphere. It steers major weather features, can contribute to strong surface winds, and determines how much heat or cold weather will move into the region.
There are two key areas of the jet stream to watch that determine how hot it will get in California.
The first is how the jet stream is shaped. There are ridges of high pressure and troughs of low pressure.
A ridge is usually when the jet stream has a bump in it that reaches towards the poles and usually has a cooresponding trough next to it, an area of the jet stream that reaches towards the equator.
The interesting thing about a ridge in the jet stream is that it allows for warm tropical air to push northward, leading to temperatures being warming than normal.
A ridge in the jet stream is a common occurence all year round. It usually leads to wonderful weather in the winter but it can leading to swealtering temperatures in the summer.
The reason for this is due to how far north the jet stream is placed. Usually in the winter it is located around the Pacific northwest or even California whereas in summer the jet stream is pushed way up into Canada.
The farther north the jet stream is placed, the easier it is for more intense warm air to move into California and the harder it is for cold polar air to affect temperatures farther south.
The second big player that drives California's heat waves are persistent north winds in the summer.
You've probably all heard about the the delta breeze. It is a cool breeze that blows from the south towards the north in the valley. This allows cooler daytime high temperatures during hot times of the year. The breeze is cool because it brings in air from the Bay Area, which is much cooler thanks to the Pacific Ocean.
However, what about a north wind? A wind that blows from the north, towards the south.
This is usually what we see during some of our strongest heat waves in Califronia.
North winds (or Northeast winds depending on where you are located) flow down the slopes of the Lower Klamath Mountains (and/or Coast Ranges) and the Sierra-Cascades into the Valley. These downslope winds which are most common and pronounced in the summer, cause temperatures to be warmer in the Valley for two reasons.
North winds that travel downslope compress and warm using the adiabatic proce3ss. Meaning that if an imaginary parcel of air were to sink, it would compress and gain internal energy. This added energy often leads to the air being warmer as it sinks and thus contributes to warmer temperatures in our area during the Summer.
Downslope sinking winds also cause the air to be drier. Clouds are formed by rising air. But downslope, sinking air prevents clouds from forming and leads to sunny skies. Sunnier weather always means warmer weather due to increased solar radiation.
Another factor that plays into high heat in cities like Chico or Redding, is called the urban heat island effect. This leads to warmer temperatures due to more pollution trapping heat plus concrete which absorbs heat.