Thermal Troughs, Adiabatic Processes, And A Warm End To The Week

Good evening everybody!

Today was beautiful! Most locations around Western Washington got into the upper 70s to lower 80s, and some areas east of the crest got much warmer! The Dalles was 96 degrees at 5 pm, but Buoy 46026 off the Columbia River was only 58. The Pacific Ocean sure cools things off!

As warm as today was, tomorrow will be even warmer. A large, upper-level ridge of high pressure will be directly over us, dramatically warming the entire atmosphere and giving us crystal clear skies. Moreover, we will have a weak thermal trough extending from Nevada into our region, which will pump warm air into our region and warm us.

Thermal troughs are local areas of low pressure that are generated by warm air masses. For us, they form when an upper-level ridge of high pressure is just to our east, forcing warm, continental air to flow eastward into our region. This air sinks as it slopes down the Cascades, warming it even further. Thus, we get a tongue of warm air extending to the northwest from the Intermountain West or Central Valley of California, and since, warm air is less dense than cool air, this tongue of warm air is associated with a trough of low pressure. They are responsible for almost all of our record breaking heat in the summer and early autumn, and tend to become more common as the summer transitions into autumn.

Offshore flow warms us up for two reasons. First off, the Pacific is really cold! I don’t care how sunny you are or how warm the temperatures are below the surface – if you are getting a stiff breeze coming off an ocean that stays in the low-mid 50s all summer long, you are not going to be hot. When you have offshore flow, the moderating effects of the Pacific are much less of a factor.

Additionally, we have a large mountain range to our east that offshore flow sinks down when coming into Western Washington. When air sinks, it warms at the “dry adiabatic lapse rate” of 9.8 degrees C per km. This is due to the compression of an air parcel as the air pressure around it increases with decreasing altitude, and while the total heat content of the parcel stays the same, the parcel becomes smaller, which means the temperature must go up in order to keep the total heat the same. A process in which no heat is exchanged with the atmosphere is called an “adiabatic process,” and the 9.8 degrees per km is known as the “dry adiabatic lapse rate.” There is another adiabatic lapse rate known as the “wet adiabatic lapse rate” which applies only when water vapor condenses into water droplets as air rises and cools. The wet adiabatic lapse rate varies with pressure but is generally around 6.5 degrees C per km, and is less because latent heat is released when water vapor condenses into water droplets, warming the atmosphere in the process.

For more, check out an old blog I wrote here on lapse rates and other thermodynamics stuff as part of a midterm study guide for one of my atmospheric science classes in college!: http://weathertogether.net/2013/11/01/atmos-301-thermodynamics-of-moist-air/

Alas, this thermal trough will have trouble generating a bunch of offshore flow because it will have trouble getting established to the west of our area over the coast, and this will keep temperatures from rising to unbearable levels. Nevertheless, tomorrow will be hot, with highs flirting with 90 degrees throughout Western Washington, and the trough will help minimize our onshore flow.

We cool off slightly next week to the low 80s as the thermal trough moves east and stronger onshore flow returns, but the long-term forecast still looks warmer and drier normal. Hopefully you can beat the heat!

BTW, I recommend doing everything in your power to try and catch the Perseid meteor shower Friday and Saturday night, especially Friday night. This annual shower is always spectacular, and this one should be especially awesome, as Jupiter’s gravity is directing more meteoroids than usual into our neck of the woods. The planets are aligned for a fantastic show!

Thanks for reading!

Charlie