Tuesday, January 13, 2015

The Hiker's Inversion

The lookout hut at the summit of Mt. Pilchuck
I am going to preach here and say that the best weather to go on a shorter day hike in is not just when it's incredibly sunny out, but also when there's the presence of a low-level temperature inversion in place. And we've actually seen this type of weather pattern here all over the Seattle region for the past few days and most of last week. I planned a trek up to Mt. Pilchuck yesterday and it was absolutely breathtaking at the top. I could see and feel the inversion in place and pretty much had a grin on my face the entire way up.

First of all, let's talk briefly about weather and then this temperature inversion. Typically, in the lowest level of the atmosphere, also known as the Troposphere, temperature decreases with altitude. So if temp is decreasing as you go higher, you should have colder air masses up there, with warmer air masses below or closer to the ground. Warm air is less dense and wants to rise into the cold air above that is simultaneously trying to sink. These motions produce convection in our atmosphere and can form (certain types of) clouds, turbulence, strong winds, etc. This process is just our atmosphere having a normal day.

However, when we get an inversion, this is all reversed. Temperatures are increasing as you go higher in altitude. There will be colder air at the surface and under warmer air aloft. This does not induce convection and the air column is stagnant. Having a stagnant air column is not a great thing in most cases because an inversion basically acts like a cap and can trap in fog or pollution. This is happening in the Seattle region right now with all this dense fog buildup, you can see it's abundance if you've been up in the early morning.

UIL Skew-T Diagram 4am Jan 14, 2015 
A good question to bring is up is how would you know there is a low-level inversion in the area? Well, you could study meteorology like I did and learn how to read a few types of weather maps and models like the skew-T diagram. An easy way, however, is to look for certain indicators, like this dense fog that has stayed around for a while. Or you could tune into weather on the news, good meteorologists would mention something about this - I certainly will! Here is a skew-T diagram, however, from the weather station out in Quillayute, WA. The red line is tracking the outdoor temperature in celsius (x-axis) with pressure altitude (y-axis) in millibars. It's a little tricky to understand but the red line has increasing temperatures with height up until about the 950 mb height, which is roughly 2,000 feet or something, with constant temperature all the way up until 650 mbs, roughly 10,000 feet. Crazy! This is the inversion! All that warm air aloft at roughly 15 C° is capping the colder air at the surface.

Enough science and onto my hike up to Pilchuck yesterday. I initially wanted to go so that I could see all of the fog trapped below by the inversion. I snapped a good photo as I climbed into the lookout hut at the top. Check it out to the right, you can see all that white fog in the valleys. It's super dense and very much trapped there. The beautiful Mt. Rainier is also peaking out in the back.

Where I stopped at the hut, the temperature also felt warmer than in Granite Falls. I was wearing three layers and freezing at the bottom. As I hiked up, the air temp became less brisk and turned to a nice mild temp. This is why the low-level inversion is a plus*. I became more comfortable as I climbed. I would guess the temp was near freezing down there, and somewhere in the upper 40s at the top, where I took off my outer layers, ate lunch, and basked in the sun.

I'm lucky I had the day off to hike up Pilchuck with this sunny weather and inversion. I encourage everyone to get out here to the lookout hut at the top, it is now one of my favorite hikes around with its full 360° view of our beautiful PNW mountain ranges. And I'd to mention that the lookout is one snazzy, well groomed hut to hangout in. ⤱

*A temperature inversion may not be ideal or safe for climbing to high altitudes where there is lots of ice or glaciers. Low-level inversions are perfect for short day hikes like the ones we have all around the northwest. 

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