May 02, 2024

A Very Unstable Spring Atmosphere

Spring is the time when our local atmosphere is most unstable, often percolating in upward and downward motions like hot cereal in your stove-top saucepan.

The visual expression of such unstable motions is cumulus convection, ranging from relatively benign fair-weather cumulus to towering cumulonimbus.  

On Tuesday, the atmosphere became unstable and a full range of cumulus clouds was evident.

During the afternoon, relatively shallow cumulus clouds had formed by 2 PM (see below from the Seattle PanoCam on the Space Needle).  No precipitation was evident underneath them.


By 5 PM, following afternoon surface heating, vertical turrets start to form, indicating greater instability and vertical development.


The 6 PM image showed even more vertical development and the formation of two cirrus anvils (the flat, elongated objects at the top of the image.)


30 minutes later the anvils had explored in size.  Very impressive!  It was raining underneath the big cumulus clouds and lightning was observed


Cumulus clouds like this are a sign of vertical instability driven by large temperature changes in the vertical.

Stability 101

Most of the time the atmosphere is stable.   You push some air upwards and it comes back down.

But if the temperature declines with height rapidly enough, the situation changes.    In this case,  a displaced air parcel can accelerate upwards.   

The explanation for this behavior is something we describe in detail in my department's classes, but the essential point is this:

When temperature cools rapidly with height it becomes easier for an air parcel to be warmer and less dense than the surrounding air.  And such warm/less dense air parcels tend to be buoyant and rise.

Think of a hot-air balloon! (see below)


Here in the Pacific Northwest, the change in temperature with height is greatest during the spring.  That is why this is the best time of the year to see impressive cumulus clouds.

But why spring, you ask?

During the spring, after a long winter, the temperatures aloft are quite cold...in fact, the coldest of the year.  To show this, below are the temperatures around 10,000 ft (700 hPa pressure) at Forks, WA, on the NW coast.

Lowest temperatures in March and April.  Quite cold in early May.


While temperatures are cold aloft, solar heating is getting very strong now, particularly on days without extensive clouds.  Below is the plot of solar radiation reaching the surface in Seattle for the last year.  Getting pretty strong lately.    Keep you sunscreen handy.

May 2 sun is roughly as intense as early August!


So with strong heating at the surface and cool temperatures aloft, there is often a very large change in temperature with height during early spring, which fosters lots of cumulus clouds....from shallow ones to deep thunderstorm clouds.  

Just to drive the point home, here is a plot of the change in observed temperature with height (termed the lapse rate) at Forks/Quillayute between the surface and 3 km (about 10,000 ft).    Very high this time of the year.


Finally, you can see the growth of cumulus during the day produced by increasing temperature change with height.   Let's start with the visible satellite image over eastern Washington on Wednesday morning around 8 AM.  No clouds over the Columbia Basin.


By 2 PM, with plenty of solar heating at the surface, the temperature change with height is greatly enhanced and instability has developed, leading to lots of cumulus clouds.












toweromg 

8 comments:

  1. Been quite the temperature swings between night and day for us in Okanogan Valley. I know your talking about difference with temps at height, but the surface swings quite dramatic too. At my place roughly 800 ft, morning are cold enough for irrigation to be covered in ice and warm enough in afternoon to tan. The wind machines been working hard to save the cherry and apple crops. Turning on around midnight and off around 7am when sun breaks over hill. Would hate to have their fuel bill. Been at least 10 or more nights they have ran since bloom started.

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  2. While we lived in the Phoenix "Valley of the Sun," for a number of years, the most dramatic air movements occurred during the July-Sept monsoon season. The result of such air movement was strong and occasionally violent thunderstorms. Just like in the PNW, when heating from the setting sun subsided, cooling moist air rushed downward, and at times created "micro-burst" thunderstorms similar to hurricane force winds resulting in downed power lines, trees and sometimes damage to structures.

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    Replies
    1. Everyplace else has more exciting weather than here. Last time I saw a decent thunderstorm here was May 2017. I heard here was one in Sept 2019 that I missed. I guess what few we get tend to the mountains.

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  3. On the 2 PM satellite view, there is a short blue line crossing the N-S line. This spot is at 47° X 120°. The cloud-free elongated region to the northwest extends toward Snoqualmie Pass. This is the Kittitas Valley. We frequently see the mountain induced clouds stream along the ridges to the north and south. Blewett Pass (Hwy 97) is to the north and Vanderbilt Gap (I-82) to the south.

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  4. Why is the dewpoint so low? According to NOAA weather radio it has been dipping into the upper 20's at times.

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  5. Nice to see your graph of the lapse rate Cliff. It is the first time I have seen the data plotted that way (what is the period of record?). I wonder how it compares to the average lapse rates of the Rockies and points east where thunderstorms are most common in summer.

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  6. Cliff: I observed this cloud formation early evening Tuesday from NE Seattle, and it was quite impressive! Thanks for sharing the images of it and writing about it.

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  7. I love cumulus clouds, they can be so pretty when the weather is fair. Back in March, I had a dream while napping, not once, but twice where the dream involved beautiful sunny weather, cumulus clouds, I think it was my mind telling me I'm ready for nice, warm weather to come.

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