How Do I Use Soundings to Forecast?

[Michael Wilkinson]

I'm learning the basics of what information is in a skew t diagram. I'd like to learn how to use them properly.

I think I understand that a sounding is just a snapshot of that particular time. I don't know how to use that information to forecast what will be happening several hours later.

I think I'm grasping how a parcel rises at the dry adiabatic rate and then rises at the moist adiabatic rate when condensation occurs.. But, I don't know the significance of the dew point line on the skew t diagram since the dew point that is important is that of the rising parcel(?) Or is the dew point line representative of the rising parcel rather than a current condition measured in the atmosphere?

If the dew point line is representative of the rising parcel, then I don't understand how a cloud can be forming all the way up to the equilibrium level when it seems that every skew t diagram I see has the dew point moving dramatically to the left side of the diagram at what appears to me to be a low level in the atmosphere.

Any help or links to educational materials that will help me understand this is appreciated.

 

[ChristofferB]

MetEd Skew-T Mastery course (free) is the best I have seen: https://www.meted.ucar.edu/mesoprim/hodograf/

 

[Michael Wilkinson]

MetEd Skew-T Mastery course (free) is the best I have seen: https://www.meted.ucar.edu/mesoprim/hodograf/

Thank you. I signed up for MetEd when I took my SkyWarn classes. I'll take the Skew-T Mastery course there too.

 

[Randy Jennings]

The best explanation of this I have seen is in Tim Vasquez's Weather Analysis & Forecasting book. I highly recommend every chaser buy and read his book. Here is Tim's explanation (note temperature is usually red and dewpoint green in colorized soundings):

"The environmental temperature profile for this day is drawn as a thick solid line (T), and the environmental dewpoint profile is drawn in thick dashed line (Td). We lift a parcel from the surface, noting its starting mixing ratio. The parcel's temperature always follows the dry adiabat until it meets this mixing ratio line. At this level, condensation occurs, and we consider this level to be the lifted condensation level (LCL). From that point, it follows the moist adiabat upward. This lifted temperature profile is compared to the environmental dewpoint profile. At levels where the lifted parcel is cooler than the environment, a negative energy area is indicated (the parcel is cooler than the surrounding air, thus it gains a sinking tendency). At levels where the lifted parcel is warmer than the environment, a positive energy area is indicated (the parcel is warmer than the surrounding air, thus it gains buoyant tendencies). Wherever the lifted parcel rises into an area of large positive energy, this level is called the level of free convection (LFC). The parcel rises until the surrounding temperature becomes warmer (typically in the stratosphere); the level at which this crossover occurs is called the equilibrium level (EL). The parcel will usually continue rising through its own momentum, and will sink back under the EL with time."

 

[Michael Wilkinson]

The best explanation of this I have seen is in Tim Vasquez's Weather Analysis & Forecasting book. I highly recommend every chaser buy and read his book. Here is Tim's explanation (note temperature is usually red and dewpoint green in colorized soundings):

"The environmental temperature profile for this day is drawn as a thick solid line (T), and the environmental dewpoint profile is drawn in thick dashed line (Td). We lift a parcel from the surface, noting its starting mixing ratio. The parcel's temperature always follows the dry adiabat until it meets this mixing ratio line. At this level, condensation occurs, and we consider this level to be the lifted condensation level (LCL). From that point, it follows the moist adiabat upward. This lifted temperature profile is compared to the environmental dewpoint profile. At levels where the lifted parcel is cooler than the environment, a negative energy area is indicated (the parcel is cooler than the surrounding air, thus it gains a sinking tendency). At levels where the lifted parcel is warmer than the environment, a positive energy area is indicated (the parcel is warmer than the surrounding air, thus it gains buoyant tendencies). Wherever the lifted parcel rises into an area of large positive energy, this level is called the level of free convection (LFC). The parcel rises until the surrounding temperature becomes warmer (typically in the stratosphere); the level at which this crossover occurs is called the equilibrium level (EL). The parcel will usually continue rising through its own momentum, and will sink back under the EL with time."

Thank you, Randy. If I understand correctly, I think that the parcel that is being lifted is always at its dew point once condensation starts, but the surrounding atmosphere (that it's being lifted into) is not at its dew point. I'm still not sure if I'm right about that or if I'm even thinking of it in correct terms.

I started the MetEd class today that deals with Skew-Ts. I'm continuing to try to understand it better. Thank you for the recommendation of Tim's book.

 

[Randy Jennings]

Yes, if a parcel cools to its dewpoint it is saturated and any further lifting will cause condensation. I should note that in some cases the lifted parcel's dewpoint can equal the environmental dewpoint at that level.

I should have taken the time to post a picture:



In this sample (tonight's OUN observed sounding on SPC website), The solid green line is the dewpoint, the solid red line is the temperature, the dashed green lines are the mixing ratio lines, and the dashed brown line is the rising parcel. I added the orange line to show how the LCL is calculated (starts at dew point and is parallel to the mixing ratio line until it crosses the dashed brown line). The parcel follows the dry adiabat until the LCL and then it follows the wet adiabat. It turns out that the dashed brown line is the temperature profile of the parcel. The dew point profile of the parcel follows my added orange line to the LCL then the dewpoint of the parcel is the same as the temperature (the dashed brown line above the LCL).

I should note that everything I talked about in my first post was based on the parcel being lifted from the surface. You should note that the picture I posted in this post has 4 LCLs listed in the table and it shows "ML LCL" (Mixed Layer LCL) on the graph. The parcel being lifted doesn't have to come from the surface. Since the starting points (temperature and dewpoint) are different, so too is the resulting LCL. The same methods apply no matter where you start.

 

[Michael Wilkinson]

I should note that everything I talked about in my first post was based on the parcel being lifted from the surface. You should note that the picture I posted in this post has 4 LCLs listed in the table and it shows "ML LCL" (Mixed Layer LCL) on the graph. The parcel being lifted doesn't have to come from the surface. Since the starting points (temperature and dewpoint) are different, so too is the resulting LCL. The same methods apply no matter where you start.

Thank you, Randy. Pointing out the 4 different LCLs is very helpful. I wasn't getting that before, but now it seems so obvious and makes sense.

I graduated high school long before there was such a thing as an online lecture. Now, trying to learn something new by watching online classes is making me realize how valuable it was to be able to raise my hand and ask the teacher to clarify something before moving on.

 

[Jeff House]

Also look for turning winds with height. Gradual turn from southeast, to south, then southwest, and perhaps almost west at jet stream level is beautiful. Increasing speed with height is preferred. I like to eyeball the actual wind barbs before looking at the hodograph.

I know the original question is about temps and dews. Just thought I'd throw in the wind part. Soundings are a powerful forecast tool.

 

[Michael Wilkinson]

Also look for turning winds with height. Gradual turn from southeast, to south, then southwest, and perhaps almost west at jet stream level is beautiful. Increasing speed with height is preferred. I like to eyeball the actual wind barbs before looking at the hodograph.

I know the original question is about temps and dews. Just thought I'd throw in the wind part. Soundings are a powerful forecast tool.

Thank you, Jeff. Tips that you've gained from your experience are appreciated. I've learned what the wind barbs are and how to read them, but that alone isn't very useful without knowing what a good setup looks like.