IL-6: Forecasting Precipitation Types

• Due Sunday by 11:59pm
• Points 6
• Submitting a text entry box or a file upload
• Available after Jan 13 at 12am

In this lab we will explore precipitation formation and how to forecast different precipitation types.

Part I Precipitation Formation

In previous activities we found that to form a cloud you have to bring the air to saturation, or to a relative humidity of 100%. The two ways this can happen is by reducing the temperature of the air or adding water vapor to the air. In the atmosphere this usually occurs by reducing the temperature of the air by lift. In previous chapters they discussed some of the lifting mechanisms in the atmosphere including fronts, convection, mountains, and convergence. All of these will make it more likely that a cloud will form. Further, having an unstable atmosphere will make it more likely that air will continue to rise and increase the changes of cloud formation.

However, to create precipitation that will actually fall to ground we have to grow cloud droplets or crystals to a large enough size so they will fall to the ground due to gravity. Condensation alone is much to slow to form precipitation sized droplets on their own, so the two ways we can get large droplets or crystals are collision/coalescence and the bergeron-findeisen processes. Watch the video below, which reviews these two processes, and when they occur:

https://www.youtube.com/watch?v=WByQ3yKxj1s (Links to an external site.)

Part II Precipitation Types

Once droplets or crystals become large enough that they begin to fall to the ground we have precipitation. However, depending on the temperature structure of the atmosphere, different types of precipitation are likely to occur. The main types of precipitation include rain, snow, freezing rain, and sleet. Hail is a particular precipitation type that forms mostly in severe thunderstorms, so we won’t be discussing it in detail here.

Watch this video that gives a brief review of when we will get these different precipitation types:

https://www.youtube.com/watch?v=y0vNZ6-oeK4 (Links to an external site.)

From previous activities you should now be familiar with stuve diagrams. These diagrams show the temperature and moisture (dew point) of the atmosphere above a certain location at a certain time, as measured from a radiosonde (also known as a weather balloon).

In the activity below you will be analyzing four stuve diagrams and using them to then forecast the most likely kind of precipitation that fell at those locations on the days the sounding was taken. You will likely need to use your textbook to help you answer some of the questions and complete the forecasts. As you complete the activity feel free to ask questions in a discussion or send me a message.

ForecastingPrecipitation-1 x

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Forecasting Precipitation Types Name __________________________

The four main precipitation types we will be investigating are rain, snow, sleet, and freezing rain.

1. On the diagram below, fill in the name of each phase change:

GAS LIQUID SOLID

2. Each of these precipitation types are produced by phase changes of water. Describe the ALL the phase changes and other processes that happen to produce each type of precipitation (for some there might be multiple ways to form them):

RAIN

SNOW

SLEET

FREEZING RAIN

3. Look at the upper air diagrams (stuve) on the following pages. For each location describe how the

(a)

temperature (how does T change as you go up, you can describe this in pressure layers) and

(b)

moisture characteristics (how close or far are T and Td as you go up, describe in pressure layers)

change as you rise through the atmosphere.

Make sure you include the surface temperature, what layers are above or below 0 degrees C, and the maximum temperature observed.

SGF Springfield

(a)

(b)

SLE Salem

(a)

(b)

TBW Tampa Bay

(a)

(b)

SLC Salt Lake City

(a)

(b)

4. Looking at the information you described in questions 1 and 2, as well as the upper air diagrams (stuve) on the following pages, provide a forecast what precipitation type you think will fall at each location. Hint: Each precipitation type will only be used once. Why did you produce that forecast?

SGF Springfield Precipitation Type ____________________

WHY?

SLE Salem Precipitation Type ____________________

WHY?

TBW Tampa Bay Precipitation Type ____________________

WHY?

SLC Salt Lake City Precipitation Type ____________________

WHY?

5. Which precipitation type do you think would have the biggest impact if it fell on a large city? WHY?

6. Imagine that a storm moves into Salt Lake City at 8am. From 8-10am the precipitation falls as rain, from 10-11am the precipitation falls as sleet, and from 11am-5pm the precipitation falls as snow. Describe and/or sketch how the vertical temperature structure of the atmosphere over Salt Lake City might have changed throughout the day.