Grow a thunderstorm
Stability
As we learned in earlier chapters, a parcel of air will not rise unless it is unstable (i.e. forced upward from the surface and/or warmer than its surrounding environment). More generally, the condition of the environment around warm, moist air makes all the difference between a clear sky, a garden-variety thunderstorm, and a severe thunderstorm with tornadoes. The crucial factors are the environment’s temperature, moisture, and wind speed and direction from the ground all the way up to the tropopause. The applet below address the impact of the environment temperature and moisture.
Whether a thunderstorm produces severe weather, such as hail, is largely dependent on the stability of the atmosphere. We can determine if an atmosphere is stable or unstable by plotting the measured temperature as a function of altitude and comparing it to the dry and moist adiabatic lapse rates. This method of determining the stability of the atmosphere is called the parcel method.
We learned in Chapter 3 that a dry parcel will cool at a rate of approximately 10 degrees C for each kilometer it is lifted. A moist parcel cools at the moist adiabatic lapse rate, or approximately 6C per km. An atmospheric condition favorable for the formation of clouds occurs when the parcel temperature is much warmer than its surroundings. This is favorable for storm development because air that is warmer than its environment rises. We know the rate at which a parcel will cool. If we plot the temperature of the environment as a function of altitude we can quickly tell if the air is favorable for vertical lifting. This is demonstrated in the following applet. Use this applet (instructions) to investigate the following:
- Describe the vertical distribution of temperature and moisture favorable for supporting a thunderstorm.
- Is there a relationship between cloud base and the temperature and moisture near the surface?
- The pink line represents the vertical displacement of a parcel of air lifted from the surface. Is there a relationship between the displacement, the temperature (or dew point temperature) and cloud top?
Remember, severe thunderstorms grow in an unstable environment that also has vertical wind shear, a change in wind speed or a change in wind direction with altitude. Environment vertical wind shear is important for severe thunderstorms as it helps to separate the updraft from the downdraft. This enables the storm to last longer and grow to more severe conditions. Without wind shear, the updraft is vertically erect, and the cloud particles' eventually fall through the updraft. The drag of the falling particles and the sinking motion enhanced by evaporation cooling, converts the updraft into a downdraft and ending the storm. Tornadoes and large hail are produced by thunderstorms that grow in unstable atmospheres with vertical wind shear.
The red line is temperature and the blue line dew point temperature. The red and blue boxes represents regions you can adjust.