What are steep lapse rates and its relation to thunderstorm development?

Question

Steep lapse rates are often used to predict whether an innocent band of convective showers without any lightning could create enough static electricity to produce lightning and hence thunderstorms. During tornado season in the United States, you will often hear the term used a lot by meteorologists when severe convective weather is imminent.

Answer 1

The lapse rate is basically the rate the temperature will change on the average as you move upwards within a layer of the atmosphere. This rate is estimated based mainly on the the temperature found or forecasted at the top of the layer and the temperature found or forecasted at the bottom of this layer. The lapse rate will steepen when the temperature aloft gets colder and/or the the air at the lower levels gets warmer. But there is one other fact that you must understand, this rate of change in temperature per unit change in elevation when moving upwards must be greater than the rate normally found with just the change in temperature due to the change in pressure as you go up. So in order to have an air parcel to continue to rise, your lapse rate must be steeper than your normal lapse rate. Also note that the rate will change once the air become satuatued. So there is a normal dry lapse rate and a moist lapse rate.

Examples of how the lapse rate will increase...

Low level warming due to solar heating during the day.
Upper level cooling due to cold air advection aloft...usually this will happen when a cold upper low moves over an area.

See the following links for better details

http://www.uwsp.edu/geo/faculty/ritter/geog101/textbook/atmospheric_moisture/lapse_rates_1.html

It is also important to note that you will need more than just steep lapse rates to get severe weather. When you steepen the lapse rate, you are just increasing the general instability for that layer. You will also need a good moisture source, a trigger to start the air moving upwards, little or no CINS or caps (a layer of stable air mass that may be strong enough to prevent the the air below from reaching the point where it finally builds enough to precipitate. So you can have a steepening lapse rate, but never get any precipitation or anything close to a thunderstorm if the cap is too strong or the air is bone dry.

In tornadic events, you will also want to look for strong low level directional and vertical wind shear. This can be found on a sounding chart.

Answer 2

Thunderstorms develop from cumulonimbus clouds and are characterised by lightning discharges.Thunder and lightning occur when large accumulations of liquid and solid water are carried to heights where the temperature is well below - 20(minus twenty) degree celcius. Large masses of condensed water can be carried to great heights only if the updrafts in the cumulonimbus cloud, are strong.The updrafts will be strong only if the lapse rate is steep.So, thunderstorms develop only when the lapse rate is steep through deep layers of air.