The weather is normally quiet; then violence erupts. Tornadoes and powerful thunderstorms occur when Mother Nature uses specific physical laws in a certain way. Why does wild weather still surprise us – and how does it work?
Weather: Stable or Unstable, That is the Question
Meteorologists are fond of talking about a ‘parcel’ of air and what happens to it. A parcel is an undefined quantity, bigger than a breadbox and smaller than a microwave, that moves as a unit. It’s more convenient to think of parcels than individual molecules, and the result is perfectly equivalent as long as you’re not interested in what makes a specific leaf on a specific tree flutter. All violent weather depends on what happens to a parcel of air when you lift it.
If it returns to where it was, the air is stable; if it keeps rising, the air is unstable.
Ideal Gas Law
Atmospheric pressure decreases with height, simply because there is less air above to push down. The Ideal Gas law relates the pressure to the temperature (yes, volume is involved, too, but let’s keep it simple): as pressure falls, temperature also falls. So as one goes up in the atmosphere, the temperature decreases. Anyone who has climbed a mountain is familiar with this phenomenon.
The rate of decrease of temperature with height is called the ‘lapse rate.’ The average lapse rate in the lower atmosphere, where weather takes place, is 3.5 degrees Fahrenheit per 1000 feet, or about 17 degrees per mile. When a passenger plane flies at an altitude of 30,000 feet, about five miles, the temperature is about 85 degrees lower than when it left the ground. This is part of the reason why they don’t make passenger planes with windows that the passengers can open.
Cooling a Parcel of Dry Air
If a parcel of dry air rises, it cools as the pressure decreases. Other things being equal, which basically means no one is heating the parcel with a blowtorch, the parcel will cool at a rate of approximately 5.4 degrees per 1000 feet. Since this dry lapse rate is greater than the normal lapse rate, the parcel will find that it is cooler than its surroundings and sink back to where it came from — the air is stable.
Cooling a Parcel of Saturated Air
It makes a difference whether air is dry (unsaturated) or wet (saturated) as it cools. Air can hold less water vapor (moisture) when it is cold than when it is warm. So as a parcel of air rises, it can cool to the point (the dew point) at which moisture begins to condense. Since it requires more energy to be a fast-moving gas molecule than a lazy liquid one, energy (latent heat) is released when water vapor molecules condense.
This energy appears as heat, which partially offsets the cooling of our rising parcel. As a result, rising saturated air cools less rapidly than rising unsaturated air. The wet lapse rate is about 2.7 degrees per 1000 feet. This is often less than the actual lapse rate; a rising parcel of saturated air can find itself warmer than the surroundings and will continue to rise.
Afternoon thunderstorms on a sunny summer day form when the sun heats the earth, the ground heats nearby parcels of air, and the parcels of air rise At first the parcels cool at the dry lapse rate, but if other parcels push upwards, some parcels can reach the dew point. Condensation occurs: a cloud forms. From this point upwards, the parcels cool at the wet lapse rate and instability takes over — a thunderstorm forms.
Instability of a Column of Air
If the air near the surface has a higher moisture content than the air above, the whole atmosphere can become unstable. If the air is saturated, the air near the surface, just by being lifted slightly, can begin to cool at the wet rate, while the drier air above cools at the dry rate. The overall lapse rate thus becomes steeper (the temperature decreases more rapidly with height) until it reaches the point at which ANY displacement of a parcel of air leads that parcel to continue to rise or fall. A condition of extreme instability results, and this instability leads to violent storms and tornadoes.
U.S. – Prime Spot for Violent Storms
The central United States is a prime spot for this kind of instability. Very humid air at low levels streams north from the Gulf of Mexico. At higher levels, a dry flow from the west persists. Add a cold front, which wedges colder air beneath the warm air ahead of it and physically lifts the whole column of air, and you have the conditions that produced the Moore, Oklahoma tornado on May 20, 2013 and the tornadoes of November 17, 2013 in a wide swath of the midwest US.
San Francisco State university. Instability. Accessed November 27, 2013.