The weather Factory

Pre Skywarn Class Training

By KD1LD

Hello to all, this is KD1LD. I have put together some material that is used in a Skywarn Class. If you are planing to go to a Skywarn Class, read on, and take the test.

A thunderstorm is a weather factory which produces thunder, lighting, rain, snow, hail and varying degrees of turbulence. There are two general types of thunderstorms, air mass thunderstorms, and frontal thunderstorms.

Air mass thunderstorms may result when the sun heats the surface causing warm moist air to rise and cool. Over land areas these storms generally accrue during the afternoon and early evening. When air moves out over a warmer body of water the lower layer of air is warmed and rises convectively. These storms may occur at night or during the day. But usually reach there most intense development at night when the land water contrast is the greatest. Air mass thunderstorms also occur when warm moist conditionally unstable air is pushed up a mountain slope. They may be isolated over mountain peaks or quite general along the range.

Cold front thunderstorms occur when a wedge of cold air moves into a region of unstable warm air supplying the required lifting action. These storms generally are concentrated along the front which is hundreds of miles long. The line seems to be continuous. Warm front thunderstorms occur when warm conditional unstable air flows up a slope of retreating colder air.

Warm front thunderstorms are difficult to locate because they are masked by other clouds. Generally they are widely spread and not as common as cold front thunderstorms. Thunderstorms very greatly in size. Those from twelve to fifteen thousand feet high are usually not as severe as the larger storms which rise up to sixty thousand feet. Smaller storms however may be severe at certain stages of development. Storms generally five to ten miles in diameter could expand to fifty miles. Also thunderstorms very in size with the season. In summer they reach maximum height. In winter thunderstorms are infrequent.

Three factors are required to form a thunderstorm. Air of high moisture content (high dew point), some type of lifting action, and conditionally unstable air.

There are only two ways by which surface air can be carried aloft to operate by the adiabatic process. Mechanical convection and thermal convection. Thermal convection is the result of instability in the atmosphere. Mechanical convection is caused by advaction ( the movement of air) and orographic lifting (like mountians and large buildings). Thermal convection is caused by heating from below. The basic principle of thermal convection is that heating from below causes surface air to expand because of it's lesser density. It is displaced upward by the surrounding air of greater density. This incoming air is in turn heated, expanded, and forced aloft.

We know that air that is not saturated, when lifted, cools at the dry cooling rate of three degrees centigrade per thousand feet of altitude. We know that saturated air, when lifted, cools at the moist cooling rate of two degrees centigrade per thousand feet of altitude. Temperature at various altitudes can be plotted for a given column of air. When the temperature decreases at a rate between the dry and the moist cooling rate the air is conditionally unstable. Lets consider a parcel of air in a column of unsaturated air about to be lifted. Essentially the parcel has the same temperature as the surrounding air, in this case twenty degrees Celsius. When the parcel is lifted it cools and expands at the dry cooling rate of three degrees centigrade per thousand feet. As it cools it gets closer to saturation. Eventually it becomes saturated. The slightest additional lifting will expand and cool the parcel further and produce condensation of liquid water on condensation nuclei which are usually in abundance in the air. As the moisture in the parcel of air condenses the heat of condensation is released, which slows down the rate of cooling of the parcel being lifted from three degrees to approximately one and one half degrees centigrade per thousand feet Until this time, the parcel has been cooler than the air through which it has been lifted. Now do to the slower cooling rate it approaches the temperature of the surrounding air. If the lifting is continued the parcel will become warmer than the surrounding air.It is then relatively lighter and is excellerated upward by the weight of the surrounding air. It will continue to rise until it is cooled by radiation and mixing to the temperature of the air around it. The release of conditional instability therefore promotes instant cloud growth. And these three factors, high moisture content, lifting action, and conditionally unstable air working together may produce a thunderstorm.

No matter how thunderstorms are formed and regardless of their size and intensity the basic characteristics are the same. Within a thunderstorm there are one or more regions of vertical drafts called cells. These cells may be as much as five miles in diameter. Draft volicity may very from a few feet to as much as one hundred feet per second. In the early stages of development, the cumulus stage, an updraft perveils through the entire cell. As it develops further into the mature stage there is both an updraft and a down-draft, and finely in the anvil stage also know as the disapating stage there is a down-draft in the lower two third's of the cell with little or no vertical motion in the upper third. In and around each draft the air flow is turbulent. Some of these drafts are very small and some are hundreds of feet in diameter. The region of the greatest turbulence is associated with the height of the freezing level. Now lets talk about the development of thunderstorms.

In the cumulus stage the updraft is fed by the general converging surface winds. Air is drawn in through the sides of the cell. The cell grows in height. As the warmer moist air rises through the colder environment condensation occurs. The droplets form and concentrate into larger drops. In time the weight of these drops becomes to heavy to be born by the updraft. They began to fall. As rain reaches the earth the cumulus stage ends and the mature stage begins.

The falling drops of water and ice particles drag on the rising air currents and create a down-draft of colder air. This air is colder than the surrounding air, it excelerates in it's downward movement. This develops both downward and horizontally at the expense of the updraft. These updrafts and down-drafts create gusts who's maximum intensities is at or above the freezing level. The down-drafts reach the earth as a cold core of air in the rain area spreading out along the surface. It extends as much as five miles in front of the advancing storm clouds reducing surface temperature and producing strong gusty surface winds. In the mature stage rain is found in the lower level, mixed rain and snow in the mid levels, and snow and ice in the upper levels. The down draft spreads through the cell until it cuts off the updraft. This ends the mature stage and begins the dissapating or anvil stage. The updraft vanishes. So does the source of rain fall and energy. Gradually the rain ceases. The down draft weakens and turbulence diminish gradually. This stage of the cell is indicated by the anvil shape. However, the appearance of an anvil cloud is no indication that it is a disapating one since there may be several cells in various stages of development. Every thunder cell passes through these three stages. Cumulus, mature, and dissapating. They produce turbulence, thunder, lighting, rain, hail, snow and ice crystals.

The intensity of any storm depends on it's stage of development. A multi cellular storm contains cells in various stages of development. Each successful cell tends to obtain a greater height because one feeds the next. A new cell absorbs saturated air from earlier cells rather than relatively dry air which surrounds an isolated cell. Thus developing some of them into new thunderstorm cells. Thunderstorms are divided into two general types. Air mass thunderstorms and frontal thunderstorms. thunderstorms are made up of one or more cells. Regions of vertical draft and gusts which produce turbulence. The anvil tops and shelf clouds indicate the general direction of thunderstorm movement.

There are four basic types of thunderstorms, single cells, multi cells, cluster, and the supercell. Multi cells can form in clusters or line. The multi cell line (squall line) also know as organized convection are common in New England. Sometimes there is a multi cluster thunderstorm. Supercells are not common in New England but do occur.

The typical duration of a single cell thunderstorm is about twenty minutes. A mid level thunderstorm lasts about thirty minutes. Severe weather occurs as down bursts, moderate size hail, flash floods and tornadoes. Severe events frequently occur near the updraft down draft interface, also know as the rain free base. A bow echo develops as part of the down burst. It forms a head and a tail. It bows out from the center. The strongest winds are in the leading edge of the bow. Tornadoes can form here, but they usually are found in the head.

The supercell is a strong storm. It's also know as a mezzo low. The shelf cloud is quite common. The rain shaft, the wall cloud and quite often a tornado occur here. Strong down burst, damaging hail, flash floods and tornadoes are common. All supercells rotate. Mostly counter clock wise.

If you are planing to go to a Skywarn Class, take the test.

It has much of the material they will be talking about.

Tip #1 .. download the test to your printer. Use the test as a cheat sheet.

Tip #2 .. Remember, you will be representing ARES at the class. The instructor will ask questions. If you know the answers, speak-up.