Types of Weather
Dust Storm
Dust storms are created by winds moving across an arid region. Many times a strong dry cold front is the mechanism that creates the dust or sand storm. As strong winds move across an arid landscape, sand or dirt is picked up from the ground. Sometimes the lighter dust particles can be lifted up to 20,000 feet high into the atmosphere. A classic example of this is a sand and dust storm that develops over the Sahara desert. Easterly winds may carry the dust particles across the Atlantic ocean to the Caribbean Islands or Florida. Severe sand and dust storms can obscure visibility to zero and last for days.
Fog
Fog is a stratus cloud on the ground. There are several types of fog. Advection fog is produced by a hortizontal motion of warm moist air over colder ground. An example of this would be a warm front moving across land with a recent snow or cold weather. Radiation fog is produced as heat from the earth surface is radiated back to space at night. A moist layer and nearly calm winds need to be present. Wind can mix in dryer air aloft, keeping the air below the saturation point. Steam fog forms when cold air moves over relatively warm water. This can be observed as wisps of "steam" rising from lakes, rivers or oceans when a cold air mass moves in. Upslope fog occurs as air is pushed up a mountainside and becomes saturated so that condensation occurs.
Freezing Rain
Freezing rain - known as "ice" occurs when a thin layer of freezing or below freezing air exists near the earth's surface. Typically this occurs when snow falls into warmer air changing the precipitation into rain. If a very thin layer of freezing air is present near the ground the rain will freeze on objects such as power lines, trees, and cars. Ice accumulations begin to cause serious problems when the ice is 1/2" or greater on objects.
Hurricane
If conditions are met sufficiently, showers and storms will start to gain organization as low level winds converge toward the center of the low pressure area. The cluster of convection will start to form bands. As the convection increases, the warm air near the surface rises and cools. As it cools the water vapor condenses. There is a tremendous amount of heat produced from the condensation of water vapor. Some of this heat warms the center region of the low. As the temperature of the air near the core rises it produces lower pressure. In response to this lower pressure, winds increase in intensity. A tropical storm forms when the surface winds reach sustained winds of 39-73 mph. A hurricane officially classified as the winds reach sustained values of 74 mph.
Hail
The exact processes that creates hail are still being studied. Traditionally, textbooks showed hailstones being formed as a water droplet was carried aloft with an updaft into subfreezing air and freezing. As the ice moved into a downdraft, more water coated the hailstone. The cycle continued with the hailstone continuing to grow with each ride up and down the updraft and downdraft.
More recent studies suggest that super cooled water droplets are carried aloft by strong updrafts in thunderstorms. A small ice particle forms and grows as "waves" of supercooled water droplets continue to bump into the ice particle. A new coat of ice grows with each cycle. The hailstone is kept aloft by the strong updraft. The stronger the updraft the greater the force to keep the heavier hailstones aloft. The hailstone eventually falls to the grown when the the weight is too great for it to remain aloft or when it gets pushed out of the updraft. Sometimes hailstones bump into each other while being bombarded with supercooler water droplets and stick together giving the hailstone a spiked appearance.
More recent studies suggest that super cooled water droplets are carried aloft by strong updrafts in thunderstorms. A small ice particle forms and grows as "waves" of supercooled water droplets continue to bump into the ice particle. A new coat of ice grows with each cycle. The hailstone is kept aloft by the strong updraft. The stronger the updraft the greater the force to keep the heavier hailstones aloft. The hailstone eventually falls to the grown when the the weight is too great for it to remain aloft or when it gets pushed out of the updraft. Sometimes hailstones bump into each other while being bombarded with supercooler water droplets and stick together giving the hailstone a spiked appearance.
Lightning
As a thundercloud develops, it is made up of many up and down drafts. Supercooled water droplets are carried high aloft to the top of the cloud and form into ice crystals. As the ascending water droplets collide with some of the ice crystals and graupel, electrons are broken off of the decending particles. Because of this, the bottom of the thunderstorm becomes negatively charged and the top positively charged. A pool of positively charged particles shadows the thunderstorm's movement. These positive charges will move up high objects like trees, buildings, and telephone poles.
A cloud to ground lightning bolt starts as the difference between the postive and negative charges grow. A "stepped leader" drops from the base of the thunderstorm. This is the channel in which the bolt will flow. This happens extremely quickly - in tens of milliseconds. As this channel decends toward the ground, streamers of positve charge to move toward this stepped leader. The two meet about 100-300 feet above the ground. This is when the charge flows to the ground and the bright flash is seen.
A cloud to ground lightning bolt starts as the difference between the postive and negative charges grow. A "stepped leader" drops from the base of the thunderstorm. This is the channel in which the bolt will flow. This happens extremely quickly - in tens of milliseconds. As this channel decends toward the ground, streamers of positve charge to move toward this stepped leader. The two meet about 100-300 feet above the ground. This is when the charge flows to the ground and the bright flash is seen.
Rain
Precipitation in the form of rain can be created in a number of different ways. First, two types of processes cause minute droplets to form rain drops. Collision coalescence is a process where tiny droplets gradually grow in size by bumping into each other and growing. This is mostly a warm cloud process where temperatures are above freezing. The second process is a cold cloud process where super cooled water droplets freeze on ice nuclei. These ice nuclei grow into ice crystals. The snowflakes fall and turn into rain drops as the temperatures go above freezing closet to the ground. This is the primary process that produces rainfall.
Sleet
Sleet is transparent frozen raindrops. Sleet or ice pellets are created when snow falls into air above freezing and melts into rain drops. If another below freezing layer below the warm air is of sufficient depth, the rain drops freeze into balls of ice or ice pellets. Any significant amount of sleet is farily rare. This is due to the fact that there needs to be just the right conditons to get a perfect layer of warm air in between the layers of cold air. There have been cases of storms producing several inches of sleet. A mix of snow and sleet and sleet and rain is more common.
Snow
Snow is the accumulation of crystal snowflakes. Snowflakes are made as water ice crystalizes. The crystals may form in many different shapes dictated by the water content and temperature in the cloud as the snowflakes are forming. Snowflakes can take the shape of columns, dendrites, plates, needles and other six sided shapes. It is possible for it to snow several degrees above freezing if the air aloft is very cold and the above freezing layer is shallow near the ground.
Tornado
Tornadoes form in generally strong wind shear conditons. This is observed as winds increase in speed and change direction (veer) with increasing altitude. Invisible horizontal tubes of spinning air are created by this process. As thunderstorms form, these tubes are titlted vertically. Thunderstorms start to rotate with diameters ranging from 2-6 miles. If the rotation is strong enough, a wall cloud develops. This is a cloud that rotates, hanging low under the updraft of the thunderstorm. If a burst of air drops from a nearby downdraft called the rear flank downdraft - it may enter the strong updraft under the wall cloud. The column of air is stretched and a tornado is formed. In this sense, most tornadoes actually form from the ground up. It is simply the highly saturated air dropping from the thunderstorm which gives the appearance of a tornado "touching down". A great deal of study is focused on the relationship of the rear flank downdraft and wall cloud. The exact formative process of tornadoes is still being studied.
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