In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media (e.g., the atmosphere and the ocean) which has the restoring force of gravity or buoyancy.
When a fluid element is displaced on an interface or internally to a region with a different density, gravity tries to restore the parcel toward equilibrium resulting in an oscillation about the equilibrium state or wave orbit. Gravity waves on an air–sea interface are called surface gravity waves or surface waves while internal gravity waves are called internal waves. Wind-generated waves on the water surface are examples of gravity waves, and tsunamis and ocean tides are others.
Wind-generated gravity waves on the free surface of the Earth's ponds, lakes, seas and oceans have a period of between 0.3 and 30 seconds (3 Hz to 0.03 Hz). Shorter waves are also affected by surface tension and are called gravity–capillary waves and (if hardly influenced by gravity) capillary waves. Alternatively, so-called infragravity waves, which are due to subharmonic nonlinear wave interaction with the wind waves, have periods longer than the accompanying wind-generated waves.
A supercell is a thunderstorm that is characterized by the presence of a mesocyclone: a deep, persistently rotating updraft.For this reason, these storms are sometimes referred to as rotating thunderstorms. Of the four classifications of thunderstorms (supercell, squall line, multi-cell, and single-cell), supercells are the overall least common and have the potential to be the most severe. Supercells are often isolated from other thunderstorms, and can dominate the local climate up to 32 kilometres (20 mi) away.
Supercells are often put into three classification types: Classic, Low-precipitation (LP) and High-precipitation (HP). LP supercells are usually found in climates that are more arid, such as the high plains of the United States, and HP supercells are most often found in moist climates. Supercells can occur anywhere in the world under the right pre-existing weather conditions, but they are most common in the Great Plains of the United States, an area known as Tornado Alley.
Characteristics
Supercells are usually found isolated from other thunderstorms, although they can sometimes be embedded in a squall line. Typically, supercells are found in the warm sector of a low pressure system propagating generally in a north easterly direction in line with the cold front of the low pressure system. Because they can last for hours, they are known as quasi-steady-state storms. Supercells have the capability to deviate from the mean wind. If they track to the right or left of the mean wind (relative to the vertical wind shear), they are said to be "right-movers" or "left-movers," respectively. Supercells can sometimes develop two separate updrafts with opposing rotations, which splits the storm into two supercells: one left-mover and one right-mover.
Supercells can be any size – large or small, low or high topped. They usually produce copious amounts of hail, torrential rainfall, strong winds, and substantial downbursts. Supercells are one of the few types of clouds that typically spawn tornadoes within the mesocyclone, although only 30% or less do so.
Effects
Supercells can produce large hail, damaging winds, deadly tornadoes, flooding, dangerous cloud-to-ground lightning, and heavy rain.
Severe events associated with a supercell almost always occur in the area of the updraft/downdraft interface. In the Northern Hemisphere, this is most often the rear flank (southwest side) of the precipitation area in LP and classic supercells, but sometimes the leading edge (southeast side) of HP supercells.
While tornadoes are perhaps the most dramatic of these severe events, all are dangerous. High winds caused by powerful outflow can reach over 148 km/h (92 mph) and downbursts can cause tornado-like damage. Flooding is the leading cause of death associated with severe weather.
Note that none of these severe events are exclusive to supercells, although these events are highly predictable once a supercell has formed.
Snowstorms are storms where large amounts of snow fall. Snow is less dense than liquid water, by a factor of approximately 10 at temperatures slightly below freezing, and even more at much colder temperatures.
Dangers of Snow
Snowstorms are usually considered less dangerous than ice storms. However, the snow can bring secondary dangers. Mountain snowstorms can produce cornices and avalanches. An additional danger, following a snowy winter, is spring flooding if the snow melts suddenly because of a dramatic rise in air temperature. Deaths can occur from hypothermia, infections brought on by frostbite or car accidents due to slippery roads. Fires and carbon monoxide poisoning can occur after a storm causes a power outage. Large amounts of snow can also significantly reduce visibility in the area, a phenomenon known as a whiteout; this can be very dangerous to those who are in densely populated areas, since the whiteout can cause major accidents on the road or while flying. There is also several cases of heart attacks caused by overexertion while shoveling heavy wet snow. It is difficult to predict what form this precipitation will take, and it may alternate between rain and snow. Therefore, weather forecasters just predict a "wintry mix". Usually, this type of precipitation occurs at temperatures between -2 and 2 °C (28.4 and 35.6 °F).
A landslide or landslip is a geological phenomenon which includes a wide range of ground movement, such as rockfalls, deep failure of slopes and shallow debris flows, which can occur in offshore, coastal and onshore environments. Although the action of gravity is the primary driving force for a landslide to occur, there are other contributing factors affecting the original slope stability. Typically, pre-conditional factors build up specific sub-surface conditions that make the area/slope prone to failure, whereas the actual landslide often requires a trigger before being released.
Causes Landslides occur when the stability of a slope changes from a stable to an unstable condition. A change in the stability of a slope can be caused by a number of factors, acting together or alone. Natural causes of landslides include:
groundwater (porewater) pressure acting to destabilize the slope
Loss or absence of vertical vegetative structure, soil nutrients, and soil structure (e.g. after a wildfire)
erosion of the toe of a slope by rivers or ocean waves
weakening of a slope through saturation by snowmelt, glaciers melting, or heavy rains
Red tide is a common name for a phenomenon also known as an algal bloom(large concentrations of aquatic microorganisms), an event in which estuarine, marine, or fresh water algae accumulate rapidly in the water column and results in discoloration of the surface water. It is usually found in coastal areas.These algae, known as phytoplankton, are single-celled protists, plant-like organisms that can form dense, visible patches near the water's surface. Certain species of phytoplankton, dinoflagellates, contain photosynthetic pigments that vary in color from green to brown to red.When the algae are present in high concentrations, the water appears to be discolored or murky, varying in color from purple to almost pink, normally being red or green. Not all algal blooms are dense enough to cause water discoloration, and not all discolored waters associated with algal blooms are red. Additionally, red tides are not typically associated with tidal movement of water, hence the preference among scientists to use the term algal bloom.Some red tides are associated with the production of natural toxins, depletion of dissolved oxygen or other harmful effects, and are generally described as harmful algal blooms. The most conspicuous effects of these kind of red tides are the associated wildlife mortalities of marine and coastal species of fish, birds, marine mammals, and other organisms.
Hurricane Katrina formed as Tropical Depression Twelve over the southeastern Bahamas on August 23, 2005 as the result of an interaction of a tropical wave and the remains of Tropical Depression Ten. The system was upgraded to tropical storm status on the morning of August 24 and at this point, the storm was given the name Katrina. The tropical storm continued to move towards Florida, and became a hurricane only two hours before it made landfall between Hallandale Beach and Aventura on the morning of August 25. The storm weakened over land, but it regained hurricane status about one hour after entering the Gulf of Mexico.
The storm rapidly intensified after entering the Gulf, growing from a Category 3 hurricane to a Category 5 hurricane in just nine hours. This rapid growth was due to the storm's movement over the "unusually warm" waters of the Loop Current, which increased wind speeds. On Saturday, August 27, the storm reached Category 3 intensity on the Saffir-Simpson Hurricane Scale, becoming the third major hurricane of the season. An eyewall replacement cycle disrupted the intensification, but caused the storm to nearly double in size. Katrina again rapidly intensified, attaining Category 5 status on the morning of August 28 and reached its peak strength at 1800 UTC that day, with maximum sustained winds of 175 mph (280 km/h) and a minimum central pressure of 902 mbar (26.6 inHg). The pressure measurement made Katrina the fourth most intense Atlantic hurricane on record at the time, only to be surpassed by Hurricanes Rita and Wilma later in the season; it was also the strongest hurricane ever recorded in the Gulf of Mexico at the time. However, this record was later broken by Hurricane Rita.
Katrina made its second landfall at 1110 UTC (6:10 a.m. CDT) on Monday, August 29 as a Category 3 hurricane with sustained winds of 125 mph (205 km/h) near Buras-Triumph, Louisiana. At landfall, hurricane-force winds extended outward 120 miles (190 km) from the center and the storm's central pressure was 920 mbar (27 inHg). After moving over southeastern Louisiana and Breton Sound, it made its third landfall near the Louisiana/Mississippi border with 120 mph (195 km/h) sustained winds, still at Category 3 intensity. Katrina maintained strength well into Mississippi, finally losing hurricane strength more than 150 miles (240 km) inland near Meridian, Mississipi. It was downgraded to a tropical depression near Clarksville, Tennessee, but its remnants were last distinguishable in the eastern Great Lakes region on August 31, when it was absorbed by a frontal boundary. The resulting extratropical storm moved rapidly to the northeast and affected eastern Canada.
An ice disc, ice circle, or ice pan is a natural phenomenon that occurs in slow moving water in cold climates. Ice circles are thin and circular slabs of ice that rotate slowly in the water. It is believed that they form in eddy currents. Ice discs have most frequently been observed in Scandinavia and NorthAmerica, but they are occasionally recorded as far south as England and Wales. An ice disc was observed in Wales in December 2008 and another was reported in England in January 2009.
Ice circles vary in size but have been reported to be more than 4 metres (13 ft) in diameter.
Ice discs form on the outer bends in a river where the accelerating water creates a force called 'rotational shear', which breaks off a chunk of ice and twists it around. As the disc rotates, it grinds against surrounding ice — smoothing into a circle. A relatively uncommon phenomenon, one of the earliest recordings is of a slowly revolving disc was spotted on the Mianus River and reported in a 1895 edition of Scientific American.
River specialist and geography professor Joe Desloges states that ice pans are "surface slabs of ice that form in the center of a lake or creek, instead of along the water’s edge. As water cools, it releases heat that turns into frazil ice" that can cluster together into a pan-shaped formation. If an ice pan accumulates enough frazil ice and the current remains slow, the pan may transform into a 'hanging dam', a heavy block of ice with high ridges low centre.
