A geomagnetic storm is a temporary disturbance of the Earth's magnetosphere caused by a disturbance in space weather. Associated with solar flares and resultant solar coronal mass ejections (CME), a geomagnetic storm is caused by a solar wind shock wave and/or cloud of magnetic field which typically strikes the Earth's magnetic field 3 days after the event. The solar wind pressure on the magnetosphere and the solar wind magnetic field will increase or decrease depending on the Sun's activity. The solar wind pressure changes modify the electric currents in the ionosphere, and the solar wind's magnetic field interacts with the Earth's magnetic field causing the entire structure to evolve. Magnetic storms usually last 24 to 48 hours, but some may last for many days.
Interactions with planetary processes
The solar wind also carries with it the magnetic field of the Sun. This field will have either a North or South orientation. If the solar wind has energetic bursts, contracting and expanding the magnetosphere, or if the solar wind takes a southward polarization, geomagnetic storms can be expected. The southward field causes magnetic reconnection of the dayside magnetopause, rapidly injecting magnetic and particle energy into the Earth's magnetosphere.
During a geomagnetic storm, the ionosphere's F2 layer will become unstable, fragment, and may even disappear. In the northern and southern pole regions of the Earth, auroras will be observable in the sky.
Geomagnetic storm effects
Drumlin – an elongated whale-shaped hill formed by glacial action.
Radiation hazards to humans
Biology
Disrupted systems
Communications
Navigation systems
Satellite hardware damage
Geologic exploration
Electric grid
Pipelines
aurora polaris
A thunderstorm, also known as an electrical storm, a lightning storm, thundershower or simply a storm is a form of weather characterized by the presence of lightning and its acoustic effect on the Earth's atmosphere known as thunder. The meteorologically-assigned cloud type associated with the thunderstorm is the cumulonimbus. Thunderstorms are usually accompanied by strong winds, heavy rain and sometimes snow, sleet, hail, or no precipitation at all. Those which cause hail to fall are known as hailstorms. Thunderstorms may line up in a series or rainband, known as a squall line. Strong or severe thunderstorms may rotate, known as supercells. While most thunderstorms move with the mean wind flow through the layer of the troposphere that they occupy, vertical wind shear causes a deviation in their course at a right angle to the wind shear direction.
Thunderstorms result from the rapid upward movement of warm, moist air. They can occur inside warm, moist air masses and at fronts. As the warm, moist air moves upward, its cools, condenses, and forms cumulonimbus clouds that can reach heights of over 20 km. As the rising air reaches its dew point, water droplets and ice form and begin falling the long distance through the clouds towards the Earth's surface. As the droplets fall, they collide with other droplets and become larger. The falling droplets create a downdraft of air that spreads out at the Earth's surface and causes strong winds associated with thunderstorms.
Thunderstorms can generally form and develop in any geographic location, perhaps most frequently within areas located at mid-latitude when warm moist air collides with cooler air. Thunderstorms are responsible for the development and formation of many severe weather phenomena. Thunderstorms, and the phenomena that occur along with them, pose great hazards to populations and landscapes. Damage that results from thunderstorms is mainly inflicted by downburst winds, large hailstones, and flash flooding caused by heavy precipitation. Stronger thunderstorm cells are capable of producing tornadoes and waterspouts.
There are four types of thunderstorms: single-cell, multicell cluster, multicell lines, and supercells. Supercell thunderstorms are the strongest and the most associated with severe weather phenomena. Mesoscale convective systems formed by favorable vertical wind shear within the tropics and subtropics are responsible for the development of hurricanes. Dry thunderstorms, with no precipitation, can cause the outbreak of wildfires with the heat generated from the cloud-to-ground lightning that accompanies them. Several methods are used to study thunderstorms, such as weather radar, weather stations, and video photography. Past civilizations held various myths concerning thunderstorms and their development as late as the Eighteenth Century. Other than within the Earth's atmosphere, thunderstorms have also been observed on Jupiter and Venus.
An alluvial fan is a fan-shaped deposit formed where a fast flowing stream flattens, slows, and spreads typically at the exit of a canyon onto a flatter plain.
Formation
Owing to the flow as stream gradient decreases, coarse-grained solid material carried by the water is dropped. As this reduces the capacity of the channel, the channel will change direction over time, gradually building up a slightly mounded or shallow conical fan shape. The deposits are usually poorly-sorted. This fan shape can also be explained with a thermodynamic justification: the system of sediment introduced at the apex of the fan will tend to a state which minimizes the sum of the transport energy involved in moving the sediment and the gravitational potential of material in the cone. There will be iso-transport energy lines forming concentric arcs about the discharge point at the apex of the fan. Thus the material will tend to be deposited equally about these lines, forming the characteristic cone shape.
In arid climates
Alluvial fans are often found in desert areas subject to periodic flash floods from nearby thunderstorms in local hills. They are common around the margins of the sedimentary basins of the Basin and Range province of southwestern North America. The typical watercourse in an arid climate has a large, funnel-shaped basin at the top, leading to a narrow defile, which opens out into an alluvial fan at the bottom. Multiple braided streams are usually present and active during water flows.
Phreatophytes are plants that are often concentrated at the base of alluvial fans, which have long tap roots 30 to 50 feet (9.1 to 15 m) to reach water. The water at this level is derived from water that has seeped through the fan and hit an impermeable layer that funneled the water to the base of the fan where it is concentrated and sometimes forms springs and seeps if the water is close enough to the surface. These stands of shrubs cling onto the soil at their bases and over time wind action often blows away sand around the bushes which form islands of habitat for many animals.
In humid climates
Alluvial fans also develop in wetter climates. In Nepal the Koshi River has built a megafan covering some 150,000 km2 (58,000 sq mi) below its exit from Himalayan foothills onto the nearly level plains the river traverses into India before joining the Ganges. Along the upper Koshi tributaries, tectonic forces elevate the Himalayas several millimeters annually. Uplift is approximately in equilibrium with erosion, so the river annually carries some 100 million cubic meters (3.5 billion cu ft) of sediment as it exits the mountains. Deposition of this magnitude over millions of years is more than sufficient to account for the megafan.In North America, streams flowing into California's Central Valley have deposited smaller but still extensive alluvial fans. That of the Kings River flowing out of the Sierra Nevada creates a low divide, turning the south end of the San Joaquin Valley into an Endorheic basin without a connection to the ocean.
The distinction between a hill and a mountain is unclear and largely subjective, but a hill is generally somewhat lower and less steep than a mountain. In the United Kingdom geographers historically regarded mountains as hills greater than 1,000 feet (300 m) above sea level, which formed the basis of the plot of the 1995 film The Englishman Who Went Up a Hill But Came Down a Mountain. In contrast, hillwalkers have tended to regard mountains as peaks 2,000 feet (610 m) above sea level: the Oxford English Dictionary also suggests a limit of 2,000 feet (610 m). This has led to Cavanal Hill in Poteau, Oklahoma, receive billing as the "World's Tallest Hill" due to its height of 1,999 feet (609 m). Mountains in Scotland are frequently referred to as "hills" no matter what their height, as reflected in names such as the Cuillin Hills and the Torridon Hills. In Wales, the distinction is more a term of land use and appearance and has nothing to do with height.
A hillock is a small hill. Other words include knoll and (in Scotland, Northern Ireland and northern England) its variant, knowe.Artificial hills may be referred to by a variety of technical names, including mound and tumulus.
Hills may form through a number of geomorphic phenomena: faulting, erosion of larger landforms, such as mountains and movement and deposition of sediment by glaciers (eg. moraines and drumlins, or by erosion exposing solid rock which then weathers down into a hill. The rounded peaks of hills results from the diffusive movement of soil and regolith covering the hill, a process known as downhill creep.
Areas that would otherwise have hills do not because of glacier cover during the Ice Age. The hills that existed before the ice age were worn down by the ice (and the rocks they carry) and/or the surrounding valleys and hollows were filled in with glacial drift, therefore leaving a level topography. The contrast between the flat plains of northern Indiana, once covered by ice, and the rugged hills of southern Indiana, where the ice never reached, is a result of this. Another example is the Driftless Zone, an island of hilly country that the ice sheets missed, surrounded by glacial plains, also in the American Midwest.
There are various specific names used to describe particular types of hill, based on appearance and method of formation. Many such names originated in one geographical region to describe a type of hill formation peculiar to that region, though the names are often adopted by geologists and used in a wider geographical context. These include:
- Drumlin – an elongated whale-shaped hill formed by glacial action.
- Butte – an isolated hill with steep sides and a small flat top, formed by weathering.
- Tor – a rock formation found on a hilltop; also used to refer to the hill itself, especially in South West England.
- Puy – used especially in the Auvergne, France, to describe a conical volcanic hill.
- Pingo – a mound of earth-covered ice found in the Arctic and Antarctica.
