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Hadley cell

A tropical atmospheric circulation that features rising air near equator, poleward airflow 10 - 15 kilometers (6-9 miles) above the surface, descending air in the subtropics (near the latitudes of 30°N and 30°S), and surface flow toward the equator. Regions of rising air expand and cool; worldwide equatorial latitudes are therefore characterized by meteorological low pressures and high rainfall. The rising air cools, eventually becoming denser and sinking; this cool air has a low relative humidity, so subtropical regions have an arid climate. Surface airflow is deflected westward by the Earth’s rotation, creating the trade winds.
See also: atmosphere, climate, trade winds


See salt

hanging valley

A tributary valley that drops abruptly into a much larger and deeper valley. Hanging valleys are most commonly associated with U-shaped valleys that form due to glacial erosion.


A physical property of minerals, specifying how hard the mineral is, and its resistance to scratching. Hardness helps us understand why some rocks are more or less resistant to weathering and erosion.
See also: Moh’s Scale of Hardness


A dense layer of soil, generally found below the topsoil layer, that is generally impervious to water.
See also: soil


A form of energy transferred from one body to another as a result of a difference in temperature or a change in phase. Heat is transmitted through solids and fluids by conduction, through fluids by convection, and through empty space by radiation.

heat island effect

A phenomenon in which cities experience higher temperatures than do surrounding rural communities.

heat wave

A period of excessively hot weather that may also accompany high humidity. Temperatures of just 3°C (6°F) to 6°C (11°F) above normal are enough to reclassify a warm period as a heat wave.
Under high humidity, the mechanism of sweating does little to cool people down because the humidity prevents sweat from evaporating and cooling off the skin.


A metric unit of area defined as 10,000 square meters.


A gaseous chemical element (He), which is the second most abundant and second lightest element in the universe. Helium is used in cryogenics, as a coolant; it is also used in industrial applications including pressurizaton, welding, and leak detection. Balloons and blimps, although probably the most well-known and visible application of helium, take up less than an eighth of its total use.


A mineral form of iron oxide (Fe2O3). The name hematite has its origins in the Greek word haimatos, meaning "blood." It is very common in Precambrian banded iron formations.
Iron from hematite is used in the manufacture of steel. The vivid red pigments that iron lends to the mineral also makes it valuable as a commercial pigment.


A soil order ; these are organic-rich soils found along lake coastal areas where poor drainage creates conditions of slow decomposition and peat (or muck) accumulates.

Hoh rock assemblage

A mélange formed from a variety of chaotically jumbled sedimentary, metamorphic, and volcanic rocks that accreted to the Olympic Peninsula during the Eocene.
See also: accretion, Eocene, mélange, metamorphic rock, sedimentary rock, volcanic


The most recent portion of the Quaternary, beginning about 11,700 years ago and continuing to the present. It is the most recent (and current) interglacial, an interval of glacial retreat.
The Holocene also encompasses the global growth and impact of the human species.

horizon (soil)

A layer in the soil, usually parallel to the surface, which has physical characteristcs (usually color and texture) that are different from the layers above and below it. Each type of soil usually contains three or four horizons.


A pointed rocky peak created by glacial erosion.


A dark silicate mineral that can occur in a variety of forms. Hornblende is a common constituent of many igneous and metamorphic rocks.


See sphenopsid

hot spot

A volcanic region thought to be fed by underlying mantle that is anomalously hot compared with the mantle elsewhere. Hot spots form from plumes of hot material rising through the mantle. Magma from the hot spot pushes its way up through the crust, creating an igneous intrusion and sometimes a volcano.
Although the hot spot remains fixed, the plates of the lithosphere continue to move above it. As a plate continues to move over the hot spot, the original volcano shifts off of the hot spot and a new intrusion or volcano is formed. This gradually produces a chain of volcanic islands such as the Hawaiian Islands. Erosion of volcanoes may eventually wear down the crust to reveal the igneous intrusions that formed the volcano’s magma chamber.


The organic component of soil; a major part of the soil horizon containing organic matter.

Huronian glaciation

A glaciation beginning about 2.4 billion years ago, that covered the entire surface of the Earth in ice for as long as 300 million years.


A rapidly rotating storm system with heavy winds, a low-pressure center, and a spiral arrangement of thunderstorms. These storms tend to form over large, warm bodies of water. Once winds have reached 119 kilometers per hour (74 miles per hour), such a storm is classified as a hurricane.
Hurricanes usually develop an eye, which is visible as a small, round, cloud-free area at the center of the storm. The eye is an area of relative calm and low atmospheric pressure. The strongest thunderstorms and winds circulate just outside the eye, in the eyewall.


hydrothermal solution

Hot, mineral -rich water moving through rocks. These solutions are often enriched in salts (such as sodium chloride, potassium chloride, and calcium chloride) and thus are called “brines.” The brine is as salty or even saltier than seawater.
Salty water can contain minute amounts of dissolved minerals such as gold, lead, copper, and zinc. The presence of salt in the water suppresses the precipitation of the metallic minerals from the brine because the chlorides in the salt preferentially bond with metals. Additionally, because the brine is hot, minerals are more easily dissolved, just as hot tea dissolves sugar more easily than cold tea.


Animals with cone-shaped shells that existed throughout the Paleozoic. Their affinities to other animals are uncertain, with some scientists classifying them as mollusks and others placing them in their own phylum.


Of high salinity.