ATMOSPHERIC WATER AND WEATHER

ORIGINS OF WATER ON THE EARTH

Water molecules came from within Earth over a period of billions of years, in a process called outgassing by which water and water vapor emerge from deep within the crust. Although water is continuously being lost (escaping to space or breaking down and forming new compounds with other elements, lost water is replaced by pristine water from within the Earth. Thus began endless cycling of water through the hydrologic system of evaporation-condensation-precipitation.

QUANTITY OF WATER

Water covers about 71% of Earth, 97.22% of it is salty seawater and 22.78% is freshwater—most of it frozen. 22% of the freshwater exists as groundwater (11.02%) or soil moisture (0.18%). Ice sheets and glaciers (99.3%) account for majority of all freshwater on the earth’s surface. All rivers combined make up 0.003% of surface water.

The present volume of water on Earth is estimated at 1.36 billion cubic kilometers (326 million cubic miles), an amount achieved roughly 2 billion years ago. Worldwide changes in sea level are called Eustasy, and are related to the change in volume of water in the oceans. Some of these changes are explained by the amount of water stored in glaciers and ice sheets. At present, sea level is rising because of increases in the temperature of the oceans and the record melting of glacial ice.

UNIQUE PROPERTIES OF WATER

Water is the most common compound on the surface of Earth. It possesses unusual solvent and heat characteristics. Part of Earth’s uniqueness is that its water exists naturally in all three states—solid, liquid, and gas—owing to Earth’s temperate position relative to the Sun. The polarity of the water molecule (positive charge for the 2 atoms of hydrogen and negative charge for the oxygen atom) explains why water dissolves many other molecules. Because of its solvent activity, pure water is rare. The bonding between water molecules is termed hydrogen bonding which is responsible for surface tension.

PHASES OF WATER

A change from one state to another is called a phase change. The change from solid to vapor is called sublimation; from liquid to solid, freezing; from solid to liquid, melting; from vapor to liquid, condensation; and from liquid to vapor, vaporization, or evaporation. For water to change from one state to another (solid, liquid, gas) heat energy must be added to it or released from it. The heat energy required for water to change phase is termed latent heat, because, once absorbed, it is hidden within the structure of the water, ice, or water vapor. For 1 g of water to become 1 g of water vapor at boiling requires addition of 540 calories, the latent heat of vaporization; at less than boiling the latent heat of evaporation describes the energy absorbed during phase change. When this 1 g of water vapor condenses, the same amount of heat energy is liberated, as 540 calories of the latent heat of condensation. The latent heat of sublimation is the energy exchanged in the phase change from ice to vapor and vapor to ice. The heat that is exchanged in the phases of water is an important driving force in producing weather. 

Uniqueness of Ice Form

As water cools, it contracts in volume like all other compounds reaching its greatest density at 4^oC (39^oF). Below this temperature, water behaves differently from other compounds and begins to expand as more hydrogen bonds form among the slower-moving molecules creating the hexagonal structures of snow and ice. This expansion continues to a temperature of -20^oF (-29^oC), with up to 9% increase in volume possible. This explains why ices is less dense than water and why ice floats. 

 HUMIDITY

Amount of water vapor in the atmosphere is termed humidity. The ability of air to hold water vapor is principally a function of the temperature of the air and of the water vapor (usually the same). Relative humidity is a percentage expression of the humidity content of the air compared with the capacity of the air to hold water vapor at a given temperature—content compared to capacity.

                                   Actual water Vapor Content of Air

Relative Humidity = Maximum water vapor capacity of the Air * 100

Relative Humidity changes because of variations in evaporation, condensation and temperature. Air is said to be saturated or filled to capacity, if it contains all the water vapor it can hold at a given temperature (100% relative humidity). Saturation indicates that any further addition of water vapor or a decrease in temperature will result in active condensation. The temperature at which air achieves saturation is called the dew-point temperature. Air is saturated when the dew-point temperature and the air temperature are the same. During a typical day, air temperature and relative humidity relate inversely, as temperature rises, relative humidity falls.

Among the various ways to express humidity and relative humidity are vapor pressure and specific humidity. Vapor pressure is that portion of the atmospheric pressure that is produced by the presence of water vapor expressed in millibars. Saturation is reached when the movement of water molecules between surface and air is in equilibrium. The maximum capacity of the air at a given temperature is termed Saturation vapor pressure. A comparison of vapor pressure with the saturation vapor pressure at any moment produces a relative humidity percentage. Specific humidity is the mass of water vapor (in grams) per mass of air (in kilograms) at any specified temperature. A comparison of specific humidity with the maximum specific humidity produces a relative humidity percentage.

Relative humidity is measured by several instruments: a) Hair Hygrometer – uses the changes in length of human hair as it absorbs (increases) or loses water in the air (shortens), b) Sling Psychrometer – uses wet and dry bulb thermometers.

ATMOSPHERIC STABILITY

Differences in temperature create changes in density within a parcel of air. Warm air has a lower density in a given volume of air; cold air has a higher density. Stability refers to the tendency of an air parcel, with its water-vapor cargo, either to remain in place or to change vertical position by ascending (rising) or descending (falling). An air parcel is stable if it resists displacement upward or, when disturbed, it tends to return to its starting place. An air parcel is unstable if it continues to rise until it reaches an altitude where the surrounding air has a density (air temperature) similar to its own.

An ascending (rising) parcel of air cools by expansion, responding to the reduced air pressure at higher altitudes. A descending (falling) parcel heats by compression. Physical laws that govern the behavior of gases explain these temperature changes internal to a moving air parcel. Temperature changes in both ascending and descending air parcels occur without any significant heat exchange between the surrounding environment and the vertically moving parcel of air. The warming and cooling rates for a parcel of expanding or compressing air are termed adiabatic.

DRY AND WET ADIABATIC LAPSE RATES

The normal lapse rate, is the average decrease in temperature with increasing altitude, a value of 6.4 C°/1000 m (3.5 F°/1000 ft). This rate of temperature change is for still, calm air. It can vary greatly under different weather conditions. Consequently, the actual lapse rate at a particular place and time is labeled the environmental lapse rate.

Adiabatic means occurring without the loss or gain of heat. The dry adiabatic rate (DAR) is the rate at which “dry” air cools by expansion (if ascending) or heats by compression (if descending). The term dry is used when air is less than saturated (relative humidity less than 100%). The DAR is 10 C°/1000 m (5.5 F°/1000 ft). The moist adiabatic rate (MAR) is the average rate at which ascending air that is moist (saturated) cools by expansion, or descending air warms by compression. The average MAR is 6 C°/1000 m (3.3 F°/1000 ft). This is roughly 4 C° (2 F°) less than the dry rate. The MAR, however, varies with moisture content and temperature and can range from 4 C° to 10 C° per 1000 m (2 F° to 5.5 F° per 1000 ft). A simple comparison of the dry adiabatic rate (DAR) and moist adiabatic rate (MAR) in a vertically moving parcel of air with that of the environmental lapse rate in the surrounding air determines the atmosphere’s stability, that is, whether it is unstable (lifting of air parcels continues) or stable (air parcels resist vertical displacement).

For a parcel of air, buoyancy force (up) and gravitational force (down) act on it. Whether or not the parcel of air may rise up or fall depends upon its density. Warm air rises while dense air falls.

CLOUDS AND FOG

A cloud is an aggregation of tiny moisture droplets and ice crystals suspended in the air. Clouds are reminders of the powerful heat-exchange system in the environment. Moisture droplets in a cloud form when saturated air and the presence of condensation nuclei lead to condensation. Horizontal clouds–flat and layered clouds are called Stratiform Clouds, Vertical clouds – puffy and globular are termed Cumuliform, and Wispy clouds – high composed of ice crystals are called Cirroform.

 

LOW CLOUDS, ranging from the surface up to 2000 m (6500 ft) in the middle latitudes, are called stratus (flat clouds, in layers) or cumulus (puffy clouds, in heaps). When stratus clouds yield precipitation, they are called nimbostratus. Clouds are identified by altitude (low, medium-high, high) and shape (flat, puffy and wispy).

 

MIDDLE_LEVEL CLOUDS are denoted by the prefix alto-. Altocumulus clouds, in particular, represent a broad category that occurs in many different styles. A cumulus cloud can develop into a towering giant called cumulonimbus (-nimbus denotes precipitation). Such clouds are called thunderheads because of their shape and their associated lightning, thunder, surface wind gusts, updrafts and downdrafts, heavy rain, and hail. HIGH ALTITUDE CLOUDS, principally composed of ice crystals, are called cirrus. Sometimes near the end of the day, lumpy, grayish, low-level clouds called stratocumulus may fill the sky in patches.

Fog is a cloud that occurs at ground level (3,300feet above the surface). Advection fog forms when air in one place migrates to another place where conditions exist that can cause saturation—for example, when warm, moist air moves over cooler ocean currents. Another type of advection fog forms when cold air flows over the warm water of a lake, ocean surface, or swimming pool. This evaporation fog, or steam fog, may form as the water molecules evaporate from the water surface into the cold overlying air. Upslope fog is produced when moist air is forced to higher elevations along a hill or mountain. Another fog caused by topography is valley fog, formed because cool, denser air settles in low-lying areas, producing fog in the chilled, saturated layer near the ground. Radiation fog is caused by radiative cooling of a surface that chills the air layer directly above the surface to the dew-point temperature, creating saturated conditions and fog.

AIR MASS OVER NORTH AMERICA

Specific conditions of humidity, stability, and cloud coverage occur in a regional, homogenous air mass. The longer an air mass remains stationary over a region, the more definite its physical attributes become. Air masses are categorized by their MOISTURE CONTENT—m for maritime (wetter) and c for continental (drier)—and their TEMPERATURE (a function of latitude)—designated A (arctic), P (polar), T (tropical), E (equatorial), and AA (antarctic).

Principal air masses of North America: 1) cP air masses, 2) cP air masses, mP air masses and mT air masses. North America is influenced by two maritime tropical air masses a) Mt Gulf Atlantic and the mT Pacific

ATMOSPHERIC LIFTING MECHANISMS

Air masses can be lifted by convergent lifting (air flows conflict, forcing some of the air to lift), convectional lifting (air passing over warm surfaces gains buoyancy), orographic lifting (passage over a topographic barrier), and frontal lifting. In North America, chinook winds (called föhn or foehn winds in Europe) are the warm, downslope air flows characteristic of the leeward side of mountains. Orographic lifting creates wetter windward slopes and drier leeward slopes situated in the rain shadow of the mountain. Conflicting air masses at a front produce a cold front (and sometimes a zone of strong wind and rain) or a warm front (of warm air mass).

LIFE CYCLE OF A MIDLATITUDE CYCLONE

Weather is the short-term condition of the atmosphere; meteorology is the scientific study of the atmosphere. The spatial implications of atmospheric phenomena and their relationship to human activities strongly link meteorology to physical geography. Analyzing and understanding patterns of wind, air pressure, temperature, and moisture conditions portrayed on daily weather maps is key to numerical (computer) weather forecasting.

A midlatitude cyclone, or wave cyclone, is a vast migrating center of low-pressure system that migrates across the continent, pulling air masses into conflict along fronts. Cyclogenesis, the birth of the low-pressure circulation, can occur off the west coast of North America, along the polar front, along the lee slopes of the Rockies, in the Gulf of Mexico, and along the East Coast. An occluded front is produced when a cold front overtakes a warm front in the maturing cyclone. These systems are guided by the jet streams of the upper troposphere along seasonally shifting storm tracks.

VIOLENT WEATHER

The violent power of some weather phenomena poses a hazard to society. Thunderstorms produce lightning (electrical discharges in the atmosphere), thunder (sonic bangs produced by the rapid expansion of air after intense heating by lightning), and hail (ice pellets formed within cumulonimbus clouds). A spinning, cyclonic column rising to mid troposphere level—a mesocyclone—is sometimes visible as the swirling mass of a cumulonimbus cloud. Dark gray funnel clouds pulse from the bottom side of the parent cloud. A tornado is formed when the funnel connects with Earth’s surface. A waterspout forms when a tornado circulation occurs over water.

Within tropical air masses, large low-pressure centers can form along easterly wave troughs. Under the right conditions, a tropical cyclone is produced. Depending on wind speeds and central pressure, a tropical cyclone can become a hurricane, typhoon, or cyclone, when winds exceed 65 knots (74 mph, 119 kmph). As forecasting and the public’s perception of weather-related hazards have improved, loss of life has decreased, although property damage continues to increase.