Basic Concepts

H₂O Phase Diagram

A phase diagram is a graphical way to depict the effects of pressure and temperature on the phase of a substance. This water phase diagram shows whether a substance exists as a vapor, liquid, or solid at a given temperature and pressure.

Source:http://wine1.sb.fsu.edu/chm1045/notes/Forces/Phase/Forces06.htm

The curves indicate the conditions of temperature and pressure under which equilibrium between different phases of a substance can exist. The triple point occurs where the three lines intersect in a phase diagram and indicates the pressure and temperature where solid, liquid, and vapor all exist in equilibrium.

The critical point is the thermodynamic state in which liquid and gas phases of a substance coexist in equilibrium at the highest possible temperature. At higher temperatures than the critical no liquid phase can exist.

For water the critical point is:

es = 2.21 x 105 mb

T = 647°K

α = 3.1 gm cm-3

where es is the saturation vapor pressure of the water vapor, T is the Kelvin temperature, and α the specific volume. (The specific volume is the volume per unit mass of a substance, and hence the reciprocal of density.)

A torr is a non-SI unit of pressure and is the pressure exerted by 1mm of mercury (Hg). A standard atmosphere of pressure equals 760 torr or 1 torr equals 0.001316 atmospheres.

QUESTION: At what temperature do ice, liquid water, and water vapor exist in equilibrium?

Hydrological Cycle

The hydrological cycle is powered by solar energy and is the continuous circulation of water from the atmosphere to earth and oceans and back again. It involves changes in the physical state of water between liquid, solid, and gas phases. The amount of water never changes, but its state and position in the cycle does change.

Source: United Nations Environment Program(UNEP)

QUESTIONS: Where are the ice and snow in the water cycle?
What is their impact on the cycle?

Hydrological Cycle Processes

Condensation: The process of water changing from a vapor to a liquid. Water vapor in the air rises mostly by convection. This means that warm, humid air will rise, while cooler air will flow downward. As the warmer air rises, the water vapor will lose energy, causing its temperature to drop. The water vapor then has a change of state into liquid or ice.

Evaporation: The process by which water changes from a liquid to a gas and is transferred from the earth’s surface to the atmosphere. Radiant energy from the sun heats water causes water molecules to become so active that some of them rise into the atmosphere as vapor. Approximately 80% of all evaporation is from the oceans, with the remaining 20% coming from inland water and vegetation.

Groundwater: The underground water trapped between layers of rock or clay. Water that infiltrates the soil flows downward until it encounters impermeable rock and then travels laterally. The locations where water moves laterally are called ‘aquifers’. Groundwater returns to the surface through these aquifers , which empty into lakes, rivers and the oceans. The flow of groundwater is much slower than run-off with speeds usually measured in centimetres per day, metres per year or even centimetres per year.

Infiltration: The portion of the precipitation that reaches the Earth's surface and seeps into the ground. The amount of water that infiltrates the soil varies with the degree of land slope, the amount and type of vegetation, soil type and rock type, and whether the soil is already saturated by water. The more openings in the surface (cracks, pores, joints), the more infiltration occurs. Water that doesn't infiltrate the soil flows on the surface as runoff.

Percolation: The gravity flow of water within soil from the surface or surface groundwater into the groundwater table. In general, water flow in which gravitational forces predominate.

Precipitation: The primary mechanism for transporting water from the atmosphere to the surface of the earth. Precipitation begins after water vapor, which has condensed in the atmosphere, becomes too heavy to remain in atmospheric air currents and falls from clouds as rain, sleet, snow, or hail. A proportion of atmospheric precipitation evaporates.

Surface Runoff: Precipitation that reaches the surface of the Earth but does not infiltrate the soil. Runoff can also come from melted snow and ice. When there is a lot of precipitation, soils become saturated with water. Additional rainfall can no longer enter it. Runoff will eventually drain into creeks, streams, and rivers, adding a large amount of water to the flow. Surface water always travels towards the lowest point possible, usually the oceans. Along the way some water evaporates, percolates into the ground, or is used for agricultural, residential, or industrial purposes.

Transpiration: The process by which water absorbed by plants, usually through the roots, is transferred as water vapor to the atmosphere from a single leaf, or the amount of water so transferred.

Transport: The movement of water through the atmosphere, specifically from over the oceans to over land. Most water is transported in the form of water vapour, which is actually the third most abundant gas in the atmosphere. Some of the earth’s moisture transport is visible as clouds, which themselves consist of ice crystals and/or tiny water droplets.
Clouds are propelled from one place to another by either the jet stream, surface-based circulations like land and sea breezes or other mechanisms. However, a typical cloud 1 km thick contains only enough water for a millimetre of rainfall, whereas the amount of moisture in the atmosphere is usually 10-50 times greater than this.

Water Table: The level below the land surface at which the subsurface material is fully saturated with water. The depth of the water table reflects the minimum level to which wells must be drilled for water extraction.

For more meteorlogical definitions click here and here

Other resources can be found here.

See animations of some of these processes.

To learn more about the world's water cycle click here.

Over 70% of the earth's surface is covered by water. However, not all of it is useable by human beings. Some sources of freshwater have long "replacement" times.

Go to The Water Cycle: A Guide for Students.

Thermal Conductivity

Heat conduction is the flow of internal energy from a region of higher temperature to one of lower temperature (temperature gradient) by the random interaction of the adjacent particles (atoms, molecules, ions, electrons, etc.) in the intervening space.

Factors affecting the rate of heat transfer by conduction are:

• temperature difference,
• length,
• cross-sectional area, and
• material.

For example, conductivities are greatest for metallic solids, lower for non-metallic solids, very low for liquids, and extremely low for gases.

Below is a table of thermal conductivity values. It includes values for the air and water, at various pressures and temperatures, and common materials used for clothing, construction and cooking.

QUESTIONS:
Which of the clothing materials is best for winter? Why?
Which of the construction materials is best for a cold climate? Why?
Which of the cooking materials would cook a fried egg the fastest?

Albedo

Source: NSIDC

Albedo is a measure of reflectivity of a surface or body. It is the ratio of electromagnetic radiation (EM radiation) reflected to the amount incident upon it. The fraction, usually expressed as a percentage from 0% to 100% (or as a dimensionless value between 0 and 1), is an important concept in climatology and astronomy.

A perfect reflector reflects all the sunlight that strikes it. It looks white and has an albedo of 1. Objects that reflect most of the light that hit them appear bright and have a high albedo.

A perfect absorber does not reflect any of the sunlight that strikes it. It looks black and has an albedo of 0. When an object absorbs most of the light that hits it, it looks dark and has a low albedo.

QUESTION: What would be the environmental consequences if a land cover changed from glacier ice to water?