Ocean currents are the continuous flow of water along a path. Flow direction and intensity depend on: (1) wind speed and direction, (2) the spinning of Earth, (3) landmasses (e.g., continents, islands, undersea ridges) that are "in the way" of current flow, and (4) the density of the water mass. Together with atmospheric flow, ocean currents regulate Earth's temperature: cooler water flows towards the equator and warmer water flows towards the poles.
Waves are disturbances that move through a medium such as air or water. In the case of most sea surface waves, the disturbance is wind and the medium is water. Wind generated waves are dependent on the force of the wind, the distance over which the wind blows, and how long it blows.
Water temperature is a measure of the heat (or kinetic energy) in the water. It is expressed as degrees Celsius (°C) or degrees Fahrenheit (°F). As oceans are warmed by the sun, seawater temperature varies with latitude and depth. Generally, seawater temperature is warmer near the equator than the poles, and warmer near the surface than down deep. However, movement by currents helps to regulate these temperature differences.
Salinity is a measure of the total amount of dissolved material, or the salt content, in water. Salinity is the number of grams (g) of material in 1000g of water. For example, if seawater has 35g of salt in 1000g of water, it has a salinity of 35‰ (parts per thousand). Practical Salinity Units (PSU) are often used to describe salinity: a salinity of 35‰ equals 35 PSU. (Because salinity is measured as "grams per grams" many oceanographers prefer a "unitless" description: "35" rather than "35 PSU" or "35‰".)
Density is weight divided by the amount of space it occupies. Density can be expressed as kilograms per cubic meter (mass divided by volume). For instance, if a cubic meter (~264 gallons) of seawater weighs 1025 kg (~1 ton), it has a density of 1025 kg/m^3. Another way you will see density expressed is as sigma-t, which is simply the density minus 1000. Seawater with a density of 1025 kg/m^3 has a sigma-t of 25.
Atmospheric visibility is basically a measure of how clear the air is. It is often defined in terms of a visual range (how far an observer can see) and is expressed in nautical miles, miles or kilometers. Precipitation, clouds, and air pollution can reduce atmospheric visibility by increasing the number of small particles in the air that absorb and scatter light.
Air temperature is a measure of the heat (or kinetic energy) in the atmosphere. It is expressed as degrees Celsius (°C) or degrees Fahrenheit (°F). Air temperature varies with latitude, elevation, season and position over land or water. Air temperature is generally warmer near the equator than the poles. However, wind and weather patterns move air to moderate these temperature differences: cooler air flows towards the equator and warmer air flows poleward.
Wind is the movement of air caused by Earth's air temperatures and rotation. Without landmasses, the wind basic pattern would look like the model at left. Of course, continents create their own weather systems and thereby complicate wind patterns. Globally, winds move to regulate Earth's temperature: cooler air flows towards the equator and warmer air flows towards the poles.
Clouds are visible masses of condensed water vapor and/or ice crystals. Cloud formation is dependent on the moisture content and temperature of the air. A general rule: when moist air cools enough for the water vapor to condense, clouds can form. We use clouds to help describe and predict weather patterns, but clouds are also vital in creating weather! Although we may think of clouds only when deciding whether to bring an umbrella, it is important to realize that clouds are integral to Earth's overall energy budget.