Page 161 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 5 global Temperatures 125
CONTINENTAL
Temperature conditions more extreme—land warms and cools rapidly
Less evaporation
(lower latent heat)
Surface is opaque
Land has a lower specific heat
Land has
no mixing
between layers
Land
MARINE
Temperature conditions more moderate—water warms and cools slowly
Greater evaporaton
(higher latent heat) Surface is transparent
Water has a higher specific heat
Water has
mobility
and mixes in vast ocean currents
Ocean
is that land heats and cools faster than water. Figure 5.7 summarizes these differences, which relate to the prin- ciples and processes of evaporation, transparency, spe- cific heat, and movement. We include ocean currents and sea-surface temperatures in this section because of their effects on temperatures in coastal locations.
Evaporation The process of evaporation dissipates sig- nificant amounts of the energy arriving at the ocean’s surface, much more than over land surfaces where less water is available. An estimated 84% of all evaporation on Earth is from the oceans. When water evaporates, it changes from liquid to vapour, absorbing heat energy in the process and storing it as latent heat.
You experience the cooling effect of evaporative heat loss by wetting the back of your hand and then blowing on the moist skin. Sensible heat energy is drawn from your skin to supply some of the energy for evaporation, and you feel the cooling as a result. As surface water evaporates, it absorbs energy from the immediate environment, result- ing in a lowering of temperatures. (Remember that the water and vapour remain the same temperature through- out the process; the vapour stores the absorbed energy as latent heat.) Land temperatures are affected less by evapo- rative cooling than are temperatures over water.
Transparency Soil and water differ in their transmis- sion of light: Solid ground is opaque; water is transpar- ent. Light striking a soil surface does not pass through, but is absorbed, heating the ground surface. That energy is accumulated during times of sunlight exposure and is rapidly lost at night or when shaded.
Maximum and minimum daily temperatures for soil surfaces generally occur at the ground surface level. Below the surface, even at shallow depths, temperatures remain about the same throughout the day. You encounter this at a beach, where surface sand may be painfully hot
to your feet, but as you dig in your toes and feel the sand a few centimetres below the surface, it is cooler, offering relief. On the MasteringGeography website, you will find a profile of daily temperatures for a column of soil and the air above it, showing the pattern just described.
In contrast, when light reaches a body of water, it pen- etrates the surface because of water’s transparency—water is clear, and light passes through it to an average depth of 60 m in the ocean. This illuminated zone occurs in some ocean waters to depths of 300 m. The transparency of water results in the distribution of available heat energy over a much greater depth and volume, forming a larger reservoir of energy storage than that which occurs on land.
Specific Heat The energy needed to increase the tem- perature of water is greater than for an equal volume of land. Overall, water can hold more heat than can soil or rock. The heat capacity of a substance is specific heat. On average, the specific heat of water is about four times that of soil. Therefore, a given volume of water represents a more substantial energy reservoir than does the same volume of soil or rock and consequently heats and cools more slowly. For this reason, day-to-day temperatures near large water bodies tend to be moderated rather than having large extremes.
Movement In contrast to the solid, rigid characteristics of land, water is fluid and capable of movement. The movement of currents results in a mixing of cooler and warmer waters, and that mixing spreads the available en- ergy over an even greater volume than if the water were still. Surface water and deeper waters mix, redistribut- ing energy in a vertical direction as well. Both ocean and land surfaces radiate longwave radiation at night, but land loses its energy more rapidly than does the moving reservoir of oceanic energy, with its more extensive dis- tribution of heat.
◀Figure 5.7 Land–water heating differences. The differential heating of land and water produces contrast- ing marine (more moderate) and continental (more extreme) tempera- ture regimes.
Insolation
Insolation