Daily Mean Temperature
Temperature determined by averaging the 24 hourly readings or by adding the maximum and minimum temperatures for a 24-hour period and dividing by 2.
Daily Temperature Range
The difference in a day’s maximum and minimum temperatures.
Monthly Mean Temperature
Calculated by adding together the daily means for each day of the month and dividing by the number of days in the month.
Annual Mean Temperature
An average of the 12 monthly means.
Annual Temperature Range
Computed by finding the difference between the warmest and codes mean temperatures.
Isotherm
A line that connects points on a map that have the same temperature (iso = equal, therm = temperature).
Temperature Gradient
The amount of temperature change per unit of distance.
Closely spaced isotherms indicate a rapid rate of temperature change, whereas more widely spaced lines indicate a more gradual rate of change.
Controls of Temperature
Factors that cause temperatures to vary from place to place and from time to time.
What are the controls of temperature?
- Variations in sun angle and length of daylight which depend on latitude - differences in the receipt of solar radiation.
- Differential heating of land and water.
- Ocean currents.
- Altitude.
- Geographic position.
- Cloud cover and albedo.
How can you tell latitude is not the only temperature control?
If it were, all places along the same parallel would have the same temperature but this is not the case.
Why are there different air temperatures over different Earth surfaces? Which varies the most?
Different Earth surfaces reflect and absorb varying amounts of incoming solar energy, which in turn cause variations in the temperature of the air above. The greatest contrast being between land and water.
Land heats more rapidly and to higher temperatures than water, and it cools more rapidly and to lower temperatures than water. Variations in air temperatures, therefore, are much greater over land and water.
Why do land and water heat and cool differently?
- Water is highly mobile. As water is heated, convection distributes the heat through a considerably larger mass. Daily temperature changes occur to depths of 20 feet or more below the surface, and yearly, oceans and deep lakes experience temperature variations through a layer between 650 and 2000 feet thick. In contrast, heat does not penetrate deeply into soil or rock; it remains near the surface. No mixing can occur on land because it is not fluid. Instead, heat must be transferred by the slow process of conduction. Consequently, daily temperature changes are small below a depth of 4 inches, although some change can occur to a depth of perhaps 3 feet. Annual temperature variations usually reach depths of 50 feet or less. Thus, as a result of the mobility of water and the lack of mobility in the solid Earth, a relatively thick layer of water is heated to moderate temperatures during the summer. On land only a thin layer is heated but to much higher temperatures. During winter the shallow layer of rock and soil that was heated cools rapidly. Water bodies, in contrast, cool slowly as they draw on the reserve of heat stored within. As the water surface cools, vertical motions are established. The chilled surface water, which is dense, sinks and is replace by warmer water from below, which is less dense. Consequently, a larger mass of water must cool before the temperature at the surface will drop appreciably.
- Because land surfaces are opaque, heat is absorbed only at the surface. Water, being more transparent, allows some solar radiation to penetrate to a depth of several meters.
- The specific heat is more than three times greater for water than for land. Thus, water requires considerably more heat to raise its temperature the same amount as an equal volume of land.
- Evaporation (a cooling process) from water bodies is greater than from land surfaces. Energy is required to evaporate water. When energy is used for evaporation, it is not available for heating.
Specific Heat
The amount of heat needed to raise the temperature of 1 gram os a substance 1*C.
Gulf Stream
An important surface ocean current in the Atlantic Ocean that flows Northward along the coast of the United States.
What force drives ocean currents?
Surface currents are set in motion by the wind. At the water surface, where the atmosphere and ocean meet, energy is passed from moving air to the water through friction. As a consequence, the drag exerted by winds blowing steadily across the ocean causes the surface layer of water to move. Thus, major horizontal movements of surface waters are closely related to the circulation of the atmosphere, which in turn is driven by the unequal heating of Earth by the Sun.
Are pollard-moving ocean currents warm or cold?
Poleward-moving ocean currents are warm, and equator ward-moving currents are cold.
Warm currents make temperatures higher than expected and cold currents make temperatures cooler than expected.
How does altitude influence average temperatures?
The higher the altitude, the lower the average temperature.
Where is daily temperature range usually greater: at the base or the top of a mountain? Explain.
The top of a mountain. The fact that high-altitude places are warmer than the value calculated using the normal lapse rate results from the adsorption and reradiating of solar energy by the ground surface.
Not only do temperatures drop with an increase in altitude but atmospheric pressure and density also diminish. Because of the reduced density at high altitudes, the overlying atmosphere absorbs and reflects a smaller portion of the incoming solar radiation. Consequently, with an increase in altitude, the intensity of solar radiation increases, resulting in relatively rapid an intense daytime heating. Conversely, rapid nighttime cooling is also the rule in high mountain locations. Therefore, stations located high in the mountains generally have a greater daily temperature range than do stations at lower elevations.
How does geographic position affect temperature?
A coastal location where prevailing winds blow from the ocean onto the shore (a windward coast) experiences the full moderating influence of the ocean - cool summers and mild winters - compared to an inland station at the same latitude. A coastal location where prevailing winds blow from the land toward the ocean (a leeward coast) will have a more continental temperature regime because the winds do not carry the ocean’s influence onshore.
How does cloud cover and albedo control temperature?
Cloud cover is important because many clouds have a high albedo and therefore reflect a significant proportion of sunlight that strikes them back to space. By reducing the amount of incoming solar radiation, daytime temperatures will be lower than if the clouds were absent. The albedo of clouds depends on the thickness of cloud cover and can vary from 25 to 80%
At night, clouds have the opposite effect as during daylight. They absorb outgoing Earth radiation and emit a portion of it toward the surface. Consequently, nighttime temperatures do not drop as low as they would on a clear night.
The effect of cloud cover is to reduce the daily temperature range by lowering the daytime maximum and raising the nighttime minimum.
Heat Wave
A prolonged period of abnormally hot and usually humid weather that typically lasts from a few days to several weeks.
Lag of the Maximum
The time of the highest daily temperature does not generally conincide with the time of maximum radiation. This delay is called the lag of the maximum.
What causes the lag of the maximum?
- Solar radiation received exceeds the rate of Earth radiation lost.
- Air is a poor absorber of most solar radiation; it is heated primarily by energy reradiated by Earth’s surface. The rate at which Earth supplies heat to the atmosphere through radiation, conduction, and other means, however, is not in balance with the rate at which the atmosphere radiates away heat. Generally, for a few hours after the period of maximum solar radiation, more heat is supplied to the atmosphere by Earth’s surface than is emitted by the atmosphere to space. As a result, most locations experience an increase in air temperature during the afternoon.
Urban Heat Island
The fact that temperatures within cities are generally higher than in rural areas.
