1.2 D ESARROLLO
1.2.4 Técnicas para la recopilación de requisitos
Temperature Variation With Altitude.
The general decrease of temperature with altitude throughout the atmosphere is known as the Environmental Lapse Rate (ELR). In the ICAO Standard Atmosphere (ISA), the ELR is 1.98°C/1 000 feet. However, under certain conditions, the ELR can vary significantly from this average, especially in the lower Troposphere.
If the temperature remains constant through a given depth of atmosphere, it is described as being isothermal, as depicted in Figure 4.12. The most striking example of an isotherm is the almost constant temperature of the lower part of the Stratosphere. But isotherms can also be found near the Earth’s surface, as depicted in Figure 4.12.
Figure 4.11 Within the Troposphere, temperature gradually decreases with altitude. In ISA, the average temperature lapse rate is 1.98ºC/1 000 feet.
Under certain circumstances, such as on clear, cloudless nights in winter, temperature may increase with altitude; this phenomenon, known as a temperature inversion, was mentioned earlier in the chapter under the heading Conduction.
Diurnal Variation.
The temperature of the air will vary greatly in a given location through a 24-hour period. This is called diurnal variation (from the Latin diurnus meaning daily.) The Sun is at its highest at noon, with the maximum incoming solar radiation occurring at that time. However, due to ‘thermal inertia’, the surface of the Earth continues to receive more incoming solar radiation than it emits as terrestrial radiation. Therefore, the actual maximum air temperature usually occurs some two to three hours after maximum solar elevation, usually at about 1500 local time. Similarly, as a result of
‘thermal inertia’, the time at which the minimum temperature is reached is around half an hour after dawn. Figure 4.14 shows diurnal variation over a twenty four hour period.
In general, temperature varies with time over a 24 hour period. Because of
“thermal inertia” the hottest time is 2 to 3 hours after the sun has reached its highest point, at local noon, and the coldest about 30 minutes after dawn.
Temperature will generally decrease with height, but it is possible for temperature to remain constant with altitude. In such a case, the temperature is said to be isothermal. Temperature may even increase with height.
This phenomenon is called a temperature inversion.
Figure 4.13 Inversion - temperature increasing with height.
Figure 4.12 An Isotherm - temperature remains constant with height.
CHAPTER 4: TEMPERATURE
Clouds and Diurnal Variation.
Clouds have a great influence on the maximum and minimum air temperatures.
Clouds reflect some incoming solar radiation during the day, thereby reducing the incoming solar radiation, which, in turn, will reduce the maximum daily temperature.
But, at night, cloud acts in a very different manner. Heat energy stored inside the cloud layer, by water vapour, is radiated back to the atmosphere, especially to that part of the atmosphere lying beneath the cloud. In this way, cloud acts like a blanket around the Earth. The release of latent heat from cloud, as it forms by the process of convection, also contributes to the warming effect.
When it is cloudy, therefore, minimum night time temperatures are higher than if skies were clear.
Cloud cover, then, will reduce the extent of diurnal variation of the temperature of the atmosphere. Note, however, that the basic form of the diurnal variation curve is almost the same, whether or not cloud is present. Figure 4.15, shows how the diurnal variation is modified by cloud cover. With cloud cover, maximum daytime temperature is lower, and the minimum temperature, occurring just after dawn, is
Figure 4.14 Diurnal Variation - the change of temperature over a 24-hour period. The temperature is highest at 15:00 local time, and lowest just after dawn.
A cloud layer will modify the diurnal variation. Cloud
will reduce the maximum temperature, and increase the minimum temperature.
Figure 4.15 Cloud cover reduces diurnal variation.
Wind and Diurnal Variation.
The wind also affects diurnal variation of temperature. By day, the effect of wind is to reduce the maximum temperature. Figure 4.16 depicts turbulent mixing of the lower parts of the atmosphere, caused by wind, bringing down cooler air from above, and mixing this cooler air with warmer air near the Earth’s surface. During the day, then, mixing causes the surface temperature to be lower than it would have been if there had been no wind.
At night, the mixing process has the opposite effect. The presence of wind keeps surface air temperatures higher than they otherwise would be. (See Figure 4.17.)
At night, if there is no wind and no cloud, the Earth’s surface will cool down rapidly, because there is no longer any incoming solar radiation. The air in direct contact with the Earth’s surface, therefore, will also cool down, while air at higher altitudes will not be as greatly affected by the cooling from the surface. This phenomenon gives rise to a temperature inversion. If wind is present, however, turbulent mixing will bring warmer air down from a higher level, averaging-out the temperature throughout an appreciable depth of the lower atmosphere.
Figure 4.17 At night, mixing causes the warmer air at altitude to mix with the surface layer of air, raising the temperature of the surface air.
Figure 4.16 Mixing: by day, the wind causes the cooler air at altitude to mix with the warmer air at the surface, reducing air temperature at the surface.
Wind will modify the diurnal variation by mixing; this reduces the maximum daily temperature and increases the minimum daily temperature.
CHAPTER 4: TEMPERATURE
Variations with Latitude.
There are also large temperature variations across the surface of the Earth. The latitude of a given location has a marked effect on the amount of energy received from the Sun. Figure 4.18, below, shows that, when considering a unit cross section of radiated energy from the Sun, the same amount of energy is spread across a greater area at higher latitudes than at the Equator. Therefore, a location on Earth, at a high latitude, near the Poles, will receive less energy from the Sun than a location at a lower latitude, nearer to the Equator.
The Equator is warmer because, at the Equator, a beam of radiated energy from the Sun meets the Earth’s surface at approximately 90º to the surface, which means that solar radiation is spread over a smaller area, than an identical amount of radiation nearer the Poles would be. Of course, the insolation at the Poles varies seasonally, too. The Pole tilting towards the Sun will receive more solar radiation over an equivalent area than the Pole tilting away from the Sun. (See Figure 4.19, below.) Seasonal Variations of Temperature.
The Earth’s spin axis is tilted at 23.5º to a line passing vertically through its orbital plane. This tilt causes seasonal variations in temperature across the Earth’s surface.
Figure 4.18 Variations of temperature due to latitude.
The angle at which the Sun’s rays hit the
Earth affects the amount of insolation in a given area.
Thus, high latitudes receive less insolation than the tropics.
Figure 4.19 The Earth’s tilt and its effect on the seasons in the Northern Hemisphere.
The degree of Insolation at any given location on
Earth varies with latitude and seasons.
From Figure 4.19, we can see that, on 21st June, the summer solstice in the Northern Hemisphere, the tilt of the Earth causes the Northern Hemisphere to receive a greater amount of radiation from the Sun, than the Southern Hemisphere. This fact defines the Northern Hemisphere summer and the Southern Hemisphere winter. In fact, from simple geometry, we may calculate that the latitude of 23.5° North marks the point of maximum solar radiation in the Northern Hemisphere summer, i.e. on 21st of June. The latitude of 23.5° North is commonly referred to as the Tropic of Cancer.
Looking back at Figure 4.19, you can see that on 21st December, the winter solstice in the Northern Hemisphere, the tilt of the Earth causes more of the Southern Hemisphere to be exposed to the Sun. In December, then, the Southern Hemisphere is warmer than the Northern Hemisphere. This defines the Southern Hemisphere summer and the Northern Hemisphere winter. The latitude of 23.5° South marks the point of maximum solar radiation on 21st December in the Southern Hemisphere.
The latitude of 23.5° South is commonly referred to as the Tropic of Capricorn.
The geographic region of the Earth between the Tropic of Cancer, in the Northern Hemisphere, and the Tropic of Capricorn, in the Southern Hemisphere, is called the Tropics. The Tropics include all the areas of the Earth where the Sun reaches a point directly overhead (90°), at least once during the year. Therefore, the Tropics represent the warmest areas on the Earth.
Seasonal temperature change in any locality on Earth is, therefore, caused by regular variations in insolation over different parts of the Earth as the seasons progress.
Temperature Variations from Land to Sea.
Compared to the land, the sea takes a longer time to heat up and cool down.
Therefore, the diurnal temperature variation of the sea is less than that of the land. This is because the sea has a larger specific heat capacity than the land.
Consequently, the sea requires more heat energy to raise its temperature than does the land, for an identical temperature increase. This effect is noticeable, both
Figure 4.20 At the summer solstice, in the Northern Hemisphere, maximum solar radiation is received at 23.5° North, the Tropic of Cancer.
CHAPTER 4: TEMPERATURE
daily and seasonally. Figure 4.21 shows that, while the temperature of the world’s oceans generally varies by only five degrees in a 24-hour period, the temperature of the land masses varies by up to three times that figure.
During the day, and in the summer months, the land will be at a higher temperature than the sea, but, during the night and in the winter months, this situation is reversed, with the sea generally being warmer than the land.
The differences in temperature between the land and the sea are the cause of sea breezes and land breezes We will examine sea and land breezes in Chapter 12.
Figure 4.21 Diurnal temperature variation of the land and sea.
The diurnal variation of the sea temperature
is less than that of land temperature. This is the underlying cause of sea breezes, by day, and land breezes, by night.