The annual mean temperature is an average of the twelve monthly means

3.1. Temperature: A Measure of Heat

Temperature is an expression of the degree of hotness or coldness of a substance.

Temperature can be measured using a thermometer. In this device, a liquid inside a glass tube expands and contracts with temperature, and the position of the liquid surface indicates the temperature. Another device measuring temperature is called thermister, in this device, the electrical resistance changes with temperature, and temperature is digitally measured.

There are three types of units for temperature:

Celsius (C ), international standard units. The reference points of this scale are the sea-level freezing and boiling points of pure water, which are 0 and 100 degrees, respectively. The difference between freezing and boiling point is 100 degrees

Fahrenheit (F): we all know that, the reference points on this scale are: 32 degree is freezing point for pure water on sea-level. 212 degree is boiling point.

Kelvin (K), mostly used as scientific research. The scale maintains a 100 degrees between the boiling and freezing points of water but no negative values.

The relationship between the three scales can be expressed

 Degree Celsius = (degree Fahrenheit –32)/1.8

Degree Celsius = degree Kelvin – 273

Since air temperature can very with height, it is measured at a standard level –1.2 m or 4 feet above ground at all weather station.

Although some weather stations report temperatures hourly, most station only reports the highest and lowest temperatures recorded during a 24-hour period.

Daily, monthly, and yearly temperature statistics for a station are produced using the daily maximum and minimum temperaure.

The mean daily temperaure is defined as the average of the maximum and minimum daily values.

The monthly mean temperature for each station is determined by averaging the daily means for each day in the calendar month.

The annual mean temperature represents the average of the 12 monthly mean.

The difference between the means of the warmest and coldest months is the annual temperature range.

3.2 Daily temperature pattern

Daily temperature pattern is decided by daily net radiation pattern.

Figure 3.3 show average curves of daily insolation, net radiation, and air temperature observed at lat. 40 to 45 N in the interior of North America.

Graph (a) shows daily insolation. There are several features in this figure. (1) at all time, the maximum insolation occurs at noon. (2) the maximum insolation changes over time during the year.  (3) the time length of insolation also changes over time during a year. At the equinox (middle) curve), insolation begins at about sunrise (6 am), rises to a peak value at noon, and declines to zero at sunset (6 pm). At the June solstice, insolation begins about two hours earlier (4 am) and ends about two hours later (8 pm). At the Decemeber solstice, insolation begins about two hours later than the equinox and ends two hours earlier.

Graph (b) shows net radiation for the surface. Net radiation is the difference between incoming radiation and outgoing radiation. If the incoming radiation is greater than outgoing radiation, then net radiation is positive or surplus, otherwise, net radiation is negative or deficit. Compare graph (b) with (a), we can see it is generally similar in shape. During the night, net radiation is deficit and the deficit lasts till the sunrise and increasingly sharply and reach maximum at noon and then decrease and to a value of zero shortly before sunset.

Although the three net radiation curves show same general daily pattern. They differ greatly in magnitude. For the June solstice, the positive values are quite large, and the area of surplus is much larger than the area of deficit. This is means the net radiation for the entire day is positive. At the December, the total deficit is larger than the surplus. This means the net radiation for the entire day is negative.

Graph (c ) shows the daily temperature pattern. The minimum daily temperature usually occurs about half an hour after sunrise. Since net radiation has been negative during the night, heat has flowed from the ground surface, and the ground has cooled the surface air layer to its lowest temperature.  As net radiation become positive, the surface warms quickly and transfers heat to the air above. Air temperature rises sharply in the morning hours and continues to rise after the noon peak of net radiation.

We should expect the air temperature to rise as long as net radiation is positive. However, another process begins in the early afternoon. Mixing of the lower air by vertical currents distributes heat upward, offsetting the temperature rise. Therefore, the temperature peak usually occurs in the mid-afternoon, from 2-4 pm depending upon the local weather conditions 

3.3. Vertical temperature patterns

The lower atmosphere can be divided into two layers:

Height:  0-13 km                                                          13-50 km        

Temperature: usually decrease with heights:                   Slightly increase mainly because of

6.4 C/1000 m (environmental lapse rate)

Water vapors, tiny dust (aerosol).                                 very little water vapor and dust because of strong wind but contains the ozone layer which absorbs solar ultraviolet radiation and thus shields earthly life from this intense, harmful form of energy.

Temperature inversion: The most prominent exception to a normal lapse-rate condition is a temperature inversion, a situation in which temperature in the troposphere increases, rather than decreases, with increasing altitude.

They occur typically on a long,cold winter night when a land surface rapidly emits long-wave radiation into a clear, calm sky. The ground is soon colder than the air above it and snow cools the air by conduction. In a relatively short time, the lowest few hundred feet of troposphere become colder than the air above and a temperature inversion is in effect.

High-mountain environment

For those of you who have climbed the high mountain, you might feel

(1)   temperature decreases with elevation;

(2)   sunburn faster

(3)   more difficult to breath;

(4)   temperature at night drops very quickly.

temperature generally drop as you go up in elevation. In addition, you may feel sunburn is a bit faster, also you may feel breathing is getting more difficult, and also, you may feel at night, it is much cold than you expected. This is because, at high elevation, the air is thin-that is , there are fewer gas molecules in a unit volume of air. Of course, oxygen is fewer as well, that is why you feel a bit difficult to breath. With fewer air molecues and dust particels to absorb and scatter the sun’s light, so the sun’s ray will feel stronger. And also, there is less carbon dioxide, so less radiation from the surface will be trapped (called the greenhouse effect). As a result, the temperature will tend to drop lower at night than you expect.

3.4. Annual temperature patterns

As the earth revolves around the sun, the tilt of earth’s axis causes an annual of variation in insolation. This cycle produces an annual cycle of net radiation, which, in turn, causes an annual cycle to occur in mean monthly air temperature.

See figure 3.12. it shows the yearly cycle of the net radiation and monthly mean temperature at four sites with different latitudes.

Graph (a) shows the annual cycle of the net radiation rate for four stations. At Manaus, located nearly on the equator, the average net radiation rate is strongly positive in every month. However, there are two minor peaks. These coincide approximately with the equinoxes, when the sun is nearly straight overhead. A look at the temperature graph of Manaus shows uniform air temperatures, averaging about 27 C (81 F) for the year. The annual temperature range, or difference between the highest and lowest, is only 2.7 C (3 F). So there are no temperature seasons near equator areas.

The next station is Aswan, Egypt, located at Lat. 24. The positive net radiation rate curve shows that a large radiation surplus exist for every month. Furthermore, the net radiation rate curve has a much stronger annual cycle, with values for June and July that are almost double those of December and January. The annual range or difference is 17 C (or 30 F) and summer (June, July and August) averaging 32 C (90 F).

Farther north to the third station, Hamberg, Germany, located lat. 54. The net radiation rate is positive for nine months and negative for three winter months. Summer months reach a maximum of just 16 C (60 F), while winter months reach a minimum of just about freezing (0 C or 32 F). The annual range is about 17 C, the same as at Aswan.

The last one is Yakutsk, Siberia, Lat. 62 N. The net radiation rate is negative for almost six months. During the winter , the air temperatures drop to extremly low levels, between 35 –45 C (-30 and –50 F). In summer, when daylight lasts most of a 24-hour day, the net radiation rates rises to a strong peak, higher than the other three stations. The summer months average temperature is about 13 C (55 F). the annual range is enormous-over 60 C (108 F).

Land-water contrast

The temperature pattern not only is affected by latitude but by the location-maritime or continental. because oceans heat and cool more slowly than continents.

Four important thermal differences between land and water surfaces account for the land-water contrast.

(1)   Water is better transmitter than rock or soils . So the radiation therefore warms a thick water surface layer only slightly and a thin land surface layer more intensely.

(2)   Water is slower to heat than dry soil or rock. It takes about five times as much heat to raise the temperature of the water one degree as it does to raise the temperature of the rock one degree.

(3)   Warm water can mix with cold water

(4)   Warm water can cool easily by evaporation.

The significance of these contrasts between land and water heating and cooling rates is that both the hottest and coldest areas of earth are found in the interiors of continents, distant from the influence of oceans. The places located well inland and far from oceans generally experience a stronger daily and annual temperature range.

Urban and rural temperature contrasts

Temperature in cities is usually higher than the surrounding rural regions. Because:

(1)difference in surface materials. Surface in city is mainly covered with pavement or structure, while rural areas are mainly covered with soil and vegetation. Because the water can be evaporated from soil and vegetation into air in the rural regions, so loss a considerable amount of latent heat, and as a result, the temperature at the surface would cool down. While in the city, evaporation takes place only in small proportion of areas such as lawns, street trees. So during the daytime, temperature is higher than the rural area.  at night, urban materials release stored heat to the surface, also keeping temperatures warmer. Because the ability of concrete, stone, and pavement to conduct and hold heat is better than the soil.

(2) in the city, fuel consumption is much greater than the countryside. Summer, air conditioner pumping heat out of building , releasing the heat to the air. In winter, furnaces warm building. So interior heat is conducted to outside walls and roofs, which radiate heat into the urban environment.

As a result, the city itself is like a heat island.

3.5. Global temperature pattern.

The gross patterns of global temperature are controlled largely by four factors-altitude, latitude, land-water contrasts, and ocean currents.

Altitude: Temperature decreases with altitude

Latitude: clearly the most evident feature of any world temperature map is the general trend that temperature decreases with latitudes.

Land-Water contrasts: the different heating and cooling characteristics of land and water also affect the global temperature patterns.

Ocean Currents: Warm ocean currents would increase the temperature.

Now let’s see the world temperature map:

Isotherms: lines drawn to connect locations having the same temperature.

Figure 3.18 shows the mean monthly temperature (C ) for January and July. Some important features can be summarized as followings:

1.      temperatures decreases from the equator to poles.

2.      Large landmasses located in the subarctic and arctic zones develop centers of extremely low temperatures in winter.

3.      Temperatures in equatorial regions change little from January to July.

4.      Highlands are always colder than surrounding lowlands.

5.      Summer temperatures are higher over continents than over the oceans, as shown by the poleward curvature of the isotherms. Winter temperatures are lower over continents than over the oceans; the isotherms bend equatorward over continents.

6.      Enormous seasonal variation in temperature occur in the interiors of high-latitude continents. At the other extreme, the average temperature fluctuates only slightly from season to season in the tropics, particularly over tropical oceans.