**4. Evapotranspiration and net radiation**

Some of the characteristics can be sorted into the four main causes of heat island formation: reduced evapotranspiration, increased heat storage, increased net radiation, and increased anthropogenic heat. The lack of vegetation and increase of surface impervious cause a reduction in evapotranspiration. Low solar reflectance and increased levels of air pollution foment increases in net radiation. Evapotranspiration is energy transmitted away from the Earth's surface by water vapor, and this is a process plants use to keep themselves cool, and it increases when there is more moisture available. On the other hand, net radiation encompasses four separate radiation processes taking place at the Earth's surface. Net radiation = Incoming solar − Reflected solar + Atmospheric radiation − Surface radiation. The first term in the equation is related with the amount of energy radiating from the sun. This varies in Puerto Rico mainly according to the time of the day, the amount of cloud cover, and the atmospheric pollution levels. The second term, reflected solar, is directly related with the "albedo," which is the amount of solar energy that bounces off a surface. The higher albedo, the greater is the amount of reflected energy. The third term is heat emitted by particles in the atmosphere, such as clouds, pollution, and Sahara dust and the last term, surface radiation, is heat radiated from a surface itself. A relatively warmer surface radiates more energy to its surroundings, which is the case of San Juan city in north Puerto Rico area. In 2002, the north area city had approximately 70% impervious surface cover, 13% grass area, and 16% tree areas [10].

A comparative study for evapotranspiration and net radiation was successfully accomplished, as part of the evidence to determine the heat island effect. Using GOES satellite data, the ground level, 1 km resolution net radiation, evapotranspiration, and rainfall parameters became available in Puerto Rico in March 2009 [11]. **Figure 8** shows that during the last 10 years, evapotranspiration measures have been higher in the area designated as rural, the difference between rural and urban is very significant, and this evidences a tendency to a separation that increases with time.

In the case of precipitation between the urban and rural areas, using 10-year historical data, it is observed that rainfall has been greater in the urban center (**Figure 9**), under conditions of similar land use. It would be assumed that the amount of precipitation increases evapotranspiration. But this is not the case when there is an UHI phenomenon.

**187**

*Urban Heat Island Effects in Tropical Climate DOI: http://dx.doi.org/10.5772/intechopen.91253*

*Evapotranspiration (mm) urban vs. rural data (2009–2019).*

*Rainfall (mm) urban vs. rural data (2009–2019).*

**Figure 8.**

**Figure 9.**

The earth in natural conditions absorbs rain, which is evaporated by the sun's rays on hot days and released to the atmosphere, cooling the environment. The lack of vegetation in the cities contributes to the heat island effect. But in the cities, the rain ends up in the sewage systems. The urban centers usually also have few trees.

Another valid theory is that the net radiation collected by an urban setting is generally greater than that collected in a rural area. The difference is due to many factors, such as, for the Puerto Rico case, lower solar reflectance of urban materials and restrictive urban geometries. Another significant factor, even though it occasionally happens, is the pollutant due to Sahara dust. However, the difference in net radiation between urban and rural areas is not significant. A possible little difference is observed in **Figure 10** where the urban area has higher values. During the 10 years of study, an average of 0.6656 MJ/m2/day represents the difference in net radiation. Another main characteristic of the climate is wind speed. Physical characteristics of urban climate such as tall buildings, paved streets, and parking lots affect wind flow. The differences of urban climate and rural climate are attributable in large part to the altering of the natural terrain through the construction of artificial structures and surfaces. In theory, the center of a city is warmer than the outlying areas. Monthly minimum temperature readings at related urban and rural sites frequently show that the urban site is 3–4°F warmer than the rural site, for this case. Two primary processes influence the formation of this "heat island." During summer, urban masonry and asphalt absorb, store, and reradiate more solar energy per unit area than vegetation and soil typical of rural areas. Furthermore, a small amount of this energy can be used for evaporation in urban areas, which characteristically exhibit greater precipitation runoff from streets and buildings. At night,

Vegetation plays a crucial role due to evapotranspiration.

*Urban Heat Island Effects in Tropical Climate DOI: http://dx.doi.org/10.5772/intechopen.91253*

*Vortex Dynamics Theories and Applications*

Rosenzweig et al. [8] analyzed the New York city heat island effect model by taking six case study areas and tested the mitigation strategies. They found that vegetation helps to keep surfaces cool more effectively than increasing the albedo. But they suggested that in order to reduce the temperature in New York city, replacement of low-albedo materials with high-albedo light-colored materials will

Sailor [9] describes that the urban heat island effect mitigation can be done in two ways. One is by increasing the albedo of the urban surface, and the other is by increasing evapotranspiration. On the other hand, white materials which have albedo greater than 0.60 instead of black materials having albedo of 0.05–0.10 can be used as roofing materials. They found that the roof temperature dropped by 25**°**C for 0.60 albedo compared to that of 0.20 albedo. More solar radiation could be reflected if the road and highway pavements were of high-albedo materials. White cement mixtures can be made for which the albedo should be higher than the most reflective gray cement mixtures. However, use of high-albedo materials for roads and highway pavement may not be so much effective because of the sky view factor. Even if, it is used,

work great as 64% of the surface area of the city can be replaced easily.

some of the reflection will be intercepted by the buildings surrounding it.

Some of the characteristics can be sorted into the four main causes of heat island formation: reduced evapotranspiration, increased heat storage, increased net radiation, and increased anthropogenic heat. The lack of vegetation and increase of surface impervious cause a reduction in evapotranspiration. Low solar reflectance and increased levels of air pollution foment increases in net radiation. Evapotranspiration is energy transmitted away from the Earth's surface by water vapor, and this is a process plants use to keep themselves cool, and it increases when there is more moisture available. On the other hand, net radiation encompasses four separate radiation processes taking place at the Earth's surface. Net radiation = Incoming solar − Reflected solar + Atmospheric radiation − Surface radiation. The first term in the equation is related with the amount of energy radiating from the sun. This varies in Puerto Rico mainly according to the time of the day, the amount of cloud cover, and the atmospheric pollution levels. The second term, reflected solar, is directly related with the "albedo," which is the amount of solar energy that bounces off a surface. The higher albedo, the greater is the amount of reflected energy. The third term is heat emitted by particles in the atmosphere, such as clouds, pollution, and Sahara dust and the last term, surface radiation, is heat radiated from a surface itself. A relatively warmer surface radiates more energy to its surroundings, which is the case of San Juan city in north Puerto Rico area. In 2002, the north area city had approximately 70% impervi-

A comparative study for evapotranspiration and net radiation was successfully accomplished, as part of the evidence to determine the heat island effect. Using GOES satellite data, the ground level, 1 km resolution net radiation, evapotranspiration, and rainfall parameters became available in Puerto Rico in March 2009 [11]. **Figure 8** shows that during the last 10 years, evapotranspiration measures have been higher in the area designated as rural, the difference between rural and urban is very significant, and this evidences a tendency to a separation that increases with time. In the case of precipitation between the urban and rural areas, using 10-year historical data, it is observed that rainfall has been greater in the urban center (**Figure 9**), under conditions of similar land use. It would be assumed that the amount of precipitation increases evapotranspiration. But this is not the case when

**4. Evapotranspiration and net radiation**

ous surface cover, 13% grass area, and 16% tree areas [10].

**186**

there is an UHI phenomenon.

**Figure 8.** *Evapotranspiration (mm) urban vs. rural data (2009–2019).*

**Figure 9.** *Rainfall (mm) urban vs. rural data (2009–2019).*

The earth in natural conditions absorbs rain, which is evaporated by the sun's rays on hot days and released to the atmosphere, cooling the environment. The lack of vegetation in the cities contributes to the heat island effect. But in the cities, the rain ends up in the sewage systems. The urban centers usually also have few trees. Vegetation plays a crucial role due to evapotranspiration.

Another valid theory is that the net radiation collected by an urban setting is generally greater than that collected in a rural area. The difference is due to many factors, such as, for the Puerto Rico case, lower solar reflectance of urban materials and restrictive urban geometries. Another significant factor, even though it occasionally happens, is the pollutant due to Sahara dust. However, the difference in net radiation between urban and rural areas is not significant. A possible little difference is observed in **Figure 10** where the urban area has higher values. During the 10 years of study, an average of 0.6656 MJ/m2/day represents the difference in net radiation.

Another main characteristic of the climate is wind speed. Physical characteristics of urban climate such as tall buildings, paved streets, and parking lots affect wind flow. The differences of urban climate and rural climate are attributable in large part to the altering of the natural terrain through the construction of artificial structures and surfaces. In theory, the center of a city is warmer than the outlying areas. Monthly minimum temperature readings at related urban and rural sites frequently show that the urban site is 3–4°F warmer than the rural site, for this case. Two primary processes influence the formation of this "heat island." During summer, urban masonry and asphalt absorb, store, and reradiate more solar energy per unit area than vegetation and soil typical of rural areas. Furthermore, a small amount of this energy can be used for evaporation in urban areas, which characteristically exhibit greater precipitation runoff from streets and buildings. At night,

**Figure 10.** *Net radiation urban vs. rural data (2009–2019).*

**Figure 11.** *Average annual wind speed - urban vs rural area.*

radiative losses from urban buildings and street materials keep the city's air warmer than that of rural areas. For this case, it is important to consider the location of the urban site and rural site. San Juan is located on the NE coast of the island of Puerto Rico. It is surrounded by waters of the Atlantic Ocean. The climate is tropical marine, slightly modified by insular influence when land breezes blow. San Juan is representative of most of the coastal localities on the island. That proximity to the coast makes wind speeds higher than in the rural area (see **Figure 11**).
