Before rapid human population of the earth all areas of the earth were rural. As humankind's population and technology increased, so did urbanization. The temperature pattern between vegetated rural areas and constructed urban areas can be quite different and is part of the reason global temperatures have increased over the last century (especially where urban hot spots with populations greater than 500,000 people have expanded closer to these temperature sensors). The difference between urban and rural temperatures is most prevalent when the winds are light, the dewpoint is moderate and the skies are clear. In this weather situation, the localized airmass building over the urban and rural areas do not mix out with each other rapidly, the difference in absorbed solar energy is greater between the two environments and evaporation produces a greater cooling in rural areas. The effect of cooler temperatures in rural areas is most noticeable at night since winds tend to be calmer at night. Below are some reasons rural areas are cooler and urban areas warmer: 1. WATER- There is more water in vegetated rural areas. Vegetation transpires water through pores. This evaporation (transpiration) helps cool the air through latent heat absorption. Soil has more moisture to evaporate than concrete also. Rainfall soaks into rural soil while in urban areas the water runoffs into storm drains. 2. ABSORBED SOLAR ENERGY- Some of the sun's energy in rural areas is used for the photosynthetic process. Solar energy is absorbed over a larger spatial area in rural areas due to vast increase in surface area produced by leaves and twigs. More solar energy in rural areas is used to evaporate water. These combine to produce less sensible heat in rural areas as compared to urban areas. 3. POLLUTION- Greater air pollution in urban areas increases the concentration of greenhouse gases. These greenhouse gases trap energy that would have radiated into space. |