Mesoscale meteorology is weather influences that occur on smaller size and time scales as compared to the synoptic scale. Mesoscale processes occur in time scales from minutes to hours and space scales of kilometers to many 10s of kilometers. The size of a major metro area can take an hour or more to drive from one side to the other and the distances can be 10s of kilometers. Thus, mesoscale influences can occur over the urban area and due to influences from the urban area. One mesoscale influence occurs due to the heat generated by the urban area. A higher temperature leads to more instability and a greater potential for rising air. When the environment is already unstable, an urban area can make it more unstable. An urban area can be the burst point for convection. In many cases convection will begin over the urban area and then drift downwind with the mean wind direction. Areas downwind from an urban area often tend to get more precipitation when the precipitation is generated from convective air mass type storms. Another mesoscale influence is the urban heat island. Concrete and metal absorb solar radiation and cause the temperature to warm more as compared to solar radiation striking vegetation. On sunny days, the urban area can end up being several degrees warmer than the surrounding countryside. Over many years this can cause the average temperature for a city to increase. Climate data is sometimes calculated for the last 30 years so that data before 30 years ago (that was a little less reliable and when the city was a much different size) does not influence the record. 30 years is used to have a sample size large enough so that a climatic average is meaningful. However, record temperatures will often be used for the entire history of temperature measurements at a location. It is common to have record high temperatures as the urban region expands. Other mesoscale influences include less moisture to evaporate into the air, gustier winds and funneling of wind due to obstructions to wind flow, the potential for severe damage to occur when severe weather strikes, more particulates in the air to develop clouds/fog when the relative humidity is high, and warmer nighttime temperatures due to the building materials releasing heat that was absorbed during the day. |