Urban Climates

located 37N of the equator (similar to that of the mediterranean) which gives it a warm dry climate. In the city, these temperatures are raised by the 800,000 strong population. Industry and homes emit a lot of heat energy as do the people whol live in the city, heat that is often trapped by the stratus cloud that tends to form overnight. The reflective surfaces of buildings concentrate the sunlight energy into the street, further raising temperatures.

Due to the temperature of the city, low pressure is created and this causes strong pressure gradients off the coast of the Pacific ocean, where the cold californian current has created high pressure. THe pressure gradient causes strong SW winds to blow inland to the city and these peak in the afternoon, when the gradient is largest, at around 20-30mph.  These winds are then often channelled into the canyons of the city's skyscrapers and streets. They are also channlled into the Bay and therefore blow across the Golden gate bridge - there have been occassions where these winds have been so strong that the bridge has had to close. Due to the cool winds blowing inland and the warm, low pressure over the city, there is often advection fog formed in the morning- especially in winter anticyclonic conditions. This fog usually burns throguh by the afternoon. 

San Fransisco is a dry region, but recieves rainfall predominantly in the form of convectional and orographic rainfall. THe warm air over the city contains a lot of moisture blown inland on the cooler maritime winds and is blown toward the Sierra Nevado mountain range - this causes it to rise, cool and condense to cause precipitiation. Similarly, the warm, moist maritime air from the south in the Pacific ocean blws eastwards over the cool Californian current, producing rainclouds that may then be blown inland over San Fransisco 

London city remperatures are warm but mild as London is situated in a temperature maritime climate. However, the 11 million people in the urban area generate a large amount of anthropogenic heat in addition to the heat generated by industry, vehicles and homes. This amount of heat energy can dramatically increase temperatures, especially in winter when the effect of heating systems is greatest. 

This heat not only raises temperatures, but generates low pressure over the city which often then cools and condense, leading to extra precipiation in the city. Similarly, this low pressure is replaced by high pressure in the early morning, leading to advection fogs and temperature inversions. These fogs are exaggerated by the presence of the Thames which has a narrow cooling effect of the immediate surrounding urban areas. It provides moisture for fog as well as cooling the warm air to cause condensation

The uplands that surround london, shield it from the common south westerly winds but within the city, shearing and canyoning between large skyscrapers are often responsible for very strong wind speeds.