Why san diego is cooler than LA?

Question

Temperature., because of population or others?

Answer 1

LA is much more inland than San Diego.  The Pacific ocean stays pretty cold throughout the summer and keeps San Diego pretty cool.  The further inland you go, the much hotter it gets.    For example, take a look at Santa Monica up near LA.  It is on the ocean and its average high temperature in August is 70 degrees.  Los Angeles, in contrast, has an average high in August of 85 degrees.  Incidentally, the average high for San Diego is 78 in August and Huntington Beach (up near LA) has an average high in August of 73.  Wind and ocean currents also have an impact.  But, the principal reason is that LA is much more inland than SD.

Answer 2

It's more a function of where the representative weather station is.  Los Angeles' weather station that's almost always referred to is the near the Civic Center/USC campus downtown, about 10 miles inland.  San Diego's is at the airport, within a mile or two of the beach.  A better comparison would be San Diego with Los Angeles International.  

Anyway, the more inland location of Los Angeles Civic Center downtown gives the illusion that it's warmer in L.A.  When you watch the news, see if they give a report for LAX and I'm sure you'd find that much more comparable.