Why is it so hot in San Diego?
Most days the cool Pacific and the marine layer keep coastal San Diego mild. It gets hot when that ocean influence is cut off: Santa Ana winds blow desert air to the coast, or the marine layer collapses under a strong high, or monsoon moisture makes the heat sticky.
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Why it's normally mild
The water off San Diego stays cool all year, and the usual afternoon sea breeze carries that cool air inland. On many summer mornings a marine layer of low cloud shades the coast too. That's why the beaches can be 70°F while inland valleys like El Cajon or Escondido are in the 90s.
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Three ways the heat gets in
- Santa Ana winds. High pressure over the Great Basin turns the wind around so it blows from the desert to the sea. The air warms as it sinks to the coast and arrives dry, and the beaches can end up hotter than inland. How Santa Anas work.
- A crushed marine layer. A strong high-pressure dome pushes the warm lid of air down until the cool marine layer is too thin to form clouds or reach far inland. The coast loses its air conditioning.
- Monsoon humidity. In late summer, moist air from Mexico can flow in from the southeast. Temperatures aren't extreme but the dew point climbs into the 60s or 70s, so it feels much hotter. About the monsoon.
Why September and October can be the hottest
San Diego's ocean is at its warmest in late summer and early fall, and fall is when Santa Anas start. Together they make September and October prime time for the coast's hottest days, often hotter than July. Air Story's first screen tells you which of these is happening right now and shows where today's air came from.
Questions people ask
- Why is it hotter at the beach than inland during a Santa Ana?
- The air warms the whole way down as it sinks from the high desert, so it's hottest when it reaches sea level, and the cooling sea breeze is blocked.
- Why is San Diego so humid sometimes?
- Monsoon flow from the southeast in late summer, or tropical moisture from storms off Mexico, can raise the dew point well above normal. The ocean alone usually keeps it moderate.