Why is the wind offshore in the afternoon?
You're out for an early morning surf session, and the offshore breeze is holding the wave’s face wide open. It might just be the day you tuck your head in for a second and spend the next week talking about barrels. Fast forward a few hours, and suddenly you're battling an onshore mess. Why does luck favor the dawn patrollers?
This daily cycle isn't random. It's all about temperature differences between land and sea coupled with the earth's rotation.
During the day, the sun heats up the land faster than the ocean. Hot air rises, creating a low-pressure void over land. The cooler air over the ocean moves to fill this area of lower pressure onshore. This movement of air from sea to land is what we call a sea breeze, and it's why afternoons often bring onshore winds.
But at night, the land cools off faster than the ocean. The air over the land becomes cooler and denser, creating a high-pressure area. This cooler air then flows towards the relatively warmer ocean, giving us those crisp offshore winds that sculpt the waves.
Okay, that explains the back-and-forth, but what about the north-and-south winds throughout the day? This is thanks to the Coriolis effect, which we discussed here.
Remember how the Earth's rotation makes moving objects appear to curve? Well, it affects our winds too. In San Diego, this means our onshore winds tend to come from the west-northwest, while our offshores often have a more southerly component.
San Diego's coastline isn't a straight line running north to south. It's got curves. Our local topography, with its mesas, canyons, and coastal mountains, can funnel and redirect these winds. So while the general pattern holds, your break can experience some unique variations.
Take places like Blacks or Sunset Cliffs. The cliffs there can amplify offshore winds, creating those glass-off conditions that make dawn patrol totally worth the early wake-up call. Meanwhile, spots like Mission Beach, with its flatter terrain, might see a more textbook sea breeze pattern.
Understanding this cycle can significantly improve your surf sessions. Early mornings and late evenings often offer the best conditions, thanks to those offshore winds. But if you're stuck with a midday session, knowing when the wind might shift can help you time your session just right.
And it's not just surfers who benefit from this knowledge. Sailors, paragliders, and even city planners use this information. Those cool sea breezes help moderate San Diego's temperatures, keeping us from baking like our inland neighbors. Our city breathes, just slower and saltier than you or me.
Further Reading:
So interesting. I’ve been trying to learn much more lately about local geographical constraints and happenings that have impacts on waves. I think a lot of people look at Surfline and don’t question any further why the waves are bad or good etc.
Nice write up!