| Month | High | Low | Rain | Wet days | Score |
|---|---|---|---|---|---|
| January | 17.9° | 10.4° | 53.1mm | 6.6 | 80 |
| February | 17.5° | 10.5° | 53.2mm | 6.2 | 80 |
| March | 17.6° | 11.7° | 30.4mm | 4.8 | 83 |
| April | 18.3° | 12.8° | 13.2mm | 2.6 | 89 |
| May | 19.3° | 14.8° | 5.9mm | 1.7 | 94 |
| June | 20.4° | 16.4° | 1.1mm | 0.8 | 98 |
| July | 22.1° | 18.2° | 0.3mm | 0.4 | 99 |
| August | 23° | 18.6° | 2.3mm | 0.4 | 99 |
| September | 23.2° | 18.1° | 2.2mm | 0.6 | 99 |
| October | 21.9° | 15.9° | 7.7mm | 2 | 96 |
| November | 20.2° | 12.8° | 16.1mm | 3.4 | 93 |
| December | 17.9° | 10.4° | 39.8mm | 5.7 | 82 |
January in Costa Mesa averages a daily high of 17.9°C with nights around 10.4°C. Expect roughly 6.6 wet days totalling 53.1mm. Comfortable conditions for being outdoors all day.
February in Costa Mesa averages a daily high of 17.5°C with nights around 10.5°C. Expect roughly 6.2 wet days totalling 53.2mm. Comfortable conditions for being outdoors all day.
March in Costa Mesa averages a daily high of 17.6°C with nights around 11.7°C. Expect roughly 4.8 wet days totalling 30.4mm. Comfortable conditions for being outdoors all day.
April in Costa Mesa averages a daily high of 18.3°C with nights around 12.8°C. Rain is rare — about 2.6 wet days and 13.2mm across the month. Comfortable conditions for being outdoors all day.
May in Costa Mesa averages a daily high of 19.3°C with nights around 14.8°C. Rain is rare — about 1.7 wet days and 5.9mm across the month. Comfortable conditions for being outdoors all day.
June in Costa Mesa averages a daily high of 20.4°C with nights around 16.4°C. Rain is rare — about 0.8 wet days and 1.1mm across the month. Comfortable conditions for being outdoors all day.
July in Costa Mesa averages a daily high of 22.1°C with nights around 18.2°C. Rain is rare — about 0.4 wet days and 0.3mm across the month. Comfortable conditions for being outdoors all day.
August in Costa Mesa averages a daily high of 23°C with nights around 18.6°C. Rain is rare — about 0.4 wet days and 2.3mm across the month. Comfortable conditions for being outdoors all day.
September in Costa Mesa averages a daily high of 23.2°C with nights around 18.1°C. Rain is rare — about 0.6 wet days and 2.2mm across the month. Comfortable conditions for being outdoors all day.
October in Costa Mesa averages a daily high of 21.9°C with nights around 15.9°C. Rain is rare — about 2 wet days and 7.7mm across the month. Comfortable conditions for being outdoors all day.
November in Costa Mesa averages a daily high of 20.2°C with nights around 12.8°C. Expect roughly 3.4 wet days totalling 16.1mm. Comfortable conditions for being outdoors all day.
December in Costa Mesa averages a daily high of 17.9°C with nights around 10.4°C. Expect roughly 5.7 wet days totalling 39.8mm. Comfortable conditions for being outdoors all day.
Each month scores out of 100. It starts at 100 and loses points for daytime highs outside a 20–27°C band, for wet days, and for nights below 2°C. Nothing else is weighted in — no crowd data, no prices, no opinion. Costa Mesa peaks at 99.
Based on 30 years of records, May–November scores highest for comfort in Costa Mesa, balancing daytime temperature against rainfall.
September is the warmest, averaging a daily high of 23.2°C.
February is the wettest, averaging 53.2mm of rain across roughly 6.2 wet days.
February is the coolest month, with average daily highs around 17.5°C.