Weather in Guanacaste in September

How September compares

September is the 9th warmest month of the year in Guanacaste and ranks 31st of 134 destinations we compare for September - see where else is hot in September. Against London's average high of 18.6°C in September, Guanacaste is 10.5°C warmer by day and 11.3°C warmer at night.

Daytime and night-time in September

The typical September day in Guanacaste peaks at 29.1°C, but that is an average: the warmest 10% of days reach 31.1°C or more. About 99% of days top 25°C and 1 in 4 pass 30°C. Nights average 22.5°C; one night in ten drops to 21.6°C or lower, so pack a layer for evenings. Between 1991 and 2020 the hottest September day here reached 34.6°C (2019-09-08) and the coldest night 20.5°C (1992-09-20).

Rain in September

Wet (more than 8 rainy days): about 26 days with 1 mm or more, 355 mm in total. Year to year it varies a lot - the wettest September was 2010 with 761 mm, the driest 1994 with 161 mm. Humidity averages 89%, which makes the heat feel heavier than the thermometer suggests.

Sea temperature in September

The sea averages 28.7°C - warm enough to swim (22°c+). Depending on the year it ranges from 28.2°C to 29.2°C. For the warmest sea here, come in May (28.9°C).

Sunshine and daylight

Around 1.6 hours of sunshine a day (estimated from cloud cover), 12.1 hours of daylight: the sun rises about 05:31 and sets about 17:43 local time in mid-September. Average cloud cover is 87%.

Is September a good time to visit Guanacaste?

It is a reasonable choice: hot days but expect showers. The best months for Guanacaste on our scoring are February, January, December; months averaging 25°C or more: Jan, Feb, Mar, Apr, May, Jun, Jul, Aug, Sep, Oct, Nov, Dec.

Hot days, sticky evenings

September is hot enough for light summer clothing, even though it is not among Guanacaste's warmest months. With 89% humidity, the heat can feel sticky, particularly when you are walking around rather than sitting in the shade. Keep strenuous outdoor plans flexible and take breaks when the heat feels uncomfortable. Nights average 22.5°C, so evenings offer limited relief rather than a cool change. Pack loose, breathable clothes and check that your accommodation has cooling you are comfortable with. A heavy raincoat is likely to feel clammy in these conditions; lightweight waterproof protection is a more practical choice for getting caught outside.

Rain is the main compromise

The catch is how much wet weather you need to accommodate: September brings 355.0 mm of rain and only about 1.6 hours of sunshine a day. That makes it a poor fit for a holiday built around sunbathing, even though temperatures stay high. Rain on most days does not mean every day is a complete washout, but you should not assume a dependable dry spell at a particular time. Leave room to move outdoor plans around, keep something dry to change into and protect anything that cannot get wet. This month makes more sense if you are content to take warm outdoor breaks when conditions allow, rather than expecting long, sunny stretches.