What the Start Line Is Actually Like
Run SEA's race research records the weather at the start for 26 races in seven countries [1]. Humidity was between 89% and 100% at every one of them, and 11 started in saturated air at 100%. Start temperatures ranged from 19°C at Hai Phong in northern Vietnam to 28°C at Rayong in Thailand, with a median of 25°C, even though most of these races start at or before 5am.
The heat then arrives quickly. Laguna Phuket was at 30°C by 8am in 2026, Noi Bai airport recorded 32°C by 10am on the morning of the 2025 Hanoi marathon, and Ho Chi Minh City's marathon reached 29°C by its cut-off [1]. That matters because most marathoners here are out for a long time: across 25 Southeast Asian marathons and 30,732 finishers in Run SEA's results data, the median finish is 5:25, and 31% of finishers take more than six hours [2].
Saturated air is the problem kit has to solve. Sweat cools the body only when it evaporates, and at 90% to 100% humidity little of it does, so clothing stays wet, skin stays wet and the body sheds less heat for the same effort. Heat acclimatisation matters more than anything worn on race day; the training side is covered at runsea.run/articles/training-marathon-heat-humidity-southeast-asia/.
Clothing: Less Fabric, Nothing New
The rule is minimal and tested. A light singlet and shorts that have already done a long run in the same conditions beat anything bought at the expo. Fabric that holds water gets heavier and rubs more over three or four hours, so synthetic fabrics that drain and dry quickly earn their place here, and cotton is best left at home.
Chafing is the common casualty of wet kit. Anti-chafe balm on the underarms, inner thighs, bra line and waistband before the start, and covered nipples on longer distances, prevent most of it. Seams that are fine on a dry morning can cut on a wet one, so the full race outfit, socks included, should be tested on a humid long run first.
Cooling Before and During the Race
Cooling has a real evidence base. A meta-analysis of 28 studies of exercise in heat above 30°C found that cooling improved performance by about 6.7% on average: cooling before exercise by about 5.7%, and cooling during exercise by about 9.9% [3]. Combining several precooling methods worked better than any single one, and an ice vest was the most effective method during exercise. The 2015 consensus recommendations on training and competing in the heat include cooling before exercise and starting well hydrated, and name one to two weeks of heat acclimatisation as the most important preparation [4].
Ice slurry is the easiest precooling method to copy. In a laboratory study at 34°C, ten men ran about 19% longer after drinking ice slurry than after drinking the same amount of cold water [5]. On race morning that means a slushy or very cold drink in the hour before the gun, in a volume already tried in training.
In the corral, a wet towel or evaporative cooling towel round the neck and a small handheld fan make the wait easier, and both can go into the bag-drop bag. Expect less from evaporative products in saturated air, because they rely on the same evaporation that sweat does. During the race, cold water over the head and neck at aid stations, and ice where it is offered, are the nearest most runners get to the in-race cooling the studies tested.
Sun: The Late Finishers' Problem
Singapore's National Environment Agency classes a UV index of 8 to 10 as very high and above 11 as extreme, and advises extra protection especially between 11am and 3pm, when readings are highest [6]. A pre-dawn start keeps most 10K and half-marathon runners out of the worst of it. Marathoners at five-hour-plus pace, which is most of them, are still running at 8, 9 or 10am with the sun well up.
Sunscreen has to survive sweat. The US Food and Drug Administration allows a water-resistant label only for 40 or 80 minutes of protection while sweating or swimming, and advises reapplying at least every two hours and more often when sweating [7]. A marathon outlasts any label, so a small stick for the neck, ears and shoulders is worth carrying. A cap or visor, sunglasses that do not slide on a wet face and light UV arm sleeves cover the rest; the NEA's own list adds sunglasses that block UVA and UVB and a broad-brimmed hat [6].
Rain: Which Races Fall in the Wet Months
Each country's rainy season is different, so Run SEA checked the dates of the 418 dated races in its calendar against each national weather service's own definition [1]. The Philippine weather bureau PAGASA puts the rainy season from June to November [8], and 95 of the 122 Philippine races (78%) fall in those months. In southern Vietnam the rains run from May to November [9], and 12 of the 19 races there fall in that window.
Elsewhere the calendar mostly dodges the rain. Thailand's rainy season ran from 15 May to around mid-October in 2026, with the east coast of the south wet until December [10]; 25 of 89 Thai races (28%) fall in it. MetMalaysia says rainfall on most of the Peninsula peaks in October and November [11], when 23 of the 69 races there are held. Indonesia's weather agency BMKG expects the 2026/27 rainy season to peak between December and February [12], which takes in 13 of 64 Indonesian races (20%). Singapore has no distinct wet season, but November to January is its rainiest period [13], and 3 of its 14 races fall then.
Rain shows up in the race notes too. Rain fell through both race mornings at Pattaya in 2026, the Bulacan Marathon had heavy rain and a pre-dawn thunderstorm that year, and the Standard Chartered Hanoi Marathon Heritage Race started at 2am in rain in 2025 [1]. Floods have moved races as well: Chaiyaphum's race was postponed to February 2027 after floods in September 2026 damaged the organiser's office and the race packs, and the Jitra Half Marathon's 2024 edition was cancelled because of flooding in Kedah [1].
Rain kit is mostly about the wait. A disposable poncho, or a bin bag with holes for the head and arms, keeps a runner dry in the corral and comes off before the start; a compact umbrella works if someone can take it at the corral gate. Once running in 25°C air, getting wet is rarely the problem, and a sealed waterproof jacket traps heat. A cap brim keeps rain out of the eyes, a waterproof pouch or zip bag protects the phone, and dry socks and a shirt in the bag drop make the finish far more comfortable.
For shoes, drainage beats waterproofing: a waterproof membrane holds water in once it comes over the collar, while open mesh drains and dries faster. Grip matters more than usual on painted road markings and metal covers after rain, so the first kilometres after a downpour are worth running carefully.
Feet: Wet Socks and Blisters
Blisters come from friction repeated over thousands of strides, and moisture raises the friction between skin and sock, although fully soaked skin is less prone than damp skin [14]. The same review found acrylic socks linked to fewer blisters in runners, and found no benefit from antiperspirants or drying powders on the feet. Synthetic socks, in the model already used in training, are the starting point.
Tape helps where a runner already knows they blister. In a randomised trial during 250km desert stage races, plain paper tape on blister-prone spots cut blisters by 40% [15]. Two hydrocolloid blister plasters weigh nothing to carry, and the review found that hydrocolloid dressings relieve pain on blisters whose roof has come off [14].
What to Carry
Check the race's aid-station spacing before deciding. The Garmin Run in Putrajaya, for example, lists eight aid stations on its 21km, roughly every 2 to 3km [1]; with spacing like that, many half-marathon runners carry only a gel or two. For a marathon, or a race with sparse aid, a small belt or soft-flask vest with an electrolyte drink is worth the weight. Sodium and fluid in the heat are covered at runsea.run/articles/electrolytes-and-sodium-beyond-just-water-in-the-heat/.
A short list covers most race mornings: cap or visor, sunglasses, a sunscreen stick, anti-chafe balm applied before the start, two blister plasters, the phone in a waterproof pouch, a poncho for the corral and dry clothes in the bag drop.
Insect repellent belongs in the bag too. The mosquitoes that spread dengue are active during the day, according to the World Health Organization, so the exposure is less the dark start than the wait in a park after sunrise and the time spent in the finish village. The WHO lists repellents containing DEET, picaridin or IR3535, and clothing that covers as much skin as possible [16].