It takes mere seconds for the thick, gnarly clouds to tumble across the sky, smothering every inch of blue. The chalky red dirt track and flanking green paddy fields, which until moments ago had looked so vibrant they seemed unreal, are now bathed in an eerie gloom.
The rain follows with equally dramatic speed. I pedal on, reminding myself to savour the moment - while also keeping an extra careful eye out for potholes and darting dogs - until our guide Alistair yells: "Everybody stop here!"
I'm grateful for the opportunity to rest. Five days into my eight-day ride with global cycling and trekking challenge specialists Discover Adventure, I've been having an uncomfortable morning.
Despite the padded shorts and chamois cream, blisters have sprouted under my bum cheeks and my legs are close to seizing up.
Stiff and sore, I'm realising why the trip brochure warned this was going to feel like a challenge and may sometimes require us to 'dig deep'.
That digging had begun on day one for some. As Alistair had said at the start: "It's not the distance or terrain that makes this trip challenging, it's the heat."
Our itinerary - starting in Vietnam's Ho Chi Minh City and finishing in Cambodia's Angkor Wat - would take us on a 510km cycle ride over eight days. The trip is manageable for people of moderate to decent fitness, though cycling experience massively helps.
Throw 40C heat and hours under a blazing sun into the mix and it's largely pot luck how you'll fare; heat exhaustion makes the first couple of days tough for some.
Demand for adventure travel has boomed in recent years, with cycling trips becoming increasingly popular. It's a trend that's set to freewheel even further, and Discover Adventure has seen a notable increase in appetite for trips in southeast Asia.
In the four years this Ho Chi Minh to Angkor Wat itinerary's been going, demand's risen by around 50 per cent year on year, leading to an additional third date being added in late 2015.
What's the appeal? A quick survey of our group sums it up.
Ranging in age from 22 to 67, we're a right mix. For some, fundraising for charities close to their hearts is their main motivation, while for others, it's simply a passion for cycling, a desire to push their limits a little and see a bit of the world in the process. As seasoned adventure-seeker Carole Fendick, 63, puts it: "Having a challenge on the horizon gives me a reason to keep training and keep it up in the gym."
The fact we're travelling in this part of the world gives our adventure an additional exotic twist - and cycling enables us to get up close and personal with the destination in a way that's not usually possible.
The itinerary also includes gaps for touristy stuff, including a morning boat ride through Vietnam's Can Tho floating market where fish, fruit and even livestock are all traded on the water; the Cu Chi Tunnels where the Vietcong hid during the Vietnam War; the Tuol Sleng Museum in Phnom Penh - a former school used as a secret prison camp by the Khmer Rouge during the Pol Pot regime in the late 1970s which saw around two million Cambodians killed; and the Killing Fields, where much of the genocide took place.
Although truly heartbreaking, these sites are important reminders of the horrors people went through.
It's on our bikes though, that we really get to see - and fall in love with - these enchanting countries.
We pedal through bustling towns and tiny villages, bounce across bridges, weave through narrow streets and cruise along endless country roads surrounded by lush rice paddies, banana and sugar plantations.
We marvel at colourfully-painted pagodas and a duck farmer guiding his flock down a river, gasp as mopeds transporting entire families - or squealing pigs - whizz by, and hours are spent chatting and sharing stories with fellow riders.
Daily distances range from 30-90km, with regular stops to refill drink bottles and top up on sun-cream and calories. Staying properly hydrated and fed is crucial and provisions are plentiful; we snack on fruit, cake, crackers and crisps and - though I don't touch the stuff back home - guzzle endless fizzy drinks, and tuck into bowls of rice and veg with meat or fish for lunch.
Two other things play a key role in keeping those pedals turning: the group banter and the endless friendly smiles, waves and cheery greetings from the locals.
Discover Adventure partners with local operators to support the in-country trip logistics, sorting everything from the bikes, accommodation and meals. We're also accompanied by a mini bus (which we travel in for some stints of the journey) and support vehicle.
They're passionate about promoting cycling in the region and giving back to the local community; bikes are donated to orphans once they get too old to be used on the challenges.
Seven days into the trip, my skin is gross, I'm bloated from all the sports drinks and I've worn nothing but grubby Lycra for a week - but I've also fallen head over heels in love with it all and can't imagine returning to a routine that doesn't start with a 6.30am wake-up call, involve seven hours on a bike and end with a beer and hobbly toddle to bed.
To soften the blow, our final day is a real treat - a short ride, which includes some super fun jungle tracks and ends in the breathtakingly beautiful ancient temples of Angkor.
* The writer was a guest of Discover Adventure.
