Khom Loi, Drawn

What it is

Paper, a hoop and a ball of fire

A khom loi is a paper drum, closed at the top and open at the bottom, on a hoop of bamboo. Two wires cross the hoop and hold the fuel in the middle: paper or cloth soaked in wax or paraffin. A Lanna description calls the burning part the luk fai, ลูกไฟ, the ball of fire, and says it was once cast from khi ya and is now toilet paper soaked in candle wax.

Thai provincial notices set the most it can be: 90 cm across, 140 cm tall, one cubic metre inside, natural materials, and at most 55 g of fuel burning for at most 8 minutes. Bigger than that, it counts as an aircraft. Chiang Mai uses the same limits.

In Lanna the word is waao, ว่าว. Yi Peng had two kinds: waao hom, ว่าวฮม, filled with smoke and let go by day, and waao fai, ว่าวไฟ, which carries fire and goes up at night. The night one is what most people now call khom loi; the airport's notices call the day one โคมควัน, khom khwan, the smoke lantern.

Yi Peng is the full moon of the second Lanna month, the same night as the full moon of the twelfth month in central Thailand, Loy Krathong. The government's public relations department writes that the lanterns honour Phra Ket Kaew Chulamani, the Buddha's hair relic in Tavatimsa heaven, and carry sorrow and bad luck away.

night airhot air inside

Why it rises

Hot air is thin air

Air is tiny bits bumping around. Heat them and they bump harder and spread out, so the same space holds fewer of them. A lantern full of hot air holds less air, by weight, than the same space of cool night air.

The lantern goes up when the night air it pushes aside weighs more than the hot air inside plus the paper, the hoop and the fuel. The difference is the lift.

lift = V × (ρ_night − ρ_inside) ρ = p ÷ (R × T)

Inside the lantern The night Lantern height

Lift–

Weight–

Air, kg per m³ (night / inside)–

The lantern drawn here is a paper drum, 62% as wide at the mouth as it is tall and a little wider at the top: 22 g/m² paper, a 12 g hoop, 30 g of fuel. T in the sum is in kelvin, °C + 273; p is 96.5 kPa, the air pressure Chiang Mai University's physics lab measured; R is 287 J/(kg·K) for air. That lab lifted a 28.7 g lantern of 0.159 m³ with a tealight in 2022. The same sum says it needs 85.6 °C inside at the lab's 27.65 °C; they worked out 85.8 °C and measured 95.0 °C with an infrared camera.

One flight

Up while it burns, down when it cools

Light the fuel and hold on. The air inside warms until the lantern tugs at your hands, and then you let go. It climbs while the fuel burns. When the fuel is gone the air inside cools in under a minute and the lantern comes down.

heightinside temperaturefuel burning

Fuel

Lantern height

Paper

The night

You hold it for–

Highest–

In the air–

Hottest inside–

Every quarter of a second the page works out four things: how much fuel burned, how much of its heat stayed in the air inside, how hard the night air pushes up, and how fast the lantern moves against the drag of the air. Twice the fuel flies about twice as long. A smaller lantern with the same fire runs hotter inside.

This flight is the page's own model, built the way TU Delft's 2017 sky-lantern model is: lift from the difference in air density, air thinning with height, fuel burning down at a steady rate, then a fall at the speed where drag matches weight. The burn rate comes from TU Delft's timed burns (100–330 s) and the 55 g, 8 minute legal limit. The share of heat kept inside and the heat lost through the paper are tuned guesses. For comparison, Dutch inspectors watched ten lanterns in 2009: 2 to 5 minutes up, 50 to 300 m high, landing 633 to 1,900 m away.

Where it lands

The wind decides

A lantern has no steering. It goes where the air goes, and the air higher up moves faster than the air by the river. Here are forty lanterns from one place, each a little different. The big ticketed releases are out near Mae Jo in San Sai.

TicketsLantern-night tickets Chiang Mai tickets & toursPaid links: Mot Dang earns a fee if you book.

came down on landin wateron the airport

Wind from

Wind near the ground

Half land within–

Farthest–

On the airport–

In the water–

All of Mueang Chiang Mai and Hang Dong, and parts of Saraphi, San Sai, Mae Rim and San Pa Tong, are closed to lanterns at Loy Krathong: 39 sub-districts in 6 districts. Tha Phae Gate is on this map to show why: the runway is 3.3 km away as the crow flies. A light wind from the east and a few minutes in the air are enough.

The airport has counted the lanterns that came down inside its fence: 1,425 in 2013, 142 in 2014 and 59 in 2015.

Map: OpenStreetMap contributors (ODbL). The wind grows with height as (height ÷ 10 m)^0.25, a usual shape on a calm night. Each lantern gets its own size, fuel and gust, and a wind up to 20° off the one you set.

A thousand lanterns

How many are up at once

Let lanterns go at a steady rate and the sky fills, until the first ones start coming down. From then on the count holds: lanterns let go each minute times the minutes each one flies.

Shops use the same sum, called Little's law: people in the shop = people coming in each minute × minutes each one stays.

up at once = λ × W

Lanterns let go For

Each flies (W)–

Most up at once–

Let go in all–

W comes from the one-flight model above: a 1 m lantern, 30 g of fuel.

Yi Peng 2026

Two nights, set hours

The full moon is Tuesday 24 November 2026 at 21:53 Chiang Mai time. The Civil Aviation Authority's notice for Chiang Mai and Lamphun, published 23 July 2026, sets the hours for lanterns:

Tue 24 Nov · 10:00–12:00daytime; in past years, the hours for smoke lanterns

Tue 24 Nov · 19:00–01:00night lanterns

Wed 25 Nov · 19:00–01:00night lanterns

In those hours Chiang Mai airport stops flying. In 2025 it cancelled 65 flights and retimed 96 over two nights; in 2023 it cancelled 101 and retimed 59.

A release needs a permit from the district chief, asked for at least 30 days ahead. Chiang Mai approved about 62,000 lanterns in 2024; requests for 2025 came to 92,313. Launching near an airport against the Air Navigation Act carries up to 5 years in prison and a 200,000 baht fine.

Chiang Mai province had not posted its 2026 notice by 3 October 2026. In past years it came out in October or November.