Weather in Chiang Mai and Northern Thailand

This webpage is a description of weather variations in Chiang Mai and Northern Thailand. It explains how it changes through the year, the causes and relationship with climate as well as extreme records and events in the region

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Other information about climate

Other information about flooding
Other information about air pollution

Meteorological variables averages and records

Temperatures

On average, temperatures in the Upper Ping range between 20.7 to 34.0 ºC with an annual mean of 27 ºC and significant variations between cold and hot seasons and daily fluctuations outside the rainy season. In the hot season, upper thirties are common while the cold season can occasionally reach below 15 ºC in river basins but significantly lower in mountainous highlands. The temperature in the rainy season is quite stable with differences mainly due to cloud cover and cooling after heavy rain; in the cold season, temperatures are mild during the day (20-ish) but can drop significantly during the night due when the air is dry. The same occurs during the hot season with 35+ ºC during the days and around 25 ºC during the night.

Monthly outlook

The beginning of the year (January-February) is the cold season. While it’s not that cold in the city (and the Chiang Mai basin as a whole), it can quickly become cold-ish (depending on your reference for cold), around 10-15ºC in the foothills and colder in the mountains (5-10ºC). However, since the air is mostly dry, these minima are only reached in the early morning. In Chiang Mai itself, when the sun shows up, it can easily increase by 10ºC. In the past decade, there are only a couple of days that have kept a whole day temperature below 15ºC in the city.

Cold nights quickly disappear in February to be replaced by milder temperatures while maximum temperatures are reached in the mid afternoon and increase from 25 to 40ºC between February and April. April and May are the heart of the hot season, with temperatures in the upper 30ºC for a good part of the day. Apparent temperature also significantly increase during that period due to increasing humidity.

Between May and July, the rainy season starts with a significant cloud cover that keeps temperatures lower and in a narrower range. While variations in the cold and hot season are in excess of 15º, daily variations are restricted to the 24 to 31ºC on average by the end of May. This small interval of temperatures is kept until October when skies are clearing up again with lower humidity, leading to a lowering of the minimum temperatures from 24ºC to 18ºC.

November-December is the transition period when temperatures are rarely hot (<30ºC) and nights are rarely in the cold range (>15ºC) with relatively low amount of humidity providing pleasant temperatures.

Figure 1. Right: Monthly distribution of maximum and minimum average temperatures with the range given for 25-75% and 10-90% of recorded data. The dashed lines are felt temperature when relative humidity is taken into account.

Left: Hourly variation of average temperature throughout the year clearly shows the duration of the cold and hot season and when (6 am; 3 pm) extreme temperatures are reached.

Records & variability

Extreme records for Chiang Mai is 42.5 ºC in May 2016 while the hottest in the region was 45.4 ºC in Tak in April 2023 and 44.6ºC in Mae Hon Song in April 2016. In Chiang Mai, the coldest temperature recorded was 3.8ºC in December 1999 while -5ºC has been observed on Doi Inthanon in the winter of 2017.

Low temperature records are achieved when cold, dry and dense air from the Siberian high pressure system, helped by a weak polar vortex, is pushed southward towards the Equatorial Low Pressure Belt. In most cases, cold surges goes towards the Pacific Ocean and the Philippines sea, but when atmospheric conditions aren’t met, and under the influence of the Tibetan high pressure system, the cold surge front is pushed towards Indochina and can reach Northern Thailand. It typically happens early in the winter monsoon, in December or January

Figure 2. Map of eastern Asia during boreal winter showing the statistical main direction of winds from the Siberian high pressure system. While most cold surges are directly towards Japan and coastal China and the South China Sea, there is around 11% of cold surges that are diverted towards continental south east Asia, including Northern Thailand (modified from Rais Abdillah et al., 2021). The dashed line is the southern limit where freezing ground temperatures can be experienced in some locations.

Extremely high temperatures above 40ºC are caused by the WPSH high pressure system developing to the south west over continental South East Asia. It produces conditions of high pressure in the upper troposphere, confining hot air in the lower troposphere as a heat dome. In addition, weak north east winds carrying dry cold air prevent oceanic moisture to reach inland as these winds are redirected towards a low pressure in the South China Sea. This situation produces meteorological conditions where the cloud cover is very limited and the soil is significantly moisture deficient, leading to a strong positive feedback of land-atmosphere coupling when the ground is heated by intense sunlight.

Figure 3. Example of some weather and environmental conditions during the hot season of 2023. Maximum temperatures during that heatwave were 45.6 ºC in Tak and 41.2ºC in Chiang Mai (modified from Lyu et al., 2024)

Precipitations

The average annual rainfall over the region is ~1100 mm/yr. It varies from 800 mm/yr long term average for the plains in the Chiang Mai – Lamphun basin to values exceeding 1500 mm per year recorded on mountain ranges. This is partly due to the elevation effect that add 0.5 mm of rain for each 100 m of altitude but also orographical rainfall enhancement during storm events.

Most (85 to 90%) of rainfall occurs during the rainy season between May and September with the wettest month in August.

Figure 4: Annual isohyets for the region surrounding Chiang Mai clearly showing the effect that mountains (particularly Doi Suthep here) have on annual rainfall averages (1988-1997) (modified from Margane & Tatong, 1999)

Monthly outlook

January is the driest month. Although some grey days with a shower can still occur as a result of the winter monsoon, the chance of experiencing any rain in January is around 2% per day. February is not very different and no rain should be expected most years.

By the end of March the risk of summer rain storms is a lot higher and 1 to 3 of those storms per month is not uncommon. April is in the same trend of sometimes violent rain storms, sometimes hail, but with the exception of these events, March, April and eventually May are mostly very dry.

In June, the summer monsoon generally starts to reach Northern Thailand causing more regular rains. It can happen early or in July depending on climate conditions. Contrary to the assumption of residents in temperate countries, the rainy, wet season in Northern Thailand is not synonym of constant rain. Rainy season weather is mostly cloudy days with daily scattered showers for an hour or two and some rainy days. The exceptions are up to week-long rainfall events caused by tropical depressions and the stalling of the monsoon trough that are more common in August and September.

In October, although these intense rainfall events are still possible, the daily chance of rain starts to decrease and in November and December, the chance of having a rainy day is one-per-week to one-per-month by the end of the year.

Figure 5. Progression of the summer monsoon front from the Malay peninsula to Siberia. Dotted lines are areas where the monsoon is not well defined.

Records & variability

Rainfall in the Upper Ping basin is recorded continuously for more than a century in a handful of stations, providing valuable information for long term trends and variability. The accuracy of historical rainfall is limited by the insufficient number of rain gauges but the role played by summer thunderstorms in April and May, the increasing influence of the South West Monsoon in August and September and the passage of tropical depressions from the South China Sea are the dominant factors in monthly rainfall.

As a result, while locations such as Samoeng (Chiang Mai) or Mae Tha (Lamphun) have a long-term average rainy season of 700 to 800 mm/yr, minimum annual rainfall can be as low as 300 mm/yr and maximum of almost 2000 mm/yr have been recorded in the past. In these locations and elsewhere, up to 10% of annual rainfall can be provided by a single tropical depression. The coincidence of the ITCZ with summer monsoonal winds can also create significant rainfall events and the stalling of such system over Northern Thailand is known to be associated with major floods such as the 2024 Chiang Mai floods (see Flooding in Chiang Mai).

The modeling of this long term variability gives a rainfall return period of 1 year to be below average and 2 years for average, 10 years with 150% rainfall, 20 years for 180% and 100 years for 200%. Long term yearly averages also show that the number of wet spells (>0.3 mm) of 2 to 4 days duration occur 16x per year; duration of 5 to 7 days occurs 3x per year and there is on average only one occurrence per year of rainy days lasting for more than a week.

Figure 6. Right: Monthly rainfall in Chiang Mai in millimeters with shaded areas representing 25-75% and 10-90% of recorded data. Left: Daily probability of rain in percent and the red line represent the average atmospheric pressure throughout the year.

Wind

The wind in Chiang Mai is heavily influenced by monsoons and people who spend significant time outdoor can notice when a season change occurs by feeling different winds blowing, particularly in February, June and October-November when major shifts occurs in direction, temperature and humidity. Due to topography however, the wind can locally be different, particularly adjacent to mountains and in deep valleys.

Monthly outlook

The average ground wind velocity in Chiang Mai does not change significantly during the year and is between 3 and 5 km/h. The direction of the wind is however strongly associated with the season.

In the first part of the rainy season (June to September), the dominant winds are from the South West, bringing the monsoon from the Indian Ocean. It shifts at the end of the season with a stronger component from the East Asia monsoon from the East. While many cyclones, particularly in the second part of the rainy season, have paths bringing them over Northern Thailand, these tropical storms are weak by the time they reach the inland region and have no specific associated winds known from stronger cyclonic systems.

In the cold season, the Siberian High produces winds mostly coming from the North-East. Eastern winds are generally more humid and associated with the winter monsoon while some northern dry winds can be linked to cold surges.

In pre-monsoon conditions of the hot season, the South Asian High is producing winds from the west and eventually south and while surface winds are relatively weak in March-April, these atmospheric flows from Tibet is one of the reason for the high level of air pollution during that period (see air pollution).

In March, April and May, the coalescence of various fronts (dry air from the South Asia High) with some moisture brought from the Indian Ocean can create optimal conditions for rain storms that are associated with gust fronts that can be quite destructive. This period also has some reports of short-lived landspout tornadoes with intensities reaching EF0.

Figure 7. Right: Wind velocity in Chiang Mai in kilometers per hour with shaded areas representing 25-75% and 10-90% of recorded data. Wind rose diagrams are shown for each month at the bottom. Left: Alternative graphical representation of the wind direction throughout the year.

Humidity

Average humidity doesn’t have a strict correlation with precipitation although it is needed for rainfall. Broadly, the climate in continental Thailand has a dry (cold & hot) and a humid (wet) season but the transition is not as abrupt as precipitations.

During the cold and the beginning of the hot season (December to April), relative humidity drops to around 50±10% in March but from May to December, it hoovers around 80±10% on a daily average.

Within a day however, variations can be quite significant and clear days in the rainy season can have >90% relative humidity in early mornings, dropping to 50% in the mid-afternoon. In the dry season, 50-60% relative humidity is reached in early mornings but down to very dry conditions at the hottest time of the day since the air is severely water undersaturated.

UV Index

The UV Index in Chiang Mai measured independently from the weather varies from 8-9 (high) in December-January, 9-10 (very high) in February and November and 12 to 14 (extreme) from March to October. These are theoretical values based on the highest position of the sun in the sky. In the tropics, these values are therefore rarely below 10.

However, The UV index value is a maximum and while 14 is possible around midday, it also starts at a value of 2-3 at sun rise, and drops progressively back to that value in the evening. The UV index also does not take the cloud cover into account. In the rainy season, with a complete thick cloud cover, the actual ground UV index is more like 3-4 rather than 14.

Air pollution, among all its negative effects, has the advantage to absorb ultraviolet frequencies and reduce the UV Index by a few units in a season where cloud cover is relatively scarce. The relationship between UV Index and PM2.5 concentration is around 1 UV Index unit per 10 mcg/m3 of PM2.5 in the atmosphere.

Figure 8. Right: Ultraviolet Index given for theoretical irradiance (purple) and green curves are UV index for 60% of records and 90% of records. Yellow to Red dotted lines are absorbance due to various AQI. Left: Solar azimuth thoughout the year. WS: Winter Solstice; SS: Summer Solstice; SP: Subsolar Point.

AQI Index

The AQI index is the air quality index. It is controlled by atmospheric conditions and human & ground activities. The AQI index steadily increases in December to February but peak to high values (200-300) in March and April at the heart of the burning season. After the first significant rains at the end of April or in May, it decreases back to rainy season background levels below 50 (See AQI).

Solar irradiance

Somewhat related to the UV index, the illuminance is the amount of light received per surface unit on the ground. It is particularly useful to forecast photovoltaic energy production. Solar irradiance reaches its peak value in March and April when the sun is already quite high, a mostly absent cloud cover and low to very low humidity. In August and September, the solar irradiance reaches its lowest and is around 30% lower than at its peak during the hot season.

Figure 9. Right: Solar irradiance in kWh with shaded areas representing 25-75% and 10-90% of recorded data. Left: Cloud cover throughout the year clearly showing the difference between the dry (cold and hot) season and the wet season.

Summary of extreme events

Extreme cold is obviously not something that you would expect in Thailand. However, since cold sensation is relative, some period can be considered as extreme for the general Thai population, particularly in conditions of high elevations. Cold surges that occur occasionally in December or January can produce temperatures near zero in the mountains. While frostbite is not possible, hypothermia and burns from sleeping too close to a source of heat is not uncommon among hilltribes.

Extreme heat could qualify when the temperature soars above 40ºC with a relative high humidity. Heatwaves occur every few years mostly between April and May and maximum temperatures are reached between 1 and 4 pm.

Extreme precipitations are common in the same April-May period in the form of rain storms. The rain front of these storms can easily reach 50 mm/hr, considered as extreme downpour. Some storms are also associated with significant hail that can cause some damage. Some rains associated with the monsoon trough or tropical depressions can also reach this rainfall intensity.

Extreme wind is also associated with these summer storms as gust fronts. These winds can reach in excess of 100 km/h and create significant damage. Although less common, non-mesocyclone tornadoes (landspouts) may appear here and there during the hot season.

Extreme haze are days during the burning season (March-April) where the particulate matter in suspension in the air is very high (>0.5 mg/m3). It is caused by several meteorological conditions including strong temperature inversion, a calm and dry weather with abundant wildfires in the direct mountainous environment of Chiang Mai.

Finally, cyclones and typhoons are not a significant risk in Chiang Mai. These atmospheric phenomena are fed by warm seas and quickly loose their wind power after landfall. The closest seas are several hundred kilometers away with significant mountain ranges breaking down any cyclonic structure. However, the rain associated with these tropical depressions is still very significant and week-long wet spells are sometimes associated with ex-typhoons.

Figure 10: Map of continental South East Asia with cyclones tracks (down to tropical storm) of the past several decades. While numerous tropical storms can reach eastern Isaan, it is very rare for tropical storms to reach Northern Thailand as anything else than a tropical depression.