How long did the Harris Mtn Fire last?

The official perimeter only gives the start date (23 July 2021). The end comes from the heat detections inside the perimeter: it burned until 8 August 2021 (17 days from 23 July 2021), the last day with detections before more than 10 days in a row without any, so it is approximate.

How much area did it burn?

The official perimeter published by United States agencies (MTBS / NIFC) is 12,856 hectares (128.6 km²). Other tallies, such as daily incident reports, may give slightly different figures.

Which counties burned?

The fire entered 2 counties: Cascade (12,685 ha, 1.8 %); Lewis and Clark (118 ha, 0.0 %).

What was the weather like?

During the fire the temperature reached 33.5 °C, humidity dropped to 7 % and gusts hit 54 km/h. The fire danger index (FWI) reached 44, class Very high.

Where does this data come from?

From the official burnt perimeter of United States agencies (MTBS / NIFC, public domain), NASA FIRMS VIIRS hotspots and the administrative boundaries of United States. It is not an official report.

US · United States · 2021-07-23 → 2021-08-08

Harris Mtn Fire, July 2021 Cascade, Lewis and Clark

Out · last detection 8 Aug 2021, 09:40 UTC

The fire started on 23 July 2021 in Cascade, United States, and satellites saw it burning for 17 days. United States agencies (MTBS / NIFC) mapped an official burnt perimeter of 12,856 hectares, spread across 2 counties.

It is the 19th largest in Montana since 2016 (of 302 with a page). Number 42 in United States in 2021, of 385.

Compare with…

Suggestions: Rock Creek · 2021

Burnt area: 12,856ha; official perimeter (MTBS / NIFC)
Duration: ≈ 17days; 23 Jul 2021 – 8 Aug 2021
Counties hit: 2; counties
Started on: 23 Jul; 2021
Heat detections: 744; in a single hotspot: 348 MW
Danger at start: 39; Very high · FWI index
Average daily power: 956MW; detected power per day, on average

HOTSPOTS BY DAY 23 Jul8 Aug

Play back 8 Aug 2021

How it evolved, day by day

23 Jul 26 Jul 29 Jul 1 Aug 4 Aug 8 Aug

Fire radiative power detected each day, in megawatts, and number of hotspots. The magenta bar marks the peak day.

The weather it burned in

34°high

54km/h · peak gust

7%lowest humidity

It had gone one day without rain (1 mm or more) when it started. Rain came after the fire: 13.6 mm on 17 Aug.

This area had burned before

No earlier perimeters overlap this fire since 2016, when the satellite record begins.

How big is it?

It burned the equivalent of 8.1% of the area of London (129 km² against 1,583 km²).

London boundary: geoBoundaries · Office for National Statistics (Open Government Licence v3.0).

This compares size, not location: both shapes are at the same scale, north up, centred on each other.

What happened to each county

Cascade where it started

12,685 ha reached on 23 Jul

1.8 %
Lewis and Clark

118 ha reached on 28 Jul

0.0 %

Burnt area inside each county, from crossing the official perimeter with administrative boundaries. The distance runs from the point marking the centre of each area to the nearest fire: staying a kilometre away does not mean it reached the houses.

People and buildings inside the perimeter

GHSL 2020

Built-up area0.25 ha

Residents< 20

Inside the perimeter there were about 0.25 hectares of built-up land and almost nobody lived there (GHSL 2020, satellite-based estimate).

This is an estimate on 100 m cells, made from satellite imagery and census data: it does not mean that homes burned or that anyone was harmed. The perimeter also contains land that never burned.

Source: Global Human Settlement Layer (GHS-BUILT-S and GHS-POP R2023A), European Commission Joint Research Centre (JRC), © European Union, CC BY 4.0. Sum inside the perimeter computed by WorldFires. human-settlement.emergency.copernicus.eu

The weather day by day

ECMWF ERA5 reanalysis at the fire location, the standard reference for past weather, and the Canadian Fire Weather Index (the same system Copernicus uses), computed by WorldFires from that data. Days after the fire run until the first rain.

Open-Meteo.com (CC BY 4.0) · Copernicus Climate Change Service (ERA5)

Day	High	Min. humidity	Peak gust	Rain	Danger (FWI)
23 Jul	27.8 °C	11 %	49 km/h	0.0 mm	Very high 39
24 Jul	30.4 °C	10 %	22 km/h	0.0 mm	High 32
25 Jul	31.8 °C	9 %	37 km/h	0.0 mm	Very high 40
26 Jul	33.5 °C	7 %	31 km/h	0.0 mm	Very high 43
27 Jul	32.7 °C	15 %	32 km/h	0.0 mm	Very high 44
28 Jul	30.0 °C	23 %	33 km/h	1.6 mm	High 22
29 Jul	30.3 °C	24 %	41 km/h	1.3 mm	High 24
30 Jul	31.9 °C	19 %	30 km/h	0.0 mm	High 32
31 Jul	32.9 °C	16 %	34 km/h	0.0 mm	Very high 38
1 Aug	32.6 °C	18 %	26 km/h	0.2 mm	High 36
2 Aug	27.5 °C	35 %	38 km/h	3.3 mm	Low 10
3 Aug	29.0 °C	21 %	32 km/h	1.1 mm	High 22
4 Aug	30.4 °C	14 %	31 km/h	0.8 mm	High 35
5 Aug	29.7 °C	17 %	23 km/h	0.0 mm	High 37
6 Aug	28.3 °C	21 %	46 km/h	1.2 mm	High 32
7 Aug	27.9 °C	21 %	54 km/h	0.0 mm	High 38
8 Aug	21.9 °C	40 %	45 km/h	13.6 mm	Very low 3
9 Aug after	20.1 °C	28 %	50 km/h	0.6 mm	Low 11
10 Aug after	26.0 °C	23 %	59 km/h	0.0 mm	High 29
11 Aug after	30.6 °C	19 %	45 km/h	0.0 mm	High 33
12 Aug after	26.1 °C	27 %	34 km/h	0.0 mm	High 29
13 Aug after	29.2 °C	16 %	23 km/h	0.0 mm	High 29
14 Aug after	31.1 °C	9 %	34 km/h	0.0 mm	Very high 41
15 Aug after	31.8 °C	13 %	46 km/h	0.0 mm	Very high 44
16 Aug after	30.4 °C	17 %	56 km/h	0.0 mm	Very high 48
17 Aug after	17.8 °C	32 %	60 km/h	13.6 mm	Very low 4

Before and after, from space

Drag the line to compare the same ground before the fire and after it.

↔

Before · 16 Jul 2021

After · 15 Aug 2021

Before After

If the day is cloudy, change the date: each satellite passes once a day. In false colour the burnt ground looks brown or red.

Detections table · 744Every satellite heat detection, one by one, with time, satellite, power and coordinates.

⬇ Download CSV Times in UTC. Each detection is a 375 m pixel: a large fire yields hundreds per pass.

Date and time	Satellite	FRP (MW)	Lat	Lon

Other fires nearby

1,338 haRock Creek Lewis and ClarkJuly 2021 · 2 days · 26 km away

All fires in United States ›

Frequently asked questions

How long did the Harris Mtn Fire last?: The official perimeter only gives the start date (23 July 2021). The end comes from the heat detections inside the perimeter: it burned until 8 August 2021 (17 days from 23 July 2021), the last day with detections before more than 10 days in a row without any, so it is approximate.
How much area did it burn?: The official perimeter published by United States agencies (MTBS / NIFC) is 12,856 hectares (128.6 km²). Other tallies, such as daily incident reports, may give slightly different figures.
Which counties burned?: The fire entered 2 counties: Cascade (12,685 ha, 1.8 %); Lewis and Clark (118 ha, 0.0 %).
What was the weather like?: During the fire the temperature reached 33.5 °C, humidity dropped to 7 % and gusts hit 54 km/h. The fire danger index (FWI) reached 44, class Very high.
Where does this data come from?: From the official burnt perimeter of United States agencies (MTBS / NIFC, public domain), NASA FIRMS VIIRS hotspots and the administrative boundaries of United States. It is not an official report.

Where to verify it

A satellite detection is not an official report. Final figures and the fire’s status are published by each country’s emergency service.

Emergency number: 911 · Emergency number and official warnings ›

Satellite cross-check

NASA FIRMS ↗
NIFC · Fire maps ↗

History of this page Changes in area, dates, name, official record or merged pages, compared every night since 24 October 2026.

No changes since 24 October 2026.

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How we know this

Perimeter: MTBS (US Geological Survey and US Forest Service), official.
The official perimeter has no end date: the end is the last day with VIIRS detections inside the perimeter before a gap of more than 10 days (150 days at most), so it is approximate. 744 Suomi-NPP VIIRS detections on those dates.
We have not matched this page with any official record.
Weather: Copernicus ERA5 reanalysis at the fire location, in cells of about 25 km.
Built-up area and population come from the JRC Global Human Settlement Layer (epoch 2020, 100 m cells), summed inside the perimeter: an estimate of what was there, not a damage count.

How we build fire pages: methodology ›

Where this data comes from

The perimeter is official and in the public domain. For past years it comes from MTBS (Monitoring Trends in Burn Severity), run by the U.S. Geological Survey and the USDA Forest Service, which map it on Landsat and Sentinel-2 imagery. For the current and last year it comes from WFIGS at the National Interagency Fire Center (NIFC): incident perimeters mapped by fire crews, aircraft and imagery. Heat detections come from the VIIRS sensors aboard Suomi-NPP and NOAA-20, distributed by NASA FIRMS at 375 m resolution. County boundaries come from geoBoundaries ADM2 (Public Domain). The area per county is computed by WorldFires by crossing the perimeter with those boundaries, so it may differ slightly from official figures.

Where it burned