Fairmont
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fairmont clear sky chart from 2005-11-06 to 2024-01-16.

This page is a summary of past forecasts. Even though it's based only on forecasts, some people use these pages as clue to a site's climate. But climate data based on actual observation should be preferred (if you can find it!).

Cloud, Transparency and Seeing forecasts are summarized individually. But because good observing conditions require a combination of minimum cloud cover, transparency and seeing, the individual forecast summaries can be difficult to interpret. The observing conditions section counts the combinations of conditions needed for common types of astronomical observation.

Observing Conditions

The following tables count the occurrence of the combinations of forecast and darkness conditions needed for different observations. Conditions are rated by by what percentage of the hours the observation would likely be attempted are usable.

Observation Type	Minimum conditions for hour to be usable				Out of hours
	Clear of Clouds	Transparency	Seeing	Sky Brightness
Planetary:	50%	(no limit)	Average	(no limit)	Nighttime Hours
Deep Sky:	70%	Average	(no limit)	5.9	Nighttime Hours
CCD Imaging:	70%	Below Average	Poor	(no limit)	Nighttime Hours

Percentages are of nighttime hours per month, or per year, when a particular observing conditions was met. Deep Sky percentages never exceed 50% because the moon is up half the time. Deep sky percentages are much lower for charts at high latitudes because of fewer dark hours in the summer.

Overall

By Month

Individual Forecasts

The length of each colored bar in the "occurrence of each Forecast" column is proportional to the number of times the chart forecast that color. The percentages are of the total number of hours per month, or per year, with a particular forecast. The colors as the same as those used in clear sky charts.

Note: Transparency probabilities are only counted for those hours that have 30% or less cloud cover. Seeing probabilities are only counted for those hours that have 80% or less cloud cover. If that's too tricky, try Observing Conditions instead.

Overall

		Occurrence of each Forecast	Data Points
Cloud Cover	Day	13% 8% 3 7% 6% 3 5% 5% 5% 7% 39%	74702
	Night	12% 7% 3 7% 6% 3 4 4 5% 6% 42%	75119
Transparency	Day	44% 7% 14% 22% 11% 1	74413
	Night	42% 6% 14% 23% 13% 2	74929
Seeing	Day	16% 10% 30% 28% 13% 3	14775
	Night	15% 9% 34% 30% 10% 1	24803

By Month

Percentage Cloud Cover SE (2006-)
Color Meanings: Overcast 90% covered 80% covered 70% covered 60% covered 50% covered 40% covered 30% covered 20% covered 10% covered Clear
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	8% 11% 6% 10% 10% 4 6% 6% 6% 7% 26%	1517
	11th to 20th	Day	10% 14% 6% 11% 8% 5% 7% 4 6% 5% 25%	1622
	21st to end	Day	10% 12% 6% 11% 9% 5% 6% 5% 5% 7% 22%	1782
February	1st to 10th	Day	9% 12% 6% 9% 8% 5% 8% 5% 6% 7% 25%	1620
	11th to 20th	Day	10% 10% 6% 8% 9% 5% 7% 6% 6% 7% 28%	1574
	21st to end	Day	14% 10% 4 8% 9% 3 5% 4 4 7% 31%	1386
March	1st to 10th	Day	15% 11% 6% 11% 6% 3 4 3 4 7% 31%	1980
	11th to 20th	Day	16% 10% 5% 8% 6% 4 4 5% 5% 7% 33%	1980
	21st to end	Day	19% 12% 4 8% 6% 3 4 5% 5% 4 28%	2178
April	1st to 10th	Day	19% 8% 3 8% 5% 3 5% 4 5% 6% 33%	2125
	11th to 20th	Day	20% 9% 4 7% 5% 3 4 3 5% 7% 33%	2181
	21st to end	Day	25% 10% 3 6% 4 2 3 5% 4 5% 32%	2340
May	1st to 10th	Day	21% 8% 3 5% 6% 3 4 5% 6% 6% 33%	2340
	11th to 20th	Day	19% 7% 3 4 5% 3 5% 4 4 7% 39%	2459
	21st to end	Day	17% 6% 3 6% 5% 3 6% 4 5% 8% 37%	2725
June	1st to 10th	Day	12% 5% 2 4 5% 4 4 5% 5% 8% 46%	2520
	11th to 20th	Day	13% 4 2 5% 6% 3 4 4 6% 9% 43%	2520
	21st to end	Day	10% 4 3 5% 5% 4 5% 5% 6% 9% 45%	2520
July	1st to 10th	Day	6% 3 2 3 4 3 5% 5% 6% 9% 52%	2520
	11th to 20th	Day	7% 3 2 3 4 3 4 6% 6% 9% 54%	2520
	21st to end	Day	6% 4 2 4 5% 3 5% 6% 6% 9% 51%	2564
August	1st to 10th	Day	9% 5% 2 6% 7% 3 6% 5% 6% 8% 42%	2328
	11th to 20th	Day	11% 5% 3 5% 5% 3 4 5% 5% 7% 46%	2301
	21st to end	Day	8% 4 2 5% 4 3 4 4 5% 8% 53%	2494
September	1st to 10th	Day	13% 5% 2 4 4 3 4 4 5% 8% 49%	2160
	11th to 20th	Day	11% 5% 2 4 5% 2 4 4 4 7% 52%	2052
	21st to end	Day	13% 6% 2 4 4 2 4 4 4 7% 48%	1943
October	1st to 10th	Day	16% 6% 2 5% 5% 3 4 4 5% 6% 45%	1800
	11th to 20th	Day	15% 9% 3 4 4 3 4 4 4 6% 46%	1800
	21st to end	Day	18% 9% 3 6% 5% 3 3 3 3 5% 41%	1980
November	1st to 10th	Day	11% 8% 3 4 5% 3 5% 4 5% 6% 46%	1610
	11th to 20th	Day	11% 9% 4 11% 7% 3 4 4 4 6% 36%	1520
	21st to end	Day	14% 9% 4 7% 5% 3 3 4 4 9% 39%	1520
December	1st to 10th	Day	12% 10% 4 10% 8% 5% 6% 5% 4 6% 29%	1520
	11th to 20th	Day	10% 14% 5% 9% 7% 4 4 4 6% 7% 30%	1918
	21st to end	Day	20% 12% 6% 17% 6% 2 4 4 4 5% 20%	2783
January	1st to 10th	Night	8% 8% 6% 10% 8% 5% 5% 6% 6% 7% 33%	2648
	11th to 20th	Night	8% 11% 6% 10% 9% 4 7% 5% 5% 6% 28%	2456
	21st to end	Night	12% 12% 6% 9% 9% 4 5% 5% 5% 8% 26%	2574
February	1st to 10th	Night	10% 11% 4 9% 8% 4 5% 5% 6% 5% 33%	2340
	11th to 20th	Night	9% 9% 4 7% 6% 4 5% 5% 6% 6% 37%	2273
	21st to end	Night	12% 9% 3 7% 9% 4 4 4 5% 7% 36%	1883
March	1st to 10th	Night	12% 9% 4 8% 6% 2 4 4 5% 7% 40%	1980
	11th to 20th	Night	14% 9% 4 7% 6% 2 5% 4 5% 6% 37%	1980
	21st to end	Night	19% 11% 4 6% 5% 3 4 5% 4 7% 32%	2178
April	1st to 10th	Night	15% 9% 3 6% 5% 3 5% 4 5% 6% 39%	1834
	11th to 20th	Night	19% 8% 3 7% 5% 3 4 5% 4 5% 38%	1724
	21st to end	Night	21% 8% 2 5% 4 2 4 4 5% 6% 39%	1620
May	1st to 10th	Night	16% 7% 3 6% 5% 3 4 4 6% 7% 40%	1620
	11th to 20th	Night	16% 5% 2 5% 4 3 4 4 5% 6% 45%	1434
	21st to end	Night	16% 6% 2 5% 5% 2 4 5% 5% 8% 41%	1551
June	1st to 10th	Night	9% 3 2 4 4 3 4 5% 6% 9% 52%	1440
	11th to 20th	Night	11% 3 2 5% 5% 3 4 6% 5% 9% 49%	1440
	21st to end	Night	9% 4 2 3 4 2 3 4 4 9% 57%	1440
July	1st to 10th	Night	5% 3 2 4 4 2 4 4 5% 9% 59%	1440
	11th to 20th	Night	3 2 2 3 3 2 5% 5% 6% 10% 61%	1440
	21st to end	Night	4 2 1 3 3 2 4 4 6% 9% 61%	1749
August	1st to 10th	Night	7% 4 2 5% 5% 3 4 4 6% 9% 51%	1611
	11th to 20th	Night	9% 3 2 3 4 3 3 4 5% 8% 58%	1600
	21st to end	Night	7% 4 2 4 4 2 3 4 3 6% 61%	1862
September	1st to 10th	Night	11% 4 1 3 3 2 3 3 5% 6% 60%	1800
	11th to 20th	Night	11% 5% 2 4 3 2 3 4 5% 8% 53%	1903
	21st to end	Night	10% 6% 2 4 4 2 4 4 4 6% 54%	1998
October	1st to 10th	Night	14% 5% 2 5% 4 3 4 5% 4 5% 48%	2160
	11th to 20th	Night	16% 7% 2 5% 4 2 4 4 4 5% 48%	2160
	21st to end	Night	16% 9% 3 6% 4 3 3 3 5% 5% 44%	2376
November	1st to 10th	Night	12% 7% 3 5% 4 3 4 5% 5% 4 47%	2462
	11th to 20th	Night	11% 8% 4 10% 5% 3 5% 5% 4 6% 40%	2660
	21st to end	Night	13% 8% 4 7% 5% 3 3 4 4 6% 42%	2660
December	1st to 10th	Night	10% 9% 4 9% 8% 4 5% 5% 5% 7% 33%	2660
	11th to 20th	Night	12% 10% 4 9% 6% 4 5% 6% 5% 7% 31%	3279
	21st to end	Night	20% 12% 6% 13% 7% 3 4 4 4 4 22%	4884

Transparency (when cloud cover <= 30%)
Color Meanings: Too cloudy to forecast Poor Below Average Average Above average Transparent
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	54% 7% 15% 15% 8%	1518
	11th to 20th	Day	59% 7% 11% 16% 7%	1622
	21st to end	Day	62% 6% 14% 12% 6% 1	1782
February	1st to 10th	Day	57% 6% 13% 16% 8% 1	1620
	11th to 20th	Day	55% 8% 11% 18% 7% 2	1583
	21st to end	Day	54% 6% 13% 16% 10% 1	1386
March	1st to 10th	Day	54% 6% 14% 19% 6% 1	1980
	11th to 20th	Day	52% 7% 12% 22% 7% 1	1980
	21st to end	Day	57% 8% 12% 16% 6% 1	2178
April	1st to 10th	Day	50% 7% 14% 19% 8% 2	2125
	11th to 20th	Day	50% 7% 12% 19% 10% 1	2181
	21st to end	Day	53% 7% 10% 21% 8% 1	2340
May	1st to 10th	Day	50% 7% 13% 19% 10%	2340
	11th to 20th	Day	45% 7% 13% 22% 12% 1	2463
	21st to end	Day	46% 8% 12% 21% 12% 1	2725
June	1st to 10th	Day	35% 9% 15% 26% 14% 1	2520
	11th to 20th	Day	37% 8% 14% 23% 16% 2	2520
	21st to end	Day	35% 8% 16% 26% 13% 2	2520
July	1st to 10th	Day	27% 10% 17% 32% 13% 1	2451
	11th to 20th	Day	27% 9% 15% 26% 19% 4	2445
	21st to end	Day	28% 8% 16% 30% 15% 2	2422
August	1st to 10th	Day	38% 11% 14% 25% 10% 2	2314
	11th to 20th	Day	36% 9% 14% 30% 11% 1	2301
	21st to end	Day	30% 7% 14% 29% 18% 2	2494
September	1st to 10th	Day	34% 6% 14% 28% 17% 1	2160
	11th to 20th	Day	32% 7% 15% 32% 14% 1	2052
	21st to end	Day	36% 6% 12% 30% 15% 1	1943
October	1st to 10th	Day	39% 6% 11% 27% 15% 1	1800
	11th to 20th	Day	39% 6% 13% 27% 15% 2	1800
	21st to end	Day	46% 4 12% 21% 15% 1	1980
November	1st to 10th	Day	38% 7% 13% 22% 18% 3	1610
	11th to 20th	Day	51% 6% 11% 21% 11%	1520
	21st to end	Day	44% 5% 18% 20% 11% 1	1520
December	1st to 10th	Day	55% 7% 14% 14% 9% 2	1520
	11th to 20th	Day	53% 6% 20% 16% 5%	1915
	21st to end	Day	64% 6% 11% 13% 5% 1	2783
January	1st to 10th	Night	49% 7% 13% 20% 10% 1	2648
	11th to 20th	Night	56% 7% 14% 16% 7%	2456
	21st to end	Night	56% 6% 13% 15% 9% 1	2574
February	1st to 10th	Night	52% 6% 13% 20% 8% 1	2340
	11th to 20th	Night	46% 6% 13% 21% 11% 2	2274
	21st to end	Night	48% 5% 13% 19% 12% 3	1883
March	1st to 10th	Night	45% 6% 17% 23% 7% 2	1980
	11th to 20th	Night	47% 6% 11% 23% 13% 1	1980
	21st to end	Night	52% 6% 14% 19% 7% 1	2178
April	1st to 10th	Night	45% 7% 10% 25% 12% 1	1834
	11th to 20th	Night	48% 6% 9% 17% 17% 2	1724
	21st to end	Night	45% 5% 10% 23% 14% 2	1620
May	1st to 10th	Night	43% 6% 15% 21% 13% 1	1620
	11th to 20th	Night	40% 7% 13% 23% 15% 2	1435
	21st to end	Night	40% 9% 13% 21% 15% 3	1552
June	1st to 10th	Night	29% 6% 17% 28% 17% 2	1440
	11th to 20th	Night	32% 8% 17% 20% 20% 3	1440
	21st to end	Night	25% 6% 18% 31% 17% 2	1440
July	1st to 10th	Night	21% 6% 16% 34% 19% 3	1400
	11th to 20th	Night	19% 8% 17% 31% 20% 5%	1392
	21st to end	Night	18% 7% 17% 34% 20% 4	1652
August	1st to 10th	Night	30% 7% 15% 29% 16% 3	1601
	11th to 20th	Night	25% 7% 14% 34% 17% 3	1600
	21st to end	Night	26% 6% 14% 29% 21% 4	1862
September	1st to 10th	Night	26% 5% 14% 31% 23% 2	1800
	11th to 20th	Night	28% 7% 16% 30% 17% 1	1903
	21st to end	Night	31% 6% 12% 31% 19% 2	1998
October	1st to 10th	Night	37% 6% 11% 25% 19% 2	2160
	11th to 20th	Night	39% 5% 11% 25% 18% 3	2160
	21st to end	Night	42% 6% 13% 24% 15% 1	2376
November	1st to 10th	Night	37% 6% 14% 23% 17% 3	2462
	11th to 20th	Night	45% 7% 12% 22% 13% 1	2660
	21st to end	Night	43% 6% 15% 24% 11% 1	2660
December	1st to 10th	Night	50% 7% 14% 21% 7%	2660
	11th to 20th	Night	50% 8% 17% 19% 6%	3281
	21st to end	Night	63% 7% 9% 16% 5%	4884

Seeing (when cloud cover <= 80%)
Color Meanings: Too cloudy to forecast Bad 1/5 Poor 2/5 Average 3/5 Good 4/5 Excellent 5/5
Month	Days in Month		Occurrence of each Forecast	Data Points
January	1st to 10th	Day	16% 15% 31% 25% 10% 2	270
	11th to 20th	Day	21% 10% 36% 26% 5% 1	255
	21st to end	Day	16% 11% 42% 24% 7% 1	264
February	1st to 10th	Day	13% 17% 45% 21% 4	240
	11th to 20th	Day	16% 16% 38% 25% 5%	240
	21st to end	Day	22% 15% 36% 24% 4	198
March	1st to 10th	Day	18% 13% 40% 25% 5%	240
	11th to 20th	Day	22% 11% 38% 22% 7% 1	240
	21st to end	Day	33% 9% 27% 23% 6% 2	264
April	1st to 10th	Day	28% 11% 31% 24% 5%	385
	11th to 20th	Day	23% 15% 28% 24% 8% 1	459
	21st to end	Day	29% 14% 28% 19% 8% 3	600
May	1st to 10th	Day	23% 12% 31% 25% 7% 2	603
	11th to 20th	Day	23% 12% 27% 22% 12% 4	625
	21st to end	Day	20% 7% 26% 30% 14% 4	679
June	1st to 10th	Day	9% 5% 27% 32% 22% 5%	630
	11th to 20th	Day	13% 7% 33% 31% 13% 3	630
	21st to end	Day	11% 7% 32% 30% 16% 4	630
July	1st to 10th	Day	6% 4 23% 35% 24% 8%	621
	11th to 20th	Day	10% 4 29% 33% 18% 6%	618
	21st to end	Day	5% 4 27% 38% 21% 5%	684
August	1st to 10th	Day	9% 3 21% 38% 23% 5%	626
	11th to 20th	Day	12% 5% 22% 30% 25% 6%	615
	21st to end	Day	10% 4 27% 34% 18% 6%	613
September	1st to 10th	Day	16% 7% 26% 30% 16% 5%	450
	11th to 20th	Day	11% 5% 35% 30% 15% 5%	342
	21st to end	Day	18% 10% 33% 28% 9% 3	270
October	1st to 10th	Day	19% 14% 28% 20% 16% 3	270
	11th to 20th	Day	16% 19% 37% 21% 7% 1	270
	21st to end	Day	24% 16% 35% 19% 5% 1	297
November	1st to 10th	Day	17% 14% 35% 24% 7% 2	270
	11th to 20th	Day	16% 21% 36% 21% 7%	270
	21st to end	Day	20% 14% 36% 24% 5%	270
December	1st to 10th	Day	13% 11% 42% 24% 9%	270
	11th to 20th	Day	16% 14% 37% 27% 6%	270
	21st to end	Day	25% 16% 36% 20% 3	297
January	1st to 10th	Night	12% 13% 40% 28% 7% 1	950
	11th to 20th	Night	13% 12% 40% 28% 6% 1	930
	21st to end	Night	18% 13% 36% 26% 6%	990
February	1st to 10th	Night	15% 11% 41% 27% 5%	900
	11th to 20th	Night	14% 14% 39% 26% 5% 1	879
	21st to end	Night	17% 11% 38% 24% 7% 2	699
March	1st to 10th	Night	15% 11% 41% 26% 8% 1	540
	11th to 20th	Night	19% 9% 38% 26% 7%	540
	21st to end	Night	24% 10% 31% 26% 8% 1	594
April	1st to 10th	Night	19% 13% 38% 25% 4	540
	11th to 20th	Night	23% 12% 33% 25% 7% 1	533
	21st to end	Night	24% 10% 33% 23% 9% 1	540
May	1st to 10th	Night	18% 9% 34% 31% 8% 1	540
	11th to 20th	Night	19% 7% 33% 29% 10% 2	531
	21st to end	Night	19% 7% 30% 32% 13%	584
June	1st to 10th	Night	8% 6% 29% 41% 15% 1	540
	11th to 20th	Night	10% 3 32% 42% 12%	540
	21st to end	Night	10% 3 28% 41% 17%	540
July	1st to 10th	Night	5% 2 22% 42% 27% 2	525
	11th to 20th	Night	2 2 29% 43% 21% 2	522
	21st to end	Night	3 2 22% 51% 21% 2	591
August	1st to 10th	Night	8% 2 19% 45% 24% 2	537
	11th to 20th	Night	10% 2 18% 39% 27% 3	534
	21st to end	Night	8% 3 25% 40% 20% 3	594
September	1st to 10th	Night	12% 3 26% 41% 16% 2	540
	11th to 20th	Night	12% 4 33% 34% 16% 2	540
	21st to end	Night	10% 12% 33% 34% 11% 1	558
October	1st to 10th	Night	15% 9% 34% 28% 12% 1	720
	11th to 20th	Night	18% 11% 36% 28% 6%	720
	21st to end	Night	21% 11% 31% 29% 8%	792
November	1st to 10th	Night	16% 12% 37% 25% 9%	875
	11th to 20th	Night	14% 14% 42% 24% 5%	950
	21st to end	Night	16% 13% 38% 25% 7%	950
December	1st to 10th	Night	15% 10% 41% 27% 6% 1	950
	11th to 20th	Night	18% 9% 38% 28% 6% 1	950
	21st to end	Night	23% 12% 38% 22% 5%	1045

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