Fermont
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Fermont clear sky chart from 2005-11-06 to 2026-03-18.

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	15% 14% 5% 12% 7% 6% 6% 4 6% 7% 18%	131429
	Night	12% 14% 5% 13% 8% 7% 6% 5% 6% 7% 17%	130355
Transparency	Day	41% 38% 6% 10% 4	134757
	Night	42% 37% 6% 10% 4 1	135118
Seeing	Day	23% 10% 26% 24% 14% 4	17789
	Night	20% 9% 28% 29% 12% 2	25540

By Month

Percentage Cloud Cover NE (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	6% 17% 10% 19% 9% 8% 5% 5% 7% 7% 6%	4093
	11th to 20th	Day	3 11% 6% 20% 11% 10% 11% 4 5% 9% 10%	4084
	21st to end	Day	2 5% 3 13% 10% 9% 8% 8% 10% 14% 18%	4743
February	1st to 10th	Day	4 11% 5% 12% 8% 7% 8% 7% 10% 11% 18%	1698
	11th to 20th	Day	3 16% 8% 16% 9% 7% 6% 5% 8% 10% 12%	1843
	21st to end	Day	4 15% 5% 10% 8% 7% 7% 5% 10% 11% 16%	1583
March	1st to 10th	Day	11% 13% 5% 11% 6% 9% 9% 7% 7% 8% 15%	4655
	11th to 20th	Day	9% 22% 9% 15% 6% 5% 5% 4 6% 6% 13%	5057
	21st to end	Day	12% 19% 7% 12% 6% 5% 11% 5% 5% 5% 14%	5466
April	1st to 10th	Day	13% 19% 6% 16% 9% 8% 6% 3 4 3 12%	2518
	11th to 20th	Day	11% 18% 5% 13% 8% 5% 5% 3 4 5% 24%	2572
	21st to end	Day	17% 18% 5% 12% 7% 5% 5% 3 4 5% 18%	2600
May	1st to 10th	Day	15% 22% 7% 12% 5% 4 4 2 4 4 22%	6554
	11th to 20th	Day	22% 13% 4 8% 5% 5% 4 3 5% 7% 24%	6775
	21st to end	Day	25% 14% 5% 8% 5% 4 4 3 4 5% 23%	7451
June	1st to 10th	Day	23% 14% 4 10% 6% 4 4 3 4 5% 24%	3000
	11th to 20th	Day	22% 14% 4 9% 5% 5% 4 4 5% 7% 21%	3000
	21st to end	Day	24% 10% 4 9% 7% 4 5% 4 6% 6% 20%	3000
July	1st to 10th	Day	16% 9% 3 7% 5% 5% 5% 4 6% 7% 33%	6820
	11th to 20th	Day	20% 10% 4 10% 6% 7% 6% 4 6% 6% 22%	6820
	21st to end	Day	15% 10% 3 11% 8% 8% 8% 6% 7% 7% 17%	7428
August	1st to 10th	Day	16% 10% 4 11% 7% 7% 6% 5% 6% 6% 22%	4721
	11th to 20th	Day	20% 10% 3 11% 7% 6% 6% 4 6% 7% 20%	4175
	21st to end	Day	18% 8% 3 8% 6% 5% 5% 4 6% 7% 31%	4675
September	1st to 10th	Day	21% 12% 4 10% 6% 6% 5% 3 6% 6% 20%	2517
	11th to 20th	Day	23% 15% 5% 12% 6% 5% 4 3 4 6% 18%	2215
	21st to end	Day	21% 21% 5% 10% 5% 5% 4 2 5% 5% 18%	2200
October	1st to 10th	Day	21% 21% 6% 9% 6% 5% 4 3 4 5% 17%	2020
	11th to 20th	Day	24% 20% 8% 11% 6% 4 3 3 3 5% 14%	1908
	21st to end	Day	21% 22% 7% 15% 6% 4 4 2 3 4 12%	1920
November	1st to 10th	Day	19% 20% 7% 19% 9% 6% 5% 3 3 3 7%	1645
	11th to 20th	Day	15% 22% 8% 18% 9% 6% 5% 3 3 4 7%	1612
	21st to end	Day	13% 20% 7% 16% 10% 9% 7% 4 4 4 7%	1470
December	1st to 10th	Day	4 15% 8% 18% 9% 9% 9% 6% 7% 7% 8%	2350
	11th to 20th	Day	12% 16% 5% 13% 13% 9% 7% 5% 6% 7% 6%	2688
	21st to end	Day	9% 13% 6% 12% 9% 10% 6% 5% 7% 10% 14%	3553
January	1st to 10th	Night	6% 18% 8% 18% 9% 8% 7% 5% 8% 8% 6%	8831
	11th to 20th	Night	5% 11% 6% 19% 11% 12% 8% 5% 6% 8% 9%	8831
	21st to end	Night	2 5% 3 12% 11% 9% 9% 7% 11% 14% 16%	9455
February	1st to 10th	Night	3 11% 4 11% 10% 10% 8% 7% 9% 13% 16%	2921
	11th to 20th	Night	5% 16% 7% 16% 9% 6% 5% 5% 8% 10% 13%	2663
	21st to end	Night	6% 13% 5% 13% 8% 7% 6% 7% 8% 9% 19%	2222
March	1st to 10th	Night	10% 14% 5% 10% 5% 6% 7% 7% 8% 9% 20%	5404
	11th to 20th	Night	12% 24% 8% 13% 5% 4 4 4 5% 5% 14%	5057
	21st to end	Night	9% 17% 7% 13% 7% 6% 8% 5% 6% 6% 17%	5466
April	1st to 10th	Night	13% 17% 6% 15% 7% 5% 5% 3 5% 5% 18%	1879
	11th to 20th	Night	13% 15% 4 11% 6% 5% 4 4 4 6% 28%	1778
	21st to end	Night	16% 15% 6% 11% 7% 5% 5% 3 4 5% 23%	1800
May	1st to 10th	Night	18% 19% 7% 11% 5% 3 3 2 4 3 26%	3448
	11th to 20th	Night	24% 11% 4 8% 4 4 4 3 5% 5% 28%	3138
	21st to end	Night	21% 15% 5% 8% 5% 4 4 3 5% 6% 23%	3446
June	1st to 10th	Night	18% 11% 5% 9% 6% 5% 5% 4 6% 4 28%	1400
	11th to 20th	Night	22% 11% 3 8% 6% 5% 5% 4 5% 6% 25%	1400
	21st to end	Night	23% 10% 2 9% 5% 5% 6% 4 6% 7% 25%	1400
July	1st to 10th	Night	16% 8% 3 6% 5% 5% 4 3 6% 6% 38%	3160
	11th to 20th	Night	18% 9% 3 10% 7% 5% 6% 4 6% 6% 26%	3160
	21st to end	Night	14% 9% 3 9% 5% 6% 6% 4 7% 9% 29%	3446
August	1st to 10th	Night	14% 7% 3 9% 6% 7% 6% 5% 8% 8% 26%	2408
	11th to 20th	Night	19% 10% 4 11% 8% 7% 6% 4 6% 8% 19%	2863
	21st to end	Night	14% 7% 3 6% 5% 5% 5% 4 7% 9% 34%	3223
September	1st to 10th	Night	19% 11% 4 9% 7% 5% 5% 3 5% 5% 26%	1883
	11th to 20th	Night	20% 13% 5% 10% 6% 5% 4 4 5% 6% 22%	2184
	21st to end	Night	20% 14% 3 11% 6% 5% 5% 3 5% 6% 22%	2200
October	1st to 10th	Night	20% 17% 4 9% 6% 4 4 4 4 6% 23%	2380
	11th to 20th	Night	23% 19% 7% 11% 5% 4 4 3 4 4 16%	2490
	21st to end	Night	23% 18% 6% 12% 6% 4 5% 4 5% 4 13%	2920
November	1st to 10th	Night	17% 21% 7% 18% 10% 5% 5% 3 4 4 8%	2873
	11th to 20th	Night	17% 21% 7% 16% 10% 7% 4 2 4 4 8%	3008
	21st to end	Night	14% 20% 6% 17% 9% 7% 6% 4 5% 5% 6%	3150
December	1st to 10th	Night	5% 15% 8% 17% 13% 11% 7% 5% 6% 7% 8%	5060
	11th to 20th	Night	11% 16% 6% 14% 11% 9% 7% 4 5% 7% 9%	5741
	21st to end	Night	8% 14% 7% 13% 9% 10% 8% 6% 7% 7% 10%	7667

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	61% 22% 3 9% 4	4094
	11th to 20th	Day	60% 23% 7% 6% 3	4110
	21st to end	Day	40% 25% 8% 17% 9% 2	4743
February	1st to 10th	Day	19% 61% 5% 10% 5%	1698
	11th to 20th	Day	27% 60% 4 6% 4	1851
	21st to end	Day	25% 60% 3 8% 4	1583
March	1st to 10th	Day	46% 29% 3 9% 12% 1	4659
	11th to 20th	Day	54% 29% 4 5% 7% 2	5057
	21st to end	Day	51% 30% 3 6% 9% 1	5466
April	1st to 10th	Day	27% 63% 4 3 3	2518
	11th to 20th	Day	25% 62% 2 4 6% 1	2572
	21st to end	Day	29% 63% 3 3 2	2600
May	1st to 10th	Day	48% 30% 6% 10% 7%	6566
	11th to 20th	Day	42% 30% 6% 18% 4	6779
	21st to end	Day	45% 29% 8% 13% 5%	7451
June	1st to 10th	Day	25% 62% 4 8% 1	3000
	11th to 20th	Day	25% 62% 4 6% 2	3000
	21st to end	Day	24% 62% 4 8% 1	3000
July	1st to 10th	Day	33% 28% 7% 25% 7%	6745
	11th to 20th	Day	45% 28% 10% 12% 5%	6739
	21st to end	Day	46% 30% 10% 13% 1	7216
August	1st to 10th	Day	37% 39% 8% 13% 3	4706
	11th to 20th	Day	40% 38% 9% 10% 2	4214
	21st to end	Day	27% 39% 14% 16% 4	4675
September	1st to 10th	Day	29% 59% 7% 5% 1	2517
	11th to 20th	Day	29% 58% 6% 6% 1	2215
	21st to end	Day	28% 57% 5% 8% 1	2200
October	1st to 10th	Day	37% 37% 9% 15% 1	3293
	11th to 20th	Day	51% 38% 4 6% 1	3085
	21st to end	Day	49% 37% 5% 8% 1	3087
November	1st to 10th	Day	42% 54% 2 1 1	1645
	11th to 20th	Day	38% 56% 4 1 2	1612
	21st to end	Day	37% 56% 4 3 1	1470
December	1st to 10th	Day	47% 38% 3 9% 3	2350
	11th to 20th	Day	54% 33% 9% 3 2	2688
	21st to end	Day	46% 28% 8% 13% 4 1	3553
January	1st to 10th	Night	59% 23% 5% 9% 5% 1	8831
	11th to 20th	Night	59% 24% 7% 7% 3	8880
	21st to end	Night	40% 26% 10% 15% 7% 2	9455
February	1st to 10th	Night	20% 61% 4 7% 7% 1	2921
	11th to 20th	Night	28% 59% 3 7% 4	2666
	21st to end	Night	26% 60% 3 6% 5% 1	2222
March	1st to 10th	Night	40% 30% 3 9% 16% 1	5413
	11th to 20th	Night	55% 29% 3 6% 6% 2	5058
	21st to end	Night	48% 29% 4 6% 12% 1	5466
April	1st to 10th	Night	25% 62% 3 4 5% 1	1879
	11th to 20th	Night	21% 62% 3 7% 6% 1	1778
	21st to end	Night	30% 62% 2 4 2	1800
May	1st to 10th	Night	45% 29% 4 13% 8% 1	3453
	11th to 20th	Night	43% 30% 4 18% 5%	3139
	21st to end	Night	45% 30% 9% 15% 2	3447
June	1st to 10th	Night	21% 64% 5% 9% 2	1400
	11th to 20th	Night	24% 62% 3 9% 1	1400
	21st to end	Night	21% 62% 4 11% 1	1400
July	1st to 10th	Night	32% 28% 10% 24% 6%	3125
	11th to 20th	Night	42% 29% 12% 11% 5%	3118
	21st to end	Night	35% 29% 12% 21% 2	3343
August	1st to 10th	Night	31% 40% 12% 15% 2	2399
	11th to 20th	Night	42% 39% 10% 8% 1	2890
	21st to end	Night	22% 39% 12% 22% 4	3223
September	1st to 10th	Night	26% 58% 6% 8% 1	1883
	11th to 20th	Night	25% 58% 7% 8% 1	2184
	21st to end	Night	23% 59% 6% 10% 2	2200
October	1st to 10th	Night	36% 37% 10% 15% 2	3887
	11th to 20th	Night	48% 38% 7% 5% 2	4052
	21st to end	Night	49% 38% 4 8% 2	4706
November	1st to 10th	Night	38% 54% 5% 3 1	2873
	11th to 20th	Night	38% 55% 3 2 1	3008
	21st to end	Night	37% 56% 4 2 1	3150
December	1st to 10th	Night	49% 37% 3 7% 4	5060
	11th to 20th	Night	53% 33% 7% 5% 2	5742
	21st to end	Night	51% 29% 6% 10% 3 2	7667

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	7% 10% 30% 30% 17% 6%	220
	11th to 20th	Day	11% 10% 29% 30% 16% 5%	220
	21st to end	Day	8% 10% 35% 29% 15% 2	242
February	1st to 10th	Day	6% 10% 31% 24% 18% 12%	238
	11th to 20th	Day	8% 9% 35% 30% 15% 4	426
	21st to end	Day	11% 9% 34% 27% 15% 4	369
March	1st to 10th	Day	13% 13% 28% 27% 15% 4	540
	11th to 20th	Day	15% 12% 28% 27% 14% 4	528
	21st to end	Day	17% 13% 33% 24% 11% 3	550
April	1st to 10th	Day	19% 13% 33% 21% 11% 3	500
	11th to 20th	Day	18% 11% 26% 26% 14% 6%	492
	21st to end	Day	24% 10% 24% 24% 12% 6%	500
May	1st to 10th	Day	30% 8% 22% 24% 11% 5%	692
	11th to 20th	Day	22% 9% 24% 21% 16% 7%	724
	21st to end	Day	29% 9% 22% 24% 13% 3	790
June	1st to 10th	Day	28% 6% 20% 27% 16% 3	730
	11th to 20th	Day	29% 8% 22% 21% 14% 5%	730
	21st to end	Day	26% 8% 26% 20% 15% 5%	730
July	1st to 10th	Day	24% 6% 23% 24% 16% 6%	720
	11th to 20th	Day	27% 8% 23% 26% 13% 4	718
	21st to end	Day	27% 7% 17% 26% 18% 6%	795
August	1st to 10th	Day	29% 7% 17% 23% 17% 5%	641
	11th to 20th	Day	24% 7% 16% 25% 20% 8%	519
	21st to end	Day	25% 6% 21% 23% 18% 7%	583
September	1st to 10th	Day	28% 13% 27% 21% 8% 3	530
	11th to 20th	Day	27% 11% 30% 21% 9% 2	530
	21st to end	Day	28% 15% 28% 18% 8% 4	530
October	1st to 10th	Day	26% 16% 31% 18% 8% 2	431
	11th to 20th	Day	34% 10% 24% 19% 9% 4	420
	21st to end	Day	29% 12% 26% 18% 12% 3	462
November	1st to 10th	Day	28% 14% 31% 20% 6% 1	425
	11th to 20th	Day	23% 12% 29% 23% 12% 1	362
	21st to end	Day	17% 13% 35% 25% 8% 2	220
December	1st to 10th	Day	11% 6% 30% 28% 19% 5%	220
	11th to 20th	Day	13% 18% 35% 22% 10% 2	220
	21st to end	Day	15% 10% 31% 24% 16% 5%	242
January	1st to 10th	Night	13% 8% 30% 31% 15% 3	950
	11th to 20th	Night	13% 9% 32% 31% 12% 3	950
	21st to end	Night	9% 8% 33% 34% 13% 2	998
February	1st to 10th	Night	6% 10% 30% 33% 19% 3	840
	11th to 20th	Night	11% 11% 32% 29% 14% 3	823
	21st to end	Night	11% 9% 31% 32% 14% 3	692
March	1st to 10th	Night	12% 9% 32% 31% 14% 2	840
	11th to 20th	Night	16% 10% 31% 28% 15% 1	829
	21st to end	Night	16% 11% 33% 28% 9% 2	880
April	1st to 10th	Night	22% 9% 31% 30% 7% 1	661
	11th to 20th	Night	19% 8% 31% 29% 10% 4	594
	21st to end	Night	25% 9% 25% 30% 9% 2	600
May	1st to 10th	Night	28% 8% 23% 29% 11% 2	527
	11th to 20th	Night	25% 7% 25% 25% 15% 3	394
	21st to end	Night	25% 10% 25% 29% 10% 1	433
June	1st to 10th	Night	24% 4 27% 36% 9% 1	400
	11th to 20th	Night	27% 8% 22% 31% 13%	400
	21st to end	Night	25% 7% 25% 28% 16% 1	400
July	1st to 10th	Night	22% 5% 23% 33% 16% 1	390
	11th to 20th	Night	23% 6% 25% 34% 11% 1	388
	21st to end	Night	21% 6% 18% 34% 19% 2	438
August	1st to 10th	Night	23% 6% 14% 37% 19% 1	398
	11th to 20th	Night	24% 5% 19% 32% 19% 1	583
	21st to end	Night	27% 6% 21% 32% 13% 2	660
September	1st to 10th	Night	26% 10% 28% 28% 7% 1	600
	11th to 20th	Night	26% 10% 29% 27% 8% 1	784
	21st to end	Night	25% 10% 32% 24% 8%	800
October	1st to 10th	Night	30% 13% 29% 23% 6%	800
	11th to 20th	Night	32% 9% 22% 25% 10% 2	800
	21st to end	Night	32% 12% 22% 23% 9% 2	913
November	1st to 10th	Night	26% 15% 29% 22% 6% 2	930
	11th to 20th	Night	27% 12% 28% 22% 9% 2	950
	21st to end	Night	23% 9% 31% 27% 9% 1	950
December	1st to 10th	Night	15% 10% 34% 28% 10% 3	950
	11th to 20th	Night	15% 11% 34% 28% 11% 1	950
	21st to end	Night	16% 11% 27% 29% 15% 3	1045

Page generated at 2026-03-15 02:23:51. Version ac07

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