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 2026-04-02.

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 8% 5% 4 4 4 5% 7% 39%	131758
	Night	12% 8% 3 9% 5% 4 4 3 5% 6% 41%	130362
Transparency	Day	43% 6% 13% 24% 12% 1	135358
	Night	43% 5% 13% 24% 14% 2	134840
Seeing	Day	16% 10% 31% 27% 12% 3	17789
	Night	15% 10% 35% 30% 10% 1	27841

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% 14% 6% 21% 9% 6% 5% 3 4 5% 19%	4691
	11th to 20th	Day	5% 11% 5% 11% 7% 7% 8% 6% 8% 6% 24%	5172
	21st to end	Day	5% 11% 6% 10% 6% 6% 7% 3 5% 6% 34%	5812
February	1st to 10th	Day	8% 13% 6% 11% 7% 7% 6% 5% 6% 7% 24%	1890
	11th to 20th	Day	9% 11% 5% 9% 7% 7% 6% 4 6% 7% 29%	1844
	21st to end	Day	13% 10% 3 10% 6% 4 4 3 5% 7% 34%	1621
March	1st to 10th	Day	13% 8% 3 8% 4 4 3 3 4 7% 44%	5046
	11th to 20th	Day	12% 8% 2 5% 3 3 3 3 5% 7% 47%	5090
	21st to end	Day	23% 11% 5% 10% 5% 4 4 3 4 5% 25%	5583
April	1st to 10th	Day	19% 9% 3 8% 4 4 4 4 5% 6% 33%	2373
	11th to 20th	Day	19% 9% 4 7% 4 4 4 3 6% 7% 33%	2427
	21st to end	Day	25% 10% 3 6% 4 3 3 4 5% 6% 32%	2600
May	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 6% 7% 46%	5862
	11th to 20th	Day	17% 6% 2 5% 3 3 3 3 4 6% 48%	6271
	21st to end	Day	18% 7% 3 7% 5% 4 4 4 6% 7% 35%	6935
June	1st to 10th	Day	12% 5% 2 5% 4 5% 4 4 5% 8% 46%	2800
	11th to 20th	Day	13% 5% 2 6% 5% 5% 4 3 7% 9% 41%	2800
	21st to end	Day	12% 5% 2 5% 4 5% 5% 4 6% 8% 43%	2800
July	1st to 10th	Day	9% 5% 2 5% 4 4 5% 4 6% 9% 45%	6360
	11th to 20th	Day	9% 4 2 4 3 3 4 3 5% 7% 56%	6360
	21st to end	Day	8% 4 2 6% 4 4 5% 4 6% 9% 47%	6468
August	1st to 10th	Day	12% 5% 2 7% 5% 4 5% 3 5% 7% 43%	4234
	11th to 20th	Day	12% 5% 3 7% 5% 5% 4 4 7% 9% 40%	4171
	21st to end	Day	9% 4 2 6% 4 5% 4 3 5% 7% 51%	4520
September	1st to 10th	Day	12% 5% 2 5% 3 4 3 4 5% 7% 50%	2400
	11th to 20th	Day	10% 5% 2 5% 4 3 3 4 4 7% 51%	2280
	21st to end	Day	12% 6% 2 5% 3 3 4 4 4 7% 50%	2158
October	1st to 10th	Day	14% 6% 2 5% 4 4 4 4 5% 5% 48%	2000
	11th to 20th	Day	15% 9% 2 5% 3 4 4 3 4 6% 45%	2000
	21st to end	Day	17% 10% 3 7% 4 4 3 2 4 5% 42%	2200
November	1st to 10th	Day	11% 8% 3 5% 5% 4 4 3 5% 7% 45%	1782
	11th to 20th	Day	11% 11% 4 11% 6% 3 4 3 4 5% 37%	1680
	21st to end	Day	13% 10% 4 9% 5% 3 3 3 5% 9% 37%	1680
December	1st to 10th	Day	7% 9% 3 11% 9% 7% 5% 4 5% 8% 32%	2690
	11th to 20th	Day	11% 15% 5% 12% 9% 7% 4 3 6% 6% 22%	3088
	21st to end	Day	20% 15% 6% 20% 5% 3 4 3 3 5% 17%	4070
January	1st to 10th	Night	7% 11% 5% 20% 8% 7% 6% 4 4 5% 23%	8232
	11th to 20th	Night	4 10% 4 12% 8% 8% 11% 6% 7% 6% 24%	7800
	21st to end	Night	6% 10% 6% 11% 5% 5% 5% 4 5% 6% 37%	8402
February	1st to 10th	Night	9% 12% 4 10% 7% 5% 5% 4 6% 5% 32%	2730
	11th to 20th	Night	8% 10% 4 9% 5% 5% 5% 4 6% 6% 38%	2663
	21st to end	Night	11% 9% 3 8% 8% 5% 4 3 5% 6% 38%	2200
March	1st to 10th	Night	13% 8% 3 6% 4 2 3 2 3 5% 52%	5029
	11th to 20th	Night	11% 6% 2 7% 4 4 4 4 6% 8% 45%	5090
	21st to end	Night	24% 10% 3 9% 4 3 4 3 4 5% 31%	5588
April	1st to 10th	Night	15% 9% 3 7% 4 4 5% 4 5% 6% 39%	2048
	11th to 20th	Night	18% 8% 3 7% 4 4 3 4 5% 5% 39%	1918
	21st to end	Night	20% 9% 2 6% 4 4 3 3 5% 6% 39%	1800
May	1st to 10th	Night	13% 5% 2 5% 3 3 3 2 4 5% 54%	4053
	11th to 20th	Night	14% 5% 2 5% 3 4 3 3 5% 6% 50%	3642
	21st to end	Night	19% 7% 3 6% 4 4 3 3 4 6% 42%	3942
June	1st to 10th	Night	9% 4 2 5% 4 3 3 4 6% 7% 52%	1600
	11th to 20th	Night	12% 4 2 6% 4 4 5% 5% 5% 8% 47%	1600
	21st to end	Night	11% 5% 1 4 3 2 3 3 5% 9% 55%	1600
July	1st to 10th	Night	8% 4 1 5% 4 4 5% 4 6% 9% 51%	3618
	11th to 20th	Night	7% 4 1 4 3 4 4 3 6% 9% 55%	3617
	21st to end	Night	10% 3 1 5% 3 4 4 2 5% 7% 54%	4398
August	1st to 10th	Night	12% 5% 2 6% 3 4 4 3 5% 7% 50%	2926
	11th to 20th	Night	12% 3 1 4 3 3 3 4 5% 8% 53%	2888
	21st to end	Night	9% 4 2 5% 4 3 3 3 5% 6% 56%	3384
September	1st to 10th	Night	10% 4 1 4 3 3 3 2 5% 6% 60%	2000
	11th to 20th	Night	10% 5% 1 4 3 3 4 3 6% 8% 52%	2115
	21st to end	Night	9% 5% 2 5% 3 3 4 4 3 6% 56%	2221
October	1st to 10th	Night	13% 5% 2 5% 3 3 4 4 4 5% 51%	2400
	11th to 20th	Night	15% 7% 2 6% 3 3 3 3 4 5% 49%	2400
	21st to end	Night	16% 8% 3 6% 3 4 3 3 5% 5% 44%	2640
November	1st to 10th	Night	12% 8% 2 6% 4 4 4 4 5% 5% 46%	2729
	11th to 20th	Night	11% 9% 4 10% 4 4 4 4 4 6% 40%	2940
	21st to end	Night	12% 9% 4 9% 5% 4 4 3 4 6% 41%	2940
December	1st to 10th	Night	6% 8% 4 12% 9% 7% 5% 4 5% 7% 35%	4720
	11th to 20th	Night	13% 11% 4 12% 7% 5% 5% 4 5% 6% 29%	5339
	21st to end	Night	19% 14% 7% 17% 6% 4 4 3 4 4 19%	7150

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	67% 4 10% 12% 6%	4692
	11th to 20th	Day	56% 4 10% 19% 12%	5172
	21st to end	Day	50% 5% 15% 22% 8%	5812
February	1st to 10th	Day	57% 5% 13% 15% 8% 1	1890
	11th to 20th	Day	53% 7% 10% 21% 7% 2	1853
	21st to end	Day	50% 6% 14% 19% 10% 1	1621
March	1st to 10th	Day	41% 5% 13% 28% 11% 2	5046
	11th to 20th	Day	36% 5% 13% 31% 13% 2	5090
	21st to end	Day	60% 6% 12% 15% 6%	5585
April	1st to 10th	Day	50% 6% 14% 21% 8% 2	2373
	11th to 20th	Day	49% 8% 12% 19% 11% 1	2427
	21st to end	Day	52% 7% 10% 21% 8% 1	2600
May	1st to 10th	Day	35% 7% 12% 28% 17%	5900
	11th to 20th	Day	37% 6% 10% 24% 18% 5%	6275
	21st to end	Day	46% 7% 10% 25% 11% 1	6949
June	1st to 10th	Day	34% 9% 15% 26% 15% 1	2800
	11th to 20th	Day	39% 8% 14% 22% 16% 2	2800
	21st to end	Day	37% 8% 16% 25% 13% 2	2800
July	1st to 10th	Day	34% 8% 17% 32% 8% 1	6291
	11th to 20th	Day	29% 6% 14% 33% 16% 2	6285
	21st to end	Day	32% 7% 16% 29% 14% 1	6285
August	1st to 10th	Day	40% 8% 12% 27% 12% 1	4220
	11th to 20th	Day	40% 8% 16% 28% 8%	4210
	21st to end	Day	32% 6% 16% 28% 15% 2	4519
September	1st to 10th	Day	33% 6% 15% 29% 16% 1	2400
	11th to 20th	Day	33% 7% 15% 32% 13% 1	2280
	21st to end	Day	34% 6% 11% 30% 18% 2	2158
October	1st to 10th	Day	26% 4 7% 33% 29% 1	3260
	11th to 20th	Day	35% 4 13% 26% 21% 2	3240
	21st to end	Day	41% 3 11% 29% 14% 1	3538
November	1st to 10th	Day	38% 7% 13% 22% 17% 3	1782
	11th to 20th	Day	50% 6% 11% 21% 11% 1	1680
	21st to end	Day	46% 5% 17% 20% 11% 1	1680
December	1st to 10th	Day	51% 6% 14% 20% 8% 1	2690
	11th to 20th	Day	63% 5% 14% 12% 5%	3085
	21st to end	Day	70% 5% 10% 11% 4	4070
January	1st to 10th	Night	63% 5% 12% 15% 6% 1	8233
	11th to 20th	Night	57% 5% 13% 14% 11% 1	7800
	21st to end	Night	47% 4 13% 24% 12%	8402
February	1st to 10th	Night	52% 6% 13% 20% 8% 2	2730
	11th to 20th	Night	45% 6% 13% 23% 11% 2	2664
	21st to end	Night	45% 5% 14% 20% 12% 3	2200
March	1st to 10th	Night	36% 4 9% 34% 15% 2	5029
	11th to 20th	Night	36% 5% 11% 31% 16% 1	5090
	21st to end	Night	56% 5% 13% 17% 8% 1	5588
April	1st to 10th	Night	45% 7% 10% 25% 12% 1	2048
	11th to 20th	Night	45% 6% 9% 19% 18% 2	1918
	21st to end	Night	46% 5% 10% 22% 14% 2	1800
May	1st to 10th	Night	33% 5% 10% 30% 21% 1	4080
	11th to 20th	Night	34% 6% 10% 22% 21% 8%	3643
	21st to end	Night	44% 6% 9% 27% 13% 1	3950
June	1st to 10th	Night	30% 6% 16% 29% 17% 3	1600
	11th to 20th	Night	34% 8% 16% 20% 19% 3	1600
	21st to end	Night	27% 5% 17% 30% 18% 2	1600
July	1st to 10th	Night	29% 6% 19% 30% 14% 1	3580
	11th to 20th	Night	27% 6% 15% 34% 16% 2	3572
	21st to end	Night	29% 6% 17% 30% 16% 2	4271
August	1st to 10th	Night	35% 5% 13% 28% 16% 3	2916
	11th to 20th	Night	30% 7% 16% 32% 13% 1	2915
	21st to end	Night	29% 6% 17% 26% 19% 2	3384
September	1st to 10th	Night	26% 5% 14% 32% 22% 2	2000
	11th to 20th	Night	28% 8% 17% 30% 17% 1	2115
	21st to end	Night	30% 5% 12% 29% 22% 2	2221
October	1st to 10th	Night	23% 3 7% 27% 36% 3	3920
	11th to 20th	Night	27% 4 12% 27% 24% 5%	3899
	21st to end	Night	41% 4 11% 28% 14% 1	4252
November	1st to 10th	Night	38% 6% 14% 23% 16% 3	2729
	11th to 20th	Night	45% 7% 12% 22% 12% 2	2940
	21st to end	Night	45% 5% 15% 24% 10% 1	2940
December	1st to 10th	Night	48% 6% 15% 24% 7%	4720
	11th to 20th	Night	55% 6% 14% 20% 5%	5341
	21st to end	Night	67% 5% 10% 13% 5%	7150

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% 14% 32% 27% 9% 2	330
	11th to 20th	Day	18% 12% 39% 25% 5% 1	330
	21st to end	Day	12% 14% 41% 23% 9% 1	363
February	1st to 10th	Day	14% 16% 45% 19% 5% 1	330
	11th to 20th	Day	14% 15% 39% 28% 5%	330
	21st to end	Day	19% 15% 40% 23% 3	273
March	1st to 10th	Day	18% 13% 42% 22% 4	330
	11th to 20th	Day	23% 12% 35% 22% 6% 1	330
	21st to end	Day	31% 11% 31% 20% 5% 2	363
April	1st to 10th	Day	28% 13% 32% 23% 5%	465
	11th to 20th	Day	23% 18% 29% 22% 7% 1	545
	21st to end	Day	29% 13% 29% 19% 8% 2	700
May	1st to 10th	Day	21% 12% 30% 25% 9% 3	703
	11th to 20th	Day	23% 11% 26% 24% 13% 3	725
	21st to end	Day	20% 8% 26% 30% 13% 3	789
June	1st to 10th	Day	9% 7% 27% 31% 21% 5%	730
	11th to 20th	Day	13% 7% 36% 29% 13% 3	730
	21st to end	Day	13% 8% 32% 28% 15% 3	730
July	1st to 10th	Day	7% 4 23% 36% 23% 7%	721
	11th to 20th	Day	9% 4 29% 33% 17% 7%	718
	21st to end	Day	5% 4 26% 39% 21% 5%	794
August	1st to 10th	Day	10% 4 21% 37% 22% 5%	726
	11th to 20th	Day	12% 5% 23% 30% 24% 6%	715
	21st to end	Day	10% 4 28% 33% 19% 6%	713
September	1st to 10th	Day	15% 8% 27% 29% 16% 5%	530
	11th to 20th	Day	9% 5% 33% 33% 16% 4	410
	21st to end	Day	15% 8% 31% 28% 14% 5%	330
October	1st to 10th	Day	16% 15% 29% 20% 16% 4	330
	11th to 20th	Day	17% 18% 38% 20% 5% 1	330
	21st to end	Day	22% 15% 37% 19% 5% 1	363
November	1st to 10th	Day	18% 15% 36% 22% 6% 2	330
	11th to 20th	Day	18% 21% 36% 19% 6%	330
	21st to end	Day	21% 16% 35% 22% 6%	330
December	1st to 10th	Day	13% 13% 41% 25% 8%	330
	11th to 20th	Day	17% 16% 37% 25% 5%	330
	21st to end	Day	26% 15% 37% 18% 3	363
January	1st to 10th	Night	12% 12% 40% 28% 7% 1	1050
	11th to 20th	Night	12% 14% 40% 27% 6% 1	1050
	21st to end	Night	16% 13% 38% 26% 6%	1155
February	1st to 10th	Night	15% 13% 41% 26% 5%	1050
	11th to 20th	Night	14% 14% 41% 26% 4 1	1029
	21st to end	Night	15% 12% 40% 25% 7% 2	816
March	1st to 10th	Night	16% 11% 41% 25% 7% 1	630
	11th to 20th	Night	19% 12% 39% 24% 7%	630
	21st to end	Night	24% 11% 32% 25% 7% 1	693
April	1st to 10th	Night	19% 14% 37% 25% 4	603
	11th to 20th	Night	22% 14% 33% 24% 6% 1	593
	21st to end	Night	24% 11% 33% 24% 8% 1	600
May	1st to 10th	Night	18% 9% 33% 31% 10% 1	600
	11th to 20th	Night	18% 7% 32% 31% 10% 2	591
	21st to end	Night	19% 7% 29% 32% 12%	650
June	1st to 10th	Night	9% 6% 29% 40% 15% 1	600
	11th to 20th	Night	11% 4 32% 42% 11%	600
	21st to end	Night	12% 4 29% 40% 15%	600
July	1st to 10th	Night	6% 2 22% 43% 26% 2	585
	11th to 20th	Night	3 2 28% 43% 22% 2	582
	21st to end	Night	4 2 21% 51% 21% 2	657
August	1st to 10th	Night	9% 2 19% 45% 23% 2	597
	11th to 20th	Night	10% 2 19% 40% 26% 3	594
	21st to end	Night	8% 3 25% 41% 21% 2	660
September	1st to 10th	Night	12% 3 26% 42% 16% 2	600
	11th to 20th	Night	11% 4 33% 35% 15% 2	600
	21st to end	Night	9% 12% 30% 35% 13% 1	621
October	1st to 10th	Night	14% 10% 34% 29% 12% 1	800
	11th to 20th	Night	18% 11% 37% 27% 6%	800
	21st to end	Night	21% 11% 32% 28% 7%	880
November	1st to 10th	Night	16% 12% 38% 24% 9%	970
	11th to 20th	Night	14% 15% 42% 24% 5%	1050
	21st to end	Night	17% 13% 38% 25% 7%	1050
December	1st to 10th	Night	14% 11% 41% 28% 6% 1	1050
	11th to 20th	Night	18% 11% 37% 28% 6% 1	1050
	21st to end	Night	23% 11% 38% 23% 5%	1155

Page generated at 2026-03-30 01:18:41. Version ac07

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