Boreal Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Boreal Skies Observatory clear sky chart from 2005-11-06 to 2025-10-19.

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	17% 10% 3 8% 5% 5% 5% 4 5% 7% 32%	126994
	Night	17% 11% 4 9% 5% 5% 4 3 5% 6% 31%	122708
Transparency	Day	50% 7% 12% 20% 10% 1	130459
	Night	53% 6% 12% 19% 10% 1	127330
Seeing	Day	22% 16% 31% 20% 9% 3	15675
	Night	22% 12% 29% 26% 9% 1	24702

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	14% 17% 7% 15% 6% 5% 4 3 4 5% 19%	3597
	11th to 20th	Day	13% 14% 7% 14% 7% 6% 4 3 5% 5% 22%	3570
	21st to end	Day	11% 16% 6% 15% 6% 6% 5% 4 5% 6% 20%	4121
February	1st to 10th	Day	13% 15% 5% 13% 7% 6% 4 4 4 5% 24%	1800
	11th to 20th	Day	13% 15% 6% 10% 6% 5% 4 5% 5% 6% 26%	1756
	21st to end	Day	19% 13% 4 9% 7% 5% 5% 4 5% 4 25%	1650
March	1st to 10th	Day	20% 14% 4 10% 5% 5% 4 4 4 5% 24%	4476
	11th to 20th	Day	13% 11% 4 9% 5% 5% 5% 4 6% 7% 30%	4976
	21st to end	Day	18% 11% 3 7% 6% 4 3 3 4 5% 36%	5776
April	1st to 10th	Day	22% 11% 3 7% 4 4 3 4 4 6% 32%	2400
	11th to 20th	Day	22% 9% 3 6% 4 4 4 3 4 5% 36%	2374
	21st to end	Day	22% 10% 3 9% 5% 4 4 3 5% 5% 32%	2547
May	1st to 10th	Day	23% 10% 3 8% 5% 5% 5% 4 6% 6% 26%	6346
	11th to 20th	Day	17% 9% 3 8% 6% 5% 5% 4 6% 7% 31%	6318
	21st to end	Day	21% 8% 3 7% 5% 4 4 3 5% 7% 33%	6949
June	1st to 10th	Day	20% 8% 3 7% 6% 5% 5% 4 6% 7% 30%	2800
	11th to 20th	Day	13% 6% 2 7% 5% 4 5% 5% 6% 8% 39%	2800
	21st to end	Day	19% 6% 3 7% 5% 5% 6% 4 7% 8% 31%	2800
July	1st to 10th	Day	14% 6% 3 7% 5% 5% 6% 5% 7% 9% 35%	6360
	11th to 20th	Day	12% 5% 2 6% 4 4 5% 4 6% 8% 43%	6360
	21st to end	Day	9% 5% 2 5% 4 4 5% 4 7% 10% 46%	6926
August	1st to 10th	Day	14% 6% 2 8% 4 5% 6% 5% 6% 8% 37%	4503
	11th to 20th	Day	16% 6% 2 6% 4 4 5% 4 7% 9% 38%	4122
	21st to end	Day	14% 5% 2 6% 4 5% 5% 3 6% 9% 40%	4312
September	1st to 10th	Day	13% 5% 2 6% 4 4 5% 4 5% 8% 44%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 5% 3 6% 7% 48%	2400
	21st to end	Day	18% 6% 2 6% 4 4 4 4 5% 8% 39%	2133
October	1st to 10th	Day	19% 8% 3 6% 4 4 4 4 6% 6% 37%	2000
	11th to 20th	Day	21% 9% 3 8% 4 5% 6% 4 6% 7% 29%	1980
	21st to end	Day	25% 9% 4 6% 5% 5% 4 3 4 6% 28%	1933
November	1st to 10th	Day	15% 9% 3 7% 5% 5% 5% 4 6% 6% 34%	1761
	11th to 20th	Day	19% 12% 3 7% 5% 5% 5% 5% 5% 6% 27%	1602
	21st to end	Day	22% 15% 4 8% 7% 5% 5% 4 5% 5% 21%	1600
December	1st to 10th	Day	22% 20% 5% 9% 5% 4 5% 4 5% 6% 14%	2600
	11th to 20th	Day	18% 16% 5% 11% 6% 5% 4 3 4 4 25%	2986
	21st to end	Day	23% 18% 4 11% 5% 4 4 3 3 4 20%	3960
January	1st to 10th	Night	14% 21% 8% 15% 5% 4 3 3 4 5% 19%	6349
	11th to 20th	Night	11% 15% 6% 14% 8% 7% 5% 3 4 5% 21%	6315
	21st to end	Night	13% 19% 5% 13% 6% 6% 5% 3 4 5% 20%	6830
February	1st to 10th	Night	14% 14% 5% 13% 6% 6% 5% 4 5% 5% 24%	2600
	11th to 20th	Night	14% 13% 5% 11% 7% 5% 4 3 4 6% 28%	2528
	21st to end	Night	16% 13% 4 8% 6% 5% 5% 4 5% 5% 29%	1980
March	1st to 10th	Night	23% 14% 4 10% 5% 4 4 3 3 4 28%	5364
	11th to 20th	Night	12% 12% 4 10% 5% 5% 4 4 6% 8% 29%	4979
	21st to end	Night	19% 10% 3 9% 5% 4 4 3 5% 6% 33%	5156
April	1st to 10th	Night	20% 11% 3 8% 5% 4 4 3 5% 5% 33%	2000
	11th to 20th	Night	19% 9% 3 7% 5% 4 4 4 4 5% 35%	1975
	21st to end	Night	21% 9% 3 7% 4 4 4 3 5% 6% 34%	1868
May	1st to 10th	Night	23% 9% 3 8% 5% 5% 6% 5% 6% 7% 25%	3594
	11th to 20th	Night	17% 7% 3 7% 5% 5% 4 4 5% 7% 37%	3575
	21st to end	Night	19% 8% 3 8% 5% 4 4 5% 5% 6% 31%	3926
June	1st to 10th	Night	15% 7% 2 8% 5% 5% 5% 5% 6% 8% 35%	1600
	11th to 20th	Night	13% 6% 2 7% 4 4 4 3 6% 8% 43%	1600
	21st to end	Night	18% 7% 3 6% 4 5% 5% 4 6% 8% 35%	1600
July	1st to 10th	Night	15% 6% 3 7% 6% 5% 6% 4 6% 8% 34%	3600
	11th to 20th	Night	13% 5% 2 7% 4 4 4 4 5% 7% 44%	3600
	21st to end	Night	9% 5% 2 6% 5% 4 5% 4 5% 7% 47%	3926
August	1st to 10th	Night	12% 8% 2 7% 4 5% 4 5% 5% 9% 39%	2643
	11th to 20th	Night	15% 6% 2 7% 4 4 4 4 5% 7% 43%	2893
	21st to end	Night	14% 5% 2 7% 5% 5% 5% 4 6% 7% 41%	3586
September	1st to 10th	Night	11% 5% 2 5% 4 5% 5% 4 5% 7% 47%	2000
	11th to 20th	Night	14% 4 2 5% 4 3 4 3 5% 6% 50%	2000
	21st to end	Night	17% 6% 2 6% 5% 4 3 3 4 6% 44%	2264
October	1st to 10th	Night	18% 7% 2 6% 4 4 4 4 5% 6% 41%	2400
	11th to 20th	Night	20% 8% 2 7% 6% 5% 5% 3 5% 6% 33%	2377
	21st to end	Night	23% 10% 3 8% 5% 5% 5% 4 4 6% 28%	2664
November	1st to 10th	Night	20% 10% 3 7% 5% 4 4 3 6% 6% 33%	2537
	11th to 20th	Night	19% 10% 4 9% 5% 5% 5% 4 4 5% 30%	2795
	21st to end	Night	22% 12% 4 9% 6% 5% 5% 3 5% 6% 24%	2800
December	1st to 10th	Night	20% 17% 5% 11% 6% 5% 5% 4 5% 6% 16%	4580
	11th to 20th	Night	23% 14% 5% 9% 6% 4 4 2 3 4 26%	5208
	21st to end	Night	26% 17% 4 9% 5% 4 4 3 4 4 21%	6996

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	65% 9% 8% 13% 4 1	3598
	11th to 20th	Day	63% 6% 11% 12% 8%	3600
	21st to end	Day	63% 8% 9% 11% 8% 1	4121
February	1st to 10th	Day	60% 6% 10% 17% 5% 2	1800
	11th to 20th	Day	56% 7% 12% 16% 8% 1	1765
	21st to end	Day	59% 8% 10% 13% 9% 2	1650
March	1st to 10th	Day	59% 7% 8% 12% 11% 3	4479
	11th to 20th	Day	50% 8% 17% 15% 8% 1	4976
	21st to end	Day	49% 5% 8% 19% 17% 2	5776
April	1st to 10th	Day	53% 6% 8% 19% 12% 3	2400
	11th to 20th	Day	50% 5% 7% 21% 14% 2	2374
	21st to end	Day	54% 6% 8% 14% 14% 4	2547
May	1st to 10th	Day	55% 7% 9% 18% 9% 2	6357
	11th to 20th	Day	50% 8% 13% 18% 10% 1	6322
	21st to end	Day	50% 7% 11% 19% 12% 1	6949
June	1st to 10th	Day	51% 9% 10% 18% 11% 1	2800
	11th to 20th	Day	41% 9% 12% 24% 13% 2	2800
	21st to end	Day	47% 9% 14% 19% 10% 1	2800
July	1st to 10th	Day	44% 6% 15% 26% 9%	6290
	11th to 20th	Day	37% 6% 13% 31% 13%	6285
	21st to end	Day	33% 6% 16% 35% 10%	6728
August	1st to 10th	Day	43% 8% 16% 23% 9%	4489
	11th to 20th	Day	41% 7% 16% 28% 8% 1	4161
	21st to end	Day	40% 5% 17% 26% 11% 1	4312
September	1st to 10th	Day	37% 8% 16% 28% 11%	2400
	11th to 20th	Day	34% 8% 13% 32% 12% 1	2400
	21st to end	Day	43% 8% 14% 25% 10%	2133
October	1st to 10th	Day	45% 6% 12% 28% 7%	3260
	11th to 20th	Day	46% 7% 15% 24% 8% 1	3220
	21st to end	Day	49% 6% 18% 20% 7%	3158
November	1st to 10th	Day	48% 8% 15% 22% 6% 1	1761
	11th to 20th	Day	55% 8% 13% 15% 8% 1	1602
	21st to end	Day	65% 8% 11% 10% 6% 1	1600
December	1st to 10th	Day	70% 6% 13% 9% 1	2600
	11th to 20th	Day	62% 7% 9% 15% 7%	2986
	21st to end	Day	68% 5% 8% 15% 4	3960
January	1st to 10th	Night	67% 7% 10% 12% 4	6349
	11th to 20th	Night	63% 8% 12% 12% 5%	6360
	21st to end	Night	65% 7% 7% 9% 11% 1	6830
February	1st to 10th	Night	60% 7% 10% 15% 7% 1	2600
	11th to 20th	Night	57% 6% 11% 18% 7% 1	2530
	21st to end	Night	55% 6% 10% 18% 10% 1	1980
March	1st to 10th	Night	59% 6% 7% 14% 11% 3	5374
	11th to 20th	Night	51% 8% 16% 15% 7% 3	4980
	21st to end	Night	50% 5% 9% 22% 12% 3	5156
April	1st to 10th	Night	52% 6% 10% 17% 13% 2	2000
	11th to 20th	Night	49% 7% 7% 20% 15% 1	1975
	21st to end	Night	51% 6% 8% 17% 15% 4	1868
May	1st to 10th	Night	54% 10% 9% 17% 7% 3	3600
	11th to 20th	Night	44% 8% 13% 19% 14% 1	3576
	21st to end	Night	49% 8% 12% 16% 14% 1	3927
June	1st to 10th	Night	45% 8% 14% 22% 10% 2	1600
	11th to 20th	Night	38% 8% 12% 24% 16% 2	1600
	21st to end	Night	45% 9% 12% 20% 13% 1	1600
July	1st to 10th	Night	47% 5% 16% 19% 11% 1	3560
	11th to 20th	Night	38% 6% 13% 32% 11%	3552
	21st to end	Night	35% 6% 13% 31% 15%	3809
August	1st to 10th	Night	42% 7% 15% 25% 10% 1	2633
	11th to 20th	Night	40% 6% 15% 30% 8%	2920
	21st to end	Night	41% 7% 15% 24% 12% 2	3586
September	1st to 10th	Night	36% 7% 13% 30% 14% 1	2000
	11th to 20th	Night	33% 8% 13% 34% 11% 1	2000
	21st to end	Night	42% 5% 12% 28% 12%	2264
October	1st to 10th	Night	39% 5% 14% 29% 12%	3920
	11th to 20th	Night	44% 6% 15% 25% 9%	3876
	21st to end	Night	48% 7% 17% 19% 9%	4389
November	1st to 10th	Night	51% 6% 15% 20% 8%	2537
	11th to 20th	Night	55% 7% 12% 16% 9% 1	2795
	21st to end	Night	60% 8% 12% 14% 5%	2800
December	1st to 10th	Night	68% 6% 14% 11% 2	4580
	11th to 20th	Night	63% 4 8% 14% 11% 1	5208
	21st to end	Night	68% 5% 6% 12% 9%	6996

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	23% 22% 33% 18% 4	200
	11th to 20th	Day	29% 15% 37% 17% 2 1	200
	21st to end	Day	21% 18% 38% 19% 5%	255
February	1st to 10th	Day	31% 22% 33% 10% 3	300
	11th to 20th	Day	28% 28% 36% 7% 1	303
	21st to end	Day	31% 20% 32% 15% 2	414
March	1st to 10th	Day	26% 19% 34% 17% 4	500
	11th to 20th	Day	25% 19% 36% 16% 3 1	500
	21st to end	Day	29% 22% 28% 15% 6% 1	550
April	1st to 10th	Day	30% 24% 26% 14% 5%	500
	11th to 20th	Day	33% 20% 23% 15% 8% 2	492
	21st to end	Day	30% 20% 33% 11% 6% 1	500
May	1st to 10th	Day	25% 17% 33% 17% 7% 2	503
	11th to 20th	Day	17% 19% 35% 19% 10% 2	527
	21st to end	Day	21% 16% 29% 23% 10% 1	573
June	1st to 10th	Day	24% 13% 32% 21% 8% 2	530
	11th to 20th	Day	15% 15% 31% 25% 11% 2	530
	21st to end	Day	21% 13% 32% 21% 10% 2	530
July	1st to 10th	Day	12% 7% 30% 33% 13% 3	525
	11th to 20th	Day	12% 10% 35% 27% 14% 3	524
	21st to end	Day	11% 9% 34% 30% 13% 4	576
August	1st to 10th	Day	14% 8% 27% 32% 16% 4	526
	11th to 20th	Day	14% 8% 24% 29% 17% 7%	519
	21st to end	Day	14% 11% 31% 24% 14% 6%	583
September	1st to 10th	Day	16% 10% 31% 19% 16% 7%	530
	11th to 20th	Day	10% 10% 24% 22% 21% 13%	530
	21st to end	Day	20% 12% 29% 22% 13% 5%	530
October	1st to 10th	Day	22% 14% 26% 21% 13% 4	530
	11th to 20th	Day	22% 20% 32% 17% 7% 1	522
	21st to end	Day	32% 14% 28% 21% 5%	456
November	1st to 10th	Day	24% 18% 30% 19% 7% 1	395
	11th to 20th	Day	28% 22% 33% 12% 4 1	202
	21st to end	Day	30% 23% 31% 12% 4 1	200
December	1st to 10th	Day	31% 18% 33% 15% 4	200
	11th to 20th	Day	30% 24% 34% 10% 2 1	200
	21st to end	Day	31% 19% 35% 10% 3 1	220
January	1st to 10th	Night	26% 16% 35% 20% 3	800
	11th to 20th	Night	25% 19% 29% 22% 5% 1	800
	21st to end	Night	19% 13% 38% 25% 5%	880
February	1st to 10th	Night	22% 16% 36% 24% 3	800
	11th to 20th	Night	21% 17% 39% 19% 4	783
	21st to end	Night	24% 17% 29% 26% 4	660
March	1st to 10th	Night	24% 15% 35% 23% 3	800
	11th to 20th	Night	23% 19% 33% 20% 5%	800
	21st to end	Night	24% 17% 28% 23% 7%	880
April	1st to 10th	Night	27% 14% 32% 23% 4	800
	11th to 20th	Night	24% 17% 29% 21% 8%	791
	21st to end	Night	26% 14% 30% 23% 7% 1	668
May	1st to 10th	Night	24% 10% 27% 30% 8% 2	400
	11th to 20th	Night	21% 12% 27% 31% 10%	394
	21st to end	Night	16% 8% 37% 28% 11%	433
June	1st to 10th	Night	19% 5% 31% 36% 10%	400
	11th to 20th	Night	16% 8% 28% 35% 12% 2	400
	21st to end	Night	22% 5% 29% 32% 13% 1	400
July	1st to 10th	Night	11% 4 27% 41% 16%	390
	11th to 20th	Night	11% 6% 21% 46% 15% 1	388
	21st to end	Night	8% 4 27% 45% 15% 1	438
August	1st to 10th	Night	12% 3 20% 42% 22%	429
	11th to 20th	Night	12% 5% 18% 39% 24% 2	599
	21st to end	Night	13% 6% 24% 36% 18% 2	880
September	1st to 10th	Night	13% 7% 28% 33% 19% 1	800
	11th to 20th	Night	15% 6% 20% 32% 23% 4	800
	21st to end	Night	19% 9% 25% 27% 19% 2	800
October	1st to 10th	Night	22% 9% 26% 26% 16% 1	800
	11th to 20th	Night	24% 12% 31% 25% 7% 1	793
	21st to end	Night	27% 13% 29% 26% 5%	836
November	1st to 10th	Night	26% 14% 27% 24% 8% 1	780
	11th to 20th	Night	24% 16% 34% 16% 10% 1	800
	21st to end	Night	29% 17% 27% 22% 6%	800
December	1st to 10th	Night	29% 16% 30% 21% 3	800
	11th to 20th	Night	28% 16% 30% 21% 5% 1	800
	21st to end	Night	34% 15% 30% 18% 4	880

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