Northern Skies Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Northern Skies Observatory clear sky chart from 2005-11-06 to 2026-05-17.

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	16% 11% 4 10% 6% 5% 5% 4 5% 7% 27%	131726
	Night	15% 11% 4 11% 7% 6% 5% 4 5% 6% 26%	131104
Transparency	Day	55% 4 11% 19% 10% 1	135207
	Night	57% 4 10% 18% 9% 1	135697
Seeing	Day	26% 14% 28% 19% 9% 3	16652
	Night	23% 12% 25% 29% 10% 1	25656

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	10% 12% 8% 21% 11% 7% 4 3 4 4 15%	4661
	11th to 20th	Day	10% 12% 6% 16% 14% 7% 7% 4 4 4 16%	4650
	21st to end	Day	7% 12% 7% 12% 10% 10% 7% 4 4 5% 21%	5124
February	1st to 10th	Day	12% 13% 5% 13% 8% 7% 6% 3 4 5% 23%	1820
	11th to 20th	Day	10% 12% 6% 14% 10% 6% 5% 4 4 6% 21%	2048
	21st to end	Day	12% 14% 6% 13% 8% 6% 4 3 5% 5% 23%	1730
March	1st to 10th	Day	18% 16% 5% 14% 6% 5% 4 3 3 4 23%	4576
	11th to 20th	Day	13% 12% 5% 10% 5% 5% 5% 4 5% 6% 30%	4779
	21st to end	Day	21% 11% 4 11% 5% 4 4 3 4 5% 27%	6049
April	1st to 10th	Day	19% 15% 3 9% 6% 5% 4 2 4 6% 28%	2520
	11th to 20th	Day	17% 12% 4 8% 5% 5% 4 4 5% 5% 31%	2505
	21st to end	Day	22% 9% 4 8% 5% 4 5% 3 5% 6% 29%	2730
May	1st to 10th	Day	30% 14% 4 8% 5% 4 4 3 4 4 20%	6033
	11th to 20th	Day	16% 11% 4 9% 7% 6% 6% 4 5% 7% 25%	6402
	21st to end	Day	18% 9% 3 6% 5% 5% 6% 5% 7% 7% 29%	6949
June	1st to 10th	Day	23% 9% 3 8% 5% 4 5% 4 6% 7% 27%	2800
	11th to 20th	Day	16% 8% 3 6% 5% 5% 5% 4 6% 7% 34%	2800
	21st to end	Day	19% 10% 3 8% 6% 5% 5% 4 6% 7% 27%	2800
July	1st to 10th	Day	12% 8% 3 7% 5% 6% 6% 5% 8% 10% 29%	6360
	11th to 20th	Day	11% 6% 3 7% 5% 6% 6% 6% 8% 10% 33%	6360
	21st to end	Day	14% 7% 3 7% 4 5% 5% 5% 6% 8% 37%	6926
August	1st to 10th	Day	14% 8% 3 8% 6% 6% 6% 5% 7% 9% 29%	4481
	11th to 20th	Day	15% 9% 3 8% 6% 6% 6% 5% 7% 9% 27%	4119
	21st to end	Day	12% 6% 3 7% 5% 5% 6% 4 7% 9% 37%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 5% 4 4 5% 8% 44%	2400
	11th to 20th	Day	14% 6% 2 6% 3 4 4 3 5% 7% 45%	2245
	21st to end	Day	15% 8% 3 6% 4 4 5% 3 7% 9% 37%	2200
October	1st to 10th	Day	22% 10% 3 8% 3 4 3 3 5% 8% 32%	2178
	11th to 20th	Day	16% 11% 3 8% 4 5% 5% 3 6% 7% 33%	1901
	21st to end	Day	20% 17% 4 8% 5% 5% 5% 2 3 4 25%	1980
November	1st to 10th	Day	14% 13% 4 8% 6% 4 4 3 6% 7% 31%	1851
	11th to 20th	Day	14% 15% 6% 10% 7% 4 5% 4 5% 5% 25%	1890
	21st to end	Day	18% 13% 5% 12% 6% 5% 4 3 5% 6% 23%	1753
December	1st to 10th	Day	13% 18% 6% 21% 9% 7% 4 3 4 4 12%	2680
	11th to 20th	Day	17% 12% 6% 12% 7% 5% 4 4 4 6% 23%	3066
	21st to end	Day	18% 15% 6% 14% 6% 4 5% 2 3 4 22%	4048
January	1st to 10th	Night	10% 13% 7% 21% 11% 7% 5% 3 4 5% 15%	8234
	11th to 20th	Night	9% 13% 6% 19% 13% 8% 6% 4 5% 5% 12%	8235
	21st to end	Night	10% 10% 4 14% 8% 10% 9% 4 5% 5% 19%	9057
February	1st to 10th	Night	11% 13% 5% 14% 8% 7% 6% 3 6% 5% 21%	2796
	11th to 20th	Night	10% 11% 5% 13% 9% 7% 7% 4 6% 6% 22%	2458
	21st to end	Night	11% 11% 5% 12% 8% 6% 5% 3 5% 6% 28%	2076
March	1st to 10th	Night	18% 15% 5% 9% 7% 7% 5% 4 5% 4 20%	5484
	11th to 20th	Night	13% 13% 5% 10% 7% 5% 5% 3 5% 6% 28%	5400
	21st to end	Night	19% 15% 4 11% 6% 4 4 3 4 4 26%	5123
April	1st to 10th	Night	20% 12% 3 8% 6% 5% 4 4 4 6% 28%	2100
	11th to 20th	Night	18% 12% 3 7% 6% 5% 4 3 4 6% 32%	2063
	21st to end	Night	20% 9% 3 7% 5% 5% 4 3 6% 6% 34%	1890
May	1st to 10th	Night	30% 13% 3 7% 4 4 4 3 5% 7% 21%	4144
	11th to 20th	Night	17% 13% 4 8% 5% 5% 5% 4 5% 6% 29%	3637
	21st to end	Night	17% 9% 3 8% 5% 5% 5% 4 6% 8% 31%	3926
June	1st to 10th	Night	21% 8% 3 6% 5% 5% 5% 4 5% 7% 32%	1600
	11th to 20th	Night	18% 7% 2 6% 5% 5% 5% 4 7% 6% 37%	1600
	21st to end	Night	19% 7% 3 6% 4 5% 5% 5% 7% 7% 32%	1600
July	1st to 10th	Night	11% 7% 3 8% 6% 6% 6% 5% 7% 10% 33%	3600
	11th to 20th	Night	15% 8% 3 7% 4 5% 5% 5% 6% 7% 36%	3600
	21st to end	Night	10% 7% 3 6% 5% 4 5% 4 6% 8% 42%	3926
August	1st to 10th	Night	17% 9% 3 8% 5% 4 4 4 6% 5% 35%	2637
	11th to 20th	Night	15% 8% 3 7% 5% 5% 4 4 6% 8% 37%	2924
	21st to end	Night	12% 6% 2 6% 5% 5% 5% 4 7% 7% 40%	3586
September	1st to 10th	Night	13% 6% 2 6% 5% 4 4 2 5% 7% 47%	2000
	11th to 20th	Night	13% 7% 2 5% 3 4 4 3 5% 6% 48%	2155
	21st to end	Night	14% 7% 2 5% 4 5% 4 4 5% 7% 42%	2200
October	1st to 10th	Night	19% 7% 2 7% 4 5% 5% 4 6% 6% 36%	2220
	11th to 20th	Night	16% 11% 3 8% 5% 5% 4 3 5% 6% 34%	2497
	21st to end	Night	21% 13% 3 8% 5% 5% 5% 4 6% 6% 25%	2860
November	1st to 10th	Night	16% 13% 3 9% 6% 5% 4 4 5% 5% 31%	2667
	11th to 20th	Night	15% 11% 6% 11% 5% 5% 4 3 5% 7% 28%	2730
	21st to end	Night	15% 13% 5% 13% 7% 5% 4 4 5% 6% 24%	2861
December	1st to 10th	Night	15% 15% 6% 18% 8% 6% 6% 4 5% 4 13%	4720
	11th to 20th	Night	20% 14% 5% 12% 7% 6% 4 4 4 5% 20%	5348
	21st to end	Night	18% 16% 5% 14% 5% 6% 5% 3 4 5% 20%	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	73% 3 7% 13% 3	4662
	11th to 20th	Day	69% 4 7% 12% 7% 2	4680
	21st to end	Day	64% 3 7% 14% 10% 1	5124
February	1st to 10th	Day	62% 4 7% 16% 8% 2	1820
	11th to 20th	Day	64% 4 9% 15% 7% 1	2057
	21st to end	Day	63% 4 8% 13% 10% 2	1730
March	1st to 10th	Day	67% 3 7% 10% 9% 4	4579
	11th to 20th	Day	55% 4 13% 16% 9% 3	4779
	21st to end	Day	60% 3 5% 15% 16% 2	6049
April	1st to 10th	Day	58% 3 8% 18% 11% 2	2520
	11th to 20th	Day	54% 3 9% 19% 14% 1	2505
	21st to end	Day	56% 4 8% 18% 11% 2	2730
May	1st to 10th	Day	67% 4 7% 16% 5% 1	6044
	11th to 20th	Day	58% 5% 13% 16% 8% 1	6406
	21st to end	Day	52% 5% 12% 22% 9% 1	6949
June	1st to 10th	Day	55% 5% 11% 20% 9%	2800
	11th to 20th	Day	47% 5% 10% 23% 14% 2	2800
	21st to end	Day	54% 5% 10% 20% 9% 1	2800
July	1st to 10th	Day	47% 4 16% 23% 10%	6290
	11th to 20th	Day	44% 5% 15% 26% 10%	6285
	21st to end	Day	43% 4 11% 31% 10%	6728
August	1st to 10th	Day	50% 5% 15% 22% 8%	4467
	11th to 20th	Day	52% 4 16% 22% 5%	4158
	21st to end	Day	42% 4 17% 22% 14% 1	4312
September	1st to 10th	Day	38% 5% 10% 32% 14% 1	2400
	11th to 20th	Day	38% 4 9% 31% 16% 1	2245
	21st to end	Day	43% 5% 14% 23% 13% 2	2200
October	1st to 10th	Day	47% 4 11% 24% 14%	3542
	11th to 20th	Day	46% 4 12% 26% 12%	3078
	21st to end	Day	50% 3 11% 25% 9% 1	3180
November	1st to 10th	Day	51% 5% 12% 20% 11% 1	1851
	11th to 20th	Day	59% 5% 10% 13% 11% 2	1890
	21st to end	Day	61% 4 15% 15% 4	1753
December	1st to 10th	Day	76% 2 10% 8% 3	2680
	11th to 20th	Day	61% 4 6% 19% 10%	3066
	21st to end	Day	67% 4 5% 11% 11% 2	4048
January	1st to 10th	Night	72% 4 8% 12% 4	8234
	11th to 20th	Night	72% 5% 7% 8% 6% 1	8280
	21st to end	Night	64% 4 7% 14% 10% 1	9057
February	1st to 10th	Night	64% 4 8% 16% 8% 1	2796
	11th to 20th	Night	60% 4 9% 18% 9% 1	2460
	21st to end	Night	56% 4 10% 18% 10% 2	2076
March	1st to 10th	Night	65% 4 7% 12% 10% 2	5494
	11th to 20th	Night	57% 4 14% 14% 10% 1	5401
	21st to end	Night	60% 4 4 16% 16% 1	5123
April	1st to 10th	Night	56% 4 8% 18% 13% 1	2100
	11th to 20th	Night	52% 4 8% 22% 13%	2063
	21st to end	Night	50% 4 10% 19% 14% 3	1890
May	1st to 10th	Night	65% 4 9% 15% 7% 1	4150
	11th to 20th	Night	55% 6% 11% 19% 7% 1	3638
	21st to end	Night	50% 4 15% 20% 10% 1	3927
June	1st to 10th	Night	50% 7% 10% 22% 11%	1600
	11th to 20th	Night	45% 5% 14% 22% 11% 2	1600
	21st to end	Night	49% 6% 12% 22% 10%	1600
July	1st to 10th	Night	46% 4 19% 22% 8%	3560
	11th to 20th	Night	47% 4 13% 26% 11%	3552
	21st to end	Night	39% 5% 15% 33% 9%	3809
August	1st to 10th	Night	50% 4 14% 24% 7%	2627
	11th to 20th	Night	45% 5% 18% 20% 10%	2951
	21st to end	Night	41% 4 17% 26% 12%	3586
September	1st to 10th	Night	36% 6% 14% 31% 12%	2000
	11th to 20th	Night	37% 4 12% 32% 14% 1	2155
	21st to end	Night	41% 5% 14% 25% 13% 2	2200
October	1st to 10th	Night	51% 3 12% 24% 9% 1	3623
	11th to 20th	Night	46% 3 11% 24% 15%	4059
	21st to end	Night	49% 3 13% 24% 10% 1	4610
November	1st to 10th	Night	52% 5% 11% 22% 9% 1	2667
	11th to 20th	Night	55% 4 9% 19% 10% 2	2730
	21st to end	Night	60% 4 13% 17% 6%	2861
December	1st to 10th	Night	73% 3 9% 13% 2	4720
	11th to 20th	Night	66% 4 7% 13% 11%	5348
	21st to end	Night	69% 3 6% 13% 8% 2	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	23% 20% 30% 19% 9%	220
	11th to 20th	Day	30% 23% 28% 14% 5% 1	220
	21st to end	Day	23% 21% 34% 14% 6% 2	242
February	1st to 10th	Day	33% 23% 30% 10% 3 1	305
	11th to 20th	Day	29% 18% 34% 15% 3	536
	21st to end	Day	28% 19% 31% 20% 2	446
March	1st to 10th	Day	32% 17% 30% 18% 4	540
	11th to 20th	Day	33% 16% 32% 13% 6% 1	540
	21st to end	Day	34% 20% 24% 14% 7% 2	594
April	1st to 10th	Day	36% 16% 25% 17% 5% 2	540
	11th to 20th	Day	35% 19% 25% 12% 7% 2	532
	21st to end	Day	35% 18% 26% 11% 7% 3	540
May	1st to 10th	Day	38% 15% 24% 16% 5% 2	543
	11th to 20th	Day	25% 14% 32% 20% 7% 2	551
	21st to end	Day	20% 17% 32% 20% 10% 1	573
June	1st to 10th	Day	27% 14% 31% 16% 10% 3	530
	11th to 20th	Day	21% 15% 30% 22% 9% 3	530
	21st to end	Day	23% 14% 33% 19% 8% 2	530
July	1st to 10th	Day	18% 10% 30% 28% 11% 3	525
	11th to 20th	Day	17% 7% 33% 28% 11% 3	524
	21st to end	Day	18% 9% 35% 23% 12% 3	576
August	1st to 10th	Day	14% 9% 27% 28% 18% 4	526
	11th to 20th	Day	17% 7% 29% 24% 15% 8%	519
	21st to end	Day	16% 11% 27% 24% 16% 6%	583
September	1st to 10th	Day	15% 11% 27% 22% 17% 8%	530
	11th to 20th	Day	16% 10% 22% 20% 20% 12%	530
	21st to end	Day	19% 11% 25% 26% 13% 7%	530
October	1st to 10th	Day	25% 12% 22% 20% 14% 7%	530
	11th to 20th	Day	25% 12% 27% 22% 10% 4	475
	21st to end	Day	35% 11% 24% 22% 7%	462
November	1st to 10th	Day	28% 16% 24% 20% 11%	425
	11th to 20th	Day	25% 20% 25% 20% 8% 1	430
	21st to end	Day	33% 16% 25% 19% 6%	293
December	1st to 10th	Day	27% 15% 32% 18% 6% 1	220
	11th to 20th	Day	37% 17% 30% 13% 3	220
	21st to end	Day	34% 14% 26% 21% 5%	242
January	1st to 10th	Night	22% 16% 31% 26% 5%	840
	11th to 20th	Night	26% 16% 27% 26% 4	840
	21st to end	Night	17% 18% 31% 28% 5%	924
February	1st to 10th	Night	20% 16% 32% 28% 5%	840
	11th to 20th	Night	21% 16% 34% 24% 4	823
	21st to end	Night	21% 14% 31% 29% 5%	692
March	1st to 10th	Night	26% 15% 29% 28% 3	840
	11th to 20th	Night	26% 16% 27% 24% 7%	840
	21st to end	Night	28% 14% 25% 24% 8% 1	924
April	1st to 10th	Night	30% 15% 25% 25% 5%	840
	11th to 20th	Night	25% 19% 24% 23% 9%	819
	21st to end	Night	27% 15% 28% 24% 6%	630
May	1st to 10th	Night	28% 10% 30% 24% 7%	630
	11th to 20th	Night	21% 8% 29% 33% 9%	420
	21st to end	Night	19% 9% 25% 35% 12%	433
June	1st to 10th	Night	25% 4 26% 37% 9%	400
	11th to 20th	Night	20% 9% 23% 34% 15%	400
	21st to end	Night	22% 4 24% 38% 11% 1	400
July	1st to 10th	Night	17% 2 17% 46% 16% 2	390
	11th to 20th	Night	19% 3 18% 38% 20% 2	388
	21st to end	Night	13% 4 19% 42% 20% 1	438
August	1st to 10th	Night	15% 3 14% 39% 30%	423
	11th to 20th	Night	18% 3 19% 34% 23% 2	618
	21st to end	Night	18% 4 17% 35% 23% 3	880
September	1st to 10th	Night	16% 6% 22% 36% 19% 2	800
	11th to 20th	Night	16% 8% 17% 31% 24% 4	800
	21st to end	Night	19% 7% 25% 29% 20% 2	800
October	1st to 10th	Night	23% 9% 19% 29% 18% 2	800
	11th to 20th	Night	23% 11% 24% 29% 13% 1	800
	21st to end	Night	29% 11% 25% 28% 8%	880
November	1st to 10th	Night	25% 13% 25% 28% 9% 1	820
	11th to 20th	Night	23% 13% 28% 26% 8% 2	840
	21st to end	Night	25% 14% 26% 26% 8%	840
December	1st to 10th	Night	27% 16% 29% 23% 5% 1	840
	11th to 20th	Night	28% 16% 25% 27% 5%	840
	21st to end	Night	31% 15% 23% 26% 5%	924

Page generated at 2026-05-14 01:58:59. Version ac07

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