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 2025-01-22.

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% 28%	105153
	Night	16% 12% 4 10% 6% 5% 5% 4 5% 6% 28%	104231
Transparency	Day	54% 4 11% 20% 10% 1	108652
	Night	56% 4 11% 19% 9% 1	108808
Seeing	Day	25% 14% 28% 20% 10% 3	14954
	Night	23% 12% 25% 29% 11% 1	23981

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	13% 13% 7% 19% 8% 5% 4 3 5% 4 19%	2597
	11th to 20th	Day	14% 12% 5% 14% 12% 7% 6% 3 4 4 18%	2600
	21st to end	Day	13% 18% 9% 12% 7% 7% 5% 3 3 5% 19%	2780
February	1st to 10th	Day	13% 14% 6% 12% 8% 6% 6% 3 4 5% 22%	1645
	11th to 20th	Day	11% 13% 7% 14% 9% 6% 5% 4 4 5% 21%	1848
	21st to end	Day	13% 14% 5% 13% 8% 6% 4 3 5% 5% 24%	1570
March	1st to 10th	Day	20% 18% 5% 10% 6% 4 4 3 3 4 22%	3116
	11th to 20th	Day	13% 15% 7% 12% 6% 5% 5% 4 4 5% 25%	3274
	21st to end	Day	17% 12% 5% 11% 5% 4 3 3 4 5% 30%	4128
April	1st to 10th	Day	19% 15% 3 9% 6% 5% 4 2 4 5% 28%	2280
	11th to 20th	Day	18% 11% 3 8% 5% 5% 4 4 5% 5% 32%	2263
	21st to end	Day	23% 10% 4 9% 5% 4 5% 3 5% 5% 27%	2470
May	1st to 10th	Day	24% 12% 4 8% 5% 5% 4 4 5% 5% 25%	4123
	11th to 20th	Day	17% 9% 4 9% 7% 6% 6% 5% 6% 7% 27%	4398
	21st to end	Day	13% 7% 3 6% 5% 5% 6% 5% 7% 8% 35%	4837
June	1st to 10th	Day	23% 8% 3 8% 5% 4 5% 4 6% 7% 27%	2660
	11th to 20th	Day	16% 8% 3 6% 5% 5% 5% 4 6% 7% 34%	2660
	21st to end	Day	20% 10% 3 8% 6% 6% 5% 4 6% 7% 26%	2660
July	1st to 10th	Day	12% 7% 3 7% 5% 6% 6% 5% 7% 10% 32%	4440
	11th to 20th	Day	11% 5% 2 7% 6% 6% 6% 6% 8% 11% 32%	4440
	21st to end	Day	15% 8% 3 7% 5% 5% 5% 4 6% 7% 34%	4814
August	1st to 10th	Day	15% 8% 3 9% 6% 6% 5% 5% 7% 9% 28%	4343
	11th to 20th	Day	15% 9% 3 8% 6% 6% 6% 5% 7% 9% 26%	3991
	21st to end	Day	12% 6% 3 7% 5% 5% 6% 4 7% 9% 37%	4180
September	1st to 10th	Day	14% 6% 2 6% 4 5% 4 4 5% 7% 43%	2280
	11th to 20th	Day	15% 6% 2 6% 3 4 4 3 5% 7% 45%	2133
	21st to end	Day	15% 8% 3 6% 4 4 5% 3 7% 9% 36%	2090
October	1st to 10th	Day	23% 10% 3 8% 4 4 3 3 5% 7% 30%	2069
	11th to 20th	Day	16% 11% 3 8% 5% 5% 5% 3 6% 6% 33%	1806
	21st to end	Day	21% 17% 4 8% 5% 5% 5% 2 3 4 26%	1881
November	1st to 10th	Day	14% 12% 4 8% 6% 4 5% 3 6% 8% 32%	1761
	11th to 20th	Day	15% 14% 5% 10% 7% 5% 5% 4 5% 5% 26%	1800
	21st to end	Day	18% 13% 4 11% 6% 5% 4 3 5% 6% 23%	1670
December	1st to 10th	Day	13% 18% 6% 22% 10% 7% 4 3 4 4 11%	2600
	11th to 20th	Day	17% 12% 6% 12% 7% 5% 5% 4 4 6% 23%	2986
	21st to end	Day	19% 15% 6% 14% 5% 4 5% 2 3 4 22%	3960
January	1st to 10th	Night	13% 15% 5% 17% 7% 5% 5% 4 5% 6% 18%	4569
	11th to 20th	Night	11% 12% 5% 17% 12% 7% 6% 4 5% 5% 15%	4580
	21st to end	Night	17% 16% 6% 15% 6% 6% 5% 3 4 4 18%	4899
February	1st to 10th	Night	12% 13% 6% 13% 8% 7% 6% 4 6% 5% 21%	2531
	11th to 20th	Night	10% 11% 5% 12% 8% 7% 7% 5% 6% 6% 23%	2218
	21st to end	Night	12% 11% 4 12% 7% 6% 5% 3 5% 6% 28%	1884
March	1st to 10th	Night	20% 16% 5% 7% 6% 5% 5% 3 5% 4 24%	3724
	11th to 20th	Night	14% 15% 6% 11% 6% 5% 4 3 4 5% 27%	3685
	21st to end	Night	17% 15% 5% 12% 5% 4 3 2 4 4 29%	3502
April	1st to 10th	Night	20% 12% 3 8% 6% 5% 4 4 4 6% 29%	1900
	11th to 20th	Night	17% 11% 3 7% 6% 5% 3 3 5% 7% 33%	1865
	21st to end	Night	21% 9% 3 6% 5% 5% 4 3 5% 6% 32%	1710
May	1st to 10th	Night	25% 12% 3 7% 4 4 4 3 5% 8% 26%	2834
	11th to 20th	Night	20% 10% 3 8% 5% 4 4 4 5% 6% 30%	2508
	21st to end	Night	14% 5% 2 6% 5% 4 5% 4 7% 9% 38%	2738
June	1st to 10th	Night	21% 8% 3 6% 5% 5% 5% 4 5% 7% 31%	1520
	11th to 20th	Night	18% 7% 2 6% 5% 5% 5% 3 7% 6% 36%	1520
	21st to end	Night	20% 7% 3 6% 4 5% 5% 5% 7% 7% 31%	1520
July	1st to 10th	Night	12% 7% 2 8% 6% 6% 6% 5% 7% 9% 33%	2520
	11th to 20th	Night	16% 6% 3 6% 4 4 4 5% 6% 8% 38%	2520
	21st to end	Night	12% 8% 3 7% 5% 4 6% 5% 6% 7% 37%	2739
August	1st to 10th	Night	18% 9% 3 8% 5% 4 4 4 6% 5% 34%	2556
	11th to 20th	Night	15% 8% 3 7% 5% 5% 4 4 6% 8% 36%	2832
	21st to end	Night	12% 7% 2 6% 5% 5% 5% 4 7% 7% 40%	3476
September	1st to 10th	Night	13% 6% 2 6% 5% 4 4 3 5% 8% 46%	1900
	11th to 20th	Night	14% 7% 2 5% 3 4 4 3 5% 6% 47%	2047
	21st to end	Night	14% 7% 2 5% 4 5% 4 5% 5% 7% 42%	2090
October	1st to 10th	Night	20% 7% 2 7% 5% 5% 5% 4 6% 6% 33%	2109
	11th to 20th	Night	16% 11% 3 8% 5% 5% 4 3 5% 6% 34%	2372
	21st to end	Night	21% 13% 3 8% 5% 5% 5% 3 6% 6% 26%	2717
November	1st to 10th	Night	16% 12% 3 9% 5% 5% 4 4 5% 6% 31%	2537
	11th to 20th	Night	15% 11% 5% 10% 6% 5% 4 3 5% 7% 29%	2600
	21st to end	Night	16% 13% 5% 13% 6% 5% 4 4 5% 6% 24%	2725
December	1st to 10th	Night	15% 15% 6% 19% 8% 6% 6% 4 5% 4 12%	4580
	11th to 20th	Night	20% 14% 5% 11% 7% 5% 4 4 4 5% 20%	5208
	21st to end	Night	18% 16% 5% 14% 5% 6% 5% 3 4 5% 20%	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	67% 5% 9% 16% 4	2598
	11th to 20th	Day	68% 5% 8% 12% 5% 1	2600
	21st to end	Day	68% 3 10% 12% 6% 1	2780
February	1st to 10th	Day	63% 4 8% 16% 7% 2	1645
	11th to 20th	Day	64% 5% 8% 15% 7% 1	1857
	21st to end	Day	62% 4 9% 13% 10% 2	1570
March	1st to 10th	Day	68% 2 8% 10% 10% 2	3119
	11th to 20th	Day	60% 5% 9% 12% 10% 4	3274
	21st to end	Day	58% 3 6% 14% 17% 2	4128
April	1st to 10th	Day	59% 3 7% 18% 10% 3	2280
	11th to 20th	Day	53% 3 10% 20% 14% 1	2263
	21st to end	Day	58% 4 9% 17% 10% 3	2470
May	1st to 10th	Day	60% 4 9% 19% 7% 1	4137
	11th to 20th	Day	54% 6% 14% 16% 8% 1	4402
	21st to end	Day	43% 6% 13% 26% 11% 1	4837
June	1st to 10th	Day	55% 5% 11% 20% 9%	2660
	11th to 20th	Day	46% 5% 10% 23% 14% 2	2660
	21st to end	Day	56% 5% 10% 20% 8% 1	2660
July	1st to 10th	Day	45% 4 15% 25% 11%	4370
	11th to 20th	Day	44% 5% 17% 29% 6%	4365
	21st to end	Day	48% 5% 10% 26% 11%	4661
August	1st to 10th	Day	51% 5% 15% 21% 8%	4329
	11th to 20th	Day	52% 4 16% 22% 5%	4030
	21st to end	Day	42% 4 17% 21% 14% 1	4180
September	1st to 10th	Day	38% 5% 11% 31% 14% 1	2280
	11th to 20th	Day	39% 4 10% 32% 15% 1	2133
	21st to end	Day	44% 5% 15% 23% 12% 2	2090
October	1st to 10th	Day	48% 3 11% 23% 13%	3433
	11th to 20th	Day	46% 4 12% 26% 12%	2983
	21st to end	Day	49% 3 11% 26% 9% 1	3081
November	1st to 10th	Day	49% 6% 13% 21% 11% 1	1761
	11th to 20th	Day	58% 6% 10% 13% 12% 2	1800
	21st to end	Day	60% 5% 15% 16% 4	1670
December	1st to 10th	Day	77% 2 10% 8% 2	2600
	11th to 20th	Day	60% 4 6% 19% 10%	2986
	21st to end	Day	67% 4 5% 11% 11% 2	3960
January	1st to 10th	Night	66% 6% 12% 13% 4	4569
	11th to 20th	Night	68% 6% 10% 10% 4 1	4580
	21st to end	Night	70% 4 8% 12% 6% 1	4899
February	1st to 10th	Night	63% 4 9% 14% 8% 1	2531
	11th to 20th	Night	59% 5% 8% 18% 9% 1	2220
	21st to end	Night	56% 4 10% 18% 9% 2	1884
March	1st to 10th	Night	63% 4 8% 15% 9% 2	3734
	11th to 20th	Night	59% 5% 8% 15% 12% 1	3686
	21st to end	Night	58% 4 5% 15% 16% 2	3502
April	1st to 10th	Night	55% 4 8% 19% 12% 1	1900
	11th to 20th	Night	50% 4 9% 23% 13%	1865
	21st to end	Night	51% 4 10% 18% 13% 3	1710
May	1st to 10th	Night	58% 5% 9% 18% 8% 1	2840
	11th to 20th	Night	54% 7% 12% 17% 9% 1	2509
	21st to end	Night	41% 6% 18% 21% 14% 2	2739
June	1st to 10th	Night	50% 7% 10% 22% 10%	1520
	11th to 20th	Night	45% 5% 15% 21% 11% 2	1520
	21st to end	Night	49% 7% 12% 22% 10%	1520
July	1st to 10th	Night	47% 4 17% 21% 10% 1	2480
	11th to 20th	Night	43% 4 15% 29% 9%	2472
	21st to end	Night	44% 6% 12% 27% 10%	2651
August	1st to 10th	Night	51% 4 14% 24% 7%	2546
	11th to 20th	Night	46% 6% 19% 20% 10%	2859
	21st to end	Night	41% 4 17% 26% 12%	3476
September	1st to 10th	Night	36% 7% 15% 30% 12%	1900
	11th to 20th	Night	38% 4 12% 32% 13% 1	2047
	21st to end	Night	41% 5% 14% 25% 13% 2	2090
October	1st to 10th	Night	52% 3 13% 24% 8%	3512
	11th to 20th	Night	46% 3 11% 24% 15%	3934
	21st to end	Night	48% 3 13% 24% 10% 1	4467
November	1st to 10th	Night	51% 5% 12% 22% 9% 1	2537
	11th to 20th	Night	53% 5% 9% 20% 11% 2	2600
	21st to end	Night	60% 5% 13% 17% 6%	2725
December	1st to 10th	Night	73% 3 10% 12% 2	4580
	11th to 20th	Night	66% 4 6% 13% 11%	5208
	21st to end	Night	68% 3 6% 13% 8% 2	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% 21% 30% 18% 9% 1	200
	11th to 20th	Day	30% 21% 28% 16% 5% 2	200
	21st to end	Day	22% 20% 35% 15% 7% 2	200
February	1st to 10th	Day	35% 22% 29% 9% 3 1	250
	11th to 20th	Day	29% 19% 33% 15% 4	456
	21st to end	Day	29% 18% 29% 21% 3 1	382
March	1st to 10th	Day	33% 13% 30% 19% 4	460
	11th to 20th	Day	32% 16% 31% 14% 7% 1	460
	21st to end	Day	31% 19% 25% 15% 8% 3	506
April	1st to 10th	Day	35% 14% 25% 19% 5% 2	460
	11th to 20th	Day	34% 19% 25% 13% 7% 2	452
	21st to end	Day	36% 16% 26% 11% 8% 3	460
May	1st to 10th	Day	34% 16% 24% 17% 6% 2	463
	11th to 20th	Day	23% 14% 33% 20% 8% 2	487
	21st to end	Day	19% 17% 33% 20% 10% 2	529
June	1st to 10th	Day	26% 15% 30% 16% 10% 3	490
	11th to 20th	Day	21% 14% 29% 23% 10% 3	490
	21st to end	Day	24% 14% 32% 20% 8% 2	490
July	1st to 10th	Day	18% 9% 29% 28% 12% 3	485
	11th to 20th	Day	17% 7% 34% 28% 11% 3	484
	21st to end	Day	18% 8% 35% 23% 12% 3	532
August	1st to 10th	Day	15% 9% 28% 27% 18% 3	486
	11th to 20th	Day	18% 7% 29% 25% 14% 7%	479
	21st to end	Day	17% 11% 26% 24% 16% 6%	539
September	1st to 10th	Day	16% 11% 26% 23% 17% 8%	490
	11th to 20th	Day	17% 10% 22% 20% 19% 12%	490
	21st to end	Day	18% 10% 26% 26% 13% 7%	490
October	1st to 10th	Day	27% 11% 22% 20% 13% 7%	490
	11th to 20th	Day	24% 13% 27% 22% 10% 4	440
	21st to end	Day	34% 12% 24% 22% 8%	429
November	1st to 10th	Day	27% 15% 24% 21% 12% 1	395
	11th to 20th	Day	25% 20% 25% 21% 9% 1	400
	21st to end	Day	34% 15% 24% 20% 7%	270
December	1st to 10th	Day	26% 16% 32% 19% 7% 2	200
	11th to 20th	Day	38% 16% 29% 14% 4	200
	21st to end	Day	34% 13% 26% 22% 5%	220
January	1st to 10th	Night	22% 17% 31% 26% 5%	800
	11th to 20th	Night	26% 15% 27% 27% 4 1	800
	21st to end	Night	18% 18% 31% 29% 5%	841
February	1st to 10th	Night	20% 16% 32% 28% 4	760
	11th to 20th	Night	21% 17% 34% 25% 4	743
	21st to end	Night	21% 15% 30% 29% 5%	628
March	1st to 10th	Night	26% 14% 28% 29% 3	760
	11th to 20th	Night	25% 16% 27% 24% 7%	760
	21st to end	Night	27% 14% 25% 25% 8% 1	836
April	1st to 10th	Night	29% 13% 25% 27% 5%	760
	11th to 20th	Night	24% 19% 23% 24% 10%	741
	21st to end	Night	28% 14% 27% 25% 6%	570
May	1st to 10th	Night	28% 11% 30% 24% 7%	570
	11th to 20th	Night	21% 8% 29% 33% 9% 1	387
	21st to end	Night	18% 9% 26% 35% 12%	411
June	1st to 10th	Night	24% 4 26% 36% 9%	380
	11th to 20th	Night	20% 8% 21% 34% 16%	380
	21st to end	Night	23% 3 23% 38% 12% 1	380
July	1st to 10th	Night	18% 2 16% 46% 16% 2	370
	11th to 20th	Night	18% 3 18% 39% 20% 2	368
	21st to end	Night	14% 4 18% 42% 21% 1	416
August	1st to 10th	Night	15% 3 14% 38% 30%	402
	11th to 20th	Night	18% 3 20% 35% 22% 2	586
	21st to end	Night	18% 4 17% 35% 24% 3	836
September	1st to 10th	Night	16% 5% 22% 36% 19% 2	760
	11th to 20th	Night	17% 8% 17% 29% 24% 4	760
	21st to end	Night	19% 8% 24% 28% 20% 2	760
October	1st to 10th	Night	24% 10% 19% 29% 18% 1	760
	11th to 20th	Night	23% 11% 24% 28% 13% 1	760
	21st to end	Night	29% 11% 25% 28% 8%	836
November	1st to 10th	Night	25% 12% 25% 28% 9% 1	780
	11th to 20th	Night	22% 13% 29% 26% 9% 2	800
	21st to end	Night	26% 14% 26% 25% 8%	800
December	1st to 10th	Night	28% 15% 29% 23% 5% 1	800
	11th to 20th	Night	28% 15% 24% 27% 5%	800
	21st to end	Night	31% 14% 24% 27% 5%	880

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