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 2024-08-30.

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 9% 6% 5% 5% 4 5% 6% 28%	95712
	Night	16% 11% 4 10% 6% 5% 5% 4 5% 6% 28%	93352
Transparency	Day	55% 5% 11% 19% 9% 1	95431
	Night	56% 5% 11% 19% 9% 1	93187
Seeing	Day	25% 14% 28% 20% 10% 3	14532
	Night	23% 12% 25% 29% 11% 1	23404

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% 7% 5% 4 3 5% 4 19%	2517
	11th to 20th	Day	15% 12% 5% 15% 12% 6% 6% 3 3 4 19%	2520
	21st to end	Day	13% 18% 9% 12% 7% 7% 5% 3 3 5% 19%	2772
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	13% 7% 3 9% 6% 5% 5% 5% 7% 8% 32%	2604
	11th to 20th	Day	14% 8% 3 7% 6% 6% 5% 4 7% 8% 31%	2406
	21st to end	Day	13% 7% 2 7% 5% 5% 6% 4 7% 8% 37%	2486
September	1st to 10th	Day	15% 6% 2 6% 4 5% 4 4 5% 7% 42%	2160
	11th to 20th	Day	15% 7% 3 6% 3 3 4 3 4 7% 45%	2021
	21st to end	Day	15% 7% 3 6% 4 4 5% 3 7% 9% 37%	1980
October	1st to 10th	Day	24% 10% 3 8% 3 4 3 3 5% 7% 29%	1961
	11th to 20th	Day	16% 11% 3 8% 5% 5% 6% 3 6% 6% 32%	1711
	21st to end	Day	22% 17% 4 8% 5% 5% 5% 2 3 4 24%	1782
November	1st to 10th	Day	14% 12% 4 7% 6% 4 5% 3 6% 7% 31%	1671
	11th to 20th	Day	16% 14% 5% 10% 7% 5% 5% 4 5% 5% 24%	1710
	21st to end	Day	18% 13% 4 11% 6% 5% 4 3 5% 6% 24%	1587
December	1st to 10th	Day	18% 18% 5% 12% 8% 6% 4 2 4 5% 16%	1520
	11th to 20th	Day	21% 15% 5% 9% 5% 4 4 3 4 6% 24%	1906
	21st to end	Day	22% 16% 5% 15% 7% 5% 5% 2 3 3 17%	2772
January	1st to 10th	Night	14% 15% 5% 17% 7% 5% 5% 4 5% 6% 18%	4429
	11th to 20th	Night	11% 12% 5% 17% 12% 7% 6% 4 5% 5% 15%	4440
	21st to end	Night	17% 16% 6% 15% 6% 6% 5% 3 4 4 18%	4884
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	13% 6% 2 6% 5% 5% 4 4 7% 7% 41%	1536
	11th to 20th	Night	13% 8% 2 5% 5% 4 4 4 6% 8% 41%	1710
	21st to end	Night	15% 6% 2 5% 4 5% 4 4 6% 7% 42%	2080
September	1st to 10th	Night	13% 6% 2 6% 5% 4 5% 3 5% 8% 45%	1800
	11th to 20th	Night	14% 7% 2 5% 3 4 4 3 5% 6% 47%	1939
	21st to end	Night	14% 7% 2 5% 4 5% 4 4 5% 7% 42%	1980
October	1st to 10th	Night	21% 7% 2 7% 4 5% 5% 3 6% 6% 34%	1998
	11th to 20th	Night	16% 11% 3 8% 5% 6% 4 3 6% 6% 33%	2247
	21st to end	Night	22% 13% 3 8% 5% 5% 5% 3 6% 6% 24%	2574
November	1st to 10th	Night	16% 12% 3 9% 6% 5% 4 4 5% 5% 31%	2407
	11th to 20th	Night	15% 11% 5% 10% 6% 5% 4 3 5% 7% 29%	2470
	21st to end	Night	16% 12% 5% 13% 7% 5% 4 4 5% 5% 24%	2588
December	1st to 10th	Night	19% 16% 5% 12% 8% 6% 6% 4 4 4 15%	2660
	11th to 20th	Night	20% 14% 5% 12% 7% 5% 5% 4 5% 5% 18%	3288
	21st to end	Night	22% 14% 5% 16% 6% 5% 4 3 4 4 16%	4884

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	2518
	11th to 20th	Day	69% 5% 8% 13% 5% 1	2520
	21st to end	Day	68% 3 10% 12% 6% 1	2772
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	47% 6% 16% 22% 10%	2590
	11th to 20th	Day	47% 5% 16% 23% 9%	2406
	21st to end	Day	42% 6% 15% 23% 13% 1	2486
September	1st to 10th	Day	39% 5% 11% 29% 14% 1	2160
	11th to 20th	Day	39% 4 9% 31% 15% 1	2021
	21st to end	Day	43% 5% 15% 23% 12% 2	1980
October	1st to 10th	Day	55% 3 10% 20% 11% 1	1961
	11th to 20th	Day	49% 5% 14% 24% 6% 1	1711
	21st to end	Day	65% 4 9% 13% 7% 2	1782
November	1st to 10th	Day	49% 5% 13% 20% 11% 1	1671
	11th to 20th	Day	58% 6% 11% 13% 10% 2	1710
	21st to end	Day	60% 4 16% 16% 4	1587
December	1st to 10th	Day	70% 3 10% 12% 4	1520
	11th to 20th	Day	61% 4 8% 19% 8% 1	1906
	21st to end	Day	73% 4 6% 10% 7%	2772
January	1st to 10th	Night	65% 6% 12% 14% 4	4429
	11th to 20th	Night	68% 6% 11% 10% 4 1	4440
	21st to end	Night	70% 4 8% 12% 5% 1	4884
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	39% 5% 17% 29% 9%	1526
	11th to 20th	Night	39% 7% 18% 24% 12%	1710
	21st to end	Night	40% 6% 15% 27% 12%	2080
September	1st to 10th	Night	36% 7% 15% 30% 11%	1800
	11th to 20th	Night	39% 4 12% 32% 13% 1	1939
	21st to end	Night	40% 5% 14% 26% 12% 2	1980
October	1st to 10th	Night	50% 5% 13% 21% 11% 1	1998
	11th to 20th	Night	51% 4 14% 23% 7%	2247
	21st to end	Night	58% 5% 12% 15% 9% 1	2574
November	1st to 10th	Night	52% 5% 11% 21% 9% 1	2407
	11th to 20th	Night	54% 5% 10% 20% 9% 2	2470
	21st to end	Night	60% 5% 13% 17% 5%	2588
December	1st to 10th	Night	70% 4 9% 13% 3	2660
	11th to 20th	Night	67% 5% 8% 12% 8%	3288
	21st to end	Night	72% 4 7% 14% 3	4884

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	22% 18% 30% 19% 10% 1	180
	11th to 20th	Day	32% 19% 26% 16% 5% 2	180
	21st to end	Day	22% 20% 35% 14% 7% 2	198
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%	532
September	1st to 10th	Day	16% 10% 26% 24% 17% 8%	450
	11th to 20th	Day	18% 11% 24% 19% 17% 10%	450
	21st to end	Day	18% 11% 26% 26% 12% 8%	450
October	1st to 10th	Day	28% 11% 20% 20% 13% 6%	450
	11th to 20th	Day	24% 12% 28% 22% 10% 3	405
	21st to end	Day	36% 12% 23% 22% 8%	396
November	1st to 10th	Day	27% 14% 24% 22% 13% 1	365
	11th to 20th	Day	24% 20% 25% 21% 8% 1	370
	21st to end	Day	32% 15% 24% 21% 8%	247
December	1st to 10th	Day	26% 17% 32% 17% 7% 2	180
	11th to 20th	Day	38% 16% 28% 14% 4	180
	21st to end	Day	36% 14% 26% 19% 6%	198
January	1st to 10th	Night	22% 15% 30% 27% 5%	760
	11th to 20th	Night	26% 16% 27% 27% 4	760
	21st to end	Night	18% 18% 31% 29% 5%	836
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	19% 4 16% 34% 24% 3	832
September	1st to 10th	Night	16% 5% 22% 36% 19% 2	720
	11th to 20th	Night	18% 9% 18% 30% 22% 4	720
	21st to end	Night	18% 8% 24% 29% 19% 2	720
October	1st to 10th	Night	24% 10% 19% 28% 17% 2	720
	11th to 20th	Night	23% 10% 24% 28% 13% 1	720
	21st to end	Night	29% 11% 25% 27% 7%	792
November	1st to 10th	Night	24% 12% 24% 28% 10% 1	740
	11th to 20th	Night	22% 13% 28% 26% 9% 2	760
	21st to end	Night	25% 15% 26% 26% 8%	760
December	1st to 10th	Night	28% 15% 29% 23% 4 1	760
	11th to 20th	Night	28% 16% 24% 28% 5%	760
	21st to end	Night	31% 14% 24% 26% 4	836

Page generated at 2024-08-27 09:49:37. Version ac07

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