Starry Knight Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Starry Knight Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	9% 9% 3 8% 5% 5% 5% 4 6% 8% 37%	131768
	Night	7% 9% 4 10% 6% 5% 5% 4 7% 8% 34%	128948
Transparency	Day	42% 7% 15% 24% 10% 1	131465
	Night	45% 7% 15% 22% 10% 1	128828
Seeing	Day	12% 7% 24% 31% 20% 6%	16940
	Night	12% 5% 27% 38% 16% 2	26174

By Month

Percentage Cloud Cover NW (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	4 11% 6% 16% 9% 6% 5% 3 5% 6% 29%	3249
	11th to 20th	Day	3 9% 4 11% 7% 6% 6% 5% 8% 9% 30%	3229
	21st to end	Day	4 9% 4 9% 6% 5% 4 3 6% 9% 40%	4020
February	1st to 10th	Day	6% 14% 4 12% 7% 7% 5% 4 7% 7% 26%	1605
	11th to 20th	Day	6% 14% 5% 12% 7% 6% 7% 5% 7% 8% 24%	1755
	21st to end	Day	4 13% 5% 11% 9% 6% 5% 5% 7% 8% 26%	1562
March	1st to 10th	Day	3 11% 5% 11% 7% 6% 6% 4 6% 10% 32%	4518
	11th to 20th	Day	5% 11% 4 10% 6% 4 4 4 6% 8% 37%	4995
	21st to end	Day	10% 19% 7% 14% 6% 4 5% 4 6% 6% 21%	5501
April	1st to 10th	Day	9% 12% 4 8% 6% 6% 5% 5% 7% 8% 31%	2489
	11th to 20th	Day	12% 12% 4 8% 5% 5% 5% 5% 7% 8% 29%	2570
	21st to end	Day	12% 11% 3 8% 7% 5% 4 4 6% 7% 32%	2600
May	1st to 10th	Day	15% 10% 3 6% 4 4 5% 4 5% 7% 38%	6286
	11th to 20th	Day	13% 11% 4 9% 5% 5% 6% 5% 7% 8% 27%	6734
	21st to end	Day	11% 6% 3 7% 6% 5% 6% 4 6% 7% 39%	7405
June	1st to 10th	Day	14% 6% 3 7% 5% 6% 5% 5% 7% 9% 34%	3000
	11th to 20th	Day	17% 8% 3 8% 6% 5% 7% 4 7% 7% 28%	3000
	21st to end	Day	8% 4 2 6% 6% 5% 6% 5% 8% 10% 41%	3000
July	1st to 10th	Day	6% 5% 2 5% 4 4 4 4 7% 9% 51%	6780
	11th to 20th	Day	11% 4 2 5% 3 5% 5% 4 8% 10% 44%	6780
	21st to end	Day	9% 4 2 6% 4 4 4 4 6% 8% 49%	7361
August	1st to 10th	Day	7% 6% 2 6% 5% 5% 6% 5% 7% 9% 41%	4541
	11th to 20th	Day	6% 5% 2 6% 5% 5% 5% 5% 8% 11% 42%	4174
	21st to end	Day	10% 4 2 4 3 4 4 3 6% 8% 51%	4664
September	1st to 10th	Day	10% 5% 2 6% 4 4 4 3 6% 8% 47%	2504
	11th to 20th	Day	15% 7% 2 5% 4 3 4 3 6% 7% 45%	2200
	21st to end	Day	8% 7% 3 6% 4 4 5% 3 6% 9% 46%	2200
October	1st to 10th	Day	7% 7% 2 8% 4 5% 5% 5% 6% 8% 43%	3407
	11th to 20th	Day	6% 7% 3 8% 5% 4 5% 4 6% 10% 42%	3043
	21st to end	Day	8% 9% 3 8% 5% 5% 5% 4 5% 6% 42%	3200
November	1st to 10th	Day	9% 16% 6% 11% 6% 4 4 4 5% 8% 26%	1668
	11th to 20th	Day	8% 12% 5% 10% 6% 4 4 4 6% 8% 33%	1611
	21st to end	Day	6% 12% 4 12% 6% 6% 5% 4 6% 8% 30%	1470
December	1st to 10th	Day	8% 13% 5% 10% 6% 7% 6% 5% 6% 8% 28%	2360
	11th to 20th	Day	6% 12% 5% 12% 9% 6% 6% 4 6% 7% 27%	2712
	21st to end	Day	3 8% 3 10% 6% 5% 6% 4 5% 8% 42%	3575
January	1st to 10th	Night	3 14% 6% 14% 8% 7% 6% 4 6% 8% 23%	6916
	11th to 20th	Night	3 10% 4 13% 7% 6% 7% 5% 6% 8% 33%	6889
	21st to end	Night	3 10% 4 11% 5% 5% 5% 4 6% 7% 39%	7223
February	1st to 10th	Night	5% 12% 5% 14% 8% 6% 5% 5% 6% 8% 27%	2794
	11th to 20th	Night	5% 13% 5% 13% 7% 6% 6% 4 8% 7% 27%	2532
	21st to end	Night	3 10% 4 10% 7% 6% 6% 5% 7% 11% 30%	2065
March	1st to 10th	Night	3 9% 4 12% 8% 6% 6% 5% 8% 10% 30%	5376
	11th to 20th	Night	5% 9% 4 11% 5% 5% 6% 6% 8% 9% 31%	5004
	21st to end	Night	7% 18% 6% 15% 6% 6% 6% 4 6% 7% 19%	5460
April	1st to 10th	Night	9% 11% 4 8% 7% 6% 5% 5% 7% 6% 32%	1910
	11th to 20th	Night	9% 10% 5% 12% 6% 6% 5% 5% 7% 8% 28%	1781
	21st to end	Night	11% 11% 3 8% 6% 5% 5% 5% 7% 7% 32%	1800
May	1st to 10th	Night	15% 10% 3 8% 5% 4 4 3 5% 7% 36%	3671
	11th to 20th	Night	11% 9% 3 9% 7% 6% 5% 5% 7% 9% 28%	3159
	21st to end	Night	12% 9% 4 10% 6% 5% 5% 5% 5% 6% 34%	3470
June	1st to 10th	Night	14% 10% 3 8% 6% 5% 6% 5% 7% 8% 30%	1400
	11th to 20th	Night	15% 8% 3 10% 7% 6% 6% 5% 7% 8% 25%	1400
	21st to end	Night	10% 6% 2 7% 7% 5% 5% 5% 6% 9% 39%	1400
July	1st to 10th	Night	6% 5% 2 7% 6% 5% 6% 4 7% 9% 44%	3180
	11th to 20th	Night	8% 5% 2 7% 5% 5% 5% 5% 7% 9% 42%	3180
	21st to end	Night	9% 5% 2 6% 4 3 4 4 6% 11% 46%	3467
August	1st to 10th	Night	9% 6% 2 7% 4 4 4 4 7% 9% 43%	2609
	11th to 20th	Night	6% 5% 2 7% 5% 5% 5% 4 8% 12% 41%	2884
	21st to end	Night	8% 4 2 6% 4 4 5% 3 6% 9% 49%	3245
September	1st to 10th	Night	10% 5% 2 5% 3 4 5% 3 6% 8% 50%	1892
	11th to 20th	Night	14% 6% 2 5% 4 4 4 4 5% 8% 45%	2200
	21st to end	Night	8% 7% 2 6% 4 4 4 3 7% 8% 47%	2200
October	1st to 10th	Night	9% 7% 2 5% 4 4 5% 4 6% 8% 46%	3787
	11th to 20th	Night	6% 9% 2 7% 5% 5% 5% 4 7% 9% 42%	4104
	21st to end	Night	6% 8% 3 9% 5% 5% 5% 5% 7% 7% 40%	4600
November	1st to 10th	Night	9% 15% 5% 8% 5% 5% 5% 3 6% 8% 31%	2829
	11th to 20th	Night	4 11% 4 10% 7% 6% 6% 4 7% 8% 32%	3008
	21st to end	Night	5% 11% 4 11% 7% 6% 5% 5% 7% 9% 31%	3150
December	1st to 10th	Night	6% 13% 5% 12% 7% 7% 6% 5% 7% 8% 26%	5040
	11th to 20th	Night	7% 12% 5% 13% 7% 7% 7% 4 7% 8% 22%	5700
	21st to end	Night	3 7% 3 9% 6% 5% 5% 4 6% 8% 42%	7623

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	55% 5% 11% 19% 9% 1	3249
	11th to 20th	Day	47% 6% 13% 14% 17% 2	3250
	21st to end	Day	40% 6% 15% 29% 10%	4020
February	1st to 10th	Day	56% 5% 13% 15% 9% 1	1605
	11th to 20th	Day	55% 7% 13% 16% 8% 1	1764
	21st to end	Day	54% 6% 11% 18% 11%	1562
March	1st to 10th	Day	46% 6% 16% 20% 11%	4518
	11th to 20th	Day	42% 8% 15% 22% 13%	4996
	21st to end	Day	62% 6% 10% 15% 6%	5501
April	1st to 10th	Day	48% 8% 17% 18% 9% 1	2489
	11th to 20th	Day	49% 7% 13% 21% 9% 1	2570
	21st to end	Day	49% 8% 13% 21% 8% 1	2600
May	1st to 10th	Day	44% 6% 10% 23% 14% 3	6286
	11th to 20th	Day	49% 9% 14% 22% 7%	6738
	21st to end	Day	40% 8% 17% 25% 9% 1	7406
June	1st to 10th	Day	44% 8% 15% 22% 10% 1	3000
	11th to 20th	Day	51% 9% 13% 20% 6% 1	3000
	21st to end	Day	35% 10% 19% 27% 8% 1	3000
July	1st to 10th	Day	27% 8% 18% 32% 14% 1	6706
	11th to 20th	Day	33% 8% 19% 30% 9%	6699
	21st to end	Day	31% 8% 17% 28% 14% 3	7154
August	1st to 10th	Day	35% 11% 16% 30% 7% 1	4524
	11th to 20th	Day	32% 9% 24% 29% 6% 1	4213
	21st to end	Day	29% 7% 21% 31% 11% 1	4663
September	1st to 10th	Day	34% 7% 16% 28% 11% 5%	2504
	11th to 20th	Day	38% 6% 14% 30% 11% 2	2200
	21st to end	Day	35% 7% 17% 27% 11% 3	2200
October	1st to 10th	Day	37% 7% 20% 27% 8% 1	3407
	11th to 20th	Day	36% 6% 18% 27% 11% 2	3043
	21st to end	Day	40% 7% 9% 33% 10% 1	3200
November	1st to 10th	Day	55% 6% 12% 17% 7% 2	1668
	11th to 20th	Day	48% 8% 13% 23% 8% 1	1611
	21st to end	Day	50% 8% 15% 20% 7% 1	1470
December	1st to 10th	Day	54% 6% 17% 17% 5% 1	2360
	11th to 20th	Day	56% 6% 16% 18% 4 1	2714
	21st to end	Day	38% 6% 12% 26% 15% 3	3575
January	1st to 10th	Night	57% 7% 12% 17% 6% 2	6916
	11th to 20th	Night	49% 6% 12% 17% 15% 3	6930
	21st to end	Night	42% 7% 16% 26% 9% 1	7223
February	1st to 10th	Night	53% 6% 11% 19% 10% 1	2794
	11th to 20th	Night	53% 7% 11% 17% 11% 1	2533
	21st to end	Night	45% 8% 14% 21% 12% 1	2065
March	1st to 10th	Night	45% 7% 19% 19% 9% 1	5376
	11th to 20th	Night	43% 8% 19% 22% 8% 1	5004
	21st to end	Night	62% 8% 10% 13% 6% 1	5460
April	1st to 10th	Night	48% 9% 13% 17% 12% 1	1910
	11th to 20th	Night	50% 7% 13% 21% 7% 1	1781
	21st to end	Night	47% 8% 12% 23% 8% 2	1800
May	1st to 10th	Night	47% 7% 8% 22% 13% 3	3668
	11th to 20th	Night	47% 9% 13% 24% 7%	3159
	21st to end	Night	47% 8% 14% 23% 7% 1	3470
June	1st to 10th	Night	49% 9% 13% 20% 9% 1	1400
	11th to 20th	Night	54% 9% 12% 17% 6% 2	1400
	21st to end	Night	39% 9% 17% 26% 8% 1	1400
July	1st to 10th	Night	33% 9% 18% 24% 15% 1	3145
	11th to 20th	Night	35% 8% 19% 31% 5% 2	3138
	21st to end	Night	32% 8% 18% 26% 13% 2	3369
August	1st to 10th	Night	34% 8% 17% 31% 9%	2601
	11th to 20th	Night	32% 10% 21% 30% 5% 1	2911
	21st to end	Night	30% 7% 21% 31% 9% 2	3245
September	1st to 10th	Night	32% 7% 15% 27% 14% 6%	1892
	11th to 20th	Night	36% 7% 15% 27% 12% 4	2200
	21st to end	Night	34% 5% 16% 28% 15% 3	2200
October	1st to 10th	Night	34% 6% 19% 28% 13% 1	3787
	11th to 20th	Night	37% 6% 16% 28% 12% 2	4104
	21st to end	Night	40% 7% 13% 30% 9% 1	4600
November	1st to 10th	Night	50% 6% 12% 21% 9% 1	2829
	11th to 20th	Night	46% 8% 15% 19% 11% 1	3008
	21st to end	Night	48% 9% 16% 19% 7% 2	3150
December	1st to 10th	Night	54% 7% 16% 17% 5% 1	5040
	11th to 20th	Night	56% 9% 16% 14% 5%	5697
	21st to end	Night	39% 6% 11% 26% 16% 3	7623

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	7% 5% 25% 33% 21% 8%	220
	11th to 20th	Day	8% 8% 30% 34% 18% 1	220
	21st to end	Day	7% 9% 26% 31% 20% 6%	242
February	1st to 10th	Day	11% 8% 25% 35% 14% 6%	203
	11th to 20th	Day	10% 6% 26% 36% 19% 3	300
	21st to end	Day	8% 5% 22% 40% 20% 5%	326
March	1st to 10th	Day	9% 6% 29% 32% 18% 6%	500
	11th to 20th	Day	8% 4 26% 37% 18% 7%	500
	21st to end	Day	13% 6% 30% 29% 16% 6%	550
April	1st to 10th	Day	11% 11% 28% 30% 14% 5%	500
	11th to 20th	Day	15% 10% 30% 27% 14% 4	492
	21st to end	Day	17% 9% 26% 30% 14% 4	500
May	1st to 10th	Day	18% 11% 21% 26% 19% 6%	603
	11th to 20th	Day	13% 8% 21% 34% 20% 4	725
	21st to end	Day	16% 6% 22% 27% 22% 7%	789
June	1st to 10th	Day	14% 9% 25% 33% 15% 4	730
	11th to 20th	Day	18% 8% 24% 30% 16% 4	730
	21st to end	Day	9% 5% 23% 31% 25% 7%	730
July	1st to 10th	Day	10% 3 18% 34% 27% 8%	721
	11th to 20th	Day	11% 4 21% 32% 24% 8%	718
	21st to end	Day	8% 3 19% 32% 27% 11%	791
August	1st to 10th	Day	10% 5% 23% 31% 20% 11%	527
	11th to 20th	Day	9% 4 20% 32% 29% 7%	517
	21st to end	Day	10% 8% 20% 28% 24% 9%	583
September	1st to 10th	Day	13% 6% 26% 28% 21% 6%	530
	11th to 20th	Day	16% 5% 22% 28% 23% 5%	530
	21st to end	Day	10% 13% 28% 28% 16% 4	530
October	1st to 10th	Day	16% 7% 21% 31% 18% 7%	415
	11th to 20th	Day	11% 8% 34% 28% 15% 5%	330
	21st to end	Day	14% 14% 31% 28% 11% 2	363
November	1st to 10th	Day	19% 7% 25% 34% 11% 3	330
	11th to 20th	Day	13% 6% 30% 33% 15% 2	293
	21st to end	Day	8% 5% 32% 29% 20% 7%	220
December	1st to 10th	Day	10% 6% 28% 31% 20% 5%	220
	11th to 20th	Day	10% 5% 36% 31% 14% 4	220
	21st to end	Day	10% 2 22% 36% 23% 7%	242
January	1st to 10th	Night	11% 6% 30% 37% 15% 2	1050
	11th to 20th	Night	11% 5% 33% 37% 13% 2	1050
	21st to end	Night	11% 8% 28% 39% 12% 2	976
February	1st to 10th	Night	10% 7% 25% 38% 18% 2	800
	11th to 20th	Night	10% 4 31% 38% 15% 1	784
	21st to end	Night	7% 4 24% 39% 22% 4	660
March	1st to 10th	Night	9% 3 27% 40% 17% 4	800
	11th to 20th	Night	8% 4 27% 42% 16% 3	800
	21st to end	Night	15% 4 27% 36% 16% 2	880
April	1st to 10th	Night	13% 6% 28% 35% 15% 2	710
	11th to 20th	Night	15% 9% 30% 35% 11% 1	593
	21st to end	Night	15% 8% 26% 37% 13% 1	600
May	1st to 10th	Night	17% 7% 24% 37% 13% 1	518
	11th to 20th	Night	13% 5% 29% 35% 17% 1	394
	21st to end	Night	17% 6% 25% 31% 19% 1	433
June	1st to 10th	Night	17% 9% 26% 34% 13% 1	400
	11th to 20th	Night	17% 3 33% 29% 18% 1	400
	21st to end	Night	13% 3 21% 40% 22% 2	400
July	1st to 10th	Night	10% 2 18% 45% 24% 1	390
	11th to 20th	Night	12% 3 18% 45% 21% 2	388
	21st to end	Night	5% 3 21% 41% 29% 2	438
August	1st to 10th	Night	11% 2 17% 42% 26% 2	398
	11th to 20th	Night	8% 3 19% 44% 24% 1	577
	21st to end	Night	9% 3 22% 42% 23%	660
September	1st to 10th	Night	13% 3 22% 42% 19% 2	656
	11th to 20th	Night	15% 4 20% 38% 22% 1	800
	21st to end	Night	11% 4 30% 38% 16% 1	800
October	1st to 10th	Night	15% 6% 25% 36% 18% 1	800
	11th to 20th	Night	12% 6% 29% 38% 13% 2	800
	21st to end	Night	10% 8% 31% 37% 13% 1	880
November	1st to 10th	Night	15% 8% 31% 36% 10% 1	984
	11th to 20th	Night	8% 7% 37% 35% 11% 2	1050
	21st to end	Night	9% 4 32% 40% 13% 2	1050
December	1st to 10th	Night	12% 7% 30% 33% 16% 2	1050
	11th to 20th	Night	10% 6% 32% 37% 13% 2	1050
	21st to end	Night	11% 4 27% 40% 15% 2	1155

Page generated at 2026-01-28 23:21:41. Version ac07

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