Digital-SF Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Digital-SF Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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	5% 7% 3 6% 4 4 4 4 6% 7% 49%	130442
	Night	5% 8% 3 7% 4 4 5% 4 6% 8% 46%	127329
Transparency	Day	29% 10% 14% 25% 17% 6%	130167
	Night	32% 11% 16% 24% 14% 4	127181
Seeing	Day	13% 22% 31% 23% 9% 2	15477
	Night	12% 18% 27% 28% 13% 2	24394

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	9% 13% 6% 11% 6% 6% 5% 5% 7% 9% 22%	3639
	11th to 20th	Day	7% 12% 3 6% 4 4 5% 4 7% 8% 40%	3616
	21st to end	Day	13% 13% 4 8% 4 4 5% 4 6% 6% 34%	4004
February	1st to 10th	Day	8% 11% 5% 11% 6% 6% 5% 4 7% 8% 29%	1790
	11th to 20th	Day	7% 12% 4 9% 6% 4 6% 4 7% 8% 34%	1755
	21st to end	Day	6% 12% 5% 10% 7% 4 5% 6% 8% 9% 29%	1627
March	1st to 10th	Day	3 8% 3 8% 7% 7% 7% 6% 8% 9% 35%	4518
	11th to 20th	Day	5% 11% 3 9% 6% 6% 4 4 6% 6% 40%	4929
	21st to end	Day	6% 10% 4 11% 8% 7% 7% 5% 7% 8% 26%	5793
April	1st to 10th	Day	7% 11% 3 8% 7% 6% 7% 5% 8% 9% 29%	2400
	11th to 20th	Day	6% 10% 3 8% 5% 5% 5% 4 7% 8% 38%	2372
	21st to end	Day	5% 7% 3 7% 5% 6% 6% 5% 8% 8% 42%	2627
May	1st to 10th	Day	5% 5% 2 5% 3 4 4 4 5% 7% 56%	6320
	11th to 20th	Day	4 5% 2 6% 4 4 5% 4 6% 9% 51%	6277
	21st to end	Day	5% 6% 2 6% 5% 5% 5% 4 6% 9% 47%	6903
June	1st to 10th	Day	7% 6% 2 6% 5% 5% 5% 4 7% 8% 44%	2800
	11th to 20th	Day	5% 3 2 5% 4 5% 6% 5% 7% 9% 50%	2800
	21st to end	Day	4 3 1 4 4 4 3 3 6% 8% 60%	2800
July	1st to 10th	Day	1 2 1 2 1 2 2 2 3 4 81%	6320
	11th to 20th	Day	1 2 1 2 2 1 2 2 3 3 81%	6320
	21st to end	Day	1 2 1 2 2 2 2 2 3 6% 76%	6874
August	1st to 10th	Day	1 2 1 3 2 3 3 2 4 6% 74%	4541
	11th to 20th	Day	2 2 1 3 3 3 4 3 6% 8% 66%	4194
	21st to end	Day	5% 5% 1 5% 4 3 3 3 4 6% 61%	4349
September	1st to 10th	Day	3 2 1 4 3 3 3 3 4 7% 67%	2400
	11th to 20th	Day	3 4 2 5% 4 4 5% 5% 6% 7% 55%	2377
	21st to end	Day	7% 5% 2 5% 4 4 4 4 6% 6% 54%	2039
October	1st to 10th	Day	4 4 2 4 3 3 4 4 6% 9% 57%	3180
	11th to 20th	Day	6% 5% 2 6% 5% 5% 5% 5% 7% 9% 44%	3161
	21st to end	Day	7% 7% 3 10% 5% 5% 5% 5% 7% 7% 39%	3163
November	1st to 10th	Day	9% 11% 4 10% 6% 5% 6% 5% 8% 7% 30%	1705
	11th to 20th	Day	8% 13% 5% 9% 6% 6% 5% 5% 7% 8% 29%	1600
	21st to end	Day	8% 13% 4 9% 6% 5% 5% 4 7% 9% 30%	1600
December	1st to 10th	Day	6% 9% 3 7% 4 4 5% 4 8% 10% 39%	2620
	11th to 20th	Day	7% 15% 7% 14% 9% 7% 6% 4 6% 5% 19%	3025
	21st to end	Day	10% 15% 6% 13% 6% 5% 5% 4 6% 7% 21%	4004
January	1st to 10th	Night	8% 15% 6% 12% 7% 6% 6% 4 7% 7% 21%	6306
	11th to 20th	Night	8% 9% 3 6% 4 4 5% 4 6% 10% 42%	6282
	21st to end	Night	10% 14% 4 9% 5% 5% 6% 4 5% 6% 33%	6952
February	1st to 10th	Night	7% 12% 4 8% 6% 5% 6% 4 8% 10% 31%	2605
	11th to 20th	Night	6% 11% 3 8% 5% 5% 4 3 7% 8% 40%	2532
	21st to end	Night	7% 12% 4 8% 6% 4 6% 4 7% 6% 36%	2002
March	1st to 10th	Night	3 6% 3 6% 5% 6% 7% 6% 9% 11% 39%	5376
	11th to 20th	Night	3 7% 3 8% 6% 5% 5% 5% 6% 7% 44%	5053
	21st to end	Night	6% 8% 3 8% 6% 6% 7% 5% 8% 9% 33%	5182
April	1st to 10th	Night	7% 6% 2 7% 6% 6% 6% 6% 8% 10% 37%	2000
	11th to 20th	Night	4 6% 3 7% 4 5% 5% 4 7% 9% 47%	1979
	21st to end	Night	4 6% 2 6% 5% 4 6% 4 8% 12% 42%	1770
May	1st to 10th	Night	4 4 1 5% 4 4 4 3 6% 7% 58%	3640
	11th to 20th	Night	3 5% 1 5% 4 4 5% 4 6% 7% 56%	3616
	21st to end	Night	4 4 2 4 3 4 6% 5% 8% 10% 48%	3972
June	1st to 10th	Night	6% 5% 2 5% 4 5% 5% 3 9% 6% 51%	1600
	11th to 20th	Night	3 5% 2 4 5% 4 5% 5% 8% 8% 52%	1600
	21st to end	Night	1 3 1 4 3 4 5% 4 6% 8% 61%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 2 3 5% 82%	3640
	11th to 20th	Night	1 1 1 1 1 2 3 5% 84%	3640
	21st to end	Night	1 1 1 2 1 1 2 2 4 5% 80%	3969
August	1st to 10th	Night	1 1 1 3 1 2 2 2 4 6% 76%	2609
	11th to 20th	Night	1 1 1 3 2 2 2 3 5% 7% 73%	2861
	21st to end	Night	3 3 1 4 3 3 3 2 4 8% 65%	3570
September	1st to 10th	Night	3 2 1 2 2 3 3 2 5% 7% 70%	2000
	11th to 20th	Night	3 3 2 5% 3 4 4 3 6% 9% 58%	1980
	21st to end	Night	4 4 1 4 3 4 5% 3 6% 8% 57%	2137
October	1st to 10th	Night	3 3 2 3 4 3 4 3 7% 10% 59%	3800
	11th to 20th	Night	4 4 2 5% 4 4 5% 6% 8% 9% 49%	3777
	21st to end	Night	7% 6% 3 7% 5% 5% 6% 5% 7% 9% 40%	4397
November	1st to 10th	Night	11% 10% 4 7% 5% 4 4 5% 7% 9% 34%	2590
	11th to 20th	Night	9% 10% 3 8% 6% 6% 6% 4 6% 8% 35%	2800
	21st to end	Night	8% 12% 4 9% 6% 5% 5% 5% 7% 7% 33%	2800
December	1st to 10th	Night	5% 8% 3 6% 4 4 5% 5% 7% 13% 40%	4560
	11th to 20th	Night	8% 16% 5% 14% 9% 7% 7% 5% 6% 6% 17%	5180
	21st to end	Night	11% 16% 7% 11% 5% 5% 5% 4 6% 7% 23%	6952

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	52% 14% 15% 15% 3	3639
	11th to 20th	Day	37% 10% 17% 27% 8%	3640
	21st to end	Day	46% 10% 11% 21% 12%	4004
February	1st to 10th	Day	49% 14% 17% 15% 4 1	1790
	11th to 20th	Day	43% 16% 16% 16% 8% 1	1764
	21st to end	Day	42% 19% 16% 15% 7% 1	1627
March	1st to 10th	Day	33% 14% 15% 18% 18% 1	4518
	11th to 20th	Day	37% 9% 11% 34% 9%	4929
	21st to end	Day	43% 17% 19% 17% 4	5793
April	1st to 10th	Day	44% 14% 19% 17% 6%	2400
	11th to 20th	Day	37% 12% 17% 21% 11% 2	2372
	21st to end	Day	32% 15% 17% 24% 11% 1	2627
May	1st to 10th	Day	22% 9% 12% 29% 25% 4	6320
	11th to 20th	Day	23% 8% 20% 32% 16%	6280
	21st to end	Day	27% 13% 21% 30% 10%	6904
June	1st to 10th	Day	32% 11% 15% 24% 14% 3	2800
	11th to 20th	Day	23% 10% 20% 29% 16% 2	2800
	21st to end	Day	19% 7% 18% 31% 20% 4	2800
July	1st to 10th	Day	9% 4 8% 23% 28% 29%	6251
	11th to 20th	Day	9% 3 4 18% 34% 33%	6245
	21st to end	Day	11% 3 6% 32% 35% 14%	6681
August	1st to 10th	Day	11% 4 7% 21% 37% 20%	4524
	11th to 20th	Day	12% 5% 12% 43% 23% 5%	4234
	21st to end	Day	22% 6% 12% 25% 30% 4	4348
September	1st to 10th	Day	16% 7% 11% 30% 27% 9%	2400
	11th to 20th	Day	24% 11% 15% 24% 21% 5%	2377
	21st to end	Day	26% 10% 18% 26% 17% 4	2039
October	1st to 10th	Day	20% 8% 13% 39% 19% 1	3180
	11th to 20th	Day	28% 14% 17% 25% 16%	3161
	21st to end	Day	33% 10% 14% 26% 11% 5%	3163
November	1st to 10th	Day	46% 14% 14% 17% 7% 1	1705
	11th to 20th	Day	50% 15% 16% 13% 6% 1	1600
	21st to end	Day	46% 15% 16% 16% 6% 1	1600
December	1st to 10th	Day	38% 11% 19% 24% 7%	2620
	11th to 20th	Day	58% 14% 14% 10% 4	3028
	21st to end	Day	56% 12% 12% 14% 5%	4004
January	1st to 10th	Night	57% 14% 15% 10% 3	6306
	11th to 20th	Night	35% 9% 17% 29% 8%	6320
	21st to end	Night	46% 12% 13% 20% 9% 1	6952
February	1st to 10th	Night	46% 15% 18% 16% 5% 1	2605
	11th to 20th	Night	40% 14% 18% 20% 7% 2	2534
	21st to end	Night	43% 16% 15% 18% 8%	2002
March	1st to 10th	Night	28% 13% 17% 24% 15% 2	5376
	11th to 20th	Night	31% 10% 19% 31% 10%	5054
	21st to end	Night	38% 16% 21% 20% 4	5182
April	1st to 10th	Night	33% 17% 20% 21% 8%	2000
	11th to 20th	Night	29% 13% 19% 23% 14% 1	1979
	21st to end	Night	29% 14% 20% 25% 11% 1	1770
May	1st to 10th	Night	20% 7% 11% 33% 26% 3	3637
	11th to 20th	Night	21% 8% 18% 36% 16% 1	3616
	21st to end	Night	20% 15% 27% 31% 7% 1	3972
June	1st to 10th	Night	28% 12% 18% 26% 14% 2	1600
	11th to 20th	Night	23% 10% 23% 27% 16% 1	1600
	21st to end	Night	17% 7% 18% 36% 18% 4	1600
July	1st to 10th	Night	7% 3 9% 28% 25% 28%	3600
	11th to 20th	Night	5% 2 5% 23% 33% 32%	3592
	21st to end	Night	7% 2 9% 33% 32% 16%	3857
August	1st to 10th	Night	10% 4 7% 20% 41% 18%	2601
	11th to 20th	Night	11% 5% 20% 38% 20% 6%	2887
	21st to end	Night	16% 6% 17% 27% 29% 5%	3570
September	1st to 10th	Night	14% 7% 13% 29% 30% 8%	2000
	11th to 20th	Night	20% 10% 15% 27% 21% 7%	1980
	21st to end	Night	21% 10% 18% 29% 17% 5%	2137
October	1st to 10th	Night	17% 6% 13% 46% 17% 1	3800
	11th to 20th	Night	23% 12% 19% 30% 15% 1	3777
	21st to end	Night	32% 9% 17% 27% 14% 1	4397
November	1st to 10th	Night	42% 14% 16% 18% 9% 1	2590
	11th to 20th	Night	43% 15% 16% 18% 7% 1	2800
	21st to end	Night	44% 13% 18% 16% 8% 1	2800
December	1st to 10th	Night	35% 13% 18% 26% 8%	4560
	11th to 20th	Night	60% 13% 14% 9% 4	5176
	21st to end	Night	55% 12% 14% 14% 4	6952

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	21% 18% 24% 25% 12% 2	200
	11th to 20th	Day	22% 15% 25% 24% 12% 3	200
	21st to end	Day	17% 16% 28% 24% 13% 2	220
February	1st to 10th	Day	19% 32% 31% 14% 4 1	390
	11th to 20th	Day	21% 29% 28% 17% 4	395
	21st to end	Day	17% 26% 33% 19% 4 1	403
March	1st to 10th	Day	17% 29% 28% 19% 6% 1	500
	11th to 20th	Day	16% 28% 25% 22% 7% 1	500
	21st to end	Day	20% 31% 29% 16% 5%	550
April	1st to 10th	Day	20% 28% 28% 16% 7% 1	500
	11th to 20th	Day	14% 33% 29% 15% 7% 2	492
	21st to end	Day	15% 28% 30% 19% 7% 1	500
May	1st to 10th	Day	14% 21% 36% 23% 5% 1	503
	11th to 20th	Day	16% 21% 30% 22% 8% 2	528
	21st to end	Day	13% 20% 30% 25% 10% 2	573
June	1st to 10th	Day	14% 24% 31% 22% 7% 1	530
	11th to 20th	Day	13% 28% 38% 15% 3 2	530
	21st to end	Day	7% 21% 33% 26% 12% 1	530
July	1st to 10th	Day	3 19% 41% 30% 7%	526
	11th to 20th	Day	5% 19% 39% 28% 8% 1	524
	21st to end	Day	4 15% 36% 31% 12% 2	575
August	1st to 10th	Day	5% 15% 33% 31% 15% 2	527
	11th to 20th	Day	4 11% 33% 34% 15% 3	517
	21st to end	Day	8% 14% 32% 30% 12% 3	583
September	1st to 10th	Day	6% 14% 35% 27% 14% 4	530
	11th to 20th	Day	8% 18% 32% 28% 12% 2	523
	21st to end	Day	13% 21% 28% 25% 11% 1	490
October	1st to 10th	Day	14% 20% 26% 24% 14% 2	490
	11th to 20th	Day	14% 21% 26% 23% 13% 3	490
	21st to end	Day	18% 24% 26% 22% 8% 3	368
November	1st to 10th	Day	21% 25% 24% 22% 7%	270
	11th to 20th	Day	20% 27% 23% 17% 11% 4	200
	21st to end	Day	17% 19% 23% 26% 12% 4	200
December	1st to 10th	Day	20% 20% 22% 18% 11% 10%	200
	11th to 20th	Day	17% 23% 26% 24% 9% 2	200
	21st to end	Day	21% 17% 26% 24% 10% 1	220
January	1st to 10th	Night	20% 19% 22% 26% 12% 1	800
	11th to 20th	Night	16% 19% 22% 28% 14% 2	800
	21st to end	Night	17% 13% 24% 30% 13% 3	880
February	1st to 10th	Night	15% 21% 25% 27% 11% 1	800
	11th to 20th	Night	12% 22% 25% 29% 11% 1	784
	21st to end	Night	17% 22% 23% 26% 11% 2	660
March	1st to 10th	Night	13% 19% 27% 30% 11% 1	800
	11th to 20th	Night	11% 18% 28% 29% 13% 1	800
	21st to end	Night	12% 24% 27% 26% 9% 1	880
April	1st to 10th	Night	11% 20% 31% 27% 9% 1	800
	11th to 20th	Night	9% 25% 31% 24% 9% 2	791
	21st to end	Night	9% 25% 31% 24% 10% 1	570
May	1st to 10th	Night	10% 24% 28% 27% 11% 2	400
	11th to 20th	Night	9% 13% 32% 30% 14% 2	394
	21st to end	Night	9% 14% 30% 35% 11% 1	433
June	1st to 10th	Night	10% 15% 38% 25% 12%	400
	11th to 20th	Night	9% 26% 38% 21% 7%	400
	21st to end	Night	4 20% 32% 30% 13% 1	400
July	1st to 10th	Night	3 13% 37% 38% 8%	390
	11th to 20th	Night	2 15% 44% 29% 11% 1	388
	21st to end	Night	2 13% 36% 34% 14% 1	438
August	1st to 10th	Night	2 12% 36% 33% 16% 1	398
	11th to 20th	Night	3 8% 28% 35% 20% 6%	565
	21st to end	Night	4 14% 30% 31% 18% 3	855
September	1st to 10th	Night	4 12% 28% 33% 20% 3	800
	11th to 20th	Night	6% 14% 24% 26% 24% 6%	792
	21st to end	Night	7% 15% 24% 31% 18% 5%	760
October	1st to 10th	Night	10% 13% 24% 31% 21% 3	760
	11th to 20th	Night	11% 16% 21% 30% 19% 4	760
	21st to end	Night	15% 19% 25% 26% 12% 3	836
November	1st to 10th	Night	16% 17% 26% 31% 10%	780
	11th to 20th	Night	16% 20% 27% 23% 12% 2	800
	21st to end	Night	15% 17% 23% 28% 13% 4	800
December	1st to 10th	Night	16% 16% 26% 25% 15% 3	800
	11th to 20th	Night	19% 15% 28% 30% 8%	800
	21st to end	Night	20% 21% 24% 29% 6%	880

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