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 2024-11-05.

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	6% 7% 3 6% 4 4 4 4 6% 7% 48%	105350
	Night	6% 8% 3 6% 4 4 5% 4 6% 8% 45%	102306
Transparency	Day	30% 11% 14% 22% 16% 7%	105117
	Night	32% 12% 16% 23% 13% 4	102185
Seeing	Day	13% 22% 30% 23% 9% 2	14398
	Night	12% 18% 27% 28% 13% 2	23353

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	11% 14% 7% 10% 6% 5% 5% 4 7% 8% 22%	2539
	11th to 20th	Day	10% 17% 5% 8% 5% 5% 5% 4 7% 8% 27%	2516
	21st to end	Day	18% 18% 6% 10% 5% 5% 5% 4 6% 6% 17%	2794
February	1st to 10th	Day	8% 11% 4 10% 6% 6% 5% 4 8% 8% 30%	1700
	11th to 20th	Day	7% 12% 4 8% 6% 4 6% 4 7% 9% 34%	1665
	21st to end	Day	6% 12% 5% 11% 7% 4 5% 5% 8% 9% 29%	1548
March	1st to 10th	Day	4 10% 3 9% 6% 6% 6% 5% 7% 9% 33%	3138
	11th to 20th	Day	5% 9% 3 7% 5% 4 3 3 5% 6% 49%	3445
	21st to end	Day	8% 11% 4 10% 7% 6% 7% 5% 6% 7% 29%	4041
April	1st to 10th	Day	7% 11% 3 8% 7% 6% 7% 5% 8% 9% 29%	2280
	11th to 20th	Day	6% 10% 4 8% 6% 6% 5% 4 7% 8% 36%	2252
	21st to end	Day	5% 8% 3 8% 5% 6% 6% 5% 8% 8% 40%	2496
May	1st to 10th	Day	6% 6% 2 5% 4 5% 4 4 5% 8% 52%	4420
	11th to 20th	Day	5% 4 2 4 3 3 3 3 6% 8% 59%	4377
	21st to end	Day	6% 8% 3 8% 6% 5% 6% 4 6% 9% 39%	4813
June	1st to 10th	Day	8% 6% 2 7% 5% 6% 5% 4 7% 8% 42%	2660
	11th to 20th	Day	5% 3 2 5% 4 5% 6% 5% 8% 9% 49%	2660
	21st to end	Day	4 3 1 4 4 4 3 4 6% 8% 59%	2660
July	1st to 10th	Day	1 1 1 2 2 2 2 2 3 4 80%	4420
	11th to 20th	Day	2 2 1 3 2 2 2 2 3 4 79%	4420
	21st to end	Day	1 2 1 2 2 2 2 2 3 4 79%	4790
August	1st to 10th	Day	1 2 1 3 2 2 3 2 4 6% 74%	4401
	11th to 20th	Day	2 2 1 3 3 3 4 3 6% 8% 66%	4063
	21st to end	Day	5% 5% 1 5% 4 3 3 3 4 6% 62%	4216
September	1st to 10th	Day	3 2 1 3 3 3 3 3 4 7% 68%	2280
	11th to 20th	Day	3 4 2 5% 4 4 5% 5% 6% 8% 54%	2280
	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% 4 6% 5% 8% 7% 30%	1654
	11th to 20th	Day	8% 13% 4 9% 6% 6% 5% 5% 7% 8% 29%	1520
	21st to end	Day	8% 14% 4 9% 6% 5% 5% 4 7% 8% 31%	1520
December	1st to 10th	Day	9% 14% 4 10% 4 5% 5% 4 6% 8% 32%	1520
	11th to 20th	Day	9% 14% 6% 13% 8% 7% 5% 4 6% 7% 21%	1925
	21st to end	Day	10% 15% 6% 15% 6% 5% 5% 4 6% 6% 22%	2794
January	1st to 10th	Night	10% 17% 6% 10% 6% 6% 5% 5% 7% 7% 23%	4406
	11th to 20th	Night	11% 13% 4 8% 5% 5% 5% 5% 6% 10% 30%	4382
	21st to end	Night	13% 19% 6% 11% 6% 5% 6% 5% 6% 6% 17%	4862
February	1st to 10th	Night	7% 13% 3 8% 5% 5% 5% 4 7% 11% 31%	2475
	11th to 20th	Night	6% 11% 3 7% 5% 5% 4 4 7% 8% 40%	2402
	21st to end	Night	7% 13% 4 8% 6% 4 6% 4 7% 6% 35%	1905
March	1st to 10th	Night	5% 8% 3 7% 5% 7% 7% 6% 9% 10% 35%	3736
	11th to 20th	Night	3 6% 2 6% 5% 4 4 4 6% 7% 55%	3530
	21st to end	Night	6% 9% 3 7% 6% 6% 7% 6% 9% 9% 32%	3612
April	1st to 10th	Night	7% 6% 2 8% 6% 6% 6% 6% 8% 10% 36%	1900
	11th to 20th	Night	4 6% 3 7% 4 5% 5% 4 6% 9% 46%	1879
	21st to end	Night	5% 7% 2 6% 5% 4 6% 4 8% 13% 40%	1681
May	1st to 10th	Night	5% 5% 2 6% 5% 4 4 3 6% 7% 53%	2540
	11th to 20th	Night	4 4 1 3 3 3 4 3 5% 7% 63%	2516
	21st to end	Night	3 5% 2 4 4 4 7% 6% 9% 10% 47%	2762
June	1st to 10th	Night	6% 5% 2 5% 4 5% 5% 3 9% 7% 49%	1520
	11th to 20th	Night	3 5% 2 4 5% 4 5% 5% 8% 8% 51%	1520
	21st to end	Night	2 3 1 4 3 4 4 4 6% 8% 61%	1520
July	1st to 10th	Night	1 1 1 1 1 1 2 2 3 4 82%	2540
	11th to 20th	Night	1 1 2 1 1 2 3 5% 83%	2540
	21st to end	Night	1 1 1 1 1 1 2 2 3 4 84%	2762
August	1st to 10th	Night	1 1 1 3 1 2 2 2 4 6% 77%	2529
	11th to 20th	Night	1 1 1 3 3 2 2 3 5% 7% 73%	2772
	21st to end	Night	3 3 1 4 3 3 3 2 4 8% 65%	3461
September	1st to 10th	Night	3 2 1 2 2 3 2 2 4 7% 71%	1900
	11th to 20th	Night	3 3 2 5% 3 4 4 3 6% 9% 58%	1900
	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 5% 5% 7% 9% 34%	2521
	11th to 20th	Night	8% 10% 3 8% 6% 6% 5% 4 6% 8% 36%	2660
	21st to end	Night	7% 12% 4 9% 6% 5% 5% 5% 7% 7% 34%	2660
December	1st to 10th	Night	9% 14% 5% 8% 5% 4 5% 4 7% 8% 31%	2660
	11th to 20th	Night	10% 15% 4 12% 9% 6% 6% 5% 7% 6% 19%	3280
	21st to end	Night	11% 17% 9% 12% 5% 4 4 3 5% 6% 23%	4862

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	56% 17% 13% 11% 3	2539
	11th to 20th	Day	51% 14% 17% 15% 3	2540
	21st to end	Day	62% 13% 13% 8% 3	2794
February	1st to 10th	Day	49% 14% 18% 14% 4 1	1700
	11th to 20th	Day	43% 16% 16% 16% 8% 1	1674
	21st to end	Day	42% 20% 16% 15% 7% 1	1548
March	1st to 10th	Day	36% 17% 15% 18% 13% 1	3138
	11th to 20th	Day	31% 10% 14% 41% 4	3445
	21st to end	Day	44% 18% 17% 16% 5%	4041
April	1st to 10th	Day	45% 14% 19% 16% 6%	2280
	11th to 20th	Day	39% 13% 18% 19% 10% 2	2252
	21st to end	Day	33% 15% 17% 22% 11% 1	2496
May	1st to 10th	Day	25% 11% 14% 22% 23% 4	4420
	11th to 20th	Day	20% 9% 20% 34% 17%	4380
	21st to end	Day	35% 17% 22% 21% 5%	4814
June	1st to 10th	Day	33% 12% 15% 24% 12% 3	2660
	11th to 20th	Day	23% 10% 21% 29% 14% 2	2660
	21st to end	Day	19% 8% 18% 31% 21% 4	2660
July	1st to 10th	Day	9% 5% 8% 16% 25% 38%	4351
	11th to 20th	Day	10% 3 5% 12% 25% 44%	4345
	21st to end	Day	9% 3 6% 23% 39% 20%	4639
August	1st to 10th	Day	11% 4 7% 20% 37% 21%	4384
	11th to 20th	Day	12% 5% 12% 44% 22% 5%	4103
	21st to end	Day	22% 6% 13% 25% 30% 4	4215
September	1st to 10th	Day	15% 7% 11% 30% 28% 9%	2280
	11th to 20th	Day	24% 11% 15% 22% 22% 6%	2280
	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	1654
	11th to 20th	Day	49% 15% 16% 13% 5% 1	1520
	21st to end	Day	46% 15% 15% 16% 7% 1	1520
December	1st to 10th	Day	50% 12% 14% 18% 6%	1520
	11th to 20th	Day	55% 16% 17% 8% 3	1928
	21st to end	Day	59% 14% 10% 11% 6%	2794
January	1st to 10th	Night	54% 17% 15% 9% 5%	4406
	11th to 20th	Night	47% 11% 18% 19% 5%	4420
	21st to end	Night	61% 15% 12% 9% 2 1	4862
February	1st to 10th	Night	45% 15% 18% 16% 4 1	2475
	11th to 20th	Night	39% 14% 18% 20% 7% 1	2404
	21st to end	Night	44% 16% 15% 17% 8%	1905
March	1st to 10th	Night	33% 16% 16% 20% 12% 2	3736
	11th to 20th	Night	24% 11% 20% 38% 6%	3531
	21st to end	Night	38% 19% 20% 20% 3	3612
April	1st to 10th	Night	34% 18% 21% 20% 8%	1900
	11th to 20th	Night	30% 13% 20% 22% 13% 1	1879
	21st to end	Night	30% 15% 21% 22% 11% 1	1681
May	1st to 10th	Night	25% 10% 15% 26% 20% 4	2540
	11th to 20th	Night	18% 9% 18% 37% 17% 1	2516
	21st to end	Night	21% 20% 28% 26% 5% 1	2762
June	1st to 10th	Night	29% 12% 19% 25% 13% 2	1520
	11th to 20th	Night	24% 11% 24% 28% 13% 1	1520
	21st to end	Night	17% 7% 17% 36% 18% 4	1520
July	1st to 10th	Night	7% 3 9% 20% 25% 35%	2500
	11th to 20th	Night	6% 3 7% 16% 26% 41%	2492
	21st to end	Night	5% 3 8% 25% 36% 23%	2674
August	1st to 10th	Night	10% 4 7% 19% 41% 19%	2521
	11th to 20th	Night	11% 5% 20% 38% 20% 5%	2798
	21st to end	Night	16% 6% 17% 27% 29% 5%	3461
September	1st to 10th	Night	13% 7% 13% 28% 31% 8%	1900
	11th to 20th	Night	21% 10% 15% 26% 21% 7%	1900
	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% 15% 18% 9% 1	2521
	11th to 20th	Night	43% 16% 16% 17% 7% 1	2660
	21st to end	Night	44% 14% 17% 16% 9% 1	2660
December	1st to 10th	Night	49% 13% 14% 18% 5%	2660
	11th to 20th	Night	57% 16% 16% 8% 3	3276
	21st to end	Night	56% 13% 14% 13% 4	4862

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% 17% 23% 23% 13% 2	180
	11th to 20th	Day	23% 16% 23% 23% 12% 3	180
	21st to end	Day	19% 17% 28% 21% 13% 2	198
February	1st to 10th	Day	21% 31% 30% 13% 4 1	360
	11th to 20th	Day	22% 30% 28% 17% 4	365
	21st to end	Day	18% 26% 32% 19% 4 1	372
March	1st to 10th	Day	18% 29% 27% 19% 6% 1	460
	11th to 20th	Day	16% 27% 25% 23% 8% 1	460
	21st to end	Day	21% 30% 29% 16% 5%	506
April	1st to 10th	Day	21% 28% 27% 16% 7% 1	460
	11th to 20th	Day	14% 34% 28% 15% 7% 2	452
	21st to end	Day	16% 30% 29% 19% 6% 1	460
May	1st to 10th	Day	13% 23% 36% 23% 4 1	463
	11th to 20th	Day	16% 20% 30% 23% 9% 2	488
	21st to end	Day	13% 21% 30% 24% 10% 2	529
June	1st to 10th	Day	14% 24% 31% 22% 7% 1	490
	11th to 20th	Day	13% 28% 39% 15% 3 2	490
	21st to end	Day	7% 21% 32% 27% 12% 1	490
July	1st to 10th	Day	3 19% 41% 31% 6%	486
	11th to 20th	Day	5% 19% 38% 29% 8% 1	484
	21st to end	Day	4 15% 36% 31% 12% 2	531
August	1st to 10th	Day	5% 15% 32% 30% 16% 2	487
	11th to 20th	Day	4 10% 32% 35% 16% 3	477
	21st to end	Day	8% 15% 33% 29% 12% 3	539
September	1st to 10th	Day	6% 15% 37% 26% 12% 3	490
	11th to 20th	Day	8% 19% 32% 28% 11% 1	490
	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	20% 27% 25% 22% 5%	255
	11th to 20th	Day	17% 26% 23% 17% 12% 4	180
	21st to end	Day	17% 19% 23% 25% 12% 4	180
December	1st to 10th	Day	21% 21% 22% 18% 11% 8%	180
	11th to 20th	Day	15% 23% 27% 23% 9% 2	180
	21st to end	Day	20% 19% 24% 24% 11% 2	198
January	1st to 10th	Night	20% 20% 22% 24% 13% 1	760
	11th to 20th	Night	17% 19% 21% 28% 14% 2	760
	21st to end	Night	18% 14% 23% 30% 13% 3	836
February	1st to 10th	Night	15% 22% 25% 26% 11% 1	760
	11th to 20th	Night	12% 22% 24% 28% 11% 1	744
	21st to end	Night	18% 22% 22% 25% 12% 2	628
March	1st to 10th	Night	13% 19% 26% 30% 11% 1	760
	11th to 20th	Night	11% 18% 27% 30% 13% 1	760
	21st to end	Night	12% 25% 27% 26% 9% 1	836
April	1st to 10th	Night	11% 21% 31% 27% 9% 1	760
	11th to 20th	Night	10% 25% 30% 24% 9% 2	751
	21st to end	Night	9% 25% 31% 24% 9% 1	541
May	1st to 10th	Night	9% 24% 29% 27% 10% 2	380
	11th to 20th	Night	8% 13% 31% 31% 15% 2	374
	21st to end	Night	8% 15% 30% 35% 11% 1	411
June	1st to 10th	Night	10% 15% 37% 25% 12%	380
	11th to 20th	Night	9% 24% 38% 22% 7%	380
	21st to end	Night	4 20% 31% 31% 12% 1	380
July	1st to 10th	Night	3 13% 37% 38% 9%	370
	11th to 20th	Night	2 14% 44% 29% 11% 1	368
	21st to end	Night	2 13% 36% 34% 15% 1	416
August	1st to 10th	Night	2 11% 34% 34% 17% 1	378
	11th to 20th	Night	3 8% 27% 36% 21% 6%	536
	21st to end	Night	4 15% 31% 29% 18% 3	812
September	1st to 10th	Night	4 12% 28% 32% 21% 3	760
	11th to 20th	Night	6% 14% 24% 26% 24% 6%	760
	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% 30% 10%	760
	11th to 20th	Night	16% 19% 27% 23% 13% 2	760
	21st to end	Night	15% 18% 23% 27% 13% 4	760
December	1st to 10th	Night	17% 16% 26% 26% 13% 2	760
	11th to 20th	Night	19% 15% 28% 30% 8%	760
	21st to end	Night	19% 21% 24% 29% 6%	836

Page generated at 2024-11-02 23:11:15. Version ac07

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