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 2024-01-12.

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% 6% 3 5% 5% 6% 8% 45%	74763
	Night	7% 8% 4 6% 6% 3 5% 5% 6% 8% 43%	74979
Transparency	Day	34% 12% 15% 21% 15% 4	74466
	Night	36% 12% 16% 21% 12% 3	74794
Seeing	Day	13% 22% 30% 23% 9% 2	13282
	Night	12% 18% 27% 28% 13% 2	22390

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% 13% 7% 10% 8% 4 6% 6% 6% 8% 20%	1519
	11th to 20th	Day	11% 12% 4 7% 7% 5% 6% 5% 7% 9% 26%	1451
	21st to end	Day	7% 11% 6% 9% 7% 4 8% 6% 8% 9% 27%	1584
February	1st to 10th	Day	8% 11% 4 9% 7% 4 6% 6% 7% 9% 30%	1611
	11th to 20th	Day	7% 11% 4 7% 7% 3 6% 5% 6% 10% 35%	1575
	21st to end	Day	6% 10% 5% 9% 9% 3 5% 6% 7% 10% 30%	1459
March	1st to 10th	Day	8% 11% 5% 8% 6% 4 7% 6% 6% 8% 30%	1800
	11th to 20th	Day	8% 11% 5% 10% 8% 4 5% 5% 6% 7% 30%	1948
	21st to end	Day	7% 9% 5% 9% 8% 4 7% 7% 7% 9% 27%	2304
April	1st to 10th	Day	8% 10% 4 8% 7% 4 7% 7% 8% 9% 29%	2160
	11th to 20th	Day	7% 10% 5% 7% 7% 4 6% 5% 7% 8% 35%	2132
	21st to end	Day	5% 7% 3 6% 6% 4 6% 6% 7% 8% 41%	2363
May	1st to 10th	Day	5% 5% 2 5% 5% 4 6% 6% 6% 9% 47%	2520
	11th to 20th	Day	8% 7% 3 6% 6% 3 6% 6% 8% 8% 39%	2477
	21st to end	Day	8% 8% 3 6% 6% 4 5% 6% 6% 9% 39%	2723
June	1st to 10th	Day	8% 6% 3 5% 6% 4 5% 6% 7% 9% 42%	2520
	11th to 20th	Day	5% 3 2 4 5% 3 6% 6% 7% 10% 49%	2520
	21st to end	Day	4 3 1 3 4 3 3 4 5% 8% 60%	2520
July	1st to 10th	Day	2 2 1 3 3 3 4 3 5% 7% 68%	2520
	11th to 20th	Day	3 2 1 3 3 2 3 3 4 5% 72%	2520
	21st to end	Day	1 1 1 1 2 1 3 2 3 4 80%	2741
August	1st to 10th	Day	2 2 1 3 3 2 3 3 4 7% 72%	2507
	11th to 20th	Day	2 2 1 2 3 2 2 3 5% 7% 72%	2323
	21st to end	Day	2 2 1 3 4 3 3 5% 5% 7% 64%	2399
September	1st to 10th	Day	3 2 1 3 3 2 3 3 4 7% 68%	2160
	11th to 20th	Day	4 4 2 4 5% 3 5% 6% 6% 8% 54%	2160
	21st to end	Day	7% 5% 2 5% 5% 4 4 4 6% 6% 53%	1932
October	1st to 10th	Day	8% 6% 2 5% 5% 3 4 5% 6% 8% 49%	1800
	11th to 20th	Day	9% 5% 3 6% 5% 3 4 4 7% 8% 46%	1800
	21st to end	Day	9% 7% 4 7% 6% 5% 5% 5% 5% 8% 39%	1818
November	1st to 10th	Day	10% 10% 4 8% 7% 3 6% 6% 7% 7% 31%	1618
	11th to 20th	Day	8% 12% 5% 8% 7% 4 5% 6% 7% 8% 30%	1520
	21st to end	Day	8% 13% 4 9% 6% 3 5% 6% 7% 8% 31%	1520
December	1st to 10th	Day	9% 12% 5% 9% 5% 3 5% 5% 6% 8% 32%	1520
	11th to 20th	Day	9% 14% 6% 10% 10% 5% 6% 5% 6% 7% 22%	1925
	21st to end	Day	10% 14% 7% 13% 8% 4 5% 5% 6% 6% 22%	2794
January	1st to 10th	Night	11% 13% 6% 9% 9% 5% 6% 6% 6% 7% 23%	2646
	11th to 20th	Night	12% 11% 5% 8% 6% 4 6% 7% 6% 9% 26%	2548
	21st to end	Night	8% 11% 4 8% 6% 4 7% 7% 8% 10% 28%	2772
February	1st to 10th	Night	7% 11% 4 7% 6% 3 5% 5% 7% 11% 32%	2344
	11th to 20th	Night	6% 9% 4 7% 6% 3 5% 4 7% 8% 41%	2272
	21st to end	Night	7% 12% 5% 7% 7% 3 6% 5% 7% 6% 36%	1796
March	1st to 10th	Night	8% 10% 5% 7% 6% 4 6% 6% 7% 8% 35%	2160
	11th to 20th	Night	5% 8% 4 7% 7% 4 6% 6% 7% 9% 37%	2007
	21st to end	Night	6% 10% 4 6% 7% 4 6% 7% 8% 10% 33%	2052
April	1st to 10th	Night	8% 5% 3 6% 7% 4 6% 7% 8% 10% 36%	1800
	11th to 20th	Night	5% 6% 3 6% 5% 4 5% 5% 6% 9% 46%	1779
	21st to end	Night	5% 6% 2 5% 6% 3 6% 5% 7% 13% 41%	1594
May	1st to 10th	Night	4 4 2 4 6% 4 4 5% 8% 10% 49%	1440
	11th to 20th	Night	7% 5% 2 4 6% 3 6% 6% 7% 9% 46%	1416
	21st to end	Night	5% 6% 2 4 6% 3 7% 7% 7% 8% 45%	1552
June	1st to 10th	Night	6% 4 2 4 5% 4 5% 5% 8% 8% 49%	1440
	11th to 20th	Night	3 6% 2 4 5% 3 5% 6% 8% 8% 51%	1440
	21st to end	Night	2 3 1 3 4 3 4 5% 6% 8% 62%	1440
July	1st to 10th	Night	2 2 1 1 2 1 3 4 4 7% 72%	1440
	11th to 20th	Night	1 1 1 3 1 2 3 4 6% 78%	1440
	21st to end	Night	1 1 1 1 1 1 2 3 5% 84%	1576
August	1st to 10th	Night	2 1 1 2 2 1 2 3 5% 7% 75%	1432
	11th to 20th	Night	1 2 1 2 2 1 2 3 4 6% 76%	1575
	21st to end	Night	2 2 1 3 3 2 3 3 5% 9% 68%	1957
September	1st to 10th	Night	3 2 1 2 2 2 2 3 4 7% 72%	1800
	11th to 20th	Night	3 4 2 4 3 3 5% 4 5% 9% 58%	1800
	21st to end	Night	4 5% 2 4 4 3 5% 4 6% 9% 57%	2024
October	1st to 10th	Night	6% 5% 2 4 5% 3 4 5% 5% 7% 54%	2160
	11th to 20th	Night	6% 5% 2 5% 6% 4 4 6% 7% 7% 47%	2160
	21st to end	Night	9% 7% 3 6% 5% 3 4 5% 6% 8% 42%	2538
November	1st to 10th	Night	11% 9% 4 6% 6% 3 5% 6% 7% 9% 34%	2457
	11th to 20th	Night	8% 9% 4 7% 6% 4 6% 5% 6% 8% 36%	2660
	21st to end	Night	8% 11% 5% 8% 6% 3 6% 6% 6% 8% 34%	2660
December	1st to 10th	Night	9% 12% 6% 7% 6% 4 5% 5% 7% 8% 31%	2660
	11th to 20th	Night	10% 15% 5% 10% 11% 4 6% 6% 7% 7% 19%	3280
	21st to end	Night	11% 16% 9% 11% 6% 3 4 4 5% 6% 24%	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	60% 16% 12% 9% 3 1	1519
	11th to 20th	Day	52% 14% 16% 14% 4 1	1451
	21st to end	Day	50% 14% 16% 14% 6%	1584
February	1st to 10th	Day	49% 14% 17% 14% 4 1	1611
	11th to 20th	Day	41% 17% 17% 17% 8% 1	1584
	21st to end	Day	42% 19% 16% 14% 7% 1	1459
March	1st to 10th	Day	44% 16% 16% 15% 9% 1	1800
	11th to 20th	Day	48% 12% 14% 21% 5%	1948
	21st to end	Day	46% 18% 18% 14% 3	2304
April	1st to 10th	Day	46% 14% 18% 16% 6%	2160
	11th to 20th	Day	40% 13% 18% 19% 9% 1	2132
	21st to end	Day	33% 15% 18% 22% 11% 1	2363
May	1st to 10th	Day	28% 13% 20% 26% 11% 1	2520
	11th to 20th	Day	35% 13% 18% 24% 10%	2480
	21st to end	Day	37% 13% 19% 22% 8%	2724
June	1st to 10th	Day	34% 12% 16% 24% 12% 3	2520
	11th to 20th	Day	24% 10% 22% 29% 13% 2	2520
	21st to end	Day	19% 8% 18% 30% 21% 4	2520
July	1st to 10th	Day	15% 8% 13% 25% 27% 11%	2451
	11th to 20th	Day	14% 5% 9% 21% 35% 16%	2445
	21st to end	Day	8% 4 6% 24% 40% 17%	2589
August	1st to 10th	Day	13% 6% 11% 27% 31% 12%	2490
	11th to 20th	Day	12% 5% 13% 28% 35% 8%	2323
	21st to end	Day	17% 9% 12% 29% 29% 5%	2399
September	1st to 10th	Day	15% 7% 12% 31% 26% 9%	2160
	11th to 20th	Day	25% 12% 15% 20% 22% 6%	2160
	21st to end	Day	27% 10% 17% 24% 17% 5%	1932
October	1st to 10th	Day	30% 11% 15% 30% 13% 2	1800
	11th to 20th	Day	32% 13% 17% 25% 13%	1800
	21st to end	Day	40% 12% 13% 23% 12%	1818
November	1st to 10th	Day	46% 15% 13% 17% 7% 1	1618
	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	58% 16% 13% 8% 4 1	2646
	11th to 20th	Night	54% 13% 13% 15% 4 1	2548
	21st to end	Night	48% 13% 18% 15% 3 1	2772
February	1st to 10th	Night	45% 15% 19% 17% 4 1	2344
	11th to 20th	Night	38% 14% 19% 20% 7% 1	2274
	21st to end	Night	44% 16% 15% 18% 8%	1796
March	1st to 10th	Night	40% 16% 16% 19% 9% 1	2160
	11th to 20th	Night	38% 14% 18% 23% 7% 1	2008
	21st to end	Night	40% 17% 21% 19% 3	2052
April	1st to 10th	Night	35% 17% 19% 21% 8%	1800
	11th to 20th	Night	31% 13% 20% 23% 13% 1	1779
	21st to end	Night	31% 15% 20% 22% 11% 1	1594
May	1st to 10th	Night	25% 11% 21% 30% 12% 1	1440
	11th to 20th	Night	29% 11% 19% 30% 10% 1	1416
	21st to end	Night	30% 13% 21% 27% 9% 1	1552
June	1st to 10th	Night	29% 12% 20% 26% 13% 1	1440
	11th to 20th	Night	25% 11% 25% 28% 11% 1	1440
	21st to end	Night	18% 7% 18% 36% 17% 5%	1440
July	1st to 10th	Night	12% 6% 17% 30% 25% 12%	1400
	11th to 20th	Night	9% 5% 12% 28% 30% 17%	1392
	21st to end	Night	5% 4 9% 23% 40% 19%	1488
August	1st to 10th	Night	10% 6% 12% 28% 31% 14%	1424
	11th to 20th	Night	9% 5% 14% 31% 32% 9%	1575
	21st to end	Night	11% 8% 14% 30% 30% 6%	1957
September	1st to 10th	Night	13% 7% 14% 29% 29% 8%	1800
	11th to 20th	Night	20% 11% 15% 24% 22% 8%	1800
	21st to end	Night	21% 10% 17% 29% 17% 5%	2024
October	1st to 10th	Night	27% 9% 13% 30% 19% 1	2160
	11th to 20th	Night	31% 12% 16% 26% 13% 2	2160
	21st to end	Night	36% 12% 14% 23% 15%	2538
November	1st to 10th	Night	42% 14% 15% 19% 9% 1	2457
	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% 24% 13% 2	180
	11th to 20th	Day	24% 14% 25% 21% 13% 3	163
	21st to end	Day	17% 18% 27% 23% 13% 2	176
February	1st to 10th	Day	21% 31% 30% 13% 4 1	331
	11th to 20th	Day	22% 30% 28% 16% 4	335
	21st to end	Day	18% 25% 34% 19% 4 1	337
March	1st to 10th	Day	19% 30% 27% 17% 6% 1	420
	11th to 20th	Day	16% 28% 26% 22% 7%	420
	21st to end	Day	20% 31% 29% 15% 5%	462
April	1st to 10th	Day	21% 28% 27% 16% 7% 1	420
	11th to 20th	Day	14% 35% 27% 15% 7% 2	412
	21st to end	Day	15% 30% 30% 19% 5% 1	420
May	1st to 10th	Day	12% 23% 36% 23% 4 1	423
	11th to 20th	Day	16% 19% 31% 23% 9% 2	448
	21st to end	Day	12% 20% 31% 25% 10% 2	485
June	1st to 10th	Day	14% 24% 32% 23% 7% 1	450
	11th to 20th	Day	12% 29% 40% 15% 3 2	450
	21st to end	Day	6% 21% 32% 27% 12% 2	450
July	1st to 10th	Day	3 17% 41% 32% 6%	446
	11th to 20th	Day	5% 20% 38% 28% 8% 1	444
	21st to end	Day	3 14% 37% 32% 12% 2	487
August	1st to 10th	Day	5% 15% 32% 30% 15% 2	447
	11th to 20th	Day	4 10% 32% 34% 16% 3	437
	21st to end	Day	7% 16% 35% 29% 12% 3	495
September	1st to 10th	Day	7% 16% 37% 27% 11% 3	450
	11th to 20th	Day	8% 18% 32% 29% 12% 2	450
	21st to end	Day	14% 21% 27% 26% 12% 1	450
October	1st to 10th	Day	13% 20% 27% 24% 14% 2	450
	11th to 20th	Day	14% 22% 25% 24% 12% 3	450
	21st to end	Day	19% 23% 23% 22% 9% 3	333
November	1st to 10th	Day	19% 26% 26% 23% 5%	243
	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% 21% 25% 13% 1	760
	11th to 20th	Night	17% 19% 20% 28% 14% 2	728
	21st to end	Night	17% 14% 23% 30% 13% 3	792
February	1st to 10th	Night	15% 22% 25% 26% 11% 1	720
	11th to 20th	Night	13% 22% 24% 29% 11% 2	704
	21st to end	Night	16% 22% 23% 26% 11% 2	592
March	1st to 10th	Night	14% 19% 26% 29% 11% 1	720
	11th to 20th	Night	11% 18% 27% 29% 12% 1	720
	21st to end	Night	13% 25% 27% 25% 9% 1	792
April	1st to 10th	Night	11% 21% 30% 27% 9% 1	720
	11th to 20th	Night	9% 25% 30% 24% 10% 2	711
	21st to end	Night	9% 25% 31% 24% 9% 1	514
May	1st to 10th	Night	8% 24% 30% 26% 11% 2	360
	11th to 20th	Night	8% 12% 31% 32% 15% 3	354
	21st to end	Night	8% 14% 31% 35% 12% 1	389
June	1st to 10th	Night	11% 16% 36% 26% 12%	360
	11th to 20th	Night	8% 23% 39% 23% 6%	360
	21st to end	Night	3 19% 31% 32% 13% 1	360
July	1st to 10th	Night	2 12% 37% 40% 9%	350
	11th to 20th	Night	2 14% 43% 30% 11% 1	348
	21st to end	Night	1 13% 35% 34% 15% 1	394
August	1st to 10th	Night	3 10% 35% 34% 18% 1	358
	11th to 20th	Night	3 7% 26% 36% 22% 7%	507
	21st to end	Night	4 14% 31% 30% 18% 3	769
September	1st to 10th	Night	5% 13% 28% 32% 19% 3	720
	11th to 20th	Night	6% 13% 22% 27% 25% 7%	720
	21st to end	Night	7% 14% 24% 31% 19% 5%	720
October	1st to 10th	Night	10% 13% 23% 31% 20% 3	720
	11th to 20th	Night	11% 16% 20% 29% 19% 5%	720
	21st to end	Night	15% 19% 24% 26% 13% 4	792
November	1st to 10th	Night	15% 18% 26% 30% 10%	740
	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

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