Dreamweaver Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Dreamweaver 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	4 3 1 3 2 2 2 2 4 6% 71%	134389
	Night	4 3 1 3 2 2 2 2 4 5% 71%	132159
Transparency	Day	16% 4 7% 17% 28% 28%	129473
	Night	16% 4 7% 17% 28% 28%	128047
Seeing	Day	7% 9% 29% 35% 15% 5%	16763
	Night	6% 3 15% 36% 33% 7%	26494

By Month

Percentage Cloud Cover SW (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 3 1 2 2 2 2 2 3 5% 74%	3662
	11th to 20th	Day	6% 3 1 3 2 2 2 2 3 6% 70%	4100
	21st to end	Day	10% 4 2 4 2 2 3 3 5% 9% 55%	4721
February	1st to 10th	Day	9% 5% 1 4 3 2 3 3 4 5% 60%	2000
	11th to 20th	Day	8% 3 1 3 4 3 2 3 5% 7% 60%	1950
	21st to end	Day	8% 5% 1 4 3 3 3 3 5% 7% 58%	1650
March	1st to 10th	Day	12% 5% 2 5% 4 3 3 3 5% 7% 53%	4518
	11th to 20th	Day	8% 4 2 4 3 3 3 3 4 7% 59%	4810
	21st to end	Day	7% 6% 2 6% 5% 4 5% 4 7% 9% 46%	5925
April	1st to 10th	Day	5% 4 1 4 3 3 2 2 5% 8% 63%	2400
	11th to 20th	Day	4 5% 1 4 3 2 3 2 4 7% 65%	2369
	21st to end	Day	3 5% 2 4 2 2 2 3 3 7% 66%	2400
May	1st to 10th	Day	4 6% 2 4 3 3 2 2 4 5% 65%	5453
	11th to 20th	Day	3 4 1 3 2 3 3 3 5% 9% 63%	5841
	21st to end	Day	2 4 1 4 3 3 3 3 4 7% 66%	6423
June	1st to 10th	Day	3 5% 2 4 3 2 3 2 6% 7% 64%	2762
	11th to 20th	Day	2 3 1 3 2 2 2 2 4 7% 73%	2800
	21st to end	Day	1 2 1 2 2 2 2 2 2 4 82%	2800
July	1st to 10th	Day	1 1 1 1 1 1 2 3 90%	6319
	11th to 20th	Day	1 1 2 1 1 1 1 3 4 84%	6202
	21st to end	Day	1 1 1 1 1 1 2 1 3 3 85%	6124
August	1st to 10th	Day	1 1 1 2 1 1 2 2 4 4 83%	5346
	11th to 20th	Day	1 1 1 1 1 1 2 92%	5364
	21st to end	Day	1 1 1 1 1 1 2 3 88%	5984
September	1st to 10th	Day	1 1 1 2 1 1 2 2 3 4 81%	2400
	11th to 20th	Day	1 2 1 2 1 2 2 2 3 4 81%	2279
	21st to end	Day	2 1 1 1 2 2 2 1 3 4 82%	2090
October	1st to 10th	Day	2 2 1 2 1 2 2 1 3 3 83%	3464
	11th to 20th	Day	2 2 1 3 2 2 3 3 5% 5% 72%	3161
	21st to end	Day	2 3 1 3 2 3 3 3 4 5% 70%	3448
November	1st to 10th	Day	3 1 1 3 2 2 2 2 3 6% 75%	1776
	11th to 20th	Day	3 3 1 3 2 2 3 3 4 5% 71%	1800
	21st to end	Day	5% 3 1 4 3 3 3 3 5% 7% 63%	1800
December	1st to 10th	Day	4 2 1 3 2 3 4 3 5% 6% 65%	2950
	11th to 20th	Day	11% 4 1 4 3 3 5% 3 5% 6% 54%	3294
	21st to end	Day	9% 4 2 5% 4 3 4 3 6% 8% 53%	4004
January	1st to 10th	Night	5% 3 1 3 2 3 2 2 3 5% 70%	6281
	11th to 20th	Night	5% 3 1 4 2 2 2 2 3 6% 69%	5880
	21st to end	Night	9% 6% 2 3 2 3 4 3 5% 6% 57%	6250
February	1st to 10th	Night	9% 5% 2 4 3 2 2 2 4 5% 62%	2400
	11th to 20th	Night	8% 4 1 3 3 2 3 3 4 6% 63%	2338
	21st to end	Night	9% 5% 1 5% 2 3 3 2 4 5% 61%	1980
March	1st to 10th	Night	8% 9% 2 5% 3 3 3 3 6% 8% 51%	5376
	11th to 20th	Night	7% 4 2 4 3 3 3 2 5% 6% 61%	5186
	21st to end	Night	10% 6% 2 4 3 3 3 2 4 5% 58%	5037
April	1st to 10th	Night	5% 4 1 3 2 2 3 3 4 6% 65%	2000
	11th to 20th	Night	3 5% 2 4 2 2 2 2 4 5% 67%	1979
	21st to end	Night	3 5% 1 3 2 2 3 2 4 7% 69%	2000
May	1st to 10th	Night	3 6% 1 5% 4 3 2 2 4 4 67%	4542
	11th to 20th	Night	1 3 1 3 3 3 4 3 4 6% 70%	4074
	21st to end	Night	1 2 1 3 2 2 3 2 3 6% 77%	4474
June	1st to 10th	Night	2 3 1 3 2 2 2 2 4 6% 72%	1632
	11th to 20th	Night	1 2 1 2 2 1 1 2 3 5% 81%	1600
	21st to end	Night	1 1 1 1 1 2 1 2 3 3 85%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 92%	3640
	11th to 20th	Night	1 1 1 1 1 1 3 4 86%	3732
	21st to end	Night	1 1 1 1 1 1 2 3 4 85%	4745
August	1st to 10th	Night	1 1 2 1 1 2 2 3 4 83%	4486
	11th to 20th	Night	1 1 1 1 2 2 92%	4506
	21st to end	Night	1 1 1 1 1 1 2 3 88%	5015
September	1st to 10th	Night	3 1 1 2 1 2 2 1 2 3 82%	2000
	11th to 20th	Night	1 1 1 1 1 2 2 2 2 3 83%	2078
	21st to end	Night	1 1 2 1 2 1 2 3 4 83%	2090
October	1st to 10th	Night	2 1 2 1 1 1 2 3 3 85%	3511
	11th to 20th	Night	2 3 1 3 2 2 2 3 5% 6% 73%	3777
	21st to end	Night	2 3 1 2 1 1 1 2 2 4 79%	4121
November	1st to 10th	Night	3 3 1 3 2 2 3 2 4 5% 72%	2521
	11th to 20th	Night	4 2 1 2 2 2 2 2 4 6% 72%	2600
	21st to end	Night	5% 4 1 3 4 3 4 3 5% 7% 63%	2600
December	1st to 10th	Night	5% 3 1 3 2 3 3 3 4 5% 68%	4240
	11th to 20th	Night	13% 4 1 3 3 3 4 4 6% 7% 53%	4916
	21st to end	Night	9% 4 1 4 4 4 5% 4 7% 10% 47%	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	14% 2 5% 22% 33% 24%	3662
	11th to 20th	Day	17% 3 10% 17% 20% 33%	4100
	21st to end	Day	26% 4 6% 18% 25% 21%	4721
February	1st to 10th	Day	26% 4 6% 15% 23% 27%	2000
	11th to 20th	Day	22% 4 7% 18% 28% 20%	1960
	21st to end	Day	25% 5% 9% 18% 24% 18%	1650
March	1st to 10th	Day	29% 5% 10% 17% 20% 19%	4518
	11th to 20th	Day	23% 6% 10% 29% 20% 12%	4810
	21st to end	Day	26% 11% 15% 23% 18% 7%	5925
April	1st to 10th	Day	20% 5% 13% 23% 26% 12%	2400
	11th to 20th	Day	20% 4 9% 23% 28% 16%	2372
	21st to end	Day	18% 5% 9% 25% 31% 12%	2400
May	1st to 10th	Day	20% 6% 9% 17% 28% 20%	5453
	11th to 20th	Day	16% 7% 11% 26% 31% 9%	5844
	21st to end	Day	14% 7% 9% 29% 32% 9%	6424
June	1st to 10th	Day	18% 5% 11% 22% 30% 15%	2762
	11th to 20th	Day	13% 3 7% 17% 32% 28%	2800
	21st to end	Day	9% 2 4 10% 35% 40%	2800
July	1st to 10th	Day	4 1 1 7% 20% 67%	6251
	11th to 20th	Day	6% 2 3 9% 36% 43%	6127
	21st to end	Day	6% 2 4 17% 39% 33%	5948
August	1st to 10th	Day	7% 2 3 10% 42% 36%	3890
	11th to 20th	Day	3 1 2 9% 34% 52%	3880
	21st to end	Day	5% 1 2 9% 31% 52%	4311
September	1st to 10th	Day	8% 3 4 10% 28% 47%	2400
	11th to 20th	Day	9% 2 4 13% 31% 40%	2279
	21st to end	Day	9% 2 4 14% 35% 37%	2090
October	1st to 10th	Day	8% 2 4 11% 20% 55%	3464
	11th to 20th	Day	11% 5% 6% 14% 36% 29%	3161
	21st to end	Day	16% 4 4 9% 33% 34%	3448
November	1st to 10th	Day	14% 3 5% 17% 29% 33%	1776
	11th to 20th	Day	16% 3 6% 13% 25% 37%	1800
	21st to end	Day	21% 4 8% 14% 22% 32%	1800
December	1st to 10th	Day	19% 4 5% 12% 28% 33%	2950
	11th to 20th	Day	30% 4 7% 16% 26% 18%	3293
	21st to end	Day	27% 6% 9% 25% 25% 9%	4004
January	1st to 10th	Night	17% 3 5% 15% 37% 23%	6281
	11th to 20th	Night	18% 5% 7% 18% 21% 31%	5880
	21st to end	Night	26% 4 6% 16% 26% 22%	6250
February	1st to 10th	Night	26% 3 6% 15% 25% 27%	2400
	11th to 20th	Night	23% 3 7% 18% 25% 24%	2339
	21st to end	Night	26% 3 7% 15% 25% 23%	1980
March	1st to 10th	Night	27% 7% 11% 18% 22% 16%	5376
	11th to 20th	Night	22% 7% 11% 26% 19% 14%	5186
	21st to end	Night	27% 5% 12% 25% 22% 7%	5037
April	1st to 10th	Night	21% 5% 11% 25% 25% 14%	2000
	11th to 20th	Night	19% 4 8% 24% 28% 17%	1979
	21st to end	Night	17% 5% 9% 24% 33% 12%	2000
May	1st to 10th	Night	19% 8% 9% 16% 29% 18%	4539
	11th to 20th	Night	11% 8% 12% 29% 27% 13%	4075
	21st to end	Night	8% 7% 11% 30% 34% 10%	4474
June	1st to 10th	Night	14% 5% 14% 24% 29% 15%	1632
	11th to 20th	Night	8% 3 7% 19% 36% 26%	1600
	21st to end	Night	6% 2 5% 13% 34% 40%	1600
July	1st to 10th	Night	3 1 1 7% 19% 68%	3600
	11th to 20th	Night	4 2 4 11% 37% 42%	3684
	21st to end	Night	5% 2 3 20% 38% 32%	4609
August	1st to 10th	Night	7% 1 2 14% 39% 37%	3256
	11th to 20th	Night	3 1 2 8% 35% 50%	3256
	21st to end	Night	4 1 9% 35% 50%	3608
September	1st to 10th	Night	9% 2 3 8% 30% 47%	2000
	11th to 20th	Night	8% 3 3 13% 34% 38%	2078
	21st to end	Night	7% 2 3 13% 34% 40%	2090
October	1st to 10th	Night	7% 2 3 10% 19% 59%	3511
	11th to 20th	Night	11% 3 5% 18% 29% 33%	3777
	21st to end	Night	13% 2 3 8% 30% 45%	4121
November	1st to 10th	Night	14% 3 6% 15% 29% 33%	2521
	11th to 20th	Night	15% 3 6% 12% 24% 39%	2600
	21st to end	Night	20% 4 7% 16% 21% 32%	2600
December	1st to 10th	Night	19% 3 4 11% 25% 39%	4240
	11th to 20th	Night	29% 5% 8% 15% 22% 21%	4916
	21st to end	Night	28% 7% 10% 25% 21% 8%	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	18% 13% 33% 25% 10%	210
	11th to 20th	Day	14% 15% 37% 28% 6% 1	400
	21st to end	Day	14% 16% 39% 22% 8% 1	487
February	1st to 10th	Day	13% 16% 38% 25% 6% 2	500
	11th to 20th	Day	12% 17% 39% 25% 6% 1	495
	21st to end	Day	13% 17% 41% 22% 6% 1	414
March	1st to 10th	Day	16% 19% 34% 24% 7%	500
	11th to 20th	Day	13% 15% 37% 27% 8% 1	500
	21st to end	Day	13% 17% 37% 25% 8%	550
April	1st to 10th	Day	10% 12% 34% 30% 11% 3	500
	11th to 20th	Day	8% 17% 37% 25% 9% 3	492
	21st to end	Day	5% 13% 34% 28% 13% 6%	500
May	1st to 10th	Day	10% 13% 30% 33% 10% 4	503
	11th to 20th	Day	6% 12% 28% 35% 15% 4	528
	21st to end	Day	7% 9% 35% 30% 16% 4	573
June	1st to 10th	Day	4 8% 25% 38% 19% 6%	530
	11th to 20th	Day	4 3 26% 38% 21% 8%	530
	21st to end	Day	2 3 21% 42% 23% 9%	530
July	1st to 10th	Day	1 1 16% 44% 26% 12%	526
	11th to 20th	Day	2 10% 43% 27% 17%	524
	21st to end	Day	2 11% 47% 23% 15%	575
August	1st to 10th	Day	1 2 20% 46% 19% 13%	527
	11th to 20th	Day	2 1 13% 48% 22% 14%	517
	21st to end	Day	1 2 19% 43% 22% 12%	583
September	1st to 10th	Day	2 3 22% 45% 19% 9%	530
	11th to 20th	Day	3 6% 30% 40% 18% 4	523
	21st to end	Day	1 6% 26% 42% 18% 7%	490
October	1st to 10th	Day	3 6% 32% 40% 17% 2	490
	11th to 20th	Day	3 7% 35% 39% 15% 2	490
	21st to end	Day	5% 9% 35% 33% 15% 3	539
November	1st to 10th	Day	5% 9% 28% 36% 20% 3	312
	11th to 20th	Day	3 15% 36% 34% 12% 1	300
	21st to end	Day	10% 11% 39% 29% 11% 1	300
December	1st to 10th	Day	11% 7% 36% 31% 13% 1	300
	11th to 20th	Day	8% 11% 36% 32% 12% 1	275
	21st to end	Day	15% 15% 32% 29% 8% 1	220
January	1st to 10th	Night	11% 4 27% 39% 13% 5%	800
	11th to 20th	Night	9% 6% 27% 42% 15% 2	800
	21st to end	Night	14% 8% 24% 39% 13% 3	880
February	1st to 10th	Night	11% 6% 24% 37% 18% 3	800
	11th to 20th	Night	9% 3 24% 40% 18% 6%	784
	21st to end	Night	13% 7% 24% 37% 17% 2	660
March	1st to 10th	Night	10% 5% 23% 43% 18% 2	800
	11th to 20th	Night	8% 5% 19% 40% 24% 4	800
	21st to end	Night	8% 4 19% 45% 20% 3	880
April	1st to 10th	Night	6% 3 16% 46% 26% 4	800
	11th to 20th	Night	6% 4 19% 41% 28% 3	791
	21st to end	Night	4 4 12% 41% 30% 8%	800
May	1st to 10th	Night	6% 2 15% 39% 34% 5%	795
	11th to 20th	Night	4 3 11% 35% 43% 4	591
	21st to end	Night	2 2 12% 40% 38% 6%	650
June	1st to 10th	Night	2 6% 34% 48% 9%	432
	11th to 20th	Night	1 4 32% 54% 9%	400
	21st to end	Night	1 3 28% 58% 10%	400
July	1st to 10th	Night	1 1 16% 67% 15%	390
	11th to 20th	Night	2 1 9% 68% 21%	428
	21st to end	Night	2 1 15% 63% 19%	777
August	1st to 10th	Night	1 2 23% 59% 14%	796
	11th to 20th	Night	1 2 15% 64% 18%	792
	21st to end	Night	1 2 19% 61% 16%	880
September	1st to 10th	Night	3 1 3 27% 51% 16%	800
	11th to 20th	Night	2 1 6% 33% 47% 12%	792
	21st to end	Night	1 6% 38% 41% 13%	760
October	1st to 10th	Night	4 10% 45% 38% 4	760
	11th to 20th	Night	4 3 12% 40% 34% 7%	760
	21st to end	Night	4 3 19% 38% 26% 10%	836
November	1st to 10th	Night	4 2 19% 41% 26% 7%	780
	11th to 20th	Night	5% 2 23% 41% 24% 5%	800
	21st to end	Night	7% 5% 28% 36% 21% 4	800
December	1st to 10th	Night	10% 6% 25% 38% 18% 4	800
	11th to 20th	Night	12% 4 27% 40% 15% 2	800
	21st to end	Night	11% 6% 24% 41% 15% 3	880

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