Williamson Backyard Observatory
Clear Sky Chart
History

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

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	2 3 1 4 3 3 4 3 5% 6% 65%	136229
	Night	2 3 1 3 3 3 3 3 5% 6% 69%	132670
Transparency	Day	18% 4 7% 14% 28% 29%	131097
	Night	16% 3 7% 14% 30% 30%	128798
Seeing	Day	4 9% 22% 30% 23% 12%	16542
	Night	3 4 17% 39% 33% 5%	26765

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	1 4 2 5% 3 3 3 3 4 6% 66%	4107
	11th to 20th	Day	1 3 1 3 2 3 3 3 3 5% 73%	4100
	21st to end	Day	5% 6% 2 6% 4 4 4 3 4 6% 55%	4510
February	1st to 10th	Day	2 5% 2 5% 2 3 3 3 5% 6% 62%	1896
	11th to 20th	Day	3 5% 2 5% 4 3 4 3 5% 7% 59%	1950
	21st to end	Day	2 4 2 5% 4 4 4 4 5% 7% 59%	1650
March	1st to 10th	Day	1 5% 2 5% 3 4 4 3 5% 6% 63%	4869
	11th to 20th	Day	2 7% 3 7% 5% 4 4 5% 6% 7% 50%	5000
	21st to end	Day	2 4 2 4 4 3 4 3 5% 7% 62%	5486
April	1st to 10th	Day	1 2 1 3 3 4 4 3 5% 7% 66%	2200
	11th to 20th	Day	2 3 1 3 3 2 2 3 4 5% 73%	2331
	21st to end	Day	1 2 1 3 3 2 3 3 5% 7% 70%	2600
May	1st to 10th	Day	4 3 2 4 3 3 3 2 4 4 69%	5880
	11th to 20th	Day	1 2 1 3 2 3 4 3 5% 6% 70%	5841
	21st to end	Day	1 1 1 2 1 2 2 2 3 5% 80%	6423
June	1st to 10th	Day	1 1 2 2 2 2 3 3 7% 76%	2600
	11th to 20th	Day	1 1 1 2 1 2 1 2 2 87%	2600
	21st to end	Day	1 2 1 3 3 2 3 3 3 5% 75%	2600
July	1st to 10th	Day	1 1 1 2 2 2 3 3 4 5% 75%	5879
	11th to 20th	Day	2 3 1 5% 4 6% 6% 6% 7% 10% 50%	5880
	21st to end	Day	3 2 1 5% 4 5% 5% 4 6% 7% 59%	6401
August	1st to 10th	Day	3 3 2 5% 5% 5% 6% 4 6% 7% 55%	5779
	11th to 20th	Day	1 1 1 3 3 4 5% 5% 7% 8% 62%	5798
	21st to end	Day	3 3 1 6% 5% 5% 5% 5% 7% 9% 51%	5950
September	1st to 10th	Day	4 4 2 4 4 4 6% 4 6% 7% 55%	2200
	11th to 20th	Day	2 1 1 2 3 2 3 2 4 5% 74%	2200
	21st to end	Day	2 2 1 3 3 3 3 3 5% 5% 69%	2200
October	1st to 10th	Day	2 2 1 4 3 2 3 2 4 4 71%	3577
	11th to 20th	Day	1 2 1 2 2 1 2 2 3 6% 79%	3250
	21st to end	Day	1 1 1 2 2 2 3 2 4 5% 78%	3548
November	1st to 10th	Day	1 2 1 3 2 3 2 3 4 5% 74%	1996
	11th to 20th	Day	1 3 1 3 2 3 3 4 4 5% 71%	1890
	21st to end	Day	4 5% 1 4 2 3 3 3 4 7% 65%	1890
December	1st to 10th	Day	2 3 1 3 2 2 2 3 4 6% 71%	3040
	11th to 20th	Day	2 4 2 5% 3 3 4 4 9% 9% 56%	3499
	21st to end	Day	4 4 2 6% 3 4 4 3 6% 9% 56%	4609
January	1st to 10th	Night	1 3 2 4 3 3 3 3 5% 6% 66%	5880
	11th to 20th	Night	1 2 1 3 2 2 3 3 4 5% 74%	5880
	21st to end	Night	5% 5% 2 6% 3 4 4 3 4 6% 56%	6468
February	1st to 10th	Night	2 4 2 4 3 3 4 3 4 5% 66%	2500
	11th to 20th	Night	2 3 1 5% 3 4 4 3 5% 6% 63%	2338
	21st to end	Night	3 4 1 5% 3 3 4 3 5% 7% 61%	1980
March	1st to 10th	Night	1 4 1 4 3 3 3 3 5% 7% 65%	5012
	11th to 20th	Night	3 5% 3 7% 5% 6% 5% 5% 6% 6% 50%	5000
	21st to end	Night	2 2 1 4 3 4 4 4 6% 8% 63%	5489
April	1st to 10th	Night	2 2 1 3 3 3 3 3 5% 6% 69%	2200
	11th to 20th	Night	1 2 1 5% 2 3 2 3 4 5% 72%	2011
	21st to end	Night	1 1 1 2 2 2 2 2 4 6% 78%	1800
May	1st to 10th	Night	3 3 1 3 2 2 3 2 4 4 72%	4100
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 4 80%	4074
	21st to end	Night	1 1 1 1 2 2 2 3 4 84%	4474
June	1st to 10th	Night	1 2 1 2 1 2 2 2 4 4 80%	1800
	11th to 20th	Night	1 1 1 1 2 1 1 3 3 86%	1800
	21st to end	Night	1 2 1 2 2 3 3 2 3 4 77%	1800
July	1st to 10th	Night	1 1 1 2 2 2 3 2 4 5% 78%	4100
	11th to 20th	Night	1 2 1 3 3 4 5% 5% 6% 8% 62%	4100
	21st to end	Night	1 2 1 3 3 3 4 4 6% 9% 63%	4472
August	1st to 10th	Night	1 2 1 3 2 3 5% 4 7% 13% 58%	4033
	11th to 20th	Night	1 1 2 2 2 2 3 5% 8% 74%	4051
	21st to end	Night	2 2 1 4 4 4 5% 4 6% 9% 59%	5038
September	1st to 10th	Night	2 3 2 3 3 4 5% 4 6% 8% 61%	2200
	11th to 20th	Night	2 2 2 2 2 2 3 4 4 78%	2200
	21st to end	Night	1 1 1 2 2 3 2 2 3 4 78%	2200
October	1st to 10th	Night	2 2 1 3 2 2 2 3 3 5% 77%	3620
	11th to 20th	Night	2 2 1 2 2 2 2 2 3 6% 75%	3898
	21st to end	Night	1 1 1 2 2 2 2 2 3 6% 78%	4252
November	1st to 10th	Night	1 2 1 2 2 3 2 3 3 5% 76%	2518
	11th to 20th	Night	1 2 1 3 3 3 3 2 4 5% 73%	2730
	21st to end	Night	3 4 2 4 3 3 3 2 5% 6% 67%	2730
December	1st to 10th	Night	2 3 1 2 2 2 4 3 5% 7% 70%	4370
	11th to 20th	Night	2 3 1 4 3 4 4 4 5% 7% 63%	4941
	21st to end	Night	3 3 1 4 3 3 3 3 5% 7% 64%	6611

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	20% 3 5% 17% 37% 18%	4108
	11th to 20th	Day	17% 2 5% 19% 20% 37%	4100
	21st to end	Day	28% 4 8% 11% 27% 23%	4510
February	1st to 10th	Day	24% 2 7% 14% 26% 26%	1896
	11th to 20th	Day	26% 3 5% 17% 26% 22%	1960
	21st to end	Day	25% 3 7% 13% 26% 25%	1650
March	1st to 10th	Day	19% 3 8% 16% 35% 20%	4869
	11th to 20th	Day	27% 7% 8% 21% 23% 14%	5000
	21st to end	Day	19% 4 7% 22% 35% 12%	5486
April	1st to 10th	Day	18% 3 6% 16% 34% 23%	2200
	11th to 20th	Day	15% 2 4 12% 34% 32%	2334
	21st to end	Day	15% 2 4 14% 37% 27%	2600
May	1st to 10th	Day	19% 3 5% 14% 35% 24%	5880
	11th to 20th	Day	13% 3 6% 9% 32% 36%	5844
	21st to end	Day	8% 1 4 6% 27% 54%	6424
June	1st to 10th	Day	11% 2 5% 8% 32% 42%	2600
	11th to 20th	Day	7% 1 3 4 20% 65%	2600
	21st to end	Day	13% 3 5% 7% 20% 52%	2600
July	1st to 10th	Day	11% 3 5% 8% 20% 54%	5816
	11th to 20th	Day	24% 8% 10% 18% 27% 12%	5811
	21st to end	Day	21% 8% 11% 11% 20% 29%	6217
August	1st to 10th	Day	34% 9% 14% 18% 12% 14%	4208
	11th to 20th	Day	20% 7% 10% 19% 27% 16%	4197
	21st to end	Day	24% 6% 9% 17% 26% 19%	4289
September	1st to 10th	Day	26% 7% 9% 13% 23% 23%	2200
	11th to 20th	Day	13% 3 6% 11% 33% 34%	2200
	21st to end	Day	16% 3 6% 11% 29% 35%	2200
October	1st to 10th	Day	18% 2 5% 10% 19% 46%	3577
	11th to 20th	Day	8% 1 5% 8% 47% 31%	3250
	21st to end	Day	10% 2 5% 9% 31% 44%	3548
November	1st to 10th	Day	15% 2 4 16% 35% 28%	1996
	11th to 20th	Day	16% 3 4 16% 28% 33%	1890
	21st to end	Day	21% 2 5% 15% 31% 26%	1890
December	1st to 10th	Day	16% 2 4 15% 36% 26%	3040
	11th to 20th	Day	21% 4 7% 16% 36% 17%	3498
	21st to end	Day	25% 5% 7% 22% 31% 10%	4609
January	1st to 10th	Night	18% 3 6% 14% 38% 19%	5880
	11th to 20th	Night	13% 3 6% 21% 25% 32%	5880
	21st to end	Night	26% 4 7% 10% 27% 26%	6468
February	1st to 10th	Night	21% 3 7% 16% 28% 25%	2500
	11th to 20th	Night	21% 4 7% 18% 28% 22%	2339
	21st to end	Night	24% 3 6% 14% 28% 26%	1980
March	1st to 10th	Night	15% 4 8% 18% 33% 23%	5012
	11th to 20th	Night	25% 5% 9% 19% 26% 15%	5000
	21st to end	Night	17% 4 9% 21% 36% 13%	5489
April	1st to 10th	Night	17% 2 7% 14% 36% 24%	2200
	11th to 20th	Night	15% 2 5% 10% 34% 34%	2011
	21st to end	Night	10% 2 5% 13% 39% 31%	1800
May	1st to 10th	Night	16% 3 5% 15% 32% 29%	4097
	11th to 20th	Night	8% 2 4 9% 37% 40%	4075
	21st to end	Night	7% 1 3 5% 26% 58%	4474
June	1st to 10th	Night	11% 2 3 7% 30% 48%	1800
	11th to 20th	Night	7% 1 3 4 22% 64%	1800
	21st to end	Night	13% 2 3 6% 18% 58%	1800
July	1st to 10th	Night	10% 3 4 7% 20% 55%	4055
	11th to 20th	Night	17% 5% 10% 23% 30% 15%	4046
	21st to end	Night	16% 5% 14% 13% 21% 31%	4344
August	1st to 10th	Night	22% 8% 19% 23% 14% 15%	2928
	11th to 20th	Night	12% 5% 13% 21% 29% 21%	2928
	21st to end	Night	16% 4 8% 20% 28% 25%	3622
September	1st to 10th	Night	20% 6% 11% 17% 21% 26%	2200
	11th to 20th	Night	11% 4 4 11% 31% 39%	2200
	21st to end	Night	11% 3 5% 11% 31% 39%	2200
October	1st to 10th	Night	12% 2 5% 9% 21% 51%	3620
	11th to 20th	Night	12% 2 4 8% 40% 34%	3898
	21st to end	Night	9% 1 5% 8% 29% 48%	4252
November	1st to 10th	Night	14% 2 5% 15% 35% 30%	2518
	11th to 20th	Night	15% 3 5% 14% 30% 33%	2730
	21st to end	Night	20% 2 5% 14% 30% 28%	2730
December	1st to 10th	Night	15% 3 4 14% 35% 29%	4370
	11th to 20th	Night	21% 3 6% 14% 37% 20%	4941
	21st to end	Night	21% 3 7% 23% 32% 14%	6611

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% 10% 24% 36% 19% 3	303
	11th to 20th	Day	7% 7% 26% 32% 22% 6%	300
	21st to end	Day	4 10% 25% 32% 20% 10%	330
February	1st to 10th	Day	5% 14% 27% 32% 16% 7%	396
	11th to 20th	Day	8% 17% 31% 27% 16% 2	495
	21st to end	Day	7% 23% 28% 28% 12% 2	414
March	1st to 10th	Day	6% 22% 32% 26% 12% 2	500
	11th to 20th	Day	7% 18% 27% 27% 17% 5%	500
	21st to end	Day	6% 25% 30% 24% 12% 4	550
April	1st to 10th	Day	5% 16% 29% 25% 17% 9%	500
	11th to 20th	Day	4 20% 34% 23% 14% 6%	492
	21st to end	Day	2 18% 31% 24% 16% 9%	500
May	1st to 10th	Day	6% 15% 30% 27% 17% 6%	503
	11th to 20th	Day	2 13% 30% 30% 17% 8%	528
	21st to end	Day	3 11% 29% 29% 20% 8%	573
June	1st to 10th	Day	1 6% 24% 34% 24% 10%	530
	11th to 20th	Day	2 5% 28% 33% 20% 11%	530
	21st to end	Day	1 2 22% 40% 24% 11%	530
July	1st to 10th	Day	1 1 14% 39% 29% 16%	526
	11th to 20th	Day	3 1 9% 32% 31% 25%	524
	21st to end	Day	4 1 8% 29% 31% 26%	575
August	1st to 10th	Day	4 1 8% 33% 31% 23%	527
	11th to 20th	Day	4 1 9% 28% 37% 21%	517
	21st to end	Day	3 2 8% 28% 31% 27%	583
September	1st to 10th	Day	3 3 12% 31% 31% 20%	530
	11th to 20th	Day	2 4 20% 35% 25% 13%	530
	21st to end	Day	4 8% 18% 31% 28% 12%	530
October	1st to 10th	Day	4 7% 25% 24% 28% 13%	520
	11th to 20th	Day	2 5% 17% 32% 30% 14%	330
	21st to end	Day	1 7% 18% 28% 32% 15%	363
November	1st to 10th	Day	2 8% 17% 34% 27% 12%	330
	11th to 20th	Day	3 10% 16% 30% 28% 14%	330
	21st to end	Day	4 11% 17% 33% 27% 8%	330
December	1st to 10th	Day	6% 5% 16% 36% 28% 8%	330
	11th to 20th	Day	1 5% 23% 35% 25% 11%	330
	21st to end	Day	9% 11% 20% 34% 22% 4	363
January	1st to 10th	Night	5% 5% 21% 40% 24% 6%	804
	11th to 20th	Night	5% 4 22% 41% 24% 5%	800
	21st to end	Night	6% 5% 24% 40% 23% 2	880
February	1st to 10th	Night	4 5% 25% 41% 22% 3	800
	11th to 20th	Night	4 6% 25% 44% 20% 2	784
	21st to end	Night	6% 8% 25% 42% 17% 2	660
March	1st to 10th	Night	4 7% 28% 43% 18% 1	800
	11th to 20th	Night	5% 7% 20% 39% 25% 4	800
	21st to end	Night	3 7% 25% 42% 21% 1	880
April	1st to 10th	Night	2 5% 26% 44% 21% 2	800
	11th to 20th	Night	3 6% 30% 38% 22% 2	702
	21st to end	Night	1 5% 29% 40% 22% 3	600
May	1st to 10th	Night	3 5% 19% 49% 24% 1	600
	11th to 20th	Night	2 2 22% 40% 32% 3	591
	21st to end	Night	2 2 18% 49% 26% 3	650
June	1st to 10th	Night	2 2 13% 47% 34% 2	600
	11th to 20th	Night	1 1 16% 46% 34% 2	600
	21st to end	Night	2 1 9% 39% 46% 4	600
July	1st to 10th	Night	1 4 36% 53% 5%	585
	11th to 20th	Night	3 2 21% 63% 10%	582
	21st to end	Night	2 1 2 25% 58% 13%	657
August	1st to 10th	Night	3 2 25% 59% 10%	597
	11th to 20th	Night	2 1 2 23% 60% 12%	594
	21st to end	Night	3 3 19% 58% 17%	815
September	1st to 10th	Night	2 1 5% 27% 54% 12%	800
	11th to 20th	Night	2 1 8% 37% 48% 5%	800
	21st to end	Night	2 1 14% 37% 39% 7%	800
October	1st to 10th	Night	3 2 16% 38% 37% 4	800
	11th to 20th	Night	1 3 14% 44% 33% 5%	800
	21st to end	Night	1 4 14% 40% 33% 8%	880
November	1st to 10th	Night	1 3 15% 45% 30% 5%	820
	11th to 20th	Night	2 3 17% 42% 30% 6%	840
	21st to end	Night	4 4 21% 39% 28% 4	840
December	1st to 10th	Night	6% 4 15% 44% 29% 2	840
	11th to 20th	Night	5% 5% 18% 43% 27% 2	840
	21st to end	Night	6% 6% 22% 44% 18% 4	924

Page generated at 2025-12-29 23:14:10. Version ac07

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