Wood Brothers Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Wood Brothers Observatory clear sky chart from 2005-11-06 to 2026-10-29.

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	7% 7% 2 6% 4 4 4 4 6% 8% 47%	159164
	Night	7% 8% 3 7% 5% 4 5% 4 6% 8% 43%	150310
Transparency	Day	34% 5% 13% 25% 16% 6%	153842
	Night	39% 5% 14% 24% 14% 4	146640
Seeing	Day	11% 9% 25% 30% 20% 5%	18214
	Night	12% 9% 30% 35% 13% 1	27462

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	7% 13% 5% 9% 5% 4 5% 4 7% 9% 31%	4124
	11th to 20th	Day	5% 12% 4 9% 6% 6% 5% 4 6% 7% 36%	4158
	21st to end	Day	3 11% 3 9% 4 4 4 3 6% 7% 46%	5468
February	1st to 10th	Day	9% 14% 4 10% 7% 5% 4 4 6% 6% 30%	1890
	11th to 20th	Day	6% 15% 5% 11% 6% 6% 7% 5% 5% 6% 29%	1845
	21st to end	Day	5% 12% 4 11% 6% 5% 6% 4 7% 8% 32%	1568
March	1st to 10th	Day	5% 8% 3 7% 5% 5% 5% 4 7% 9% 41%	4920
	11th to 20th	Day	3 7% 3 8% 7% 6% 6% 5% 7% 8% 39%	5110
	21st to end	Day	10% 15% 6% 12% 6% 6% 6% 5% 6% 7% 22%	5607
April	1st to 10th	Day	8% 10% 3 8% 5% 5% 6% 6% 6% 7% 36%	2486
	11th to 20th	Day	12% 9% 4 8% 5% 5% 5% 4 7% 7% 35%	2697
	21st to end	Day	12% 9% 3 7% 5% 4 5% 4 7% 8% 37%	2730
May	1st to 10th	Day	13% 8% 3 5% 3 4 4 3 6% 8% 43%	7917
	11th to 20th	Day	13% 8% 2 7% 5% 4 5% 4 6% 8% 37%	8191
	21st to end	Day	10% 6% 2 6% 4 4 4 4 7% 8% 45%	9452
June	1st to 10th	Day	12% 5% 2 6% 4 4 5% 4 7% 8% 43%	3150
	11th to 20th	Day	13% 5% 2 6% 5% 5% 5% 5% 6% 9% 39%	3150
	21st to end	Day	5% 3 1 5% 4 4 5% 5% 6% 9% 53%	3150
July	1st to 10th	Day	3 2 1 4 3 4 4 4 6% 9% 60%	8819
	11th to 20th	Day	5% 2 1 3 2 3 3 3 5% 8% 64%	8820
	21st to end	Day	2 2 1 3 2 3 3 3 5% 8% 68%	9263
August	1st to 10th	Day	4 3 1 4 3 3 4 3 6% 9% 58%	7549
	11th to 20th	Day	5% 3 1 4 3 3 4 4 7% 8% 59%	7565
	21st to end	Day	6% 4 2 5% 3 3 3 3 6% 7% 58%	8374
September	1st to 10th	Day	8% 4 1 4 2 3 4 3 5% 7% 58%	2541
	11th to 20th	Day	9% 4 2 6% 5% 4 4 4 6% 7% 49%	2359
	21st to end	Day	10% 4 2 4 2 2 3 3 5% 7% 58%	2310
October	1st to 10th	Day	7% 5% 2 5% 3 4 3 3 4 7% 56%	3375
	11th to 20th	Day	5% 6% 2 5% 4 3 4 4 6% 8% 53%	3350
	21st to end	Day	5% 7% 2 6% 4 4 5% 4 7% 8% 49%	3411
November	1st to 10th	Day	11% 12% 3 8% 5% 5% 3 4 6% 7% 37%	1654
	11th to 20th	Day	5% 9% 3 9% 7% 6% 5% 4 7% 9% 36%	1680
	21st to end	Day	5% 11% 4 8% 6% 5% 5% 4 6% 9% 37%	1680
December	1st to 10th	Day	5% 12% 4 8% 6% 6% 7% 5% 7% 8% 32%	2511
	11th to 20th	Day	6% 11% 4 9% 5% 5% 5% 4 6% 8% 38%	2715
	21st to end	Day	7% 11% 4 9% 5% 4 5% 4 5% 6% 38%	3575
January	1st to 10th	Night	6% 12% 5% 10% 5% 5% 5% 4 6% 9% 35%	8768
	11th to 20th	Night	5% 11% 4 10% 6% 6% 5% 4 5% 7% 38%	8820
	21st to end	Night	4 9% 4 11% 5% 4 4 4 5% 7% 42%	8738
February	1st to 10th	Night	9% 14% 5% 11% 7% 5% 6% 4 6% 6% 28%	2730
	11th to 20th	Night	7% 15% 5% 10% 6% 5% 6% 4 6% 6% 30%	2662
	21st to end	Night	5% 13% 5% 10% 6% 5% 5% 3 6% 8% 34%	2233
March	1st to 10th	Night	4 9% 4 8% 6% 6% 6% 5% 7% 8% 38%	5193
	11th to 20th	Night	3 9% 3 8% 5% 5% 5% 4 6% 9% 42%	5110
	21st to end	Night	12% 17% 5% 10% 6% 4 5% 4 6% 7% 23%	5610
April	1st to 10th	Night	9% 11% 3 7% 6% 5% 5% 4 7% 8% 35%	2131
	11th to 20th	Night	11% 12% 3 8% 5% 5% 5% 4 6% 8% 32%	1871
	21st to end	Night	12% 8% 2 7% 4 5% 5% 5% 7% 8% 37%	1890
May	1st to 10th	Night	15% 8% 2 5% 4 3 4 3 5% 6% 45%	5028
	11th to 20th	Night	11% 9% 3 8% 6% 5% 5% 4 6% 9% 33%	4701
	21st to end	Night	11% 5% 2 6% 4 4 5% 5% 7% 9% 42%	4727
June	1st to 10th	Night	14% 4 2 6% 4 5% 6% 4 7% 9% 39%	1470
	11th to 20th	Night	14% 6% 2 5% 5% 5% 5% 4 7% 7% 40%	1470
	21st to end	Night	6% 3 2 5% 5% 4 5% 5% 6% 9% 52%	1470
July	1st to 10th	Night	4 2 1 5% 4 4 4 4 5% 8% 59%	4140
	11th to 20th	Night	6% 3 1 3 2 3 3 3 5% 7% 63%	4140
	21st to end	Night	3 3 1 4 4 4 5% 4 7% 8% 57%	4807
August	1st to 10th	Night	6% 3 2 5% 4 4 4 4 5% 8% 55%	5273
	11th to 20th	Night	7% 3 1 5% 4 4 5% 4 7% 10% 50%	5291
	21st to end	Night	9% 4 2 4 3 4 4 3 5% 7% 55%	5842
September	1st to 10th	Night	9% 4 2 4 3 3 3 3 5% 8% 56%	2079
	11th to 20th	Night	11% 6% 2 5% 5% 3 4 3 6% 7% 48%	2257
	21st to end	Night	10% 4 1 4 3 4 4 4 6% 6% 53%	2310
October	1st to 10th	Night	7% 5% 2 5% 3 3 3 2 4 6% 58%	4035
	11th to 20th	Night	8% 5% 2 6% 4 4 4 3 5% 7% 53%	4018
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 6% 9% 49%	4562
November	1st to 10th	Night	12% 10% 3 6% 4 5% 5% 4 5% 7% 38%	2843
	11th to 20th	Night	6% 9% 3 8% 5% 6% 5% 4 5% 7% 41%	2940
	21st to end	Night	5% 13% 3 7% 5% 4 5% 4 6% 8% 41%	2940
December	1st to 10th	Night	4 11% 4 9% 6% 6% 5% 4 7% 8% 37%	4892
	11th to 20th	Night	6% 12% 5% 10% 6% 5% 5% 4 5% 7% 35%	5696
	21st to end	Night	5% 10% 4 8% 5% 5% 5% 4 6% 8% 41%	7623

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	48% 6% 18% 23% 4 1	4125
	11th to 20th	Day	47% 5% 13% 20% 12% 2	4158
	21st to end	Day	37% 5% 19% 28% 10% 1	5468
February	1st to 10th	Day	52% 7% 13% 17% 9% 1	1890
	11th to 20th	Day	52% 10% 12% 18% 7% 1	1854
	21st to end	Day	47% 7% 15% 20% 9% 2	1568
March	1st to 10th	Day	34% 6% 13% 28% 16% 2	4920
	11th to 20th	Day	38% 8% 18% 24% 11% 1	5110
	21st to end	Day	58% 7% 14% 17% 4	5607
April	1st to 10th	Day	42% 10% 16% 23% 8% 1	2486
	11th to 20th	Day	45% 7% 15% 22% 10% 1	2700
	21st to end	Day	42% 7% 14% 26% 10% 1	2730
May	1st to 10th	Day	39% 4 11% 27% 16% 3	7917
	11th to 20th	Day	42% 6% 13% 23% 13% 4	8195
	21st to end	Day	34% 7% 11% 29% 18% 1	9453
June	1st to 10th	Day	36% 8% 18% 26% 11% 1	3150
	11th to 20th	Day	39% 8% 13% 22% 14% 4	3150
	21st to end	Day	25% 7% 15% 32% 18% 3	3150
July	1st to 10th	Day	22% 2 13% 29% 25% 8%	8746
	11th to 20th	Day	20% 2 9% 24% 28% 17%	8739
	21st to end	Day	16% 2 11% 19% 26% 25%	9055
August	1st to 10th	Day	21% 2 8% 25% 28% 17%	5978
	11th to 20th	Day	28% 2 15% 34% 17% 4	5964
	21st to end	Day	25% 2 8% 32% 27% 6%	6569
September	1st to 10th	Day	24% 5% 11% 28% 20% 11%	2541
	11th to 20th	Day	31% 7% 14% 27% 16% 4	2359
	21st to end	Day	26% 4 11% 31% 21% 7%	2310
October	1st to 10th	Day	29% 4 9% 33% 20% 5%	3375
	11th to 20th	Day	29% 5% 12% 34% 17% 3	3350
	21st to end	Day	33% 5% 12% 29% 18% 3	3411
November	1st to 10th	Day	44% 8% 14% 22% 9% 3	1654
	11th to 20th	Day	42% 8% 18% 23% 8% 1	1680
	21st to end	Day	43% 7% 21% 20% 8% 2	1680
December	1st to 10th	Day	48% 6% 18% 22% 6% 1	2511
	11th to 20th	Day	43% 5% 25% 21% 5% 1	2714
	21st to end	Day	44% 6% 12% 20% 14% 4	3575
January	1st to 10th	Night	45% 5% 17% 25% 7% 1	8769
	11th to 20th	Night	47% 4 18% 19% 11% 2	8820
	21st to end	Night	41% 5% 16% 28% 9% 1	8738
February	1st to 10th	Night	54% 6% 14% 16% 9% 1	2730
	11th to 20th	Night	51% 9% 14% 17% 8% 1	2663
	21st to end	Night	47% 6% 15% 21% 9% 2	2233
March	1st to 10th	Night	39% 7% 14% 27% 13% 1	5193
	11th to 20th	Night	36% 7% 17% 30% 10% 1	5110
	21st to end	Night	57% 7% 16% 15% 5%	5610
April	1st to 10th	Night	44% 7% 16% 23% 8% 1	2131
	11th to 20th	Night	48% 7% 13% 21% 9% 1	1871
	21st to end	Night	41% 7% 15% 25% 12% 1	1890
May	1st to 10th	Night	40% 4 8% 26% 20% 2	5026
	11th to 20th	Night	45% 7% 13% 20% 14% 1	4702
	21st to end	Night	34% 6% 13% 29% 16% 1	4728
June	1st to 10th	Night	38% 9% 18% 23% 11% 2	1470
	11th to 20th	Night	39% 7% 13% 23% 14% 4	1470
	21st to end	Night	28% 7% 14% 28% 19% 3	1470
July	1st to 10th	Night	26% 2 11% 27% 26% 9%	4105
	11th to 20th	Night	23% 2 7% 25% 27% 15%	4098
	21st to end	Night	24% 3 9% 18% 25% 21%	4703
August	1st to 10th	Night	24% 2 9% 20% 24% 20%	4168
	11th to 20th	Night	35% 3 15% 29% 14% 4	4168
	21st to end	Night	29% 3 8% 32% 22% 7%	4578
September	1st to 10th	Night	26% 4 9% 27% 22% 12%	2079
	11th to 20th	Night	34% 5% 12% 24% 18% 6%	2257
	21st to end	Night	30% 5% 12% 24% 22% 6%	2310
October	1st to 10th	Night	28% 4 8% 32% 23% 5%	4035
	11th to 20th	Night	32% 4 12% 29% 21% 2	4018
	21st to end	Night	31% 5% 12% 29% 20% 3	4562
November	1st to 10th	Night	43% 7% 14% 22% 11% 3	2843
	11th to 20th	Night	40% 8% 18% 24% 10% 1	2940
	21st to end	Night	40% 6% 19% 24% 9% 2	2940
December	1st to 10th	Night	43% 7% 20% 20% 9% 2	4892
	11th to 20th	Night	47% 6% 22% 18% 5% 1	5697
	21st to end	Night	40% 6% 11% 26% 14% 3	7623

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	12% 15% 32% 18% 15% 8%	220
	11th to 20th	Day	12% 12% 32% 25% 13% 6%	220
	21st to end	Day	10% 10% 35% 32% 12% 2	294
February	1st to 10th	Day	17% 13% 32% 30% 5% 2	330
	11th to 20th	Day	12% 12% 27% 32% 14% 3	330
	21st to end	Day	10% 13% 35% 26% 14% 2	284
March	1st to 10th	Day	12% 11% 31% 29% 15% 3	540
	11th to 20th	Day	11% 13% 30% 31% 10% 5%	540
	21st to end	Day	12% 12% 30% 27% 15% 4	594
April	1st to 10th	Day	14% 13% 26% 34% 12% 1	540
	11th to 20th	Day	16% 12% 29% 29% 11% 3	532
	21st to end	Day	15% 12% 27% 26% 17% 4	540
May	1st to 10th	Day	13% 10% 22% 27% 22% 6%	641
	11th to 20th	Day	16% 7% 21% 29% 21% 6%	775
	21st to end	Day	16% 9% 18% 26% 23% 8%	844
June	1st to 10th	Day	14% 9% 23% 27% 20% 6%	780
	11th to 20th	Day	16% 11% 30% 26% 14% 3	780
	21st to end	Day	8% 4 22% 37% 23% 5%	780
July	1st to 10th	Day	6% 3 18% 36% 31% 6%	771
	11th to 20th	Day	4 4 21% 41% 24% 4	768
	21st to end	Day	3 2 17% 41% 29% 7%	815
August	1st to 10th	Day	3 5% 20% 37% 27% 8%	567
	11th to 20th	Day	7% 4 13% 34% 31% 11%	557
	21st to end	Day	5% 6% 21% 31% 27% 11%	627
September	1st to 10th	Day	8% 7% 20% 32% 26% 7%	570
	11th to 20th	Day	12% 7% 21% 26% 25% 9%	570
	21st to end	Day	13% 10% 22% 27% 19% 9%	570
October	1st to 10th	Day	12% 6% 21% 29% 25% 8%	365
	11th to 20th	Day	14% 6% 28% 27% 21% 4	360
	21st to end	Day	16% 11% 30% 24% 15% 3	387
November	1st to 10th	Day	22% 12% 27% 27% 12% 2	330
	11th to 20th	Day	10% 12% 40% 25% 11% 2	330
	21st to end	Day	10% 12% 32% 33% 12% 1	330
December	1st to 10th	Day	14% 14% 34% 21% 15% 2	271
	11th to 20th	Day	17% 15% 31% 23% 13% 2	220
	21st to end	Day	12% 10% 26% 32% 16% 3	242
January	1st to 10th	Night	13% 12% 37% 28% 9% 1	1050
	11th to 20th	Night	14% 11% 38% 28% 8% 1	1050
	21st to end	Night	11% 12% 39% 28% 9% 1	936
February	1st to 10th	Night	16% 11% 28% 33% 10% 2	840
	11th to 20th	Night	13% 8% 35% 35% 9%	824
	21st to end	Night	11% 11% 31% 32% 13% 1	692
March	1st to 10th	Night	11% 10% 32% 33% 13% 1	840
	11th to 20th	Night	9% 9% 35% 35% 12% 1	840
	21st to end	Night	16% 9% 32% 31% 10% 2	924
April	1st to 10th	Night	12% 11% 33% 35% 9%	677
	11th to 20th	Night	16% 10% 30% 34% 9% 1	623
	21st to end	Night	16% 10% 27% 38% 9%	630
May	1st to 10th	Night	14% 8% 23% 36% 17% 1	630
	11th to 20th	Night	14% 8% 21% 39% 18% 1	621
	21st to end	Night	20% 7% 22% 30% 20% 2	528
June	1st to 10th	Night	15% 9% 26% 32% 16% 2	420
	11th to 20th	Night	16% 8% 32% 34% 10%	420
	21st to end	Night	8% 6% 23% 44% 18% 1	420
July	1st to 10th	Night	7% 3 21% 47% 21% 1	410
	11th to 20th	Night	5% 3 27% 50% 14%	408
	21st to end	Night	5% 4 22% 50% 19%	533
August	1st to 10th	Night	7% 4 23% 48% 18% 1	627
	11th to 20th	Night	8% 4 19% 46% 22% 1	624
	21st to end	Night	11% 5% 22% 43% 19% 1	693
September	1st to 10th	Night	10% 4 22% 47% 16% 1	630
	11th to 20th	Night	15% 5% 20% 37% 22% 1	787
	21st to end	Night	13% 6% 25% 37% 18% 2	840
October	1st to 10th	Night	14% 6% 24% 38% 18% 1	840
	11th to 20th	Night	14% 7% 31% 33% 14% 1	840
	21st to end	Night	12% 9% 35% 33% 11%	912
November	1st to 10th	Night	18% 9% 33% 32% 8%	998
	11th to 20th	Night	11% 14% 39% 27% 9%	1050
	21st to end	Night	9% 10% 36% 37% 8%	1050
December	1st to 10th	Night	11% 12% 34% 29% 12% 2	1050
	11th to 20th	Night	11% 11% 37% 30% 10% 1	1050
	21st to end	Night	14% 9% 36% 30% 10% 1	1155

Page generated at 2026-10-26 02:37:48. Version ac07

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