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 2025-07-06.

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% 8% 3 7% 4 4 5% 4 6% 8% 45%	123743
	Night	7% 9% 3 8% 5% 5% 5% 4 6% 7% 42%	123879
Transparency	Day	35% 6% 13% 25% 16% 5%	123470
	Night	39% 6% 14% 24% 14% 3	123740
Seeing	Day	11% 9% 25% 30% 20% 5%	16417
	Night	12% 9% 30% 34% 13% 1	25782

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	9% 17% 6% 11% 6% 5% 5% 4 6% 7% 25%	3179
	11th to 20th	Day	6% 12% 4 9% 7% 6% 5% 3 5% 7% 35%	3198
	21st to end	Day	3 12% 3 8% 4 3 4 4 6% 8% 47%	4216
February	1st to 10th	Day	9% 15% 4 11% 8% 5% 5% 4 6% 6% 29%	1800
	11th to 20th	Day	6% 15% 5% 11% 6% 6% 7% 5% 5% 6% 28%	1755
	21st to end	Day	5% 12% 4 11% 6% 5% 5% 4 7% 8% 32%	1495
March	1st to 10th	Day	5% 8% 3 7% 5% 5% 5% 5% 7% 9% 41%	4814
	11th to 20th	Day	3 7% 3 8% 7% 6% 6% 5% 7% 8% 40%	5000
	21st to end	Day	10% 15% 6% 12% 6% 5% 6% 5% 6% 7% 22%	5486
April	1st to 10th	Day	8% 11% 3 8% 5% 5% 6% 6% 6% 7% 35%	2367
	11th to 20th	Day	12% 10% 4 8% 5% 5% 5% 4 7% 7% 34%	2567
	21st to end	Day	12% 9% 2 7% 5% 4 5% 4 7% 8% 37%	2600
May	1st to 10th	Day	15% 9% 3 6% 3 3 4 3 5% 7% 42%	6103
	11th to 20th	Day	13% 8% 3 7% 5% 5% 5% 5% 7% 8% 35%	6298
	21st to end	Day	10% 6% 2 7% 4 4 5% 4 6% 7% 44%	7260
June	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 7% 8% 44%	3000
	11th to 20th	Day	13% 5% 3 6% 5% 6% 5% 4 6% 9% 38%	3000
	21st to end	Day	5% 2 1 4 4 4 5% 5% 7% 9% 55%	3000
July	1st to 10th	Day	4 3 1 4 4 4 4 4 6% 9% 58%	4816
	11th to 20th	Day	2 2 1 3 2 2 2 2 4 7% 74%	4740
	21st to end	Day	2 2 1 2 2 2 3 3 5% 7% 71%	4973
August	1st to 10th	Day	6% 4 1 5% 3 3 4 3 6% 9% 55%	4092
	11th to 20th	Day	4 3 1 4 3 3 4 4 7% 8% 57%	4028
	21st to end	Day	5% 3 1 4 3 2 3 3 6% 7% 63%	4521
September	1st to 10th	Day	8% 4 1 3 2 3 4 3 5% 7% 59%	2299
	11th to 20th	Day	9% 5% 2 6% 5% 3 4 4 5% 7% 49%	2135
	21st to end	Day	11% 4 2 4 2 3 3 3 5% 7% 56%	2090
October	1st to 10th	Day	7% 6% 2 5% 3 4 3 3 5% 7% 55%	3175
	11th to 20th	Day	5% 5% 2 5% 4 3 4 4 6% 8% 54%	3150
	21st to end	Day	5% 7% 2 6% 4 4 5% 4 7% 8% 48%	3232
November	1st to 10th	Day	11% 11% 3 7% 5% 5% 3 4 6% 7% 37%	1574
	11th to 20th	Day	5% 9% 3 8% 7% 6% 5% 3 7% 9% 37%	1600
	21st to end	Day	5% 11% 4 8% 6% 5% 5% 4 6% 9% 37%	1600
December	1st to 10th	Day	5% 12% 4 8% 6% 6% 7% 5% 6% 8% 32%	2437
	11th to 20th	Day	6% 11% 4 9% 5% 5% 5% 4 6% 8% 39%	2645
	21st to end	Day	7% 12% 4 9% 5% 4 5% 4 6% 6% 38%	3498
January	1st to 10th	Night	7% 14% 5% 10% 5% 4 5% 4 6% 8% 31%	6766
	11th to 20th	Night	5% 13% 5% 11% 6% 6% 5% 3 5% 6% 35%	6780
	21st to end	Night	4 9% 4 10% 5% 4 4 4 5% 7% 45%	6760
February	1st to 10th	Night	9% 14% 5% 12% 7% 5% 5% 4 5% 6% 28%	2600
	11th to 20th	Night	7% 15% 5% 10% 6% 5% 6% 4 6% 6% 30%	2532
	21st to end	Night	5% 13% 5% 10% 7% 5% 5% 4 6% 7% 33%	2130
March	1st to 10th	Night	4 9% 4 8% 6% 6% 6% 5% 7% 8% 38%	5079
	11th to 20th	Night	3 9% 3 8% 5% 5% 5% 4 6% 9% 43%	5000
	21st to end	Night	12% 17% 5% 10% 6% 4 5% 4 6% 7% 23%	5489
April	1st to 10th	Night	9% 11% 3 7% 6% 5% 5% 4 7% 8% 35%	2030
	11th to 20th	Night	11% 12% 3 8% 5% 5% 5% 4 6% 8% 32%	1781
	21st to end	Night	12% 8% 2 7% 5% 5% 5% 5% 7% 8% 37%	1800
May	1st to 10th	Night	16% 8% 2 4 3 3 4 3 5% 6% 45%	3875
	11th to 20th	Night	12% 9% 3 9% 6% 5% 6% 4 6% 9% 30%	3617
	21st to end	Night	11% 6% 2 6% 4 4 4 4 6% 9% 43%	3616
June	1st to 10th	Night	14% 4 2 6% 4 5% 5% 4 7% 8% 40%	1400
	11th to 20th	Night	13% 6% 2 5% 5% 5% 5% 4 7% 7% 41%	1400
	21st to end	Night	5% 4 1 4 5% 4 4 5% 6% 9% 53%	1400
July	1st to 10th	Night	4 3 2 5% 4 4 5% 4 5% 9% 57%	2255
	11th to 20th	Night	2 2 1 2 2 2 3 2 5% 6% 74%	2220
	21st to end	Night	3 2 1 4 4 4 5% 4 6% 7% 62%	2580
August	1st to 10th	Night	7% 4 2 5% 4 3 4 4 5% 8% 54%	2847
	11th to 20th	Night	6% 3 2 5% 5% 5% 6% 5% 8% 10% 47%	2811
	21st to end	Night	10% 4 1 4 3 3 4 3 5% 7% 55%	3146
September	1st to 10th	Night	9% 4 2 4 3 3 4 3 5% 7% 56%	1881
	11th to 20th	Night	12% 6% 1 5% 5% 3 4 3 5% 7% 49%	2042
	21st to end	Night	11% 4 1 4 3 4 4 4 6% 6% 52%	2090
October	1st to 10th	Night	7% 5% 2 5% 3 3 3 2 4 6% 58%	3795
	11th to 20th	Night	8% 5% 2 6% 4 4 4 3 5% 6% 54%	3778
	21st to end	Night	6% 5% 2 6% 4 4 5% 4 6% 9% 49%	4324
November	1st to 10th	Night	12% 10% 3 6% 4 5% 5% 4 5% 7% 38%	2704
	11th to 20th	Night	6% 9% 3 8% 5% 6% 5% 4 5% 7% 40%	2800
	21st to end	Night	5% 13% 3 7% 5% 4 4 4 6% 8% 41%	2800
December	1st to 10th	Night	3 10% 4 9% 6% 6% 5% 4 7% 8% 38%	4747
	11th to 20th	Night	5% 12% 5% 10% 6% 5% 5% 4 5% 7% 35%	5546
	21st to end	Night	5% 10% 4 8% 5% 5% 5% 4 6% 7% 41%	7458

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	57% 7% 15% 19% 2	3179
	11th to 20th	Day	50% 7% 13% 17% 14% 1	3198
	21st to end	Day	35% 6% 19% 31% 8% 1	4216
February	1st to 10th	Day	54% 8% 13% 16% 8% 1	1800
	11th to 20th	Day	53% 11% 13% 17% 6% 1	1764
	21st to end	Day	47% 7% 15% 20% 9% 1	1495
March	1st to 10th	Day	34% 7% 13% 29% 16% 2	4814
	11th to 20th	Day	38% 8% 18% 23% 11% 2	5000
	21st to end	Day	58% 7% 14% 17% 3	5486
April	1st to 10th	Day	43% 10% 16% 22% 9% 1	2367
	11th to 20th	Day	46% 8% 15% 21% 10% 1	2570
	21st to end	Day	42% 8% 14% 26% 9% 1	2600
May	1st to 10th	Day	41% 5% 10% 26% 14% 4	6103
	11th to 20th	Day	41% 8% 16% 24% 10% 1	6302
	21st to end	Day	34% 9% 12% 27% 17% 1	7261
June	1st to 10th	Day	35% 9% 18% 26% 11% 1	3000
	11th to 20th	Day	40% 8% 14% 21% 14% 4	3000
	21st to end	Day	23% 7% 15% 33% 19% 3	3000
July	1st to 10th	Day	24% 4 9% 30% 24% 9%	4742
	11th to 20th	Day	13% 3 6% 21% 34% 23%	4659
	21st to end	Day	13% 4 6% 19% 33% 27%	4815
August	1st to 10th	Day	27% 3 10% 24% 25% 11%	4078
	11th to 20th	Day	24% 3 11% 35% 21% 5%	4067
	21st to end	Day	21% 3 9% 29% 30% 8%	4520
September	1st to 10th	Day	24% 6% 11% 27% 21% 12%	2299
	11th to 20th	Day	31% 8% 14% 26% 17% 5%	2135
	21st to end	Day	27% 5% 12% 30% 20% 7%	2090
October	1st to 10th	Day	30% 4 9% 33% 20% 5%	3175
	11th to 20th	Day	27% 5% 13% 35% 17% 3	3150
	21st to end	Day	33% 6% 11% 29% 18% 3	3232
November	1st to 10th	Day	44% 8% 14% 22% 9% 2	1574
	11th to 20th	Day	42% 8% 18% 22% 9% 1	1600
	21st to end	Day	43% 7% 21% 20% 8% 2	1600
December	1st to 10th	Day	48% 6% 17% 22% 6% 1	2437
	11th to 20th	Day	43% 5% 25% 21% 5% 1	2644
	21st to end	Day	45% 6% 11% 19% 14% 4	3498
January	1st to 10th	Night	48% 7% 17% 22% 6%	6766
	11th to 20th	Night	51% 5% 16% 16% 12% 1	6780
	21st to end	Night	38% 7% 16% 29% 9% 2	6760
February	1st to 10th	Night	55% 7% 14% 15% 8% 1	2600
	11th to 20th	Night	52% 9% 14% 17% 8% 1	2533
	21st to end	Night	48% 6% 15% 19% 9% 2	2130
March	1st to 10th	Night	38% 7% 14% 27% 13% 1	5079
	11th to 20th	Night	36% 8% 17% 30% 9% 1	5000
	21st to end	Night	57% 7% 16% 14% 4	5489
April	1st to 10th	Night	45% 8% 15% 23% 8% 1	2030
	11th to 20th	Night	48% 7% 13% 21% 10% 1	1781
	21st to end	Night	41% 7% 16% 25% 11% 1	1800
May	1st to 10th	Night	40% 5% 9% 26% 19% 2	3872
	11th to 20th	Night	46% 9% 16% 19% 10% 1	3618
	21st to end	Night	33% 8% 14% 26% 17% 2	3616
June	1st to 10th	Night	37% 9% 19% 23% 10% 2	1400
	11th to 20th	Night	38% 8% 14% 23% 14% 4	1400
	21st to end	Night	26% 7% 15% 29% 20% 3	1400
July	1st to 10th	Night	26% 3 8% 30% 25% 8%	2220
	11th to 20th	Night	13% 4 4 25% 34% 20%	2178
	21st to end	Night	19% 4 5% 20% 30% 22%	2500
August	1st to 10th	Night	29% 4 11% 23% 21% 12%	2838
	11th to 20th	Night	32% 4 12% 27% 19% 6%	2838
	21st to end	Night	29% 4 9% 29% 23% 7%	3146
September	1st to 10th	Night	27% 5% 9% 24% 22% 14%	1881
	11th to 20th	Night	33% 6% 12% 24% 19% 7%	2042
	21st to end	Night	31% 6% 13% 24% 20% 6%	2090
October	1st to 10th	Night	28% 4 8% 32% 23% 5%	3795
	11th to 20th	Night	32% 4 13% 29% 21% 2	3778
	21st to end	Night	32% 5% 11% 28% 21% 3	4324
November	1st to 10th	Night	43% 8% 14% 21% 11% 3	2704
	11th to 20th	Night	40% 8% 17% 23% 11% 1	2800
	21st to end	Night	40% 6% 19% 24% 9% 2	2800
December	1st to 10th	Night	42% 7% 20% 20% 9% 2	4747
	11th to 20th	Night	47% 7% 22% 19% 5% 1	5547
	21st to end	Night	40% 6% 11% 26% 14% 4	7458

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% 19% 16% 9%	200
	11th to 20th	Day	11% 10% 32% 27% 15% 7%	200
	21st to end	Day	10% 9% 36% 30% 12% 2	267
February	1st to 10th	Day	18% 13% 30% 32% 5% 2	300
	11th to 20th	Day	12% 12% 28% 30% 14% 4	300
	21st to end	Day	9% 13% 34% 27% 15% 2	259
March	1st to 10th	Day	11% 11% 31% 29% 15% 3	500
	11th to 20th	Day	10% 11% 31% 32% 11% 6%	500
	21st to end	Day	11% 11% 30% 28% 15% 5%	550
April	1st to 10th	Day	13% 13% 27% 34% 11% 1	500
	11th to 20th	Day	17% 12% 29% 29% 11% 2	492
	21st to end	Day	16% 12% 26% 26% 17% 3	500
May	1st to 10th	Day	14% 9% 22% 27% 22% 7%	596
	11th to 20th	Day	16% 6% 21% 30% 22% 7%	725
	21st to end	Day	16% 9% 19% 25% 23% 8%	789
June	1st to 10th	Day	13% 9% 22% 27% 21% 7%	730
	11th to 20th	Day	17% 11% 30% 25% 14% 3	730
	21st to end	Day	8% 5% 22% 36% 24% 6%	730
July	1st to 10th	Day	6% 3 18% 36% 31% 6%	697
	11th to 20th	Day	4 5% 21% 41% 25% 4	668
	21st to end	Day	3 3 17% 41% 28% 7%	709
August	1st to 10th	Day	3 6% 20% 37% 26% 8%	487
	11th to 20th	Day	7% 4 13% 34% 30% 12%	477
	21st to end	Day	5% 6% 21% 29% 29% 11%	539
September	1st to 10th	Day	8% 7% 19% 33% 26% 7%	490
	11th to 20th	Day	11% 8% 22% 26% 24% 9%	490
	21st to end	Day	15% 10% 23% 26% 18% 8%	490
October	1st to 10th	Day	12% 6% 21% 29% 25% 8%	305
	11th to 20th	Day	12% 7% 28% 28% 21% 5%	300
	21st to end	Day	17% 12% 31% 23% 15% 3	330
November	1st to 10th	Day	22% 10% 25% 29% 12% 2	300
	11th to 20th	Day	9% 12% 42% 26% 10% 1	300
	21st to end	Day	9% 11% 33% 33% 12% 1	300
December	1st to 10th	Day	13% 13% 34% 21% 16% 2	247
	11th to 20th	Day	15% 14% 30% 25% 14% 2	200
	21st to end	Day	14% 10% 27% 31% 15% 3	220
January	1st to 10th	Night	13% 12% 35% 29% 10% 1	1000
	11th to 20th	Night	14% 10% 38% 29% 8% 1	1000
	21st to end	Night	11% 12% 40% 28% 9% 1	892
February	1st to 10th	Night	16% 12% 28% 32% 10% 2	800
	11th to 20th	Night	13% 9% 35% 34% 9% 1	784
	21st to end	Night	11% 11% 31% 32% 14% 1	660
March	1st to 10th	Night	11% 10% 32% 33% 13% 1	800
	11th to 20th	Night	9% 9% 34% 35% 12% 1	800
	21st to end	Night	16% 9% 33% 31% 10% 2	880
April	1st to 10th	Night	12% 11% 33% 35% 9%	645
	11th to 20th	Night	16% 10% 31% 33% 10% 1	593
	21st to end	Night	16% 10% 27% 38% 10% 1	600
May	1st to 10th	Night	15% 9% 23% 36% 17% 1	600
	11th to 20th	Night	14% 7% 20% 39% 18% 1	591
	21st to end	Night	20% 7% 22% 29% 20% 3	502
June	1st to 10th	Night	15% 8% 26% 32% 17% 2	400
	11th to 20th	Night	16% 9% 32% 34% 10% 1	400
	21st to end	Night	7% 6% 23% 45% 19% 1	400
July	1st to 10th	Night	6% 3 22% 46% 21% 1	380
	11th to 20th	Night	5% 4 27% 51% 14%	368
	21st to end	Night	5% 4 22% 50% 19%	482
August	1st to 10th	Night	7% 3 23% 47% 19% 1	567
	11th to 20th	Night	8% 3 20% 45% 23% 2	564
	21st to end	Night	11% 4 22% 42% 20% 1	627
September	1st to 10th	Night	11% 4 21% 47% 16% 1	570
	11th to 20th	Night	15% 5% 20% 36% 22% 1	712
	21st to end	Night	13% 6% 25% 35% 18% 2	760
October	1st to 10th	Night	15% 6% 24% 35% 19% 1	760
	11th to 20th	Night	14% 7% 31% 33% 14% 1	760
	21st to end	Night	13% 10% 35% 31% 11%	836
November	1st to 10th	Night	18% 9% 33% 32% 8%	949
	11th to 20th	Night	11% 14% 40% 26% 9%	1000
	21st to end	Night	9% 10% 37% 37% 8%	1000
December	1st to 10th	Night	10% 13% 34% 30% 12% 2	1000
	11th to 20th	Night	11% 10% 37% 31% 10% 1	1000
	21st to end	Night	14% 9% 36% 30% 11% 1	1100

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