Bush Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Bush Observatory clear sky chart from 2005-11-06 to 2026-07-16.

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	12% 8% 3 7% 4 4 4 3 5% 7% 43%	138144
	Night	11% 7% 3 7% 4 4 4 3 5% 6% 46%	134858
Transparency	Day	39% 6% 13% 25% 16% 2	141751
	Night	37% 5% 12% 25% 18% 3	139330
Seeing	Day	15% 10% 29% 28% 15% 4	18084
	Night	14% 9% 32% 32% 12% 1	27911

By Month

Percentage Cloud Cover SE (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	12% 10% 4 9% 5% 4 4 3 5% 7% 37%	5148
	11th to 20th	Day	6% 8% 3 9% 6% 6% 7% 4 4 7% 40%	5310
	21st to end	Day	10% 13% 6% 12% 5% 5% 4 3 4 5% 33%	5812
February	1st to 10th	Day	11% 11% 4 8% 6% 5% 5% 4 6% 8% 33%	1890
	11th to 20th	Day	12% 11% 4 7% 5% 3 3 4 5% 7% 38%	1844
	21st to end	Day	11% 9% 3 7% 4 4 4 3 5% 6% 45%	1734
March	1st to 10th	Day	16% 9% 3 6% 3 3 3 2 3 4 47%	5046
	11th to 20th	Day	10% 7% 2 5% 4 4 4 3 4 8% 49%	5090
	21st to end	Day	16% 11% 5% 8% 5% 5% 5% 3 5% 7% 31%	5583
April	1st to 10th	Day	16% 8% 3 7% 4 4 3 3 5% 6% 40%	2457
	11th to 20th	Day	15% 9% 3 7% 4 4 4 4 4 7% 39%	2494
	21st to end	Day	22% 7% 2 6% 5% 4 5% 4 5% 7% 34%	2679
May	1st to 10th	Day	13% 7% 3 6% 4 4 4 3 6% 7% 42%	7617
	11th to 20th	Day	14% 6% 2 5% 4 4 4 3 5% 7% 47%	7627
	21st to end	Day	18% 8% 3 7% 4 4 4 3 5% 7% 37%	9047
June	1st to 10th	Day	7% 5% 2 6% 6% 4 5% 4 6% 8% 46%	2940
	11th to 20th	Day	7% 6% 2 6% 5% 4 4 4 7% 9% 46%	2940
	21st to end	Day	8% 5% 2 6% 5% 4 5% 5% 6% 8% 46%	2940
July	1st to 10th	Day	8% 6% 2 6% 4 4 5% 4 7% 10% 43%	6455
	11th to 20th	Day	8% 6% 2 6% 4 5% 5% 4 6% 8% 46%	5966
	21st to end	Day	7% 5% 2 5% 4 4 4 4 7% 10% 48%	6422
August	1st to 10th	Day	6% 4 1 5% 4 4 5% 5% 7% 11% 48%	4234
	11th to 20th	Day	11% 5% 2 6% 4 4 5% 4 6% 8% 45%	4171
	21st to end	Day	6% 4 2 6% 4 4 5% 4 6% 9% 51%	4414
September	1st to 10th	Day	12% 4 2 4 3 4 3 3 5% 7% 53%	2400
	11th to 20th	Day	9% 5% 2 5% 4 4 4 4 5% 9% 49%	2263
	21st to end	Day	11% 3 2 5% 3 3 3 3 5% 7% 55%	2163
October	1st to 10th	Day	13% 7% 2 5% 4 3 3 3 4 6% 51%	2000
	11th to 20th	Day	13% 7% 3 4 2 3 3 3 4 5% 54%	2000
	21st to end	Day	14% 9% 3 6% 3 3 3 4 5% 6% 45%	2200
November	1st to 10th	Day	9% 7% 2 6% 3 3 3 3 4 7% 53%	2034
	11th to 20th	Day	13% 8% 2 5% 4 4 3 2 5% 6% 48%	1696
	21st to end	Day	17% 10% 3 6% 3 2 3 4 5% 6% 41%	1680
December	1st to 10th	Day	6% 6% 2 7% 4 4 4 3 5% 6% 54%	2690
	11th to 20th	Day	11% 10% 3 9% 5% 4 4 3 4 5% 42%	3088
	21st to end	Day	19% 15% 6% 12% 5% 4 4 2 4 6% 23%	4070
January	1st to 10th	Night	10% 9% 5% 15% 6% 5% 5% 3 5% 5% 32%	7770
	11th to 20th	Night	6% 7% 4 8% 5% 5% 6% 4 6% 6% 43%	7680
	21st to end	Night	11% 13% 5% 11% 5% 4 3 2 3 4 38%	8402
February	1st to 10th	Night	11% 12% 3 8% 6% 4 4 3 5% 6% 38%	2730
	11th to 20th	Night	12% 9% 3 7% 5% 4 4 3 5% 6% 42%	2663
	21st to end	Night	10% 9% 3 6% 4 4 4 3 4 4 48%	2090
March	1st to 10th	Night	17% 7% 2 6% 3 3 3 2 4 4 49%	5029
	11th to 20th	Night	11% 6% 2 4 3 3 3 3 4 6% 54%	5090
	21st to end	Night	16% 10% 4 9% 4 4 4 3 5% 6% 34%	5588
April	1st to 10th	Night	13% 7% 3 5% 3 4 4 3 5% 7% 46%	2162
	11th to 20th	Night	14% 7% 2 6% 4 3 4 3 5% 6% 46%	2075
	21st to end	Night	17% 9% 3 6% 4 4 4 3 5% 6% 41%	1937
May	1st to 10th	Night	10% 7% 2 6% 5% 4 4 3 5% 7% 48%	5266
	11th to 20th	Night	14% 5% 2 5% 3 3 3 2 4 7% 51%	5248
	21st to end	Night	16% 7% 3 6% 4 3 4 3 5% 6% 41%	5139
June	1st to 10th	Night	6% 4 1 4 4 4 5% 4 7% 10% 52%	1680
	11th to 20th	Night	6% 3 2 5% 4 4 4 4 7% 11% 52%	1680
	21st to end	Night	5% 4 1 5% 3 4 4 4 5% 9% 55%	1680
July	1st to 10th	Night	8% 5% 1 6% 4 3 4 4 6% 9% 50%	3738
	11th to 20th	Night	5% 4 2 6% 4 4 5% 4 6% 8% 53%	4122
	21st to end	Night	8% 4 2 5% 4 4 5% 4 7% 11% 46%	4449
August	1st to 10th	Night	3 3 1 4 3 3 5% 4 6% 9% 61%	2926
	11th to 20th	Night	12% 6% 2 4 3 3 3 3 4 7% 53%	2888
	21st to end	Night	5% 4 2 5% 4 3 4 3 5% 7% 59%	3485
September	1st to 10th	Night	9% 4 2 4 3 3 3 3 4 7% 58%	2000
	11th to 20th	Night	10% 4 2 5% 3 3 4 4 5% 8% 53%	2133
	21st to end	Night	8% 4 2 4 4 2 4 4 4 7% 57%	2215
October	1st to 10th	Night	12% 6% 2 4 3 3 3 3 5% 6% 53%	2400
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 3 5% 54%	2400
	21st to end	Night	15% 7% 2 5% 3 3 3 3 4 4 50%	2640
November	1st to 10th	Night	9% 4 2 4 3 3 3 3 5% 7% 58%	2481
	11th to 20th	Night	11% 6% 2 5% 5% 3 4 3 4 7% 50%	2923
	21st to end	Night	15% 8% 3 6% 3 3 3 3 4 6% 46%	2940
December	1st to 10th	Night	7% 7% 3 6% 4 4 4 3 5% 6% 51%	4720
	11th to 20th	Night	11% 8% 3 8% 4 4 4 3 5% 6% 44%	5339
	21st to end	Night	20% 16% 6% 11% 4 3 3 3 4 4 27%	7150

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	47% 5% 13% 21% 12% 1	5149
	11th to 20th	Day	44% 4 11% 21% 15% 5%	5310
	21st to end	Day	54% 4 12% 20% 10% 1	5812
February	1st to 10th	Day	49% 6% 15% 19% 10% 1	1890
	11th to 20th	Day	44% 6% 13% 22% 13% 2	1853
	21st to end	Day	41% 5% 12% 25% 16% 2	1734
March	1st to 10th	Day	41% 4 8% 24% 19% 4	5046
	11th to 20th	Day	34% 5% 9% 25% 23% 4	5090
	21st to end	Day	52% 7% 17% 18% 5% 1	5585
April	1st to 10th	Day	43% 6% 11% 25% 13% 1	2457
	11th to 20th	Day	44% 6% 12% 23% 14% 1	2494
	21st to end	Day	48% 7% 11% 24% 10% 1	2679
May	1st to 10th	Day	39% 7% 12% 25% 17% 1	7655
	11th to 20th	Day	36% 7% 11% 28% 16% 2	7631
	21st to end	Day	44% 7% 10% 23% 15% 1	9061
June	1st to 10th	Day	33% 9% 16% 27% 13% 2	2940
	11th to 20th	Day	32% 8% 14% 25% 19% 1	2940
	21st to end	Day	32% 8% 17% 26% 14% 3	2940
July	1st to 10th	Day	34% 7% 15% 30% 13% 1	6386
	11th to 20th	Day	34% 8% 13% 25% 18% 2	5897
	21st to end	Day	29% 9% 17% 33% 11% 1	6240
August	1st to 10th	Day	28% 8% 16% 30% 17% 1	4220
	11th to 20th	Day	36% 7% 16% 26% 15% 1	4210
	21st to end	Day	30% 7% 16% 29% 14% 5%	4413
September	1st to 10th	Day	31% 6% 13% 27% 19% 4	2400
	11th to 20th	Day	32% 6% 13% 28% 20% 1	2263
	21st to end	Day	30% 5% 10% 30% 23% 2	2163
October	1st to 10th	Day	22% 3 6% 20% 46% 2	3260
	11th to 20th	Day	22% 3 7% 27% 33% 8%	3240
	21st to end	Day	35% 5% 10% 26% 22% 3	3538
November	1st to 10th	Day	32% 5% 15% 26% 19% 3	2034
	11th to 20th	Day	38% 4 12% 28% 16% 2	1696
	21st to end	Day	43% 6% 12% 25% 14%	1680
December	1st to 10th	Day	31% 5% 19% 30% 14% 1	2690
	11th to 20th	Day	44% 4 15% 25% 10% 1	3085
	21st to end	Day	64% 5% 13% 13% 5% 1	4070
January	1st to 10th	Night	52% 7% 12% 19% 9% 1	7771
	11th to 20th	Night	39% 5% 10% 24% 18% 3	7680
	21st to end	Night	50% 4 12% 17% 14% 2	8402
February	1st to 10th	Night	46% 6% 12% 21% 13% 2	2730
	11th to 20th	Night	43% 5% 12% 23% 15% 2	2664
	21st to end	Night	39% 4 10% 22% 20% 5%	2090
March	1st to 10th	Night	40% 4 8% 23% 22% 3	5029
	11th to 20th	Night	31% 5% 7% 29% 22% 7%	5090
	21st to end	Night	49% 7% 15% 21% 7% 2	5588
April	1st to 10th	Night	37% 7% 11% 25% 17% 3	2162
	11th to 20th	Night	38% 6% 12% 26% 15% 4	2075
	21st to end	Night	44% 7% 10% 23% 15% 1	1937
May	1st to 10th	Night	36% 6% 10% 27% 19% 3	5293
	11th to 20th	Night	34% 5% 12% 24% 21% 4	5249
	21st to end	Night	42% 7% 10% 23% 16% 1	5149
June	1st to 10th	Night	24% 9% 20% 29% 14% 3	1680
	11th to 20th	Night	25% 8% 14% 29% 20% 4	1680
	21st to end	Night	26% 7% 16% 29% 17% 4	1680
July	1st to 10th	Night	29% 6% 14% 26% 20% 4	3700
	11th to 20th	Night	28% 7% 17% 27% 20% 2	4071
	21st to end	Night	29% 9% 19% 30% 13% 1	4320
August	1st to 10th	Night	20% 7% 17% 32% 23% 1	2916
	11th to 20th	Night	32% 5% 12% 29% 20% 1	2915
	21st to end	Night	25% 6% 16% 29% 18% 5%	3485
September	1st to 10th	Night	26% 6% 11% 26% 24% 7%	2000
	11th to 20th	Night	30% 6% 11% 30% 20% 3	2133
	21st to end	Night	27% 5% 10% 28% 27% 3	2215
October	1st to 10th	Night	20% 3 6% 18% 49% 3	3920
	11th to 20th	Night	23% 3 7% 24% 33% 9%	3899
	21st to end	Night	37% 4 8% 26% 23% 2	4252
November	1st to 10th	Night	26% 5% 15% 27% 22% 5%	2481
	11th to 20th	Night	35% 4 9% 31% 17% 3	2923
	21st to end	Night	40% 5% 10% 27% 16% 2	2940
December	1st to 10th	Night	34% 4 12% 36% 13% 1	4720
	11th to 20th	Night	40% 5% 14% 28% 12% 1	5341
	21st to end	Night	61% 4 11% 18% 5% 1	7150

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	21% 15% 32% 24% 7%	330
	11th to 20th	Day	18% 18% 42% 19% 4	330
	21st to end	Day	15% 12% 42% 26% 3	363
February	1st to 10th	Day	15% 20% 42% 20% 3 1	330
	11th to 20th	Day	19% 19% 38% 18% 5% 1	330
	21st to end	Day	14% 21% 44% 18% 2 1	273
March	1st to 10th	Day	19% 15% 43% 19% 3	330
	11th to 20th	Day	26% 14% 34% 22% 4 1	330
	21st to end	Day	32% 21% 31% 13% 3	363
April	1st to 10th	Day	23% 17% 37% 16% 6% 1	477
	11th to 20th	Day	19% 16% 36% 23% 5%	532
	21st to end	Day	24% 16% 34% 17% 8% 1	699
May	1st to 10th	Day	19% 12% 31% 28% 8% 2	753
	11th to 20th	Day	18% 11% 31% 26% 12% 2	775
	21st to end	Day	18% 10% 28% 27% 15% 3	844
June	1st to 10th	Day	8% 6% 23% 33% 23% 7%	780
	11th to 20th	Day	9% 5% 27% 37% 18% 4	780
	21st to end	Day	8% 7% 36% 32% 15% 2	780
July	1st to 10th	Day	10% 4 14% 32% 30% 11%	771
	11th to 20th	Day	7% 3 20% 38% 24% 8%	744
	21st to end	Day	7% 2 16% 40% 29% 6%	794
August	1st to 10th	Day	8% 2 20% 38% 25% 6%	726
	11th to 20th	Day	10% 5% 17% 35% 28% 6%	715
	21st to end	Day	9% 3 17% 34% 28% 10%	646
September	1st to 10th	Day	13% 2 21% 29% 29% 5%	530
	11th to 20th	Day	9% 2 19% 37% 28% 6%	393
	21st to end	Day	13% 8% 25% 30% 18% 7%	330
October	1st to 10th	Day	15% 7% 32% 26% 12% 7%	330
	11th to 20th	Day	13% 19% 39% 21% 7% 1	330
	21st to end	Day	18% 16% 36% 22% 6% 2	363
November	1st to 10th	Day	15% 12% 43% 20% 9% 2	330
	11th to 20th	Day	18% 14% 38% 21% 8% 1	330
	21st to end	Day	22% 13% 35% 22% 8%	330
December	1st to 10th	Day	11% 17% 37% 25% 8% 2	330
	11th to 20th	Day	18% 18% 35% 23% 6% 1	330
	21st to end	Day	23% 17% 38% 19% 3	363
January	1st to 10th	Night	12% 13% 39% 29% 7%	1050
	11th to 20th	Night	14% 14% 40% 26% 6%	1050
	21st to end	Night	14% 12% 42% 28% 4	1155
February	1st to 10th	Night	17% 12% 41% 26% 4	1050
	11th to 20th	Night	16% 14% 39% 27% 4	1029
	21st to end	Night	16% 15% 43% 21% 4 1	706
March	1st to 10th	Night	16% 14% 38% 26% 5%	630
	11th to 20th	Night	19% 9% 38% 27% 6%	630
	21st to end	Night	25% 12% 40% 19% 4	693
April	1st to 10th	Night	17% 13% 41% 26% 4	630
	11th to 20th	Night	16% 13% 38% 27% 5%	623
	21st to end	Night	21% 10% 35% 28% 5% 1	630
May	1st to 10th	Night	14% 11% 34% 34% 6%	630
	11th to 20th	Night	16% 7% 30% 34% 11% 1	621
	21st to end	Night	14% 7% 27% 36% 16%	683
June	1st to 10th	Night	6% 3 24% 43% 21% 1	630
	11th to 20th	Night	7% 3 27% 42% 18% 1	630
	21st to end	Night	7% 3 30% 45% 14%	630
July	1st to 10th	Night	6% 2 15% 40% 33% 3	615
	11th to 20th	Night	6% 2 16% 47% 28% 2	597
	21st to end	Night	6% 2 15% 47% 28% 2	657
August	1st to 10th	Night	5% 1 20% 47% 26% 1	597
	11th to 20th	Night	8% 2 15% 43% 30% 2	594
	21st to end	Night	7% 2 15% 44% 30% 3	660
September	1st to 10th	Night	11% 2 19% 40% 25% 3	600
	11th to 20th	Night	12% 3 18% 43% 23% 2	600
	21st to end	Night	9% 5% 24% 40% 21% 1	615
October	1st to 10th	Night	15% 6% 28% 32% 18% 2	800
	11th to 20th	Night	17% 8% 36% 30% 8%	800
	21st to end	Night	19% 10% 36% 27% 8%	880
November	1st to 10th	Night	10% 10% 35% 34% 10%	841
	11th to 20th	Night	13% 13% 39% 28% 7%	1050
	21st to end	Night	20% 11% 34% 28% 7%	1050
December	1st to 10th	Night	16% 11% 38% 28% 6%	1050
	11th to 20th	Night	18% 10% 37% 29% 6%	1050
	21st to end	Night	21% 13% 35% 28% 3	1155

Page generated at 2026-07-13 02:14:47. Version ac07

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