Buehler Trust Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Buehler Trust 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	16% 6% 2 6% 4 4 4 4 6% 8% 40%	149577
	Night	17% 7% 2 6% 4 4 4 3 5% 6% 42%	142488
Transparency	Day	41% 6% 12% 22% 16% 2	153093
	Night	42% 5% 11% 22% 17% 3	147048
Seeing	Day	21% 15% 31% 22% 10% 3	17552
	Night	20% 11% 31% 28% 9% 1	26630

By Month

Percentage Cloud Cover NE (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% 6% 2 5% 4 4 4 3 6% 7% 46%	4661
	11th to 20th	Day	16% 8% 2 6% 4 4 4 3 4 5% 44%	4650
	21st to end	Day	16% 7% 3 6% 4 3 4 4 5% 6% 42%	5574
February	1st to 10th	Day	17% 7% 3 7% 4 4 5% 3 5% 6% 40%	1896
	11th to 20th	Day	15% 8% 3 6% 4 4 4 3 5% 5% 43%	2048
	21st to end	Day	19% 8% 2 5% 4 3 5% 3 5% 5% 40%	1730
March	1st to 10th	Day	22% 6% 2 5% 4 4 4 3 4 5% 42%	4576
	11th to 20th	Day	12% 7% 2 6% 5% 4 4 4 6% 7% 42%	4887
	21st to end	Day	18% 6% 2 5% 4 4 4 3 5% 6% 42%	6054
April	1st to 10th	Day	18% 8% 3 6% 4 4 4 3 5% 7% 39%	2520
	11th to 20th	Day	17% 6% 2 5% 4 4 4 3 5% 7% 42%	2494
	21st to end	Day	20% 6% 2 5% 4 4 3 4 5% 8% 39%	2546
May	1st to 10th	Day	26% 7% 2 5% 3 3 4 4 5% 7% 33%	7639
	11th to 20th	Day	20% 8% 2 6% 4 4 4 3 4 7% 38%	8180
	21st to end	Day	23% 6% 2 5% 4 4 5% 4 6% 8% 33%	9060
June	1st to 10th	Day	16% 5% 2 6% 4 4 4 5% 7% 9% 37%	2940
	11th to 20th	Day	16% 6% 3 5% 5% 4 5% 4 6% 9% 36%	2940
	21st to end	Day	12% 6% 2 6% 5% 4 5% 4 6% 10% 38%	2940
July	1st to 10th	Day	13% 6% 3 7% 4 5% 5% 5% 6% 10% 37%	8280
	11th to 20th	Day	9% 6% 2 6% 4 5% 5% 5% 8% 11% 37%	8280
	21st to end	Day	13% 7% 2 6% 4 4 5% 4 6% 8% 40%	8993
August	1st to 10th	Day	16% 6% 2 8% 5% 6% 6% 5% 9% 10% 28%	6124
	11th to 20th	Day	12% 5% 2 6% 4 4 4 4 6% 10% 43%	5488
	21st to end	Day	11% 6% 2 6% 4 4 5% 4 6% 9% 43%	6080
September	1st to 10th	Day	15% 5% 2 6% 5% 4 4 4 5% 6% 44%	2520
	11th to 20th	Day	13% 5% 2 5% 3 4 4 3 5% 7% 49%	2431
	21st to end	Day	17% 6% 3 6% 5% 5% 5% 3 5% 6% 38%	2310
October	1st to 10th	Day	19% 6% 2 5% 4 3 4 3 5% 7% 43%	2190
	11th to 20th	Day	16% 6% 2 4 4 3 3 3 4 6% 48%	2100
	21st to end	Day	20% 4 2 4 3 4 3 4 4 6% 45%	2114
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 3 5% 5% 52%	1851
	11th to 20th	Day	18% 6% 2 4 3 3 4 3 5% 6% 47%	1890
	21st to end	Day	16% 7% 2 5% 4 4 4 3 6% 6% 44%	1797
December	1st to 10th	Day	16% 7% 2 6% 4 4 3 3 6% 7% 43%	2680
	11th to 20th	Day	23% 6% 2 4 3 2 3 2 4 6% 45%	3066
	21st to end	Day	18% 7% 3 7% 4 4 4 3 5% 8% 37%	4048
January	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 5% 6% 44%	8234
	11th to 20th	Night	16% 8% 3 6% 4 3 4 3 4 5% 44%	8235
	21st to end	Night	15% 8% 2 5% 3 4 4 3 5% 6% 44%	8604
February	1st to 10th	Night	17% 8% 3 6% 5% 4 4 3 4 5% 42%	2722
	11th to 20th	Night	17% 8% 2 6% 4 4 4 4 5% 6% 40%	2458
	21st to end	Night	17% 8% 3 6% 5% 5% 3 3 4 5% 41%	2076
March	1st to 10th	Night	20% 8% 2 5% 3 3 3 2 4 6% 43%	5484
	11th to 20th	Night	15% 10% 2 5% 3 3 4 3 4 5% 45%	5288
	21st to end	Night	19% 9% 2 7% 4 3 3 4 4 6% 40%	5119
April	1st to 10th	Night	20% 7% 2 6% 4 4 4 3 5% 6% 37%	2100
	11th to 20th	Night	19% 7% 2 6% 3 3 5% 3 5% 7% 40%	2075
	21st to end	Night	20% 7% 2 6% 4 3 3 3 4 7% 42%	2071
May	1st to 10th	Night	25% 9% 3 7% 4 4 4 3 5% 7% 30%	5259
	11th to 20th	Night	20% 9% 2 6% 4 3 4 3 5% 6% 37%	4722
	21st to end	Night	24% 8% 3 7% 3 4 4 3 5% 6% 33%	5114
June	1st to 10th	Night	16% 6% 2 4 4 4 5% 4 5% 8% 43%	1680
	11th to 20th	Night	14% 6% 3 5% 4 4 4 4 6% 10% 40%	1680
	21st to end	Night	12% 5% 2 6% 6% 4 5% 4 6% 10% 42%	1680
July	1st to 10th	Night	13% 5% 3 7% 5% 5% 5% 4 6% 9% 37%	4680
	11th to 20th	Night	12% 8% 3 7% 4 4 5% 4 8% 9% 36%	4680
	21st to end	Night	14% 6% 2 5% 4 4 4 3 6% 8% 43%	5084
August	1st to 10th	Night	20% 6% 3 7% 4 5% 5% 4 5% 8% 32%	3961
	11th to 20th	Night	13% 6% 2 7% 4 5% 4 4 6% 7% 39%	4557
	21st to end	Night	12% 5% 2 6% 4 4 4 4 5% 7% 48%	5045
September	1st to 10th	Night	15% 7% 2 5% 4 4 3 4 5% 6% 45%	2100
	11th to 20th	Night	13% 6% 2 4 4 3 4 3 4 7% 51%	2189
	21st to end	Night	19% 7% 2 6% 4 4 3 3 4 6% 43%	2310
October	1st to 10th	Night	19% 7% 2 6% 4 3 3 3 4 7% 44%	2425
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 4 6% 50%	2520
	21st to end	Night	20% 6% 2 5% 3 3 4 3 4 6% 45%	2902
November	1st to 10th	Night	13% 5% 2 4 3 3 4 4 5% 6% 51%	2667
	11th to 20th	Night	19% 6% 2 4 3 2 3 3 5% 5% 48%	2730
	21st to end	Night	15% 6% 2 5% 4 3 3 3 5% 6% 47%	2819
December	1st to 10th	Night	15% 7% 2 6% 3 3 3 3 5% 6% 48%	4720
	11th to 20th	Night	25% 7% 2 4 3 3 3 3 4 4 43%	5348
	21st to end	Night	19% 9% 3 6% 3 4 3 3 4 5% 40%	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	36% 4 9% 30% 18% 2	4662
	11th to 20th	Day	42% 4 9% 21% 17% 6%	4680
	21st to end	Day	43% 4 11% 25% 17% 1	5574
February	1st to 10th	Day	44% 4 13% 23% 13% 2	1896
	11th to 20th	Day	43% 4 9% 25% 16% 3	2057
	21st to end	Day	45% 5% 11% 21% 15% 3	1730
March	1st to 10th	Day	44% 4 11% 19% 14% 8%	4579
	11th to 20th	Day	38% 7% 12% 27% 12% 4	4887
	21st to end	Day	43% 5% 10% 13% 24% 5%	6054
April	1st to 10th	Day	45% 5% 10% 19% 16% 5%	2520
	11th to 20th	Day	42% 5% 12% 20% 18% 3	2494
	21st to end	Day	42% 6% 9% 22% 16% 4	2546
May	1st to 10th	Day	50% 6% 12% 17% 14% 1	7651
	11th to 20th	Day	45% 6% 10% 23% 15% 1	8184
	21st to end	Day	47% 6% 11% 21% 14% 1	9061
June	1st to 10th	Day	39% 8% 13% 20% 17% 2	2940
	11th to 20th	Day	43% 8% 11% 23% 13% 2	2940
	21st to end	Day	39% 8% 14% 22% 16% 1	2940
July	1st to 10th	Day	42% 9% 13% 20% 16% 1	8210
	11th to 20th	Day	37% 9% 16% 25% 13% 1	8205
	21st to end	Day	40% 7% 11% 22% 19% 1	8795
August	1st to 10th	Day	47% 10% 17% 17% 9%	6110
	11th to 20th	Day	35% 8% 16% 30% 10% 1	5524
	21st to end	Day	36% 7% 12% 27% 17% 2	6080
September	1st to 10th	Day	39% 7% 10% 22% 19% 3	2520
	11th to 20th	Day	35% 5% 11% 23% 24% 3	2431
	21st to end	Day	46% 6% 9% 18% 17% 3	2310
October	1st to 10th	Day	37% 6% 9% 19% 24% 6%	3502
	11th to 20th	Day	26% 4 11% 23% 31% 4	3340
	21st to end	Day	27% 4 13% 25% 26% 4	3339
November	1st to 10th	Day	35% 4 12% 23% 23% 3	1851
	11th to 20th	Day	38% 4 9% 24% 21% 4	1890
	21st to end	Day	40% 5% 13% 29% 11% 2	1797
December	1st to 10th	Day	39% 4 15% 29% 12% 1	2680
	11th to 20th	Day	41% 4 14% 22% 16% 3	3066
	21st to end	Day	45% 6% 8% 23% 15% 2	4048
January	1st to 10th	Night	39% 5% 11% 24% 18% 3	8234
	11th to 20th	Night	42% 4 12% 20% 17% 6%	8280
	21st to end	Night	42% 4 8% 21% 21% 4	8604
February	1st to 10th	Night	44% 5% 12% 25% 13% 2	2722
	11th to 20th	Night	45% 4 10% 19% 17% 4	2460
	21st to end	Night	44% 5% 7% 24% 16% 3	2076
March	1st to 10th	Night	43% 5% 10% 21% 13% 8%	5494
	11th to 20th	Night	39% 5% 14% 24% 13% 4	5289
	21st to end	Night	46% 4 7% 15% 23% 5%	5119
April	1st to 10th	Night	46% 5% 11% 19% 15% 4	2100
	11th to 20th	Night	44% 5% 10% 19% 19% 2	2075
	21st to end	Night	42% 5% 8% 20% 19% 4	2071
May	1st to 10th	Night	53% 6% 11% 17% 12% 1	5265
	11th to 20th	Night	47% 6% 12% 20% 13% 1	4723
	21st to end	Night	51% 6% 12% 21% 10% 2	5115
June	1st to 10th	Night	40% 6% 10% 21% 21% 2	1680
	11th to 20th	Night	41% 8% 12% 26% 12% 1	1680
	21st to end	Night	36% 8% 14% 24% 16% 2	1680
July	1st to 10th	Night	43% 8% 13% 20% 15% 2	4640
	11th to 20th	Night	42% 8% 13% 24% 12% 1	4632
	21st to end	Night	38% 7% 13% 22% 18% 1	4967
August	1st to 10th	Night	49% 8% 15% 17% 10% 1	3951
	11th to 20th	Night	40% 9% 15% 28% 8%	4587
	21st to end	Night	35% 6% 12% 28% 18% 1	5045
September	1st to 10th	Night	40% 6% 11% 24% 18% 1	2100
	11th to 20th	Night	34% 6% 11% 21% 25% 3	2189
	21st to end	Night	44% 5% 9% 21% 20% 1	2310
October	1st to 10th	Night	37% 6% 12% 22% 21% 3	3893
	11th to 20th	Night	26% 4 9% 27% 30% 4	4019
	21st to end	Night	30% 4 12% 29% 23% 2	4614
November	1st to 10th	Night	33% 6% 11% 23% 22% 4	2667
	11th to 20th	Night	39% 3 9% 24% 20% 5%	2730
	21st to end	Night	38% 5% 12% 28% 15% 1	2819
December	1st to 10th	Night	39% 4 12% 30% 12% 2	4720
	11th to 20th	Night	47% 3 9% 22% 16% 4	5348
	21st to end	Night	46% 5% 10% 18% 17% 4	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	23% 28% 31% 15% 3	220
	11th to 20th	Day	28% 27% 35% 7% 2	220
	21st to end	Day	20% 24% 40% 10% 5% 2	325
February	1st to 10th	Day	26% 18% 40% 13% 2	336
	11th to 20th	Day	25% 22% 34% 16% 2	536
	21st to end	Day	25% 18% 36% 17% 4	446
March	1st to 10th	Day	25% 22% 31% 19% 3	540
	11th to 20th	Day	23% 21% 34% 16% 5% 1	540
	21st to end	Day	28% 24% 29% 13% 5% 1	594
April	1st to 10th	Day	28% 21% 31% 12% 6% 1	540
	11th to 20th	Day	24% 22% 32% 14% 7% 1	532
	21st to end	Day	28% 18% 32% 16% 4 2	540
May	1st to 10th	Day	31% 19% 28% 14% 5% 2	543
	11th to 20th	Day	23% 14% 34% 21% 7% 1	567
	21st to end	Day	23% 10% 28% 24% 12% 2	617
June	1st to 10th	Day	17% 11% 31% 27% 12% 3	570
	11th to 20th	Day	20% 14% 30% 24% 11% 1	570
	21st to end	Day	15% 12% 31% 26% 13% 2	570
July	1st to 10th	Day	14% 6% 27% 32% 18% 4	565
	11th to 20th	Day	10% 5% 30% 31% 20% 4	564
	21st to end	Day	13% 8% 28% 33% 16% 2	620
August	1st to 10th	Day	12% 7% 25% 34% 16% 7%	566
	11th to 20th	Day	14% 5% 24% 35% 18% 5%	559
	21st to end	Day	11% 8% 31% 28% 15% 6%	627
September	1st to 10th	Day	17% 9% 29% 24% 15% 6%	570
	11th to 20th	Day	17% 5% 28% 25% 14% 10%	570
	21st to end	Day	22% 11% 30% 21% 10% 5%	570
October	1st to 10th	Day	23% 9% 29% 20% 14% 5%	570
	11th to 20th	Day	20% 16% 30% 21% 10% 2	570
	21st to end	Day	23% 12% 33% 20% 9% 2	521
November	1st to 10th	Day	17% 14% 33% 23% 10% 3	425
	11th to 20th	Day	24% 18% 30% 22% 4 1	430
	21st to end	Day	19% 23% 33% 19% 4 1	337
December	1st to 10th	Day	25% 23% 30% 15% 6%	220
	11th to 20th	Day	32% 21% 31% 14% 2	220
	21st to end	Day	28% 17% 36% 12% 7%	242
January	1st to 10th	Night	19% 16% 40% 22% 4	840
	11th to 20th	Night	24% 17% 33% 23% 3	840
	21st to end	Night	20% 15% 40% 23% 2	924
February	1st to 10th	Night	23% 12% 37% 26% 2	840
	11th to 20th	Night	23% 17% 39% 19% 1	823
	21st to end	Night	23% 16% 36% 21% 4	692
March	1st to 10th	Night	23% 15% 37% 21% 5%	840
	11th to 20th	Night	22% 15% 37% 22% 4 1	840
	21st to end	Night	23% 14% 38% 20% 5%	924
April	1st to 10th	Night	24% 16% 35% 20% 5%	840
	11th to 20th	Night	23% 14% 35% 23% 5%	831
	21st to end	Night	22% 13% 34% 26% 5%	811
May	1st to 10th	Night	24% 13% 34% 23% 7%	630
	11th to 20th	Night	23% 11% 33% 28% 6%	447
	21st to end	Night	22% 5% 22% 39% 10% 1	455
June	1st to 10th	Night	17% 6% 27% 38% 11%	420
	11th to 20th	Night	15% 7% 31% 36% 11%	420
	21st to end	Night	12% 4 25% 48% 10%	420
July	1st to 10th	Night	12% 4 25% 41% 17% 1	410
	11th to 20th	Night	13% 4 20% 42% 19% 1	408
	21st to end	Night	15% 3 20% 46% 14% 2	460
August	1st to 10th	Night	12% 3 21% 43% 19% 2	544
	11th to 20th	Night	14% 4 18% 40% 22% 1	830
	21st to end	Night	15% 4 20% 39% 20% 2	924
September	1st to 10th	Night	16% 5% 24% 36% 17% 2	840
	11th to 20th	Night	16% 4 23% 35% 19% 4	840
	21st to end	Night	22% 5% 24% 33% 15% 1	840
October	1st to 10th	Night	21% 7% 28% 26% 15% 2	840
	11th to 20th	Night	18% 11% 29% 31% 11%	840
	21st to end	Night	22% 9% 34% 25% 9%	913
November	1st to 10th	Night	16% 10% 33% 29% 11% 1	820
	11th to 20th	Night	22% 13% 34% 24% 7%	840
	21st to end	Night	19% 15% 33% 26% 6% 1	840
December	1st to 10th	Night	24% 13% 34% 24% 5% 1	840
	11th to 20th	Night	23% 14% 35% 25% 3	840
	21st to end	Night	24% 13% 35% 25% 2	924

Page generated at 2026-10-26 04:37:49. Version ac07

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