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 2025-06-10.

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	17% 7% 2 6% 4 4 4 4 5% 7% 39%	119463
	Night	18% 7% 2 6% 4 4 4 3 5% 6% 42%	118742
Transparency	Day	42% 6% 11% 22% 16% 3	123022
	Night	42% 5% 11% 22% 17% 3	123331
Seeing	Day	21% 14% 31% 21% 10% 3	15701
	Night	20% 11% 31% 28% 9% 1	24912

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	13% 6% 2 5% 3 3 4 3 6% 7% 48%	3597
	11th to 20th	Day	14% 9% 3 6% 5% 5% 5% 3 4 5% 41%	3570
	21st to end	Day	18% 8% 3 7% 4 3 3 4 4 4 40%	4302
February	1st to 10th	Day	18% 7% 3 6% 4 4 5% 4 5% 7% 39%	1805
	11th to 20th	Day	15% 8% 3 6% 4 4 4 3 5% 5% 44%	1948
	21st to end	Day	19% 8% 2 5% 4 3 5% 3 5% 5% 40%	1650
March	1st to 10th	Day	22% 6% 2 5% 4 4 4 3 4 5% 42%	4476
	11th to 20th	Day	12% 7% 2 6% 5% 4 4 4 6% 7% 42%	4782
	21st to end	Day	18% 7% 2 5% 4 4 4 3 5% 6% 42%	5923
April	1st to 10th	Day	18% 8% 3 6% 4 4 4 3 5% 7% 38%	2400
	11th to 20th	Day	17% 7% 2 6% 4 4 4 4 5% 7% 41%	2374
	21st to end	Day	20% 6% 2 5% 4 4 3 4 5% 8% 40%	2425
May	1st to 10th	Day	30% 8% 2 5% 3 3 4 4 5% 8% 28%	5886
	11th to 20th	Day	23% 9% 2 7% 4 3 4 3 4 6% 34%	6290
	21st to end	Day	21% 6% 2 5% 4 4 5% 4 6% 8% 35%	6949
June	1st to 10th	Day	16% 5% 2 6% 5% 4 5% 5% 7% 9% 36%	2786
	11th to 20th	Day	16% 6% 3 5% 5% 4 6% 4 6% 9% 36%	2660
	21st to end	Day	12% 6% 2 6% 6% 4 5% 5% 6% 9% 38%	2660
July	1st to 10th	Day	9% 7% 3 7% 5% 5% 6% 5% 7% 10% 35%	4440
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 5% 7% 10% 37%	4440
	21st to end	Day	14% 9% 3 8% 6% 5% 6% 4 6% 8% 32%	4814
August	1st to 10th	Day	20% 6% 2 7% 5% 6% 5% 4 7% 8% 31%	4169
	11th to 20th	Day	13% 6% 3 6% 4 4 4 4 6% 9% 41%	3730
	21st to end	Day	10% 6% 2 6% 4 4 4 4 6% 9% 46%	4180
September	1st to 10th	Day	15% 5% 2 6% 5% 5% 4 4 5% 6% 44%	2280
	11th to 20th	Day	13% 5% 2 5% 3 4 4 3 5% 7% 49%	2199
	21st to end	Day	18% 6% 3 6% 5% 5% 4 3 5% 6% 39%	2090
October	1st to 10th	Day	21% 7% 2 5% 4 3 4 3 5% 7% 41%	1982
	11th to 20th	Day	16% 5% 2 4 4 3 3 3 4 6% 49%	1900
	21st to end	Day	21% 4 2 4 3 4 3 4 4 6% 43%	1937
November	1st to 10th	Day	14% 6% 2 5% 3 3 2 3 5% 5% 52%	1761
	11th to 20th	Day	18% 6% 2 4 3 2 3 3 5% 6% 48%	1800
	21st to end	Day	16% 7% 2 5% 4 4 4 4 6% 6% 44%	1712
December	1st to 10th	Day	16% 7% 2 6% 4 3 3 3 6% 7% 44%	2600
	11th to 20th	Day	23% 6% 2 4 3 2 3 2 4 6% 45%	2986
	21st to end	Day	18% 7% 3 7% 4 4 4 3 5% 8% 37%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 4 4 3 5% 6% 48%	6349
	11th to 20th	Night	14% 7% 3 7% 4 4 4 3 4 6% 46%	6315
	21st to end	Night	18% 9% 2 6% 3 4 4 3 5% 6% 40%	6651
February	1st to 10th	Night	17% 8% 2 6% 5% 4 4 3 4 5% 42%	2593
	11th to 20th	Night	17% 8% 2 6% 4 4 4 4 4 6% 41%	2338
	21st to end	Night	18% 8% 3 6% 5% 5% 3 3 4 5% 41%	1980
March	1st to 10th	Night	20% 8% 2 5% 3 3 3 2 4 6% 43%	5364
	11th to 20th	Night	15% 10% 2 5% 3 3 4 3 4 5% 46%	5173
	21st to end	Night	19% 9% 2 7% 4 3 3 3 4 6% 40%	5008
April	1st to 10th	Night	20% 7% 2 6% 4 4 4 3 5% 6% 37%	2000
	11th to 20th	Night	20% 7% 2 6% 3 3 5% 3 5% 6% 40%	1975
	21st to end	Night	20% 6% 1 6% 4 3 4 3 4 7% 42%	1972
May	1st to 10th	Night	29% 9% 3 7% 4 4 4 3 5% 6% 27%	4054
	11th to 20th	Night	23% 10% 3 6% 4 4 4 3 4 5% 36%	3632
	21st to end	Night	22% 8% 3 6% 3 3 4 3 5% 7% 36%	3926
June	1st to 10th	Night	17% 6% 2 4 4 4 5% 4 5% 8% 41%	1598
	11th to 20th	Night	13% 6% 3 5% 4 4 4 4 6% 10% 41%	1520
	21st to end	Night	12% 4 2 5% 5% 4 5% 4 6% 10% 42%	1520
July	1st to 10th	Night	10% 5% 3 7% 5% 6% 5% 5% 7% 9% 38%	2520
	11th to 20th	Night	13% 7% 2 5% 4 4 6% 4 8% 10% 36%	2520
	21st to end	Night	19% 8% 2 5% 4 4 5% 4 6% 7% 36%	2739
August	1st to 10th	Night	23% 6% 3 6% 4 5% 5% 4 5% 7% 32%	2711
	11th to 20th	Night	14% 7% 2 7% 5% 5% 4 4 5% 7% 39%	3097
	21st to end	Night	10% 5% 2 5% 3 3 4 4 5% 7% 52%	3476
September	1st to 10th	Night	15% 7% 2 5% 4 4 3 4 5% 6% 45%	1900
	11th to 20th	Night	14% 6% 1 4 3 3 3 3 4 7% 52%	1981
	21st to end	Night	18% 7% 2 6% 4 4 3 3 4 6% 45%	2090
October	1st to 10th	Night	20% 7% 2 6% 4 3 3 3 4 6% 42%	2194
	11th to 20th	Night	16% 5% 2 5% 3 3 3 3 4 6% 50%	2280
	21st to end	Night	21% 7% 2 5% 3 3 4 3 4 6% 44%	2660
November	1st to 10th	Night	13% 4 2 4 3 3 4 4 5% 5% 52%	2537
	11th to 20th	Night	20% 6% 2 4 3 2 3 3 4 5% 48%	2600
	21st to end	Night	15% 6% 2 5% 4 3 4 3 5% 6% 47%	2685
December	1st to 10th	Night	15% 7% 2 6% 3 3 3 3 5% 5% 48%	4580
	11th to 20th	Night	26% 7% 2 4 3 3 3 3 4 3 42%	5208
	21st to end	Night	19% 9% 3 6% 3 4 3 3 4 5% 40%	6996

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	34% 4 8% 32% 19% 3	3598
	11th to 20th	Day	43% 5% 9% 21% 16% 6%	3600
	21st to end	Day	47% 4 9% 23% 15% 1	4302
February	1st to 10th	Day	45% 5% 13% 23% 13% 2	1805
	11th to 20th	Day	43% 4 9% 24% 17% 3	1957
	21st to end	Day	45% 4 11% 21% 14% 3	1650
March	1st to 10th	Day	43% 4 11% 20% 14% 8%	4479
	11th to 20th	Day	38% 7% 12% 27% 12% 4	4782
	21st to end	Day	43% 5% 10% 13% 24% 5%	5923
April	1st to 10th	Day	45% 5% 10% 19% 16% 5%	2400
	11th to 20th	Day	43% 5% 11% 21% 18% 3	2374
	21st to end	Day	42% 6% 9% 22% 17% 4	2425
May	1st to 10th	Day	54% 6% 13% 15% 11% 1	5897
	11th to 20th	Day	50% 6% 9% 19% 14% 1	6294
	21st to end	Day	44% 7% 10% 23% 15% 1	6949
June	1st to 10th	Day	40% 9% 13% 21% 16% 2	2786
	11th to 20th	Day	43% 9% 11% 23% 12% 2	2660
	21st to end	Day	39% 8% 13% 22% 16% 1	2660
July	1st to 10th	Day	41% 10% 12% 21% 14% 1	4370
	11th to 20th	Day	38% 9% 16% 21% 14% 1	4365
	21st to end	Day	48% 8% 11% 16% 15% 2	4661
August	1st to 10th	Day	48% 8% 14% 20% 9%	4155
	11th to 20th	Day	37% 8% 14% 28% 13% 1	3766
	21st to end	Day	34% 6% 11% 28% 19% 2	4180
September	1st to 10th	Day	40% 7% 10% 21% 19% 3	2280
	11th to 20th	Day	34% 5% 11% 23% 24% 3	2199
	21st to end	Day	45% 6% 8% 19% 18% 4	2090
October	1st to 10th	Day	38% 6% 9% 18% 23% 6%	3294
	11th to 20th	Day	26% 4 11% 23% 32% 4	3140
	21st to end	Day	28% 4 14% 25% 26% 4	3162
November	1st to 10th	Day	35% 4 12% 23% 23% 3	1761
	11th to 20th	Day	38% 4 9% 24% 21% 4	1800
	21st to end	Day	39% 5% 13% 30% 11% 2	1712
December	1st to 10th	Day	39% 4 15% 29% 12% 1	2600
	11th to 20th	Day	42% 4 14% 22% 16% 3	2986
	21st to end	Day	45% 6% 8% 23% 16% 2	3960
January	1st to 10th	Night	36% 5% 9% 26% 21% 4	6349
	11th to 20th	Night	40% 4 14% 20% 16% 5%	6360
	21st to end	Night	45% 4 9% 20% 17% 5%	6651
February	1st to 10th	Night	44% 5% 12% 24% 12% 2	2593
	11th to 20th	Night	45% 4 9% 19% 18% 5%	2340
	21st to end	Night	44% 5% 7% 24% 17% 4	1980
March	1st to 10th	Night	43% 5% 10% 21% 13% 8%	5374
	11th to 20th	Night	39% 5% 15% 24% 13% 4	5174
	21st to end	Night	46% 4 7% 15% 23% 5%	5008
April	1st to 10th	Night	46% 5% 10% 19% 15% 4	2000
	11th to 20th	Night	44% 5% 10% 19% 19% 2	1975
	21st to end	Night	42% 5% 8% 21% 19% 4	1972
May	1st to 10th	Night	57% 6% 12% 14% 10% 1	4060
	11th to 20th	Night	51% 6% 12% 14% 15% 2	3633
	21st to end	Night	47% 6% 13% 21% 10% 2	3927
June	1st to 10th	Night	41% 6% 10% 20% 20% 2	1598
	11th to 20th	Night	39% 8% 13% 27% 12% 1	1520
	21st to end	Night	35% 8% 15% 25% 16% 1	1520
July	1st to 10th	Night	41% 9% 16% 18% 14% 2	2480
	11th to 20th	Night	41% 9% 14% 22% 12% 1	2472
	21st to end	Night	44% 8% 14% 15% 18% 2	2651
August	1st to 10th	Night	50% 9% 15% 16% 9% 1	2701
	11th to 20th	Night	41% 9% 12% 27% 10% 1	3127
	21st to end	Night	31% 6% 12% 31% 19% 1	3476
September	1st to 10th	Night	40% 7% 10% 24% 19% 1	1900
	11th to 20th	Night	33% 6% 10% 23% 25% 3	1981
	21st to end	Night	42% 5% 9% 22% 22% 2	2090
October	1st to 10th	Night	38% 6% 12% 22% 20% 2	3662
	11th to 20th	Night	25% 4 9% 28% 30% 4	3779
	21st to end	Night	31% 4 12% 29% 22% 2	4372
November	1st to 10th	Night	33% 6% 11% 23% 22% 4	2537
	11th to 20th	Night	39% 3 9% 24% 20% 5%	2600
	21st to end	Night	38% 5% 13% 28% 15% 2	2685
December	1st to 10th	Night	39% 4 12% 30% 12% 2	4580
	11th to 20th	Night	47% 3 10% 22% 15% 3	5208
	21st to end	Night	46% 5% 10% 18% 17% 4	6996

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	24% 29% 29% 16% 3	200
	11th to 20th	Day	27% 26% 37% 8% 3	200
	21st to end	Day	20% 23% 39% 11% 5% 2	295
February	1st to 10th	Day	28% 20% 39% 11% 2	305
	11th to 20th	Day	26% 23% 34% 16% 2	496
	21st to end	Day	25% 19% 36% 16% 4	414
March	1st to 10th	Day	24% 23% 31% 19% 3	500
	11th to 20th	Day	21% 22% 35% 16% 5% 1	500
	21st to end	Day	28% 22% 30% 14% 5% 1	550
April	1st to 10th	Day	28% 21% 31% 12% 6% 1	500
	11th to 20th	Day	25% 22% 32% 14% 7% 1	492
	21st to end	Day	28% 17% 32% 16% 4 2	500
May	1st to 10th	Day	32% 19% 27% 15% 6% 2	503
	11th to 20th	Day	24% 14% 34% 20% 7% 1	527
	21st to end	Day	21% 10% 28% 26% 13% 2	573
June	1st to 10th	Day	17% 11% 31% 26% 12% 2	526
	11th to 20th	Day	18% 12% 31% 26% 12% 1	490
	21st to end	Day	15% 12% 31% 26% 14% 2	490
July	1st to 10th	Day	14% 6% 27% 31% 19% 4	485
	11th to 20th	Day	11% 4 29% 33% 21% 3	484
	21st to end	Day	14% 8% 29% 32% 16% 2	532
August	1st to 10th	Day	13% 7% 25% 34% 15% 7%	486
	11th to 20th	Day	15% 5% 23% 33% 19% 5%	479
	21st to end	Day	12% 7% 29% 29% 16% 7%	539
September	1st to 10th	Day	17% 9% 27% 24% 17% 7%	490
	11th to 20th	Day	17% 5% 28% 26% 13% 11%	490
	21st to end	Day	22% 10% 31% 21% 10% 5%	490
October	1st to 10th	Day	25% 8% 28% 20% 14% 4	490
	11th to 20th	Day	18% 16% 30% 22% 11% 2	490
	21st to end	Day	25% 12% 32% 20% 10% 2	458
November	1st to 10th	Day	16% 13% 33% 23% 11% 3	395
	11th to 20th	Day	24% 17% 31% 23% 5% 1	400
	21st to end	Day	19% 21% 34% 20% 5% 1	312
December	1st to 10th	Day	25% 24% 30% 16% 7%	200
	11th to 20th	Day	32% 21% 32% 13% 3	200
	21st to end	Day	26% 17% 36% 12% 8%	220
January	1st to 10th	Night	19% 16% 40% 21% 4	800
	11th to 20th	Night	24% 16% 33% 24% 3	800
	21st to end	Night	20% 15% 39% 23% 2	880
February	1st to 10th	Night	23% 12% 37% 25% 2	800
	11th to 20th	Night	23% 17% 40% 19% 1	783
	21st to end	Night	23% 16% 36% 21% 4	660
March	1st to 10th	Night	22% 15% 37% 21% 5%	800
	11th to 20th	Night	23% 15% 36% 22% 4 1	800
	21st to end	Night	23% 14% 38% 20% 5%	880
April	1st to 10th	Night	23% 16% 36% 20% 4	800
	11th to 20th	Night	23% 14% 34% 23% 5%	791
	21st to end	Night	22% 13% 34% 25% 6% 1	772
May	1st to 10th	Night	24% 13% 33% 23% 7%	600
	11th to 20th	Night	23% 10% 32% 28% 6%	425
	21st to end	Night	21% 5% 22% 39% 11% 1	433
June	1st to 10th	Night	19% 5% 27% 38% 12%	400
	11th to 20th	Night	14% 5% 30% 39% 12%	380
	21st to end	Night	11% 3 26% 49% 10%	380
July	1st to 10th	Night	12% 4 24% 40% 19% 1	370
	11th to 20th	Night	13% 4 20% 43% 20% 1	368
	21st to end	Night	16% 3 20% 46% 13% 1	416
August	1st to 10th	Night	12% 3 22% 43% 19% 2	492
	11th to 20th	Night	15% 4 17% 39% 23% 1	750
	21st to end	Night	15% 4 19% 39% 21% 2	836
September	1st to 10th	Night	16% 5% 24% 36% 18% 2	760
	11th to 20th	Night	17% 4 22% 34% 18% 4	760
	21st to end	Night	21% 5% 24% 33% 15% 1	760
October	1st to 10th	Night	23% 8% 27% 26% 14% 2	760
	11th to 20th	Night	18% 10% 30% 31% 11% 1	760
	21st to end	Night	23% 10% 33% 25% 8%	836
November	1st to 10th	Night	16% 9% 33% 29% 11% 1	780
	11th to 20th	Night	22% 12% 34% 24% 7%	800
	21st to end	Night	19% 14% 34% 25% 6% 1	800
December	1st to 10th	Night	24% 13% 33% 24% 5% 1	800
	11th to 20th	Night	23% 13% 35% 26% 3	800
	21st to end	Night	23% 13% 35% 26% 3	880

Page generated at 2025-06-07 10:28:13. Version ac07

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