Buena Vista Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Buena Vista Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	9% 3 1 3 2 2 2 2 4 5% 66%	140302
	Night	11% 4 1 4 2 2 3 2 4 5% 61%	139326
Transparency	Day	23% 2 7% 18% 25% 25%	135251
	Night	28% 3 8% 19% 23% 21%	135361
Seeing	Day	10% 5% 19% 32% 26% 8%	17004
	Night	12% 3 11% 37% 31% 6%	26444

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	23% 7% 2 4 2 3 3 3 4 6% 44%	4722
	11th to 20th	Day	14% 5% 2 4 3 2 3 2 3 4 59%	5196
	21st to end	Day	15% 6% 2 6% 4 3 4 3 5% 7% 44%	5843
February	1st to 10th	Day	21% 6% 1 5% 3 3 4 3 4 5% 46%	1890
	11th to 20th	Day	19% 7% 2 5% 3 3 3 4 4 6% 44%	1994
	21st to end	Day	20% 7% 2 5% 3 2 3 3 4 5% 47%	1730
March	1st to 10th	Day	17% 11% 3 7% 4 4 4 4 5% 6% 35%	4742
	11th to 20th	Day	18% 6% 2 4 3 3 3 2 4 6% 49%	5110
	21st to end	Day	18% 8% 3 6% 4 4 4 4 6% 7% 37%	5844
April	1st to 10th	Day	15% 7% 2 4 3 3 4 4 5% 7% 48%	2520
	11th to 20th	Day	13% 4 1 4 3 3 3 3 5% 7% 54%	2442
	21st to end	Day	10% 3 1 3 2 3 3 2 5% 7% 60%	2400
May	1st to 10th	Day	6% 2 1 2 2 1 2 1 3 5% 75%	5522
	11th to 20th	Day	3 2 1 2 2 1 2 2 3 4 78%	6277
	21st to end	Day	3 2 1 1 1 1 2 2 3 5% 80%	6902
June	1st to 10th	Day	5% 3 1 3 2 2 2 2 5% 5% 69%	2800
	11th to 20th	Day	1 1 1 2 1 2 2 2 3 4 82%	2800
	21st to end	Day	2 2 2 1 1 1 1 2 3 85%	2800
July	1st to 10th	Day	1 1 1 1 1 2 93%	6319
	11th to 20th	Day	1 1 1 1 1 1 1 1 2 89%	6320
	21st to end	Day	1 1 2 1 1 2 2 3 3 84%	6446
August	1st to 10th	Day	1 1 2 1 1 1 1 2 3 85%	5777
	11th to 20th	Day	1 1 1 1 1 2 3 91%	5411
	21st to end	Day	2 1 1 1 1 1 2 2 2 3 84%	5984
September	1st to 10th	Day	2 1 1 1 1 1 2 3 88%	2400
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 4 76%	2400
	21st to end	Day	3 1 1 2 1 2 2 1 3 4 79%	2166
October	1st to 10th	Day	3 1 1 3 2 2 2 3 4 5% 74%	3280
	11th to 20th	Day	5% 2 1 2 2 2 3 2 3 4 75%	3261
	21st to end	Day	5% 3 1 4 3 3 3 3 4 6% 65%	3558
November	1st to 10th	Day	11% 4 1 3 2 3 3 2 6% 7% 56%	1888
	11th to 20th	Day	14% 5% 2 5% 4 3 3 3 4 7% 50%	1890
	21st to end	Day	13% 4 2 3 2 3 2 3 4 7% 56%	1768
December	1st to 10th	Day	9% 3 1 4 3 3 3 3 4 7% 59%	2700
	11th to 20th	Day	28% 7% 2 5% 3 3 4 3 4 6% 35%	3108
	21st to end	Day	25% 7% 2 6% 3 4 5% 3 7% 8% 30%	4092
January	1st to 10th	Night	20% 8% 2 5% 3 3 3 3 5% 8% 41%	8172
	11th to 20th	Night	12% 4 1 4 2 3 3 2 4 5% 60%	7785
	21st to end	Night	16% 6% 2 6% 4 3 4 3 4 6% 46%	8371
February	1st to 10th	Night	20% 6% 1 4 2 3 2 2 4 5% 49%	2730
	11th to 20th	Night	17% 6% 1 5% 4 3 3 3 5% 7% 44%	2512
	21st to end	Night	17% 7% 1 3 3 3 4 3 4 5% 49%	2076
March	1st to 10th	Night	21% 10% 3 6% 4 4 3 3 4 6% 38%	5366
	11th to 20th	Night	23% 7% 2 4 2 2 2 2 3 5% 48%	5110
	21st to end	Night	21% 8% 2 5% 3 3 4 4 5% 7% 39%	5371
April	1st to 10th	Night	13% 6% 2 5% 3 3 4 3 4 6% 49%	2100
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 6% 55%	2039
	21st to end	Night	8% 4 1 3 3 2 3 3 5% 7% 62%	2000
May	1st to 10th	Night	4 3 1 3 2 2 2 2 3 5% 74%	4468
	11th to 20th	Night	3 2 1 2 2 2 2 2 3 5% 76%	3616
	21st to end	Night	2 1 2 1 2 2 2 3 4 82%	3972
June	1st to 10th	Night	5% 3 1 4 3 2 2 3 5% 6% 68%	1600
	11th to 20th	Night	1 1 1 2 2 2 2 1 3 4 82%	1600
	21st to end	Night	2 1 1 1 2 1 1 2 3 85%	1600
July	1st to 10th	Night	1 1 1 1 1 2 1 94%	3640
	11th to 20th	Night	1 1 1 2 1 1 1 1 1 3 86%	3640
	21st to end	Night	1 1 1 3 1 2 2 2 3 5% 79%	4421
August	1st to 10th	Night	2 1 1 2 1 1 1 2 3 3 83%	4035
	11th to 20th	Night	1 1 1 1 1 2 2 91%	4456
	21st to end	Night	2 2 1 2 1 1 2 1 2 3 83%	5015
September	1st to 10th	Night	2 1 1 1 1 1 1 2 4 87%	2000
	11th to 20th	Night	3 2 1 3 1 2 2 2 3 3 78%	2000
	21st to end	Night	2 1 1 1 2 2 2 2 3 4 80%	2229
October	1st to 10th	Night	3 1 1 3 2 2 2 2 3 3 77%	3920
	11th to 20th	Night	4 2 1 2 1 1 2 2 3 5% 78%	3897
	21st to end	Night	7% 2 1 3 3 3 3 3 3 5% 66%	4253
November	1st to 10th	Night	12% 6% 1 3 2 2 2 3 5% 6% 58%	2630
	11th to 20th	Night	12% 5% 2 4 3 2 3 3 4 7% 54%	2730
	21st to end	Night	13% 4 2 3 3 3 3 3 4 7% 57%	2850
December	1st to 10th	Night	10% 3 1 3 2 3 3 3 4 6% 62%	4700
	11th to 20th	Night	27% 7% 2 5% 3 4 4 4 6% 9% 31%	5316
	21st to end	Night	27% 8% 3 7% 4 4 4 3 4 6% 30%	7106

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	43% 3 14% 22% 13% 5%	4723
	11th to 20th	Day	32% 2 5% 13% 25% 23%	5196
	21st to end	Day	41% 3 7% 17% 22% 11%	5843
February	1st to 10th	Day	42% 3 8% 20% 17% 10%	1890
	11th to 20th	Day	41% 4 9% 20% 19% 6%	2004
	21st to end	Day	40% 2 11% 22% 17% 7%	1730
March	1st to 10th	Day	49% 4 9% 18% 18% 2	4742
	11th to 20th	Day	38% 3 12% 29% 11% 8%	5110
	21st to end	Day	44% 5% 14% 24% 10% 2	5844
April	1st to 10th	Day	36% 4 12% 25% 18% 5%	2520
	11th to 20th	Day	31% 4 13% 27% 19% 6%	2445
	21st to end	Day	24% 3 14% 30% 25% 4	2400
May	1st to 10th	Day	15% 2 7% 27% 34% 15%	5522
	11th to 20th	Day	12% 2 5% 31% 40% 10%	6280
	21st to end	Day	10% 2 8% 28% 43% 9%	6904
June	1st to 10th	Day	18% 3 10% 26% 31% 12%	2800
	11th to 20th	Day	9% 1 7% 20% 34% 28%	2800
	21st to end	Day	8% 1 2 12% 35% 41%	2800
July	1st to 10th	Day	3 1 1 6% 17% 73%	6251
	11th to 20th	Day	5% 1 2 5% 11% 77%	6245
	21st to end	Day	8% 1 3 13% 26% 50%	6259
August	1st to 10th	Day	11% 1 2 6% 21% 59%	4206
	11th to 20th	Day	4 1 1 6% 34% 53%	3915
	21st to end	Day	6% 1 1 8% 37% 47%	4311
September	1st to 10th	Day	5% 1 2 9% 28% 54%	2400
	11th to 20th	Day	13% 2 4 21% 38% 23%	2400
	21st to end	Day	12% 2 4 18% 35% 29%	2166
October	1st to 10th	Day	14% 2 4 12% 22% 46%	3280
	11th to 20th	Day	15% 2 4 19% 40% 19%	3261
	21st to end	Day	22% 2 6% 18% 36% 15%	3558
November	1st to 10th	Day	28% 3 10% 24% 23% 13%	1888
	11th to 20th	Day	35% 4 10% 22% 22% 7%	1890
	21st to end	Day	29% 3 9% 23% 25% 11%	1768
December	1st to 10th	Day	26% 2 8% 26% 26% 12%	2700
	11th to 20th	Day	52% 2 8% 17% 14% 6%	3108
	21st to end	Day	52% 4 15% 17% 8% 3	4092
January	1st to 10th	Night	43% 3 17% 20% 11% 6%	8172
	11th to 20th	Night	29% 2 7% 14% 28% 20%	7785
	21st to end	Night	41% 2 6% 16% 20% 14%	8371
February	1st to 10th	Night	39% 2 7% 20% 20% 10%	2730
	11th to 20th	Night	40% 4 7% 22% 20% 7%	2513
	21st to end	Night	37% 3 9% 26% 17% 7%	2076
March	1st to 10th	Night	49% 3 8% 19% 15% 6%	5366
	11th to 20th	Night	41% 3 10% 24% 14% 9%	5110
	21st to end	Night	45% 5% 12% 23% 12% 3	5371
April	1st to 10th	Night	37% 4 11% 22% 21% 5%	2100
	11th to 20th	Night	31% 4 14% 25% 16% 10%	2039
	21st to end	Night	22% 5% 12% 33% 23% 6%	2000
May	1st to 10th	Night	15% 3 12% 25% 27% 18%	4465
	11th to 20th	Night	14% 2 7% 30% 38% 9%	3616
	21st to end	Night	9% 2 9% 32% 41% 7%	3972
June	1st to 10th	Night	19% 3 11% 29% 28% 11%	1600
	11th to 20th	Night	9% 1 7% 21% 33% 29%	1600
	21st to end	Night	9% 1 3 13% 34% 40%	1600
July	1st to 10th	Night	3 1 1 6% 23% 67%	3600
	11th to 20th	Night	7% 1 2 4 10% 76%	3592
	21st to end	Night	10% 2 2 14% 25% 48%	4296
August	1st to 10th	Night	12% 1 2 8% 25% 53%	2929
	11th to 20th	Night	5% 1 2 7% 36% 48%	3219
	21st to end	Night	8% 1 1 10% 36% 44%	3608
September	1st to 10th	Night	6% 1 2 9% 30% 53%	2000
	11th to 20th	Night	13% 2 5% 19% 39% 22%	2000
	21st to end	Night	10% 1 6% 18% 31% 33%	2229
October	1st to 10th	Night	14% 2 3 11% 21% 49%	3920
	11th to 20th	Night	12% 2 5% 25% 34% 22%	3897
	21st to end	Night	23% 2 5% 17% 34% 20%	4253
November	1st to 10th	Night	28% 3 10% 23% 20% 16%	2630
	11th to 20th	Night	30% 3 10% 24% 23% 10%	2730
	21st to end	Night	30% 3 7% 22% 25% 12%	2850
December	1st to 10th	Night	25% 2 8% 25% 26% 14%	4700
	11th to 20th	Night	50% 4 8% 15% 17% 6%	5316
	21st to end	Night	56% 4 12% 16% 8% 4	7106

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	35% 10% 15% 22% 15% 5%	220
	11th to 20th	Day	20% 5% 16% 30% 23% 7%	377
	21st to end	Day	19% 5% 20% 27% 23% 5%	473
February	1st to 10th	Day	25% 9% 14% 23% 23% 6%	430
	11th to 20th	Day	22% 11% 24% 25% 15% 3	506
	21st to end	Day	18% 8% 24% 33% 15% 2	446
March	1st to 10th	Day	22% 13% 26% 24% 13% 2	540
	11th to 20th	Day	22% 11% 23% 22% 18% 4	540
	21st to end	Day	19% 8% 28% 25% 18% 3	594
April	1st to 10th	Day	17% 9% 26% 24% 18% 6%	540
	11th to 20th	Day	15% 9% 26% 26% 20% 5%	517
	21st to end	Day	9% 7% 21% 32% 21% 10%	500
May	1st to 10th	Day	5% 10% 29% 28% 20% 7%	503
	11th to 20th	Day	9% 6% 23% 28% 24% 10%	528
	21st to end	Day	6% 5% 19% 31% 27% 11%	573
June	1st to 10th	Day	4 5% 20% 35% 27% 9%	530
	11th to 20th	Day	2 5% 24% 37% 26% 5%	530
	21st to end	Day	1 3 17% 35% 36% 8%	530
July	1st to 10th	Day	1 1 13% 44% 33% 8%	526
	11th to 20th	Day	1 1 9% 40% 42% 8%	524
	21st to end	Day	1 1 9% 43% 37% 10%	575
August	1st to 10th	Day	3 2 12% 42% 31% 10%	527
	11th to 20th	Day	1 1 16% 39% 31% 11%	517
	21st to end	Day	1 1 11% 39% 38% 10%	583
September	1st to 10th	Day	2 3 14% 44% 29% 9%	530
	11th to 20th	Day	5% 5% 17% 36% 30% 7%	530
	21st to end	Day	3 3 20% 36% 31% 8%	530
October	1st to 10th	Day	4 5% 20% 31% 28% 12%	530
	11th to 20th	Day	6% 6% 16% 27% 36% 10%	530
	21st to end	Day	11% 5% 17% 29% 29% 9%	583
November	1st to 10th	Day	14% 6% 15% 24% 31% 11%	364
	11th to 20th	Day	16% 3 18% 28% 25% 9%	330
	21st to end	Day	14% 3 16% 30% 27% 9%	266
December	1st to 10th	Day	15% 5% 16% 29% 24% 11%	220
	11th to 20th	Day	20% 8% 16% 25% 20% 11%	220
	21st to end	Day	25% 7% 17% 24% 21% 5%	242
January	1st to 10th	Night	23% 4 14% 31% 20% 8%	840
	11th to 20th	Night	15% 4 12% 38% 25% 7%	840
	21st to end	Night	20% 3 11% 37% 22% 7%	924
February	1st to 10th	Night	22% 3 14% 36% 21% 5%	840
	11th to 20th	Night	20% 5% 15% 35% 20% 5%	824
	21st to end	Night	19% 4 14% 43% 17% 3	692
March	1st to 10th	Night	24% 5% 14% 33% 20% 4	840
	11th to 20th	Night	21% 4 14% 32% 25% 5%	840
	21st to end	Night	19% 5% 16% 38% 20% 3	924
April	1st to 10th	Night	14% 3 15% 40% 26% 2	840
	11th to 20th	Night	12% 6% 15% 36% 26% 5%	815
	21st to end	Night	8% 3 11% 43% 29% 6%	800
May	1st to 10th	Night	8% 3 18% 38% 27% 5%	760
	11th to 20th	Night	7% 2 9% 41% 38% 4	394
	21st to end	Night	5% 1 11% 41% 38% 3	433
June	1st to 10th	Night	5% 1 11% 40% 38% 5%	400
	11th to 20th	Night	1 1 12% 45% 39% 2	400
	21st to end	Night	3 5% 46% 42% 4	400
July	1st to 10th	Night	1 3 40% 53% 3	390
	11th to 20th	Night	2 1 3 42% 50% 2	388
	21st to end	Night	1 2 31% 59% 8%	632
August	1st to 10th	Night	3 1 4 36% 53% 4	597
	11th to 20th	Night	1 1 5% 43% 45% 6%	767
	21st to end	Night	2 1 3 36% 54% 4	880
September	1st to 10th	Night	2 1 5% 37% 50% 6%	800
	11th to 20th	Night	4 1 7% 38% 42% 8%	800
	21st to end	Night	3 1 9% 39% 38% 10%	800
October	1st to 10th	Night	5% 2 9% 41% 33% 10%	800
	11th to 20th	Night	8% 2 10% 34% 41% 6%	800
	21st to end	Night	7% 2 11% 37% 30% 13%	880
November	1st to 10th	Night	13% 3 9% 35% 31% 10%	820
	11th to 20th	Night	15% 3 11% 39% 25% 7%	840
	21st to end	Night	15% 1 14% 39% 24% 8%	840
December	1st to 10th	Night	18% 3 13% 35% 25% 8%	840
	11th to 20th	Night	28% 3 10% 32% 22% 5%	840
	21st to end	Night	26% 5% 15% 33% 16% 4	924

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