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 2024-07-16.

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	10% 4 1 3 2 2 3 2 4 5% 62%	89847
	Night	13% 5% 1 4 3 3 3 3 4 5% 57%	89798
Transparency	Day	26% 3 8% 21% 23% 19%	89561
	Night	30% 3 9% 20% 21% 17%	89605
Seeing	Day	10% 5% 19% 31% 27% 8%	14748
	Night	12% 3 10% 37% 32% 6%	24086

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	26% 9% 2 4 3 3 3 3 4 5% 39%	2539
	11th to 20th	Day	26% 9% 3 8% 5% 3 4 2 4 4 33%	2772
	21st to end	Day	24% 9% 3 6% 4 4 3 3 5% 7% 31%	3146
February	1st to 10th	Day	21% 6% 1 5% 3 3 4 3 4 5% 46%	1710
	11th to 20th	Day	18% 6% 2 5% 3 3 3 4 4 6% 46%	1798
	21st to end	Day	20% 7% 2 5% 2 2 3 3 4 4 47%	1570
March	1st to 10th	Day	19% 13% 3 7% 5% 4 5% 4 5% 5% 30%	3221
	11th to 20th	Day	15% 4 2 4 2 2 3 2 4 6% 56%	3490
	21st to end	Day	17% 8% 3 6% 4 5% 5% 5% 6% 8% 34%	3992
April	1st to 10th	Day	15% 7% 2 4 3 3 3 4 4 7% 47%	2280
	11th to 20th	Day	14% 4 1 3 3 3 3 3 5% 7% 55%	2249
	21st to end	Day	10% 3 1 3 2 3 3 2 4 7% 61%	2280
May	1st to 10th	Day	8% 2 1 2 2 1 2 2 3 5% 72%	3856
	11th to 20th	Day	3 2 1 2 2 1 2 2 2 3 80%	4377
	21st to end	Day	4 2 1 2 2 2 2 2 4 5% 74%	4813
June	1st to 10th	Day	5% 3 1 3 3 2 3 2 5% 6% 67%	2660
	11th to 20th	Day	1 1 1 2 1 2 2 2 3 4 81%	2660
	21st to end	Day	2 2 2 1 1 1 1 3 3 84%	2660
July	1st to 10th	Day	1 1 1 1 2 1 3 3 88%	2660
	11th to 20th	Day	2 1 1 1 2 2 2 2 3 85%	2593
	21st to end	Day	1 1 2 2 2 2 2 3 3 83%	2564
August	1st to 10th	Day	2 2 2 1 1 2 1 2 3 83%	2328
	11th to 20th	Day	1 1 1 1 1 2 1 2 3 87%	2144
	21st to end	Day	1 1 1 1 1 1 2 2 2 90%	2376
September	1st to 10th	Day	2 1 1 1 1 2 3 88%	2160
	11th to 20th	Day	3 2 1 2 2 2 2 2 3 4 76%	2160
	21st to end	Day	3 1 1 2 1 2 2 1 3 4 79%	1951
October	1st to 10th	Day	5% 2 1 3 2 2 3 3 3 5% 71%	1800
	11th to 20th	Day	6% 2 1 3 2 3 4 3 4 5% 69%	1800
	21st to end	Day	8% 3 1 4 2 3 2 2 4 5% 65%	1980
November	1st to 10th	Day	12% 4 2 3 2 3 4 2 5% 7% 55%	1706
	11th to 20th	Day	12% 4 2 4 4 3 3 3 4 7% 53%	1710
	21st to end	Day	12% 4 2 4 2 3 3 3 4 7% 56%	1600
December	1st to 10th	Day	17% 5% 2 5% 3 4 4 3 3 6% 48%	1520
	11th to 20th	Day	30% 7% 3 6% 4 2 4 3 4 5% 33%	1928
	21st to end	Day	15% 7% 3 6% 4 4 6% 4 7% 9% 36%	2794
January	1st to 10th	Night	18% 9% 2 5% 4 3 3 3 5% 7% 41%	4406
	11th to 20th	Night	23% 7% 3 7% 4 4 4 3 5% 5% 35%	4193
	21st to end	Night	25% 9% 2 7% 4 3 4 3 5% 5% 32%	4521
February	1st to 10th	Night	20% 6% 1 4 2 3 2 2 4 5% 50%	2470
	11th to 20th	Night	16% 6% 1 5% 3 3 3 3 5% 7% 46%	2268
	21st to end	Night	18% 7% 2 3 3 3 3 2 4 5% 50%	1884
March	1st to 10th	Night	25% 11% 3 7% 4 3 3 3 4 4 33%	3648
	11th to 20th	Night	18% 5% 1 3 2 2 2 2 4 4 56%	3490
	21st to end	Night	22% 8% 2 5% 4 3 3 4 5% 6% 37%	3659
April	1st to 10th	Night	13% 6% 2 5% 4 4 4 3 4 6% 49%	1900
	11th to 20th	Night	12% 5% 2 5% 3 3 3 3 5% 5% 55%	1879
	21st to end	Night	8% 4 1 2 3 2 3 3 5% 8% 62%	1900
May	1st to 10th	Night	6% 4 1 3 2 2 2 2 3 5% 70%	3116
	11th to 20th	Night	4 2 1 1 1 1 1 2 3 5% 79%	2516
	21st to end	Night	3 1 2 1 2 2 2 3 4 79%	2762
June	1st to 10th	Night	5% 3 1 4 3 2 2 3 4 6% 67%	1520
	11th to 20th	Night	1 1 1 2 2 2 2 1 3 4 82%	1520
	21st to end	Night	2 1 1 1 2 2 1 2 3 84%	1520
July	1st to 10th	Night	1 1 1 1 1 1 3 2 89%	1520
	11th to 20th	Night	2 1 1 2 1 1 2 2 2 4 84%	1488
	21st to end	Night	1 2 1 3 2 1 2 2 4 5% 78%	1750
August	1st to 10th	Night	2 1 1 2 2 2 3 2 3 3 80%	1611
	11th to 20th	Night	1 1 1 1 2 2 2 4 4 81%	1757
	21st to end	Night	1 1 1 2 1 1 1 1 1 2 88%	1980
September	1st to 10th	Night	2 1 1 1 1 1 1 2 4 88%	1800
	11th to 20th	Night	3 2 2 1 2 2 2 2 4 80%	1800
	21st to end	Night	2 1 1 1 2 2 2 2 3 5% 80%	2005
October	1st to 10th	Night	6% 2 1 3 3 2 3 3 3 4 72%	2160
	11th to 20th	Night	6% 2 1 2 2 2 2 2 4 5% 71%	2160
	21st to end	Night	7% 2 1 3 2 2 2 2 3 6% 70%	2376
November	1st to 10th	Night	13% 6% 1 3 3 2 2 3 4 6% 57%	2373
	11th to 20th	Night	11% 4 1 4 3 2 3 3 4 7% 57%	2470
	21st to end	Night	13% 4 2 3 3 3 3 2 3 6% 57%	2578
December	1st to 10th	Night	18% 5% 2 4 3 3 3 2 4 6% 50%	2660
	11th to 20th	Night	29% 8% 2 4 4 3 3 2 5% 6% 34%	3276
	21st to end	Night	21% 7% 3 6% 5% 5% 4 4 5% 6% 34%	4862

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	48% 3 10% 25% 12% 2	2539
	11th to 20th	Day	56% 3 8% 14% 11% 8%	2772
	21st to end	Day	54% 4 10% 17% 12% 4	3146
February	1st to 10th	Day	42% 3 9% 20% 17% 9%	1710
	11th to 20th	Day	39% 4 10% 20% 21% 7%	1808
	21st to end	Day	41% 2 11% 23% 17% 6%	1570
March	1st to 10th	Day	54% 5% 11% 15% 12% 2	3221
	11th to 20th	Day	31% 3 9% 33% 14% 11%	3490
	21st to end	Day	45% 5% 16% 22% 9% 3	3992
April	1st to 10th	Day	36% 4 12% 25% 16% 6%	2280
	11th to 20th	Day	30% 4 13% 27% 20% 6%	2252
	21st to end	Day	23% 3 14% 30% 25% 4	2280
May	1st to 10th	Day	17% 2 9% 30% 26% 16%	3856
	11th to 20th	Day	12% 2 5% 28% 41% 12%	4380
	21st to end	Day	14% 2 11% 36% 30% 6%	4814
June	1st to 10th	Day	19% 3 11% 26% 29% 13%	2660
	11th to 20th	Day	9% 1 8% 21% 35% 26%	2660
	21st to end	Day	9% 1 2 13% 34% 41%	2660
July	1st to 10th	Day	5% 1 1 9% 24% 60%	2591
	11th to 20th	Day	8% 1 3 11% 25% 52%	2518
	21st to end	Day	9% 2 2 7% 25% 56%	2421
August	1st to 10th	Day	10% 1 3 10% 28% 49%	2312
	11th to 20th	Day	5% 2 2 9% 33% 50%	2144
	21st to end	Day	5% 2 1 10% 35% 48%	2376
September	1st to 10th	Day	5% 1 2 8% 28% 56%	2160
	11th to 20th	Day	13% 2 4 20% 37% 24%	2160
	21st to end	Day	12% 2 4 19% 36% 28%	1951
October	1st to 10th	Day	17% 3 6% 20% 27% 28%	1800
	11th to 20th	Day	19% 3 6% 21% 24% 28%	1800
	21st to end	Day	23% 2 6% 20% 26% 22%	1980
November	1st to 10th	Day	29% 3 9% 23% 22% 14%	1706
	11th to 20th	Day	32% 4 9% 23% 25% 8%	1710
	21st to end	Day	29% 3 9% 22% 26% 10%	1600
December	1st to 10th	Day	39% 3 9% 21% 18% 10%	1520
	11th to 20th	Day	55% 3 8% 16% 14% 5%	1928
	21st to end	Day	44% 5% 18% 21% 10% 4	2794
January	1st to 10th	Night	43% 3 16% 23% 10% 4	4406
	11th to 20th	Night	50% 4 13% 13% 11% 9%	4193
	21st to end	Night	54% 4 10% 15% 12% 6%	4521
February	1st to 10th	Night	39% 2 8% 21% 20% 10%	2470
	11th to 20th	Night	38% 4 7% 23% 21% 8%	2269
	21st to end	Night	37% 3 9% 26% 17% 7%	1884
March	1st to 10th	Night	55% 4 11% 15% 12% 4	3648
	11th to 20th	Night	33% 2 8% 27% 17% 12%	3490
	21st to end	Night	48% 6% 11% 21% 10% 4	3659
April	1st to 10th	Night	38% 4 11% 22% 20% 6%	1900
	11th to 20th	Night	30% 4 14% 26% 16% 10%	1879
	21st to end	Night	21% 5% 12% 33% 23% 6%	1900
May	1st to 10th	Night	18% 3 14% 21% 25% 18%	3116
	11th to 20th	Night	10% 2 7% 27% 42% 12%	2516
	21st to end	Night	11% 3 12% 38% 30% 6%	2762
June	1st to 10th	Night	19% 3 12% 28% 28% 11%	1520
	11th to 20th	Night	9% 1 8% 22% 33% 28%	1520
	21st to end	Night	9% 1 3 13% 33% 41%	1520
July	1st to 10th	Night	5% 2 2 11% 26% 56%	1480
	11th to 20th	Night	8% 2 3 10% 22% 55%	1440
	21st to end	Night	10% 2 2 8% 24% 53%	1653
August	1st to 10th	Night	11% 1 2 14% 26% 46%	1602
	11th to 20th	Night	8% 2 2 11% 30% 48%	1757
	21st to end	Night	7% 1 1 10% 32% 50%	1980
September	1st to 10th	Night	6% 1 2 7% 30% 54%	1800
	11th to 20th	Night	12% 2 4 18% 41% 24%	1800
	21st to end	Night	10% 1 6% 19% 31% 32%	2005
October	1st to 10th	Night	18% 3 5% 17% 27% 29%	2160
	11th to 20th	Night	16% 3 4 19% 26% 30%	2160
	21st to end	Night	19% 3 5% 20% 28% 25%	2376
November	1st to 10th	Night	29% 3 9% 22% 19% 17%	2373
	11th to 20th	Night	28% 3 10% 25% 24% 11%	2470
	21st to end	Night	30% 3 7% 22% 25% 13%	2578
December	1st to 10th	Night	37% 3 10% 21% 17% 12%	2660
	11th to 20th	Night	52% 3 7% 16% 16% 6%	3276
	21st to end	Night	50% 5% 13% 18% 10% 5%	4862

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	36% 9% 13% 22% 16% 4	180
	11th to 20th	Day	24% 5% 16% 31% 20% 6%	321
	21st to end	Day	21% 6% 19% 25% 24% 6%	407
February	1st to 10th	Day	25% 8% 13% 25% 23% 6%	370
	11th to 20th	Day	21% 10% 23% 26% 17% 3	430
	21st to end	Day	19% 9% 24% 32% 15% 2	382
March	1st to 10th	Day	23% 13% 26% 23% 12% 2	460
	11th to 20th	Day	22% 11% 24% 21% 18% 4	460
	21st to end	Day	19% 9% 29% 25% 16% 2	506
April	1st to 10th	Day	17% 10% 29% 22% 17% 5%	460
	11th to 20th	Day	15% 9% 27% 24% 20% 5%	452
	21st to end	Day	9% 7% 21% 31% 22% 10%	460
May	1st to 10th	Day	6% 10% 30% 28% 20% 6%	463
	11th to 20th	Day	8% 5% 23% 28% 26% 10%	488
	21st to end	Day	7% 5% 18% 30% 28% 12%	529
June	1st to 10th	Day	5% 6% 21% 32% 27% 9%	490
	11th to 20th	Day	2 5% 24% 36% 27% 6%	490
	21st to end	Day	1 2 17% 35% 36% 9%	490
July	1st to 10th	Day	1 10% 45% 35% 8%	486
	11th to 20th	Day	1 1 9% 38% 43% 8%	465
	21st to end	Day	1 1 9% 41% 37% 10%	487
August	1st to 10th	Day	3 2 12% 42% 32% 9%	447
	11th to 20th	Day	1 1 15% 39% 32% 12%	437
	21st to end	Day	1 1 11% 40% 37% 10%	495
September	1st to 10th	Day	2 2 11% 45% 31% 9%	450
	11th to 20th	Day	5% 4 17% 37% 31% 7%	450
	21st to end	Day	3 3 20% 35% 32% 7%	450
October	1st to 10th	Day	4 5% 21% 32% 28% 11%	450
	11th to 20th	Day	6% 5% 15% 28% 37% 10%	450
	21st to end	Day	11% 5% 15% 29% 30% 10%	495
November	1st to 10th	Day	15% 7% 12% 24% 31% 11%	302
	11th to 20th	Day	12% 3 18% 27% 29% 11%	270
	21st to end	Day	12% 3 16% 31% 30% 7%	218
December	1st to 10th	Day	18% 6% 16% 27% 23% 10%	180
	11th to 20th	Day	19% 8% 18% 26% 19% 11%	180
	21st to end	Day	20% 8% 20% 24% 24% 5%	198
January	1st to 10th	Night	23% 3 13% 31% 21% 9%	760
	11th to 20th	Night	16% 5% 11% 37% 23% 7%	760
	21st to end	Night	21% 4 11% 35% 21% 8%	836
February	1st to 10th	Night	21% 3 14% 34% 23% 5%	760
	11th to 20th	Night	19% 5% 14% 35% 22% 6%	744
	21st to end	Night	19% 5% 14% 41% 18% 3	628
March	1st to 10th	Night	25% 5% 12% 33% 20% 4	760
	11th to 20th	Night	21% 4 14% 30% 26% 5%	760
	21st to end	Night	19% 5% 17% 36% 20% 3	836
April	1st to 10th	Night	14% 3 14% 40% 26% 2	760
	11th to 20th	Night	12% 6% 14% 35% 27% 6%	751
	21st to end	Night	8% 3 11% 43% 29% 6%	760
May	1st to 10th	Night	9% 3 18% 38% 27% 6%	722
	11th to 20th	Night	7% 2 7% 40% 39% 4	374
	21st to end	Night	5% 1 11% 40% 39% 4	411
June	1st to 10th	Night	6% 1 12% 39% 37% 5%	380
	11th to 20th	Night	1 1 12% 45% 39% 2	380
	21st to end	Night	3 5% 46% 42% 4	380
July	1st to 10th	Night	1 2 40% 54% 3	370
	11th to 20th	Night	2 1 3 43% 50% 3	360
	21st to end	Night	1 2 30% 59% 8%	568
August	1st to 10th	Night	3 4 36% 53% 4	537
	11th to 20th	Night	1 1 4 42% 46% 7%	689
	21st to end	Night	2 1 2 36% 55% 4	792
September	1st to 10th	Night	2 4 35% 52% 7%	720
	11th to 20th	Night	4 1 7% 36% 44% 8%	720
	21st to end	Night	3 1 9% 38% 39% 11%	720
October	1st to 10th	Night	6% 2 9% 41% 33% 10%	720
	11th to 20th	Night	8% 1 9% 33% 42% 6%	720
	21st to end	Night	7% 2 11% 35% 31% 14%	792
November	1st to 10th	Night	13% 3 8% 35% 31% 10%	740
	11th to 20th	Night	13% 2 10% 40% 27% 8%	760
	21st to end	Night	15% 1 13% 38% 24% 8%	760
December	1st to 10th	Night	19% 3 13% 35% 23% 7%	760
	11th to 20th	Night	28% 3 10% 33% 22% 5%	760
	21st to end	Night	24% 5% 15% 34% 18% 4	836

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