Donnell Vista
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Donnell Vista clear sky chart from 2005-11-06 to 2026-03-03.

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	4 7% 2 4 2 2 3 2 4 5% 64%	139624
	Night	4 8% 2 4 2 2 3 3 4 6% 62%	138976
Transparency	Day	23% 4 7% 18% 26% 22%	134613
	Night	25% 4 7% 19% 26% 20%	134967
Seeing	Day	13% 18% 34% 24% 9% 2	16791
	Night	11% 10% 25% 33% 18% 3	26304

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	7% 16% 3 4 2 2 3 3 4 6% 49%	4722
	11th to 20th	Day	4 9% 1 4 3 3 3 2 3 5% 63%	4998
	21st to end	Day	6% 10% 3 5% 3 3 4 4 5% 8% 48%	5843
February	1st to 10th	Day	9% 12% 4 6% 3 2 3 3 5% 5% 48%	1920
	11th to 20th	Day	5% 14% 4 6% 3 3 4 3 4 6% 48%	2050
	21st to end	Day	6% 14% 5% 7% 5% 3 3 3 4 5% 45%	1730
March	1st to 10th	Day	5% 17% 7% 11% 3 3 3 3 5% 6% 35%	4539
	11th to 20th	Day	5% 15% 4 6% 2 3 3 3 5% 6% 48%	4946
	21st to end	Day	7% 21% 6% 6% 4 3 3 3 4 5% 39%	5831
April	1st to 10th	Day	7% 12% 4 5% 4 3 3 3 5% 7% 47%	2400
	11th to 20th	Day	7% 14% 2 5% 3 3 3 3 5% 6% 50%	2369
	21st to end	Day	7% 11% 2 3 3 3 3 3 5% 6% 54%	2400
May	1st to 10th	Day	5% 7% 2 3 2 2 2 2 3 5% 68%	5633
	11th to 20th	Day	3 6% 1 4 3 2 2 3 4 5% 67%	6090
	21st to end	Day	4 5% 1 3 2 2 2 2 3 5% 71%	6902
June	1st to 10th	Day	6% 4 1 4 3 3 4 3 5% 7% 60%	2800
	11th to 20th	Day	3 3 1 1 2 2 2 2 3 6% 76%	2800
	21st to end	Day	2 2 1 2 2 2 3 2 3 4 76%	2800
July	1st to 10th	Day	1 1 1 1 1 2 2 91%	6319
	11th to 20th	Day	1 1 1 2 2 2 2 2 3 4 81%	6320
	21st to end	Day	1 1 1 1 1 2 2 3 4 82%	6623
August	1st to 10th	Day	1 1 1 2 1 1 1 2 2 3 84%	5654
	11th to 20th	Day	1 1 1 1 1 1 2 2 90%	5364
	21st to end	Day	2 2 1 3 3 2 3 2 3 4 75%	5984
September	1st to 10th	Day	2 1 2 2 2 1 1 3 4 81%	2400
	11th to 20th	Day	4 2 1 3 3 2 3 3 3 5% 71%	2365
	21st to end	Day	3 4 1 3 2 3 2 2 3 5% 73%	2200
October	1st to 10th	Day	3 4 1 3 2 2 2 2 4 6% 71%	3344
	11th to 20th	Day	5% 2 1 2 2 2 2 2 3 3 76%	3261
	21st to end	Day	4 5% 1 3 2 3 3 3 5% 6% 67%	3504
November	1st to 10th	Day	7% 7% 2 3 3 3 3 3 5% 7% 56%	1854
	11th to 20th	Day	6% 7% 2 3 3 4 4 4 4 6% 57%	1890
	21st to end	Day	6% 7% 3 4 2 2 3 2 5% 8% 57%	1869
December	1st to 10th	Day	6% 4 1 3 1 2 3 4 6% 9% 61%	2700
	11th to 20th	Day	13% 13% 3 6% 3 4 4 3 5% 6% 39%	3108
	21st to end	Day	13% 12% 3 5% 2 3 4 4 7% 9% 39%	4092
January	1st to 10th	Night	7% 10% 3 5% 3 3 3 3 5% 7% 50%	8172
	11th to 20th	Night	5% 8% 2 3 2 3 3 3 4 6% 60%	7957
	21st to end	Night	4 11% 3 5% 4 4 4 3 5% 6% 50%	8371
February	1st to 10th	Night	9% 12% 3 5% 3 3 3 3 5% 5% 49%	2698
	11th to 20th	Night	4 13% 3 6% 4 3 4 3 5% 8% 46%	2458
	21st to end	Night	7% 12% 3 7% 3 2 3 2 4 6% 49%	2076
March	1st to 10th	Night	5% 17% 7% 10% 3 3 3 3 5% 6% 38%	5400
	11th to 20th	Night	6% 16% 4 6% 3 3 3 3 4 6% 48%	5050
	21st to end	Night	6% 20% 5% 6% 3 3 3 3 4 6% 38%	5143
April	1st to 10th	Night	6% 11% 3 5% 3 3 3 3 5% 7% 51%	2000
	11th to 20th	Night	4 11% 2 5% 3 3 3 2 5% 7% 56%	1979
	21st to end	Night	4 9% 1 4 2 3 3 3 5% 6% 60%	2000
May	1st to 10th	Night	2 3 2 4 2 3 2 2 3 5% 73%	4344
	11th to 20th	Night	1 4 2 3 2 2 2 2 4 5% 73%	3808
	21st to end	Night	1 3 1 2 2 2 2 2 3 5% 78%	3972
June	1st to 10th	Night	4 4 1 4 2 2 3 3 4 5% 67%	1600
	11th to 20th	Night	1 1 1 1 2 1 2 2 3 4 82%	1600
	21st to end	Night	2 2 1 2 2 2 2 1 3 2 83%	1600
July	1st to 10th	Night	1 1 1 1 2 94%	3640
	11th to 20th	Night	2 2 1 2 1 2 2 1 2 3 81%	3640
	21st to end	Night	2 1 2 1 2 2 2 3 4 81%	4236
August	1st to 10th	Night	1 1 1 2 1 2 1 1 3 3 84%	4164
	11th to 20th	Night	1 1 1 1 1 1 1 2 2 89%	4506
	21st to end	Night	2 1 1 3 2 2 2 2 2 3 80%	5015
September	1st to 10th	Night	1 1 1 1 1 1 1 1 3 88%	2000
	11th to 20th	Night	2 2 1 2 1 2 2 2 3 5% 77%	2034
	21st to end	Night	2 2 1 2 2 2 2 2 4 5% 76%	2200
October	1st to 10th	Night	2 3 1 2 1 1 2 2 3 4 78%	3851
	11th to 20th	Night	3 2 1 3 2 2 2 2 3 4 78%	3897
	21st to end	Night	5% 4 1 2 1 2 3 3 5% 6% 68%	4302
November	1st to 10th	Night	7% 8% 2 2 2 2 3 3 5% 7% 60%	2665
	11th to 20th	Night	6% 6% 1 3 3 4 5% 4 6% 8% 57%	2730
	21st to end	Night	6% 8% 2 4 3 3 3 3 5% 6% 58%	2746
December	1st to 10th	Night	6% 6% 1 3 2 3 3 3 4 6% 64%	4700
	11th to 20th	Night	10% 15% 3 4 3 3 4 4 7% 10% 37%	5316
	21st to end	Night	10% 11% 4 7% 4 5% 5% 4 6% 8% 37%	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	34% 6% 12% 19% 21% 8%	4723
	11th to 20th	Day	26% 4 5% 12% 21% 32%	4998
	21st to end	Day	34% 5% 6% 16% 22% 16%	5843
February	1st to 10th	Day	39% 4 9% 18% 16% 15%	1920
	11th to 20th	Day	37% 4 8% 20% 21% 9%	2060
	21st to end	Day	43% 4 7% 19% 20% 7%	1730
March	1st to 10th	Day	48% 6% 8% 12% 18% 7%	4539
	11th to 20th	Day	35% 7% 12% 27% 15% 4	4946
	21st to end	Day	45% 7% 11% 22% 11% 3	5831
April	1st to 10th	Day	36% 6% 14% 20% 18% 6%	2400
	11th to 20th	Day	34% 6% 12% 24% 17% 6%	2372
	21st to end	Day	30% 6% 16% 26% 19% 3	2400
May	1st to 10th	Day	21% 3 9% 30% 27% 10%	5633
	11th to 20th	Day	19% 5% 10% 31% 29% 7%	6093
	21st to end	Day	16% 4 6% 32% 33% 8%	6904
June	1st to 10th	Day	24% 5% 12% 26% 26% 7%	2800
	11th to 20th	Day	12% 3 8% 25% 33% 18%	2800
	21st to end	Day	11% 4 5% 15% 38% 28%	2800
July	1st to 10th	Day	4 2 2 5% 21% 67%	6251
	11th to 20th	Day	9% 3 2 7% 18% 61%	6245
	21st to end	Day	8% 3 3 15% 35% 37%	6431
August	1st to 10th	Day	11% 2 4 10% 31% 43%	4116
	11th to 20th	Day	6% 2 2 7% 26% 56%	3880
	21st to end	Day	9% 1 2 10% 41% 35%	4311
September	1st to 10th	Day	9% 2 4 13% 36% 35%	2400
	11th to 20th	Day	17% 3 6% 22% 35% 17%	2365
	21st to end	Day	17% 3 5% 19% 36% 21%	2200
October	1st to 10th	Day	16% 2 4 10% 38% 30%	3344
	11th to 20th	Day	15% 2 4 17% 41% 21%	3261
	21st to end	Day	20% 3 6% 22% 36% 13%	3504
November	1st to 10th	Day	28% 5% 8% 23% 23% 13%	1854
	11th to 20th	Day	28% 4 11% 20% 25% 12%	1890
	21st to end	Day	28% 3 9% 18% 28% 14%	1869
December	1st to 10th	Day	21% 3 8% 22% 27% 19%	2700
	11th to 20th	Day	46% 4 8% 14% 16% 11%	3108
	21st to end	Day	39% 8% 13% 17% 15% 8%	4092
January	1st to 10th	Night	31% 6% 14% 22% 19% 8%	8172
	11th to 20th	Night	26% 4 5% 10% 23% 32%	7957
	21st to end	Night	35% 4 5% 15% 23% 17%	8371
February	1st to 10th	Night	37% 4 8% 16% 19% 16%	2698
	11th to 20th	Night	36% 5% 8% 20% 19% 11%	2459
	21st to end	Night	37% 4 9% 21% 19% 10%	2076
March	1st to 10th	Night	45% 5% 7% 13% 21% 9%	5400
	11th to 20th	Night	37% 5% 8% 29% 15% 6%	5050
	21st to end	Night	45% 7% 14% 20% 11% 3	5143
April	1st to 10th	Night	33% 7% 14% 21% 20% 6%	2000
	11th to 20th	Night	28% 7% 12% 24% 20% 9%	1979
	21st to end	Night	24% 6% 14% 34% 19% 3	2000
May	1st to 10th	Night	16% 4 10% 35% 29% 8%	4341
	11th to 20th	Night	15% 5% 9% 32% 32% 8%	3808
	21st to end	Night	12% 3 7% 30% 40% 8%	3972
June	1st to 10th	Night	19% 6% 11% 26% 30% 8%	1600
	11th to 20th	Night	8% 3 8% 25% 35% 21%	1600
	21st to end	Night	9% 3 3 16% 39% 30%	1600
July	1st to 10th	Night	2 1 1 5% 23% 67%	3600
	11th to 20th	Night	11% 1 2 7% 21% 58%	3592
	21st to end	Night	9% 2 2 19% 30% 38%	4116
August	1st to 10th	Night	11% 1 3 11% 33% 40%	3022
	11th to 20th	Night	5% 2 2 9% 26% 55%	3256
	21st to end	Night	9% 1 2 12% 43% 34%	3608
September	1st to 10th	Night	5% 3 2 14% 40% 36%	2000
	11th to 20th	Night	11% 3 6% 25% 37% 19%	2034
	21st to end	Night	12% 3 6% 23% 36% 22%	2200
October	1st to 10th	Night	13% 3 4 11% 39% 31%	3851
	11th to 20th	Night	12% 2 4 17% 44% 21%	3897
	21st to end	Night	18% 2 5% 21% 38% 16%	4302
November	1st to 10th	Night	25% 4 8% 24% 24% 16%	2665
	11th to 20th	Night	25% 5% 9% 21% 27% 13%	2730
	21st to end	Night	28% 3 8% 19% 27% 15%	2746
December	1st to 10th	Night	23% 3 8% 20% 22% 23%	4700
	11th to 20th	Night	41% 4 9% 16% 22% 9%	5316
	21st to end	Night	43% 8% 9% 15% 16% 10%	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% 16% 26% 17% 4 1	220
	11th to 20th	Day	18% 19% 25% 25% 11% 2	309
	21st to end	Day	18% 20% 28% 23% 11% 2	473
February	1st to 10th	Day	25% 20% 25% 21% 7% 2	447
	11th to 20th	Day	24% 24% 29% 17% 6%	535
	21st to end	Day	18% 25% 34% 18% 4	446
March	1st to 10th	Day	25% 26% 28% 15% 5% 1	509
	11th to 20th	Day	25% 23% 26% 18% 8% 1	500
	21st to end	Day	23% 25% 32% 15% 5%	550
April	1st to 10th	Day	16% 24% 33% 20% 7%	500
	11th to 20th	Day	21% 22% 30% 19% 6% 1	492
	21st to end	Day	13% 19% 34% 23% 8% 2	500
May	1st to 10th	Day	16% 20% 36% 19% 6% 2	503
	11th to 20th	Day	17% 17% 33% 22% 10% 1	528
	21st to end	Day	14% 16% 31% 27% 12% 2	573
June	1st to 10th	Day	6% 18% 34% 29% 9% 4	530
	11th to 20th	Day	6% 20% 41% 22% 9% 2	530
	21st to end	Day	3 15% 39% 30% 9% 4	530
July	1st to 10th	Day	3 12% 43% 30% 10% 2	526
	11th to 20th	Day	2 8% 44% 31% 12% 3	524
	21st to end	Day	2 9% 45% 27% 13% 4	575
August	1st to 10th	Day	3 14% 45% 22% 12% 3	527
	11th to 20th	Day	3 14% 44% 24% 10% 5%	517
	21st to end	Day	4 8% 46% 28% 10% 3	583
September	1st to 10th	Day	2 13% 43% 30% 11% 2	530
	11th to 20th	Day	8% 19% 37% 28% 7% 2	530
	21st to end	Day	7% 17% 37% 31% 8% 1	530
October	1st to 10th	Day	8% 23% 29% 28% 10% 1	530
	11th to 20th	Day	9% 15% 35% 28% 11% 2	530
	21st to end	Day	12% 18% 29% 28% 11% 2	553
November	1st to 10th	Day	15% 15% 25% 25% 15% 4	330
	11th to 20th	Day	17% 18% 26% 25% 11% 3	330
	21st to end	Day	13% 19% 29% 24% 13% 2	319
December	1st to 10th	Day	17% 15% 25% 20% 14% 9%	220
	11th to 20th	Day	17% 25% 25% 17% 12% 4	220
	21st to end	Day	28% 19% 23% 21% 7% 2	242
January	1st to 10th	Night	19% 17% 26% 21% 12% 5%	840
	11th to 20th	Night	13% 14% 34% 29% 7% 3	840
	21st to end	Night	15% 15% 30% 25% 12% 2	924
February	1st to 10th	Night	19% 14% 28% 24% 13% 2	840
	11th to 20th	Night	16% 16% 28% 24% 12% 4	824
	21st to end	Night	18% 14% 32% 25% 9% 2	692
March	1st to 10th	Night	21% 14% 29% 24% 10% 2	808
	11th to 20th	Night	19% 13% 24% 24% 17% 3	800
	21st to end	Night	16% 16% 31% 26% 10% 1	880
April	1st to 10th	Night	15% 12% 28% 32% 12% 2	800
	11th to 20th	Night	11% 16% 30% 28% 12% 3	791
	21st to end	Night	10% 12% 29% 29% 16% 4	800
May	1st to 10th	Night	10% 11% 30% 31% 16% 3	714
	11th to 20th	Night	6% 8% 27% 36% 20% 2	482
	21st to end	Night	6% 6% 25% 32% 27% 3	433
June	1st to 10th	Night	6% 5% 23% 38% 26% 2	400
	11th to 20th	Night	1 5% 27% 41% 24% 2	400
	21st to end	Night	2 2 23% 46% 26% 2	400
July	1st to 10th	Night	1 1 17% 47% 33% 2	390
	11th to 20th	Night	1 2 14% 48% 34% 1	388
	21st to end	Night	3 2 12% 38% 40% 6%	551
August	1st to 10th	Night	2 1 15% 47% 32% 3	651
	11th to 20th	Night	2 1 19% 46% 28% 4	792
	21st to end	Night	2 2 14% 50% 30% 3	880
September	1st to 10th	Night	1 3 18% 48% 27% 4	800
	11th to 20th	Night	3 4 21% 41% 26% 5%	800
	21st to end	Night	4 6% 22% 39% 24% 6%	800
October	1st to 10th	Night	6% 8% 24% 39% 19% 4	800
	11th to 20th	Night	8% 8% 23% 38% 19% 5%	800
	21st to end	Night	7% 9% 25% 31% 21% 7%	880
November	1st to 10th	Night	12% 9% 20% 35% 20% 4	820
	11th to 20th	Night	11% 12% 28% 32% 13% 4	840
	21st to end	Night	12% 14% 31% 26% 14% 3	840
December	1st to 10th	Night	17% 12% 28% 28% 11% 4	840
	11th to 20th	Night	20% 13% 29% 25% 11% 2	840
	21st to end	Night	21% 16% 31% 23% 8% 1	924

Page generated at 2026-02-27 22:40:26. Version ac07

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