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 2024-12-02.

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	5% 7% 2 4 2 2 3 3 4 5% 63%	105320
	Night	4 8% 2 4 3 3 3 3 4 5% 61%	103053
Transparency	Day	24% 5% 7% 19% 25% 21%	105067
	Night	25% 4 7% 19% 26% 19%	102895
Seeing	Day	12% 18% 34% 24% 9% 2	15230
	Night	11% 10% 24% 33% 19% 3	24650

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% 17% 5% 5% 3 2 3 3 4 5% 45%	2539
	11th to 20th	Day	8% 17% 2 6% 5% 5% 4 3 4 6% 39%	2672
	21st to end	Day	9% 16% 4 6% 3 4 4 4 5% 7% 39%	3146
February	1st to 10th	Day	8% 12% 4 6% 3 2 3 3 5% 5% 48%	1736
	11th to 20th	Day	5% 13% 4 6% 3 3 4 3 4 6% 49%	1850
	21st to end	Day	6% 16% 6% 7% 5% 3 3 3 4 4 44%	1570
March	1st to 10th	Day	6% 17% 8% 13% 4 4 3 3 5% 7% 30%	3138
	11th to 20th	Day	5% 12% 3 6% 2 3 2 2 4 6% 54%	3449
	21st to end	Day	6% 22% 8% 7% 4 3 3 3 4 6% 35%	4066
April	1st to 10th	Day	7% 13% 4 5% 4 3 3 3 5% 7% 47%	2280
	11th to 20th	Day	7% 14% 2 5% 3 3 3 3 5% 6% 49%	2249
	21st to end	Day	7% 11% 2 3 3 3 3 3 5% 6% 55%	2280
May	1st to 10th	Day	5% 7% 1 3 2 2 2 2 4 5% 67%	3941
	11th to 20th	Day	4 5% 1 3 3 2 2 3 4 5% 68%	4242
	21st to end	Day	5% 6% 1 3 2 2 3 2 3 6% 66%	4813
June	1st to 10th	Day	6% 4 1 4 3 3 4 3 5% 7% 60%	2660
	11th to 20th	Day	3 3 1 1 2 2 2 2 3 6% 75%	2660
	21st to end	Day	2 2 1 2 2 2 3 2 3 5% 76%	2660
July	1st to 10th	Day	1 1 1 1 1 1 1 2 2 90%	4420
	11th to 20th	Day	2 2 1 3 2 3 3 3 4 6% 73%	4420
	21st to end	Day	1 1 1 2 1 1 2 2 3 4 83%	4611
August	1st to 10th	Day	2 1 1 2 2 2 2 3 3 4 79%	4004
	11th to 20th	Day	1 1 1 1 1 1 2 2 3 86%	3724
	21st to end	Day	2 1 2 1 1 1 1 2 3 85%	4180
September	1st to 10th	Day	2 1 2 2 2 1 1 3 4 82%	2280
	11th to 20th	Day	3 2 1 3 3 2 3 3 3 5% 71%	2247
	21st to end	Day	3 4 1 3 2 2 2 2 3 5% 74%	2090
October	1st to 10th	Day	3 4 1 3 2 2 2 2 4 6% 71%	3242
	11th to 20th	Day	5% 2 1 2 2 2 2 2 3 3 77%	3161
	21st to end	Day	3 5% 1 3 2 4 3 3 5% 6% 67%	3396
November	1st to 10th	Day	6% 8% 2 4 3 3 3 3 5% 7% 56%	1764
	11th to 20th	Day	6% 6% 2 3 3 4 3 4 4 6% 59%	1800
	21st to end	Day	7% 8% 3 4 3 2 3 2 5% 8% 56%	1780
December	1st to 10th	Day	10% 8% 2 5% 2 3 3 5% 4 8% 51%	1528
	11th to 20th	Day	13% 15% 3 8% 3 4 5% 3 5% 5% 35%	1928
	21st to end	Day	7% 9% 3 6% 3 3 4 4 7% 9% 44%	2794
January	1st to 10th	Night	5% 11% 3 7% 4 3 3 3 5% 7% 49%	4406
	11th to 20th	Night	9% 15% 3 6% 4 4 5% 4 6% 7% 36%	4287
	21st to end	Night	6% 17% 4 6% 4 4 4 3 6% 7% 38%	4521
February	1st to 10th	Night	8% 12% 3 5% 3 3 3 3 5% 6% 49%	2442
	11th to 20th	Night	4 13% 3 6% 3 3 3 3 5% 9% 48%	2218
	21st to end	Night	6% 13% 4 8% 3 2 3 2 4 5% 48%	1884
March	1st to 10th	Night	4 19% 8% 10% 4 3 3 3 5% 5% 36%	3736
	11th to 20th	Night	6% 14% 2 3 2 2 2 2 4 6% 56%	3527
	21st to end	Night	6% 21% 6% 6% 3 4 4 3 5% 6% 36%	3586
April	1st to 10th	Night	6% 11% 3 5% 3 3 3 4 5% 7% 50%	1900
	11th to 20th	Night	5% 11% 2 4 3 3 3 2 5% 7% 55%	1879
	21st to end	Night	4 8% 1 4 2 3 3 3 5% 6% 60%	1900
May	1st to 10th	Night	2 4 2 4 2 2 2 2 3 6% 70%	3034
	11th to 20th	Night	2 4 1 3 2 2 2 2 4 5% 73%	2652
	21st to end	Night	2 5% 1 2 2 2 2 2 3 6% 73%	2762
June	1st to 10th	Night	4 5% 1 4 2 2 3 3 4 5% 66%	1520
	11th to 20th	Night	1 1 1 1 2 1 2 2 3 4 82%	1520
	21st to end	Night	2 2 1 2 2 2 2 1 3 2 82%	1520
July	1st to 10th	Night	1 1 1 1 1 2 92%	2540
	11th to 20th	Night	2 3 1 3 2 3 2 2 3 4 75%	2540
	21st to end	Night	2 1 1 2 1 2 2 2 3 3 82%	2943
August	1st to 10th	Night	2 2 1 3 2 2 2 1 4 4 78%	2938
	11th to 20th	Night	1 1 1 1 1 1 2 2 3 87%	3126
	21st to end	Night	1 1 2 2 1 1 1 1 2 88%	3498
September	1st to 10th	Night	1 1 1 1 1 1 1 1 3 88%	1900
	11th to 20th	Night	2 2 1 2 1 2 2 2 3 5% 78%	1932
	21st to end	Night	2 2 1 2 2 2 2 2 4 5% 77%	2090
October	1st to 10th	Night	2 3 1 2 1 1 2 2 3 4 78%	3733
	11th to 20th	Night	3 2 3 2 2 2 2 3 4 78%	3777
	21st to end	Night	5% 4 1 2 1 2 3 3 5% 6% 68%	4168
November	1st to 10th	Night	7% 8% 2 3 2 2 3 3 5% 7% 60%	2535
	11th to 20th	Night	5% 6% 1 2 3 3 5% 3 5% 7% 59%	2600
	21st to end	Night	7% 8% 2 4 3 3 3 3 5% 6% 57%	2615
December	1st to 10th	Night	10% 10% 2 4 3 3 3 3 5% 7% 51%	2686
	11th to 20th	Night	11% 18% 3 5% 4 3 4 4 6% 6% 36%	3276
	21st to end	Night	7% 10% 4 7% 5% 5% 5% 4 6% 7% 40%	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	38% 9% 10% 18% 19% 5%	2539
	11th to 20th	Day	45% 7% 9% 17% 13% 9%	2672
	21st to end	Day	43% 8% 9% 17% 16% 6%	3146
February	1st to 10th	Day	38% 4 9% 19% 17% 13%	1736
	11th to 20th	Day	36% 4 9% 20% 21% 9%	1860
	21st to end	Day	45% 4 7% 18% 21% 6%	1570
March	1st to 10th	Day	52% 7% 10% 13% 11% 7%	3138
	11th to 20th	Day	29% 8% 9% 29% 19% 6%	3449
	21st to end	Day	49% 8% 12% 19% 10% 2	4066
April	1st to 10th	Day	36% 6% 14% 20% 18% 6%	2280
	11th to 20th	Day	35% 6% 12% 24% 17% 6%	2252
	21st to end	Day	29% 6% 16% 26% 19% 3	2280
May	1st to 10th	Day	20% 4 11% 29% 24% 12%	3941
	11th to 20th	Day	17% 5% 10% 30% 31% 7%	4245
	21st to end	Day	20% 5% 9% 38% 25% 3	4814
June	1st to 10th	Day	24% 5% 12% 25% 26% 7%	2660
	11th to 20th	Day	13% 4 8% 26% 34% 15%	2660
	21st to end	Day	11% 4 5% 15% 37% 28%	2660
July	1st to 10th	Day	4 2 2 7% 17% 68%	4351
	11th to 20th	Day	12% 4 3 10% 19% 52%	4345
	21st to end	Day	8% 3 3 10% 31% 45%	4465
August	1st to 10th	Day	11% 2 4 10% 30% 43%	3989
	11th to 20th	Day	6% 2 2 7% 25% 57%	3760
	21st to end	Day	9% 1 2 10% 42% 36%	4179
September	1st to 10th	Day	9% 2 4 12% 36% 37%	2280
	11th to 20th	Day	16% 3 6% 22% 34% 18%	2247
	21st to end	Day	16% 3 5% 19% 37% 21%	2090
October	1st to 10th	Day	16% 2 4 10% 38% 30%	3242
	11th to 20th	Day	15% 2 4 17% 41% 21%	3161
	21st to end	Day	19% 3 6% 23% 36% 14%	3396
November	1st to 10th	Day	29% 5% 8% 23% 22% 13%	1764
	11th to 20th	Day	26% 4 11% 21% 26% 13%	1800
	21st to end	Day	29% 3 10% 18% 26% 14%	1780
December	1st to 10th	Day	33% 5% 9% 19% 20% 13%	1528
	11th to 20th	Day	51% 5% 7% 13% 15% 9%	1928
	21st to end	Day	34% 9% 13% 19% 16% 10%	2794
January	1st to 10th	Night	33% 7% 11% 26% 17% 7%	4406
	11th to 20th	Night	45% 7% 8% 14% 16% 10%	4287
	21st to end	Night	45% 7% 9% 16% 16% 8%	4521
February	1st to 10th	Night	37% 4 8% 17% 20% 15%	2442
	11th to 20th	Night	34% 4 9% 21% 19% 12%	2219
	21st to end	Night	38% 5% 9% 21% 19% 9%	1884
March	1st to 10th	Night	48% 6% 9% 14% 16% 7%	3736
	11th to 20th	Night	31% 4 7% 30% 20% 8%	3527
	21st to end	Night	46% 7% 11% 19% 13% 3	3586
April	1st to 10th	Night	34% 7% 13% 21% 19% 6%	1900
	11th to 20th	Night	29% 7% 12% 24% 20% 9%	1879
	21st to end	Night	23% 6% 14% 34% 19% 3	1900
May	1st to 10th	Night	17% 4 11% 29% 30% 9%	3034
	11th to 20th	Night	13% 5% 8% 27% 37% 10%	2652
	21st to end	Night	14% 4 10% 39% 29% 3	2762
June	1st to 10th	Night	20% 6% 11% 25% 30% 8%	1520
	11th to 20th	Night	8% 3 8% 27% 35% 18%	1520
	21st to end	Night	10% 3 3 17% 37% 30%	1520
July	1st to 10th	Night	3 2 2 6% 20% 67%	2500
	11th to 20th	Night	16% 1 3 10% 19% 51%	2492
	21st to end	Night	10% 2 2 12% 28% 46%	2851
August	1st to 10th	Night	12% 2 3 11% 33% 40%	2929
	11th to 20th	Night	5% 2 2 9% 26% 55%	3156
	21st to end	Night	8% 1 2 11% 43% 35%	3498
September	1st to 10th	Night	5% 3 2 12% 40% 38%	1900
	11th to 20th	Night	11% 3 6% 24% 36% 20%	1932
	21st to end	Night	12% 3 6% 23% 36% 22%	2090
October	1st to 10th	Night	13% 3 4 11% 39% 31%	3733
	11th to 20th	Night	11% 2 4 17% 44% 21%	3777
	21st to end	Night	18% 2 5% 21% 37% 16%	4168
November	1st to 10th	Night	26% 4 7% 23% 24% 16%	2535
	11th to 20th	Night	24% 5% 9% 22% 28% 13%	2600
	21st to end	Night	29% 4 8% 19% 26% 15%	2615
December	1st to 10th	Night	34% 4 8% 18% 21% 15%	2686
	11th to 20th	Night	47% 5% 7% 12% 19% 9%	3276
	21st to end	Night	40% 9% 9% 14% 17% 11%	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	34% 17% 26% 19% 3	180
	11th to 20th	Day	20% 20% 25% 23% 11% 2	260
	21st to end	Day	19% 20% 28% 22% 11% 1	407
February	1st to 10th	Day	24% 22% 26% 20% 7% 2	383
	11th to 20th	Day	21% 24% 29% 19% 6%	455
	21st to end	Day	19% 26% 33% 18% 4	382
March	1st to 10th	Day	25% 27% 28% 14% 5% 1	460
	11th to 20th	Day	23% 23% 26% 19% 8% 2	460
	21st to end	Day	22% 26% 34% 15% 4	506
April	1st to 10th	Day	16% 24% 33% 20% 7%	460
	11th to 20th	Day	23% 23% 29% 19% 6%	452
	21st to end	Day	13% 19% 34% 24% 8% 2	460
May	1st to 10th	Day	15% 21% 37% 20% 6% 1	463
	11th to 20th	Day	15% 17% 33% 24% 10% 1	488
	21st to end	Day	14% 14% 30% 27% 12% 2	529
June	1st to 10th	Day	7% 18% 34% 29% 9% 3	490
	11th to 20th	Day	6% 19% 41% 23% 10% 2	490
	21st to end	Day	3 15% 39% 31% 9% 3	490
July	1st to 10th	Day	3 10% 43% 32% 10% 2	486
	11th to 20th	Day	2 7% 43% 33% 12% 2	484
	21st to end	Day	2 8% 44% 28% 13% 4	531
August	1st to 10th	Day	3 14% 46% 23% 12% 2	487
	11th to 20th	Day	3 14% 44% 25% 11% 4	477
	21st to end	Day	4 9% 46% 28% 10% 3	539
September	1st to 10th	Day	2 12% 44% 31% 10% 1	490
	11th to 20th	Day	8% 19% 37% 29% 6% 1	490
	21st to end	Day	7% 16% 38% 32% 7% 1	490
October	1st to 10th	Day	6% 22% 30% 30% 10% 1	490
	11th to 20th	Day	8% 15% 36% 28% 11% 2	490
	21st to end	Day	11% 19% 29% 28% 10% 2	511
November	1st to 10th	Day	16% 17% 25% 24% 15% 4	300
	11th to 20th	Day	13% 17% 27% 27% 12% 4	300
	21st to end	Day	14% 20% 27% 25% 13% 2	290
December	1st to 10th	Day	20% 14% 26% 20% 13% 7%	182
	11th to 20th	Day	14% 27% 27% 17% 13% 2	180
	21st to end	Day	23% 22% 22% 22% 8% 3	198
January	1st to 10th	Night	18% 16% 25% 22% 14% 6%	760
	11th to 20th	Night	14% 14% 32% 29% 8% 3	760
	21st to end	Night	16% 16% 28% 25% 13% 3	836
February	1st to 10th	Night	18% 15% 27% 23% 14% 3	760
	11th to 20th	Night	16% 17% 26% 25% 13% 4	744
	21st to end	Night	18% 15% 28% 26% 10% 2	628
March	1st to 10th	Night	22% 14% 28% 24% 10% 2	760
	11th to 20th	Night	18% 13% 24% 24% 18% 3	760
	21st to end	Night	16% 17% 31% 26% 10% 1	836
April	1st to 10th	Night	15% 12% 27% 33% 12% 2	760
	11th to 20th	Night	12% 16% 28% 28% 13% 3	751
	21st to end	Night	10% 12% 29% 28% 16% 4	760
May	1st to 10th	Night	9% 11% 29% 31% 16% 3	679
	11th to 20th	Night	6% 8% 26% 37% 21% 2	458
	21st to end	Night	7% 7% 24% 32% 28% 3	411
June	1st to 10th	Night	6% 5% 24% 37% 26% 2	380
	11th to 20th	Night	1 5% 27% 39% 25% 2	380
	21st to end	Night	2 2 22% 45% 27% 2	380
July	1st to 10th	Night	1 1 16% 46% 34% 2	370
	11th to 20th	Night	1 2 15% 47% 34% 1	368
	21st to end	Night	3 2 12% 36% 41% 6%	523
August	1st to 10th	Night	2 1 15% 46% 33% 3	618
	11th to 20th	Night	2 1 20% 46% 28% 4	752
	21st to end	Night	2 2 14% 49% 30% 3	836
September	1st to 10th	Night	1 3 17% 48% 27% 4	760
	11th to 20th	Night	3 4 21% 41% 26% 6%	760
	21st to end	Night	4 6% 21% 38% 24% 6%	760
October	1st to 10th	Night	5% 8% 24% 39% 20% 4	760
	11th to 20th	Night	8% 8% 22% 38% 19% 5%	760
	21st to end	Night	8% 9% 24% 31% 21% 8%	836
November	1st to 10th	Night	12% 9% 20% 35% 19% 4	780
	11th to 20th	Night	10% 12% 28% 32% 14% 4	800
	21st to end	Night	13% 15% 29% 26% 15% 3	800
December	1st to 10th	Night	19% 13% 27% 27% 11% 4	768
	11th to 20th	Night	20% 14% 27% 25% 11% 3	760
	21st to end	Night	20% 17% 29% 25% 8% 1	836

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