Altair Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Altair Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	2 3 1 4 3 4 4 4 6% 7% 62%	156541
	Night	3 3 1 4 3 4 4 4 6% 8% 60%	152793
Transparency	Day	19% 5% 9% 19% 28% 20%	151570
	Night	20% 5% 9% 18% 28% 19%	148771
Seeing	Day	7% 11% 26% 29% 21% 7%	17513
	Night	5% 8% 28% 38% 20% 2	27631

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	1 6% 2 4 3 3 4 4 5% 7% 60%	5318
	11th to 20th	Day	1 2 1 3 2 2 2 2 3 5% 77%	5340
	21st to end	Day	4 5% 2 4 3 3 3 3 6% 5% 61%	5844
February	1st to 10th	Day	2 4 1 3 3 2 3 3 6% 7% 65%	1941
	11th to 20th	Day	3 5% 1 4 4 3 3 3 4 6% 63%	2049
	21st to end	Day	2 4 1 3 2 3 3 2 4 6% 70%	1730
March	1st to 10th	Day	1 3 1 5% 3 3 3 3 6% 7% 65%	4921
	11th to 20th	Day	3 7% 1 3 2 3 3 4 5% 6% 61%	5110
	21st to end	Day	1 3 2 4 3 4 4 3 5% 8% 62%	5758
April	1st to 10th	Day	2 3 1 3 3 3 4 4 5% 7% 67%	2520
	11th to 20th	Day	3 3 1 4 3 3 3 2 4 6% 68%	2491
	21st to end	Day	2 3 1 3 2 3 3 2 4 5% 73%	2520
May	1st to 10th	Day	1 2 1 4 3 4 4 4 6% 7% 63%	7057
	11th to 20th	Day	2 2 1 2 2 3 3 3 4 6% 72%	7088
	21st to end	Day	1 1 1 3 3 3 4 3 5% 6% 71%	8788
June	1st to 10th	Day	1 2 1 3 3 3 4 4 7% 9% 60%	2940
	11th to 20th	Day	2 2 1 2 2 3 3 3 4 6% 73%	2940
	21st to end	Day	4 4 2 4 4 4 5% 4 7% 7% 54%	2940
July	1st to 10th	Day	2 3 1 5% 5% 5% 5% 4 7% 8% 57%	7695
	11th to 20th	Day	2 2 1 5% 5% 6% 7% 6% 9% 12% 46%	7650
	21st to end	Day	3 4 2 7% 6% 7% 7% 6% 9% 12% 39%	8304
August	1st to 10th	Day	2 2 1 4 3 5% 6% 5% 8% 10% 56%	7275
	11th to 20th	Day	2 2 1 5% 5% 6% 7% 5% 8% 10% 49%	7003
	21st to end	Day	2 2 1 5% 4 5% 6% 6% 8% 12% 47%	7751
September	1st to 10th	Day	6% 5% 1 6% 5% 4 6% 5% 5% 8% 49%	2520
	11th to 20th	Day	6% 4 1 4 3 4 4 4 6% 6% 58%	2520
	21st to end	Day	4 2 1 4 3 2 3 3 4 6% 67%	2278
October	1st to 10th	Day	3 2 1 3 2 2 2 2 3 3 77%	3370
	11th to 20th	Day	4 2 1 3 2 2 3 4 5% 6% 67%	3280
	21st to end	Day	2 2 1 1 1 1 2 1 3 4 82%	3548
November	1st to 10th	Day	2 2 1 3 2 3 3 2 5% 7% 72%	2059
	11th to 20th	Day	2 2 1 2 2 2 4 2 4 5% 74%	1904
	21st to end	Day	3 5% 2 3 3 2 3 3 4 6% 67%	1890
December	1st to 10th	Day	2 2 1 2 2 2 2 2 4 6% 75%	2842
	11th to 20th	Day	2 3 1 4 4 4 3 3 5% 6% 67%	3106
	21st to end	Day	2 3 1 3 2 3 3 2 4 6% 72%	4251
January	1st to 10th	Night	2 6% 2 6% 4 4 4 4 5% 7% 56%	7605
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 4 79%	7650
	21st to end	Night	4 6% 2 3 3 3 4 3 6% 8% 60%	8370
February	1st to 10th	Night	1 5% 1 3 2 3 3 3 5% 8% 68%	2677
	11th to 20th	Night	3 4 2 4 3 4 4 4 5% 7% 60%	2458
	21st to end	Night	2 4 1 4 3 3 3 2 4 7% 68%	2076
March	1st to 10th	Night	1 3 1 4 3 3 4 3 5% 6% 67%	5184
	11th to 20th	Night	7% 5% 2 4 3 3 3 3 5% 6% 61%	5110
	21st to end	Night	2 2 1 3 3 3 3 3 6% 7% 68%	5452
April	1st to 10th	Night	3 2 1 4 3 3 3 3 5% 6% 68%	2100
	11th to 20th	Night	3 3 1 4 3 3 4 3 5% 6% 65%	2077
	21st to end	Night	2 3 1 3 2 3 3 2 4 6% 70%	2100
May	1st to 10th	Night	4 5% 2 6% 4 4 5% 4 5% 6% 56%	5890
	11th to 20th	Night	2 2 1 3 3 3 3 3 4 6% 68%	5787
	21st to end	Night	2 2 1 3 3 3 3 3 5% 6% 68%	5401
June	1st to 10th	Night	3 4 2 5% 3 5% 4 5% 6% 9% 55%	1680
	11th to 20th	Night	3 3 1 3 3 3 3 3 5% 7% 66%	1680
	21st to end	Night	6% 5% 2 5% 4 4 6% 5% 8% 9% 47%	1680
July	1st to 10th	Night	3 3 2 6% 6% 5% 6% 5% 7% 9% 49%	5272
	11th to 20th	Night	3 4 2 6% 5% 6% 7% 6% 8% 13% 41%	5340
	21st to end	Night	3 4 2 7% 5% 6% 7% 6% 10% 12% 35%	5805
August	1st to 10th	Night	1 3 2 6% 6% 6% 7% 6% 10% 13% 39%	5545
	11th to 20th	Night	3 4 2 7% 6% 6% 6% 6% 10% 13% 36%	5853
	21st to end	Night	3 4 3 8% 6% 7% 8% 6% 10% 15% 30%	6465
September	1st to 10th	Night	6% 4 2 5% 5% 5% 6% 4 7% 8% 48%	2100
	11th to 20th	Night	6% 4 1 5% 3 3 4 3 6% 8% 56%	2100
	21st to end	Night	5% 4 1 3 3 3 3 3 5% 6% 64%	2334
October	1st to 10th	Night	3 2 1 3 2 2 2 2 4 5% 74%	4040
	11th to 20th	Night	3 2 1 3 2 3 3 2 4 5% 70%	3934
	21st to end	Night	2 1 1 2 2 2 2 2 4 5% 77%	4252
November	1st to 10th	Night	2 1 1 1 2 2 3 3 4 6% 75%	2460
	11th to 20th	Night	1 2 1 3 2 3 3 2 5% 5% 73%	2693
	21st to end	Night	5% 4 2 3 3 2 3 3 4 6% 66%	2730
December	1st to 10th	Night	2 2 1 2 2 2 2 2 4 6% 74%	4562
	11th to 20th	Night	2 2 1 3 3 3 3 3 5% 8% 67%	5321
	21st to end	Night	2 3 1 3 3 3 3 2 4 6% 70%	7010

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	22% 3 7% 25% 36% 7%	5321
	11th to 20th	Day	11% 3 7% 20% 38% 21%	5340
	21st to end	Day	24% 3 5% 13% 33% 23%	5844
February	1st to 10th	Day	18% 3 8% 19% 32% 20%	1941
	11th to 20th	Day	23% 3 6% 20% 31% 17%	2059
	21st to end	Day	18% 2 4 15% 34% 28%	1730
March	1st to 10th	Day	16% 2 5% 10% 33% 33%	4921
	11th to 20th	Day	20% 5% 6% 16% 30% 23%	5110
	21st to end	Day	19% 3 6% 18% 39% 15%	5758
April	1st to 10th	Day	17% 3 5% 14% 37% 23%	2520
	11th to 20th	Day	18% 2 5% 14% 30% 31%	2494
	21st to end	Day	14% 2 5% 17% 38% 23%	2520
May	1st to 10th	Day	15% 7% 9% 17% 30% 22%	7057
	11th to 20th	Day	13% 3 6% 12% 33% 34%	7091
	21st to end	Day	13% 4 7% 12% 19% 46%	8789
June	1st to 10th	Day	17% 5% 10% 17% 25% 25%	2940
	11th to 20th	Day	12% 3 5% 11% 23% 46%	2940
	21st to end	Day	25% 5% 10% 15% 22% 23%	2940
July	1st to 10th	Day	21% 8% 10% 12% 22% 27%	7632
	11th to 20th	Day	22% 11% 21% 25% 14% 6%	7581
	21st to end	Day	30% 12% 18% 25% 12% 3	8120
August	1st to 10th	Day	20% 9% 15% 34% 17% 6%	5759
	11th to 20th	Day	22% 10% 16% 25% 20% 7%	5519
	21st to end	Day	25% 10% 16% 30% 17% 3	6078
September	1st to 10th	Day	30% 6% 11% 19% 22% 12%	2520
	11th to 20th	Day	24% 7% 9% 17% 25% 18%	2520
	21st to end	Day	17% 4 7% 21% 30% 20%	2278
October	1st to 10th	Day	14% 2 5% 13% 33% 33%	3370
	11th to 20th	Day	16% 4 6% 16% 33% 24%	3280
	21st to end	Day	8% 2 3 16% 40% 32%	3548
November	1st to 10th	Day	13% 2 5% 19% 41% 21%	2059
	11th to 20th	Day	14% 2 4 22% 38% 19%	1904
	21st to end	Day	20% 2 4 23% 33% 18%	1890
December	1st to 10th	Day	12% 3 4 28% 35% 19%	2842
	11th to 20th	Day	19% 3 6% 19% 40% 14%	3104
	21st to end	Day	15% 2 6% 26% 35% 17%	4251
January	1st to 10th	Night	26% 3 7% 22% 34% 8%	7605
	11th to 20th	Night	10% 2 5% 18% 45% 19%	7650
	21st to end	Night	22% 4 5% 12% 31% 25%	8370
February	1st to 10th	Night	16% 3 8% 19% 31% 22%	2677
	11th to 20th	Night	22% 5% 6% 20% 30% 17%	2458
	21st to end	Night	19% 2 4 15% 35% 26%	2076
March	1st to 10th	Night	17% 2 6% 11% 37% 27%	5184
	11th to 20th	Night	21% 5% 7% 17% 31% 19%	5110
	21st to end	Night	14% 3 6% 22% 39% 15%	5452
April	1st to 10th	Night	18% 2 5% 16% 36% 23%	2100
	11th to 20th	Night	19% 3 5% 13% 27% 31%	2077
	21st to end	Night	17% 3 7% 17% 35% 23%	2100
May	1st to 10th	Night	24% 5% 9% 16% 28% 18%	5888
	11th to 20th	Night	15% 4 8% 12% 31% 30%	5788
	21st to end	Night	16% 4 7% 10% 17% 46%	5402
June	1st to 10th	Night	23% 6% 11% 16% 17% 26%	1680
	11th to 20th	Night	17% 3 7% 14% 18% 40%	1680
	21st to end	Night	28% 7% 12% 15% 14% 24%	1680
July	1st to 10th	Night	27% 9% 12% 15% 18% 20%	5227
	11th to 20th	Night	29% 11% 22% 24% 11% 4	5286
	21st to end	Night	33% 11% 21% 24% 10% 1	5677
August	1st to 10th	Night	28% 11% 19% 26% 12% 5%	4385
	11th to 20th	Night	30% 11% 18% 22% 15% 5%	4613
	21st to end	Night	34% 12% 17% 23% 11% 2	5069
September	1st to 10th	Night	30% 8% 13% 19% 20% 10%	2100
	11th to 20th	Night	24% 6% 12% 16% 22% 19%	2100
	21st to end	Night	20% 4 9% 16% 30% 21%	2334
October	1st to 10th	Night	14% 3 6% 11% 32% 33%	4040
	11th to 20th	Night	16% 4 7% 16% 27% 29%	3934
	21st to end	Night	10% 2 4 15% 36% 32%	4252
November	1st to 10th	Night	10% 2 5% 20% 40% 22%	2460
	11th to 20th	Night	14% 3 4 19% 38% 22%	2693
	21st to end	Night	20% 3 4 18% 35% 18%	2730
December	1st to 10th	Night	12% 3 5% 29% 29% 21%	4562
	11th to 20th	Night	16% 3 6% 18% 38% 19%	5322
	21st to end	Night	17% 3 5% 23% 37% 15%	7010

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	5% 18% 40% 26% 10%	330
	11th to 20th	Day	9% 14% 36% 23% 15% 2	330
	21st to end	Day	6% 12% 38% 29% 10% 4	363
February	1st to 10th	Day	5% 18% 34% 28% 15% 1	330
	11th to 20th	Day	12% 20% 42% 22% 5%	330
	21st to end	Day	11% 26% 34% 19% 9% 1	273
March	1st to 10th	Day	10% 19% 37% 22% 10% 1	470
	11th to 20th	Day	13% 20% 34% 21% 9% 2	540
	21st to end	Day	11% 23% 36% 19% 9% 2	594
April	1st to 10th	Day	9% 19% 35% 24% 10% 2	540
	11th to 20th	Day	11% 21% 41% 20% 7%	532
	21st to end	Day	9% 21% 38% 21% 9% 1	540
May	1st to 10th	Day	6% 17% 39% 28% 8% 2	543
	11th to 20th	Day	7% 15% 33% 27% 14% 4	568
	21st to end	Day	4 10% 34% 33% 17% 2	762
June	1st to 10th	Day	3 4 23% 39% 26% 4	780
	11th to 20th	Day	2 5% 23% 38% 27% 4	780
	21st to end	Day	6% 4 16% 36% 29% 9%	780
July	1st to 10th	Day	3 2 10% 35% 34% 16%	586
	11th to 20th	Day	4 1 5% 23% 40% 28%	564
	21st to end	Day	5% 2 4 22% 40% 27%	619
August	1st to 10th	Day	2 1 6% 28% 43% 19%	567
	11th to 20th	Day	5% 2 9% 29% 37% 18%	557
	21st to end	Day	6% 1 6% 33% 38% 16%	627
September	1st to 10th	Day	7% 2 12% 34% 31% 15%	570
	11th to 20th	Day	5% 4 16% 36% 32% 8%	570
	21st to end	Day	7% 6% 19% 31% 28% 8%	393
October	1st to 10th	Day	6% 8% 21% 33% 24% 8%	360
	11th to 20th	Day	9% 9% 26% 29% 22% 5%	339
	21st to end	Day	9% 12% 23% 31% 20% 6%	363
November	1st to 10th	Day	7% 11% 35% 26% 17% 4	330
	11th to 20th	Day	5% 11% 27% 32% 19% 6%	330
	21st to end	Day	9% 15% 35% 27% 12% 2	330
December	1st to 10th	Day	4 15% 38% 25% 15% 2	330
	11th to 20th	Day	7% 13% 38% 28% 13% 2	330
	21st to end	Day	10% 18% 36% 26% 10% 1	363
January	1st to 10th	Night	5% 9% 32% 38% 16% 1	840
	11th to 20th	Night	5% 9% 36% 34% 15% 2	840
	21st to end	Night	6% 10% 33% 36% 13% 2	924
February	1st to 10th	Night	4 10% 36% 36% 12% 1	840
	11th to 20th	Night	7% 13% 39% 34% 7%	824
	21st to end	Night	6% 12% 45% 33% 4	692
March	1st to 10th	Night	5% 13% 40% 32% 9% 1	840
	11th to 20th	Night	7% 13% 33% 34% 12%	840
	21st to end	Night	5% 12% 40% 34% 9%	856
April	1st to 10th	Night	6% 11% 45% 30% 8%	630
	11th to 20th	Night	5% 15% 41% 32% 6%	623
	21st to end	Night	5% 12% 44% 32% 7% 1	630
May	1st to 10th	Night	4 10% 41% 37% 9%	630
	11th to 20th	Night	5% 6% 33% 40% 15% 1	621
	21st to end	Night	3 6% 35% 44% 12%	683
June	1st to 10th	Night	4 5% 26% 46% 19%	630
	11th to 20th	Night	4 2 30% 47% 16% 1	630
	21st to end	Night	6% 2 17% 46% 26% 3	630
July	1st to 10th	Night	7% 3 8% 43% 35% 5%	615
	11th to 20th	Night	5% 2 5% 34% 47% 7%	612
	21st to end	Night	5% 2 4 29% 51% 9%	690
August	1st to 10th	Night	3 1 7% 31% 48% 10%	627
	11th to 20th	Night	5% 1 7% 35% 47% 4	624
	21st to end	Night	4 2 5% 35% 49% 5%	693
September	1st to 10th	Night	7% 2 9% 42% 35% 4	630
	11th to 20th	Night	8% 2 11% 49% 29% 1	630
	21st to end	Night	7% 3 17% 42% 28% 3	690
October	1st to 10th	Night	5% 4 21% 46% 21% 3	840
	11th to 20th	Night	5% 6% 24% 44% 20% 2	812
	21st to end	Night	4 7% 27% 36% 22% 4	880
November	1st to 10th	Night	3 5% 26% 42% 21% 3	820
	11th to 20th	Night	3 7% 29% 39% 19% 2	1013
	21st to end	Night	6% 11% 28% 34% 19% 2	1050
December	1st to 10th	Night	5% 9% 32% 38% 15% 1	1050
	11th to 20th	Night	6% 9% 34% 38% 13%	1050
	21st to end	Night	6% 13% 34% 38% 9% 1	1102

Page generated at 2026-10-11 03:06:31. Version ac07

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