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 2024-11-02.

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	3 4 1 4 3 3 4 4 6% 7% 61%	100674
	Night	3 4 1 4 3 4 4 3 5% 7% 60%	98502
Transparency	Day	20% 5% 9% 19% 28% 19%	100437
	Night	21% 5% 9% 19% 28% 18%	98328
Seeing	Day	7% 10% 25% 29% 21% 8%	14899
	Night	5% 8% 27% 37% 20% 2	25064

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% 8% 60%	2858
	11th to 20th	Day	2 3 1 2 2 2 2 2 5% 7% 72%	2860
	21st to end	Day	4 7% 2 5% 4 3 4 4 7% 6% 53%	3146
February	1st to 10th	Day	2 5% 1 4 3 2 3 3 6% 7% 64%	1755
	11th to 20th	Day	4 5% 1 4 4 3 3 3 4 6% 64%	1849
	21st to end	Day	2 5% 1 4 2 3 3 2 4 6% 69%	1570
March	1st to 10th	Day	1 4 1 5% 3 3 4 4 6% 7% 62%	3346
	11th to 20th	Day	5% 10% 2 5% 3 4 4 4 5% 6% 52%	3490
	21st to end	Day	1 3 2 5% 3 4 4 3 6% 8% 60%	3934
April	1st to 10th	Day	2 3 1 3 3 3 4 3 5% 7% 67%	2280
	11th to 20th	Day	3 3 1 3 2 3 3 2 4 6% 68%	2251
	21st to end	Day	2 3 1 3 3 3 3 2 4 5% 72%	2280
May	1st to 10th	Day	1 2 1 2 2 2 3 2 4 6% 76%	3800
	11th to 20th	Day	3 3 1 3 4 4 4 4 5% 7% 62%	3795
	21st to end	Day	1 1 2 2 2 2 2 4 5% 80%	4699
June	1st to 10th	Day	2 3 1 3 3 3 4 4 7% 9% 61%	2660
	11th to 20th	Day	2 2 1 2 2 3 2 3 4 6% 72%	2660
	21st to end	Day	5% 5% 2 5% 4 4 5% 4 7% 7% 53%	2660
July	1st to 10th	Day	3 2 1 4 4 5% 5% 5% 8% 9% 55%	4128
	11th to 20th	Day	3 3 1 5% 4 5% 6% 6% 9% 10% 48%	4110
	21st to end	Day	4 4 2 6% 5% 6% 6% 6% 9% 12% 38%	4454
August	1st to 10th	Day	2 3 1 5% 4 5% 6% 5% 9% 11% 48%	3943
	11th to 20th	Day	2 3 1 3 4 5% 6% 4 7% 8% 57%	3726
	21st to end	Day	3 3 2 6% 5% 5% 6% 6% 8% 11% 44%	4180
September	1st to 10th	Day	6% 5% 1 6% 5% 4 5% 5% 5% 8% 50%	2280
	11th to 20th	Day	6% 4 1 4 3 4 4 4 6% 6% 59%	2280
	21st to end	Day	3 2 1 3 3 2 3 3 4 6% 70%	2061
October	1st to 10th	Day	5% 4 1 4 3 3 3 2 4 5% 65%	1900
	11th to 20th	Day	4 2 1 3 2 2 3 2 4 4 74%	1900
	21st to end	Day	3 3 1 2 1 2 2 2 3 4 77%	2090
November	1st to 10th	Day	2 2 1 3 2 3 3 2 5% 7% 72%	1869
	11th to 20th	Day	1 2 1 2 2 2 4 3 4 5% 74%	1724
	21st to end	Day	3 5% 2 3 3 2 3 3 4 5% 66%	1710
December	1st to 10th	Day	3 4 2 3 3 2 3 3 4 6% 68%	1606
	11th to 20th	Day	3 5% 2 5% 4 4 4 4 6% 8% 56%	1926
	21st to end	Day	3 4 1 3 2 3 3 2 4 7% 66%	2894
January	1st to 10th	Night	2 5% 2 6% 4 4 4 3 5% 6% 59%	4097
	11th to 20th	Night	2 3 1 3 3 3 3 2 4 5% 72%	4110
	21st to end	Night	4 7% 2 4 3 4 5% 4 6% 7% 53%	4521
February	1st to 10th	Night	1 6% 1 3 2 3 3 3 5% 7% 67%	2423
	11th to 20th	Night	3 5% 2 4 3 4 3 4 5% 7% 61%	2218
	21st to end	Night	2 4 1 4 3 3 3 2 4 7% 66%	1884
March	1st to 10th	Night	2 4 2 4 3 3 3 3 4 5% 68%	3522
	11th to 20th	Night	9% 7% 2 4 3 2 3 2 4 5% 56%	3490
	21st to end	Night	2 2 1 3 3 3 4 3 6% 7% 66%	3715
April	1st to 10th	Night	3 2 1 3 3 2 3 3 5% 6% 69%	1900
	11th to 20th	Night	4 3 1 3 3 3 3 3 5% 6% 66%	1877
	21st to end	Night	2 3 1 3 2 3 4 3 4 6% 69%	1900
May	1st to 10th	Night	2 3 1 4 3 2 3 3 4 5% 70%	3170
	11th to 20th	Night	4 3 1 4 4 4 4 3 5% 7% 60%	3093
	21st to end	Night	2 1 1 2 2 2 2 2 3 5% 78%	2870
June	1st to 10th	Night	3 3 2 5% 3 5% 4 5% 6% 10% 55%	1520
	11th to 20th	Night	3 3 1 3 3 3 3 3 5% 7% 67%	1520
	21st to end	Night	6% 5% 2 5% 4 4 5% 5% 7% 9% 47%	1520
July	1st to 10th	Night	4 3 1 6% 6% 4 5% 5% 7% 10% 49%	2820
	11th to 20th	Night	4 4 2 6% 5% 5% 6% 5% 7% 12% 43%	2860
	21st to end	Night	5% 5% 2 6% 5% 6% 7% 6% 10% 12% 37%	3109
August	1st to 10th	Night	2 3 2 6% 6% 5% 7% 7% 10% 13% 40%	2994
	11th to 20th	Night	4 3 2 6% 4 5% 5% 5% 8% 12% 44%	3113
	21st to end	Night	3 4 3 8% 6% 6% 8% 6% 10% 11% 35%	3487
September	1st to 10th	Night	6% 4 1 5% 5% 5% 5% 4 7% 9% 50%	1900
	11th to 20th	Night	6% 4 1 4 3 3 4 3 6% 8% 57%	1900
	21st to end	Night	4 3 1 3 3 3 3 3 4 6% 68%	2111
October	1st to 10th	Night	5% 4 1 4 3 3 3 3 5% 6% 62%	2280
	11th to 20th	Night	3 2 1 3 2 3 3 2 4 4 75%	2280
	21st to end	Night	3 2 1 3 2 2 2 2 4 5% 74%	2508
November	1st to 10th	Night	2 1 1 1 2 2 3 3 4 7% 75%	2244
	11th to 20th	Night	1 2 1 3 2 3 3 2 5% 5% 74%	2436
	21st to end	Night	5% 4 2 3 3 2 3 3 4 6% 66%	2470
December	1st to 10th	Night	4 3 1 3 2 3 3 3 5% 6% 67%	2569
	11th to 20th	Night	3 4 1 4 3 4 4 4 6% 9% 58%	3271
	21st to end	Night	3 4 2 4 3 3 3 2 4 6% 65%	4800

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	21% 2 9% 24% 37% 8%	2859
	11th to 20th	Day	13% 3 10% 32% 35% 7%	2860
	21st to end	Day	30% 4 8% 18% 32% 8%	3146
February	1st to 10th	Day	19% 3 9% 20% 31% 18%	1755
	11th to 20th	Day	22% 3 6% 20% 30% 19%	1859
	21st to end	Day	19% 2 4 15% 34% 27%	1570
March	1st to 10th	Day	20% 2 7% 13% 35% 23%	3346
	11th to 20th	Day	27% 7% 8% 19% 26% 13%	3490
	21st to end	Day	20% 3 7% 19% 36% 15%	3934
April	1st to 10th	Day	17% 3 4 15% 37% 24%	2280
	11th to 20th	Day	18% 2 5% 14% 30% 31%	2254
	21st to end	Day	15% 3 6% 17% 38% 21%	2280
May	1st to 10th	Day	10% 2 6% 12% 37% 33%	3800
	11th to 20th	Day	19% 4 10% 14% 32% 22%	3798
	21st to end	Day	9% 2 6% 10% 20% 54%	4699
June	1st to 10th	Day	17% 5% 10% 17% 26% 25%	2660
	11th to 20th	Day	13% 3 5% 11% 22% 46%	2660
	21st to end	Day	26% 5% 10% 15% 21% 22%	2660
July	1st to 10th	Day	21% 9% 13% 13% 20% 24%	4064
	11th to 20th	Day	24% 10% 17% 28% 17% 5%	4041
	21st to end	Day	31% 11% 18% 28% 12% 1	4314
August	1st to 10th	Day	23% 10% 17% 27% 17% 7%	3929
	11th to 20th	Day	20% 8% 11% 24% 27% 10%	3762
	21st to end	Day	28% 9% 16% 25% 19% 3	4179
September	1st to 10th	Day	30% 6% 10% 19% 23% 13%	2280
	11th to 20th	Day	23% 7% 9% 16% 26% 19%	2280
	21st to end	Day	15% 4 7% 22% 31% 22%	2061
October	1st to 10th	Day	22% 3 7% 17% 27% 23%	1900
	11th to 20th	Day	14% 3 4 13% 32% 34%	1900
	21st to end	Day	12% 2 5% 19% 41% 21%	2090
November	1st to 10th	Day	13% 2 5% 20% 39% 21%	1869
	11th to 20th	Day	14% 2 4 23% 38% 18%	1724
	21st to end	Day	21% 2 4 22% 33% 19%	1710
December	1st to 10th	Day	19% 3 6% 29% 29% 14%	1606
	11th to 20th	Day	25% 4 9% 20% 30% 12%	1924
	21st to end	Day	19% 2 7% 18% 34% 19%	2894
January	1st to 10th	Night	25% 4 9% 19% 32% 11%	4097
	11th to 20th	Night	16% 4 7% 28% 36% 9%	4110
	21st to end	Night	27% 6% 9% 18% 30% 9%	4521
February	1st to 10th	Night	17% 3 9% 20% 31% 20%	2423
	11th to 20th	Night	21% 4 6% 20% 30% 19%	2218
	21st to end	Night	20% 2 4 15% 34% 25%	1884
March	1st to 10th	Night	18% 2 6% 14% 42% 18%	3522
	11th to 20th	Night	26% 7% 8% 21% 27% 12%	3490
	21st to end	Night	16% 4 7% 23% 37% 14%	3715
April	1st to 10th	Night	16% 2 5% 15% 37% 24%	1900
	11th to 20th	Night	19% 3 5% 13% 28% 31%	1877
	21st to end	Night	17% 3 7% 17% 35% 20%	1900
May	1st to 10th	Night	14% 2 7% 10% 37% 30%	3170
	11th to 20th	Night	19% 6% 12% 14% 27% 21%	3094
	21st to end	Night	11% 2 5% 9% 17% 56%	2870
June	1st to 10th	Night	22% 6% 12% 16% 18% 27%	1520
	11th to 20th	Night	17% 3 7% 13% 18% 42%	1520
	21st to end	Night	29% 7% 11% 15% 15% 23%	1520
July	1st to 10th	Night	27% 9% 14% 14% 16% 20%	2775
	11th to 20th	Night	29% 8% 15% 30% 14% 4	2806
	21st to end	Night	32% 10% 19% 30% 8% 1	3011
August	1st to 10th	Night	28% 10% 19% 21% 15% 7%	2985
	11th to 20th	Night	26% 10% 14% 23% 21% 7%	3143
	21st to end	Night	34% 11% 17% 21% 13% 3	3487
September	1st to 10th	Night	28% 8% 13% 19% 21% 11%	1900
	11th to 20th	Night	24% 5% 11% 16% 23% 20%	1900
	21st to end	Night	18% 4 8% 16% 32% 22%	2111
October	1st to 10th	Night	23% 4 9% 16% 25% 22%	2280
	11th to 20th	Night	15% 3 5% 13% 30% 34%	2280
	21st to end	Night	14% 3 6% 16% 38% 23%	2508
November	1st to 10th	Night	10% 3 5% 22% 39% 22%	2244
	11th to 20th	Night	13% 3 4 20% 38% 22%	2436
	21st to end	Night	21% 3 4 17% 36% 19%	2470
December	1st to 10th	Night	18% 4 7% 26% 31% 14%	2569
	11th to 20th	Night	22% 4 9% 21% 31% 13%	3272
	21st to end	Night	21% 4 5% 21% 32% 17%	4800

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% 17% 38% 29% 11%	270
	11th to 20th	Day	10% 15% 35% 24% 14% 1	270
	21st to end	Day	5% 13% 36% 30% 11% 4	297
February	1st to 10th	Day	6% 19% 33% 27% 14%	270
	11th to 20th	Day	10% 20% 40% 23% 6%	270
	21st to end	Day	13% 29% 32% 18% 6%	225
March	1st to 10th	Day	10% 19% 37% 22% 10% 1	396
	11th to 20th	Day	13% 20% 33% 23% 9% 2	460
	21st to end	Day	13% 24% 37% 18% 7% 1	506
April	1st to 10th	Day	10% 19% 37% 23% 8% 3	460
	11th to 20th	Day	12% 20% 39% 21% 8%	452
	21st to end	Day	10% 20% 38% 23% 9% 1	460
May	1st to 10th	Day	6% 16% 40% 28% 8% 2	463
	11th to 20th	Day	6% 14% 31% 29% 15% 5%	488
	21st to end	Day	4 11% 34% 32% 17% 2	660
June	1st to 10th	Day	3 4 21% 40% 28% 5%	680
	11th to 20th	Day	3 5% 23% 37% 28% 4	680
	21st to end	Day	7% 3 14% 36% 29% 10%	680
July	1st to 10th	Day	4 2 9% 36% 34% 17%	504
	11th to 20th	Day	5% 1 4 21% 39% 30%	484
	21st to end	Day	5% 1 4 23% 39% 28%	531
August	1st to 10th	Day	2 1 6% 26% 44% 21%	487
	11th to 20th	Day	6% 2 9% 29% 37% 17%	477
	21st to end	Day	6% 1 6% 33% 37% 16%	539
September	1st to 10th	Day	7% 2 11% 33% 31% 16%	490
	11th to 20th	Day	5% 2 16% 36% 34% 8%	490
	21st to end	Day	6% 6% 18% 31% 30% 9%	330
October	1st to 10th	Day	7% 9% 22% 31% 24% 8%	300
	11th to 20th	Day	9% 8% 22% 32% 24% 6%	300
	21st to end	Day	10% 12% 22% 30% 19% 6%	330
November	1st to 10th	Day	8% 12% 35% 27% 15% 3	273
	11th to 20th	Day	3 10% 28% 33% 20% 6%	270
	21st to end	Day	10% 17% 34% 26% 11% 2	270
December	1st to 10th	Day	5% 16% 36% 27% 14% 1	270
	11th to 20th	Day	8% 15% 37% 31% 9%	270
	21st to end	Day	11% 18% 36% 25% 9% 1	297
January	1st to 10th	Night	5% 8% 32% 37% 17% 1	760
	11th to 20th	Night	5% 10% 34% 34% 15% 2	760
	21st to end	Night	6% 10% 32% 36% 14% 2	836
February	1st to 10th	Night	4 11% 36% 36% 12% 1	760
	11th to 20th	Night	6% 11% 38% 36% 8%	744
	21st to end	Night	7% 13% 46% 30% 4	628
March	1st to 10th	Night	5% 13% 40% 32% 10% 1	760
	11th to 20th	Night	7% 12% 33% 35% 13% 1	760
	21st to end	Night	5% 13% 41% 33% 8%	774
April	1st to 10th	Night	6% 11% 45% 31% 7% 1	570
	11th to 20th	Night	6% 15% 39% 32% 7% 1	563
	21st to end	Night	5% 12% 43% 32% 7% 1	570
May	1st to 10th	Night	4 9% 42% 37% 8%	570
	11th to 20th	Night	6% 5% 30% 41% 16% 1	561
	21st to end	Night	3 6% 35% 43% 12%	617
June	1st to 10th	Night	4 5% 24% 46% 20% 1	570
	11th to 20th	Night	4 2 29% 47% 17% 1	570
	21st to end	Night	7% 2 15% 46% 28% 3	570
July	1st to 10th	Night	7% 3 8% 42% 35% 5%	555
	11th to 20th	Night	5% 2 5% 31% 50% 8%	552
	21st to end	Night	5% 2 4 27% 53% 9%	624
August	1st to 10th	Night	3 1 5% 29% 50% 11%	567
	11th to 20th	Night	5% 2 6% 35% 48% 4	564
	21st to end	Night	4 1 4 36% 50% 5%	627
September	1st to 10th	Night	7% 2 10% 42% 36% 4	570
	11th to 20th	Night	8% 2 11% 48% 29% 1	570
	21st to end	Night	6% 3 18% 42% 29% 4	624
October	1st to 10th	Night	6% 4 22% 44% 21% 3	760
	11th to 20th	Night	5% 6% 22% 45% 21% 2	760
	21st to end	Night	4 8% 28% 34% 22% 4	836
November	1st to 10th	Night	3 5% 26% 42% 22% 3	748
	11th to 20th	Night	3 6% 29% 40% 20% 2	916
	21st to end	Night	6% 11% 26% 34% 20% 2	950
December	1st to 10th	Night	5% 9% 31% 38% 16% 1	950
	11th to 20th	Night	6% 10% 33% 37% 13%	950
	21st to end	Night	6% 13% 33% 37% 9% 1	998

Page generated at 2024-10-30 11:19:23. Version ac07

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