Xanadu Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Xanadu Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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 2 1 3 3 3 3 3 5% 6% 68%	140976
	Night	4 3 1 3 2 3 3 3 5% 6% 67%	139062
Transparency	Day	16% 4 6% 11% 25% 37%	135870
	Night	17% 3 6% 12% 26% 36%	135185
Seeing	Day	6% 9% 25% 33% 20% 6%	16834
	Night	5% 3 17% 41% 29% 5%	27018

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% 5% 2 4 2 3 3 3 5% 6% 60%	5318
	11th to 20th	Day	3 2 2 2 2 2 1 2 3 81%	5340
	21st to end	Day	7% 4 1 4 3 4 4 3 5% 7% 57%	5844
February	1st to 10th	Day	6% 4 2 3 2 2 2 3 4 6% 66%	1922
	11th to 20th	Day	7% 4 1 4 3 3 3 3 5% 5% 60%	2050
	21st to end	Day	7% 4 1 3 2 3 2 3 3 5% 66%	1846
March	1st to 10th	Day	6% 4 1 3 2 2 3 3 4 6% 67%	5064
	11th to 20th	Day	5% 3 1 3 2 3 3 3 4 5% 67%	5110
	21st to end	Day	3 4 1 4 2 3 3 3 4 6% 67%	5607
April	1st to 10th	Day	2 2 1 2 3 2 2 3 4 7% 73%	2310
	11th to 20th	Day	2 2 2 2 2 2 2 3 4 79%	2454
	21st to end	Day	1 1 2 2 2 2 2 3 5% 80%	2521
May	1st to 10th	Day	1 1 1 1 1 2 1 2 3 86%	5894
	11th to 20th	Day	1 1 2 2 2 2 2 3 4 81%	5841
	21st to end	Day	1 1 1 2 1 1 2 2 2 3 83%	6423
June	1st to 10th	Day	2 2 1 2 2 2 2 2 4 7% 74%	2600
	11th to 20th	Day	1 1 2 1 1 2 2 3 4 83%	2600
	21st to end	Day	2 2 1 3 4 4 4 4 7% 7% 64%	2600
July	1st to 10th	Day	1 1 1 3 3 4 3 4 5% 6% 67%	5879
	11th to 20th	Day	2 2 1 4 4 5% 6% 5% 9% 11% 51%	5880
	21st to end	Day	3 3 1 4 4 5% 6% 7% 10% 12% 45%	6401
August	1st to 10th	Day	2 2 1 3 4 5% 5% 5% 7% 11% 55%	5779
	11th to 20th	Day	2 2 1 4 3 4 5% 5% 9% 13% 53%	5761
	21st to end	Day	2 2 1 3 3 4 5% 5% 7% 9% 60%	5908
September	1st to 10th	Day	5% 4 1 6% 4 4 4 4 5% 8% 54%	2200
	11th to 20th	Day	2 2 1 4 3 3 4 4 5% 6% 67%	2200
	21st to end	Day	3 2 1 3 3 3 3 3 4 5% 70%	2200
October	1st to 10th	Day	1 1 1 2 2 2 2 2 4 5% 78%	3440
	11th to 20th	Day	2 1 1 1 1 1 2 3 4 83%	3250
	21st to end	Day	1 1 1 1 2 1 1 4 4 83%	3548
November	1st to 10th	Day	2 2 1 1 2 1 3 3 5% 5% 76%	2059
	11th to 20th	Day	3 2 1 3 2 2 3 3 4 4 72%	2089
	21st to end	Day	6% 4 1 4 3 3 4 3 4 5% 63%	1890
December	1st to 10th	Day	4 2 1 2 2 2 2 2 4 4 76%	3040
	11th to 20th	Day	5% 4 2 4 2 2 2 3 5% 7% 64%	3499
	21st to end	Day	5% 4 1 3 3 3 3 2 4 5% 67%	4609
January	1st to 10th	Night	7% 5% 2 4 3 3 3 3 5% 6% 59%	7605
	11th to 20th	Night	2 1 1 1 1 1 1 1 2 4 84%	7650
	21st to end	Night	10% 5% 2 5% 3 4 4 3 5% 6% 54%	8370
February	1st to 10th	Night	6% 4 2 3 3 3 3 3 4 5% 66%	2693
	11th to 20th	Night	6% 5% 1 3 3 3 4 3 5% 6% 60%	2458
	21st to end	Night	6% 4 1 4 2 3 3 3 4 5% 64%	1957
March	1st to 10th	Night	4 2 1 3 2 3 3 3 4 6% 69%	5050
	11th to 20th	Night	9% 3 1 2 2 2 3 2 4 5% 68%	5110
	21st to end	Night	5% 4 1 4 3 2 3 3 4 6% 67%	5610
April	1st to 10th	Night	2 2 1 2 3 3 2 3 4 6% 73%	2310
	11th to 20th	Night	2 2 1 2 2 2 3 2 3 4 77%	2112
	21st to end	Night	2 2 1 2 1 1 1 1 3 4 82%	2094
May	1st to 10th	Night	1 1 1 1 1 1 2 3 4 85%	4109
	11th to 20th	Night	1 1 1 2 2 2 2 4 5% 79%	4074
	21st to end	Night	1 1 1 1 1 1 2 2 3 87%	4474
June	1st to 10th	Night	3 2 2 1 2 2 3 4 6% 75%	1800
	11th to 20th	Night	1 1 2 1 2 1 1 2 5% 83%	1800
	21st to end	Night	2 3 1 3 4 2 4 3 4 6% 67%	1800
July	1st to 10th	Night	1 2 1 4 3 4 4 4 6% 8% 62%	4100
	11th to 20th	Night	1 2 1 4 5% 5% 7% 7% 10% 14% 45%	4100
	21st to end	Night	2 2 1 4 4 4 6% 6% 12% 15% 43%	4472
August	1st to 10th	Night	2 2 1 4 4 5% 6% 6% 8% 10% 51%	4033
	11th to 20th	Night	1 2 1 4 4 5% 6% 6% 9% 13% 48%	4085
	21st to end	Night	2 2 1 4 3 4 5% 5% 7% 11% 56%	5055
September	1st to 10th	Night	4 4 2 6% 4 4 5% 3 6% 8% 54%	2200
	11th to 20th	Night	2 3 1 3 3 3 3 3 4 6% 70%	2200
	21st to end	Night	2 2 1 3 3 2 4 2 4 5% 71%	2200
October	1st to 10th	Night	1 1 1 2 2 2 3 3 4 6% 76%	3754
	11th to 20th	Night	2 1 1 2 2 1 2 2 3 3 81%	3898
	21st to end	Night	2 1 1 2 1 2 1 3 3 84%	4252
November	1st to 10th	Night	2 1 1 3 2 2 2 3 4 5% 76%	2460
	11th to 20th	Night	2 3 1 3 2 2 2 3 4 6% 73%	2525
	21st to end	Night	7% 4 1 4 3 3 3 3 4 6% 62%	2730
December	1st to 10th	Night	4 2 1 2 1 1 2 2 3 5% 78%	4370
	11th to 20th	Night	5% 4 2 4 3 3 3 2 4 5% 66%	4941
	21st to end	Night	6% 4 1 3 3 3 4 3 4 5% 65%	6611

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	24% 3 6% 12% 29% 25%	5321
	11th to 20th	Day	12% 2 2 6% 29% 49%	5340
	21st to end	Day	26% 3 6% 11% 24% 30%	5844
February	1st to 10th	Day	21% 2 4 11% 23% 39%	1922
	11th to 20th	Day	26% 3 6% 13% 25% 27%	2060
	21st to end	Day	21% 3 3 9% 28% 35%	1846
March	1st to 10th	Day	18% 2 5% 6% 35% 33%	5064
	11th to 20th	Day	16% 4 7% 13% 32% 28%	5110
	21st to end	Day	19% 3 4 10% 34% 30%	5607
April	1st to 10th	Day	13% 2 3 6% 37% 39%	2310
	11th to 20th	Day	11% 2 3 6% 27% 52%	2457
	21st to end	Day	9% 1 3 7% 29% 50%	2521
May	1st to 10th	Day	6% 1 2 5% 24% 62%	5894
	11th to 20th	Day	9% 2 3 5% 23% 57%	5844
	21st to end	Day	8% 1 2 3 13% 73%	6424
June	1st to 10th	Day	11% 1 4 4 25% 55%	2600
	11th to 20th	Day	7% 1 3 2 16% 71%	2600
	21st to end	Day	18% 4 6% 9% 18% 45%	2600
July	1st to 10th	Day	16% 5% 7% 14% 25% 33%	5816
	11th to 20th	Day	21% 8% 14% 28% 18% 11%	5811
	21st to end	Day	25% 9% 17% 20% 13% 16%	6217
August	1st to 10th	Day	26% 10% 20% 19% 15% 10%	4208
	11th to 20th	Day	21% 8% 16% 23% 23% 9%	4172
	21st to end	Day	21% 7% 14% 18% 25% 16%	4259
September	1st to 10th	Day	28% 6% 9% 18% 17% 23%	2200
	11th to 20th	Day	17% 4 6% 12% 27% 33%	2200
	21st to end	Day	15% 4 5% 11% 30% 35%	2200
October	1st to 10th	Day	8% 3 4 10% 16% 59%	3440
	11th to 20th	Day	7% 2 3 7% 28% 53%	3250
	21st to end	Day	7% 1 3 6% 22% 60%	3548
November	1st to 10th	Day	10% 2 4 9% 37% 37%	2059
	11th to 20th	Day	16% 3 4 10% 30% 38%	2089
	21st to end	Day	23% 3 5% 11% 24% 33%	1890
December	1st to 10th	Day	13% 2 3 11% 36% 35%	3040
	11th to 20th	Day	21% 2 4 10% 23% 40%	3498
	21st to end	Day	19% 3 6% 15% 34% 23%	4609
January	1st to 10th	Night	24% 3 7% 11% 35% 20%	7605
	11th to 20th	Night	8% 1 2 10% 31% 47%	7650
	21st to end	Night	29% 4 5% 12% 23% 27%	8370
February	1st to 10th	Night	21% 2 5% 11% 22% 39%	2693
	11th to 20th	Night	25% 2 7% 13% 28% 26%	2459
	21st to end	Night	21% 3 4 10% 28% 34%	1957
March	1st to 10th	Night	16% 3 5% 9% 36% 32%	5050
	11th to 20th	Night	17% 3 5% 15% 33% 27%	5110
	21st to end	Night	19% 3 5% 12% 34% 27%	5610
April	1st to 10th	Night	14% 2 4 9% 36% 36%	2310
	11th to 20th	Night	13% 2 2 7% 26% 50%	2112
	21st to end	Night	8% 2 2 8% 31% 49%	2094
May	1st to 10th	Night	6% 1 3 8% 32% 50%	4106
	11th to 20th	Night	8% 2 5% 6% 21% 59%	4075
	21st to end	Night	5% 1 2 3 15% 74%	4474
June	1st to 10th	Night	11% 2 3 5% 23% 55%	1800
	11th to 20th	Night	8% 1 3 3 14% 71%	1800
	21st to end	Night	18% 3 6% 7% 17% 50%	1800
July	1st to 10th	Night	18% 5% 9% 13% 24% 32%	4055
	11th to 20th	Night	23% 10% 16% 24% 18% 9%	4046
	21st to end	Night	23% 10% 21% 21% 10% 15%	4344
August	1st to 10th	Night	27% 10% 19% 20% 14% 10%	2928
	11th to 20th	Night	24% 8% 14% 23% 20% 11%	2950
	21st to end	Night	22% 7% 15% 19% 22% 16%	3646
September	1st to 10th	Night	27% 7% 8% 19% 17% 22%	2200
	11th to 20th	Night	15% 5% 6% 15% 28% 32%	2200
	21st to end	Night	15% 3 5% 14% 31% 33%	2200
October	1st to 10th	Night	9% 3 4 10% 17% 56%	3754
	11th to 20th	Night	8% 3 3 10% 22% 54%	3898
	21st to end	Night	7% 2 2 7% 22% 61%	4252
November	1st to 10th	Night	12% 2 3 11% 34% 38%	2460
	11th to 20th	Night	14% 3 4 12% 30% 37%	2525
	21st to end	Night	22% 3 5% 12% 27% 31%	2730
December	1st to 10th	Night	12% 2 3 12% 36% 35%	4370
	11th to 20th	Night	22% 2 3 10% 24% 38%	4941
	21st to end	Night	22% 3 5% 12% 33% 26%	6611

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	16% 6% 31% 36% 10%	330
	11th to 20th	Day	9% 5% 35% 35% 13% 3	330
	21st to end	Day	9% 8% 35% 35% 13% 1	363
February	1st to 10th	Day	8% 9% 33% 35% 12% 4	362
	11th to 20th	Day	15% 17% 33% 27% 7%	535
	21st to end	Day	15% 17% 35% 25% 6% 1	446
March	1st to 10th	Day	12% 18% 34% 28% 8% 1	540
	11th to 20th	Day	10% 15% 31% 28% 12% 3	540
	21st to end	Day	9% 21% 34% 22% 11% 3	594
April	1st to 10th	Day	5% 17% 35% 23% 16% 4	540
	11th to 20th	Day	6% 22% 34% 24% 12% 3	532
	21st to end	Day	6% 20% 30% 28% 13% 4	540
May	1st to 10th	Day	4 15% 35% 28% 15% 3	508
	11th to 20th	Day	2 12% 35% 31% 15% 4	528
	21st to end	Day	3 11% 34% 35% 14% 3	573
June	1st to 10th	Day	3 7% 30% 35% 18% 6%	530
	11th to 20th	Day	2 6% 30% 38% 20% 4	530
	21st to end	Day	4 1 25% 39% 24% 7%	530
July	1st to 10th	Day	2 1 11% 39% 34% 12%	526
	11th to 20th	Day	4 2 6% 35% 37% 16%	524
	21st to end	Day	7% 2 4 32% 39% 15%	575
August	1st to 10th	Day	5% 1 4 35% 37% 18%	527
	11th to 20th	Day	5% 1 6% 38% 33% 16%	517
	21st to end	Day	3 2 5% 35% 38% 16%	583
September	1st to 10th	Day	8% 2 15% 39% 26% 10%	530
	11th to 20th	Day	3 3 17% 42% 25% 10%	530
	21st to end	Day	5% 8% 22% 34% 24% 9%	530
October	1st to 10th	Day	3 5% 25% 34% 25% 8%	435
	11th to 20th	Day	5% 5% 25% 35% 24% 7%	330
	21st to end	Day	3 5% 23% 36% 26% 7%	363
November	1st to 10th	Day	4 9% 25% 41% 17% 4	330
	11th to 20th	Day	5% 7% 29% 36% 20% 3	330
	21st to end	Day	10% 11% 28% 36% 14% 2	330
December	1st to 10th	Day	8% 5% 31% 35% 19% 2	330
	11th to 20th	Day	6% 8% 35% 33% 15% 3	330
	21st to end	Day	13% 9% 32% 31% 11% 3	363
January	1st to 10th	Night	9% 3 24% 47% 15% 2	840
	11th to 20th	Night	7% 5% 23% 44% 20% 2	840
	21st to end	Night	11% 4 23% 43% 17% 2	924
February	1st to 10th	Night	8% 3 23% 42% 21% 3	840
	11th to 20th	Night	9% 5% 26% 44% 15% 1	824
	21st to end	Night	9% 4 29% 43% 14% 1	692
March	1st to 10th	Night	6% 6% 30% 42% 15% 2	840
	11th to 20th	Night	8% 4 23% 36% 25% 5%	840
	21st to end	Night	6% 4 20% 45% 21% 3	924
April	1st to 10th	Night	4 4 20% 50% 21% 2	840
	11th to 20th	Night	3 6% 25% 44% 21% 1	658
	21st to end	Night	2 4 24% 47% 20% 2	630
May	1st to 10th	Night	2 4 20% 49% 21% 3	603
	11th to 20th	Night	1 3 20% 46% 26% 4	591
	21st to end	Night	1 2 18% 51% 26% 2	650
June	1st to 10th	Night	3 2 11% 47% 34% 3	600
	11th to 20th	Night	2 1 11% 49% 35% 3	600
	21st to end	Night	2 2 9% 37% 45% 6%	600
July	1st to 10th	Night	2 2 4 30% 54% 8%	585
	11th to 20th	Night	2 2 3 22% 60% 11%	582
	21st to end	Night	4 2 2 23% 54% 15%	657
August	1st to 10th	Night	2 1 1 21% 60% 15%	597
	11th to 20th	Night	4 1 2 22% 58% 13%	594
	21st to end	Night	4 3 23% 57% 13%	683
September	1st to 10th	Night	6% 1 7% 32% 42% 12%	800
	11th to 20th	Night	3 1 9% 38% 41% 9%	800
	21st to end	Night	3 2 11% 42% 36% 7%	800
October	1st to 10th	Night	2 1 15% 47% 31% 3	800
	11th to 20th	Night	4 2 15% 43% 31% 6%	800
	21st to end	Night	3 3 14% 45% 28% 7%	880
November	1st to 10th	Night	4 2 17% 43% 28% 6%	820
	11th to 20th	Night	4 3 20% 44% 27% 2	840
	21st to end	Night	9% 4 25% 41% 18% 4	840
December	1st to 10th	Night	10% 2 22% 46% 19% 2	840
	11th to 20th	Night	9% 4 21% 45% 20% 2	840
	21st to end	Night	11% 4 23% 41% 18% 3	924

Page generated at 2026-04-28 23:21:41. Version ac07

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