Top Of The World Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Top Of The World 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 4 2 5% 4 4 4 4 5% 7% 59%	140207
	Night	2 4 2 5% 3 3 4 3 5% 7% 61%	139785
Transparency	Day	24% 5% 6% 16% 27% 21%	135158
	Night	22% 4 7% 18% 29% 19%	135851
Seeing	Day	7% 11% 23% 27% 21% 11%	16374
	Night	4 6% 24% 38% 24% 3	26731

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 7% 4 8% 5% 4 4 3 5% 7% 53%	5318
	11th to 20th	Day	1 3 2 2 1 2 2 2 3 5% 77%	5340
	21st to end	Day	3 9% 5% 7% 3 3 3 3 5% 5% 56%	5844
February	1st to 10th	Day	1 5% 2 4 4 3 3 3 5% 8% 62%	1890
	11th to 20th	Day	2 7% 2 5% 3 3 4 2 6% 7% 59%	1944
	21st to end	Day	1 7% 2 4 3 3 3 3 5% 6% 63%	1845
March	1st to 10th	Day	1 4 3 4 3 3 3 3 5% 7% 62%	5064
	11th to 20th	Day	2 6% 3 6% 4 4 5% 4 6% 8% 53%	5110
	21st to end	Day	1 5% 2 5% 3 3 4 4 6% 8% 59%	5607
April	1st to 10th	Day	2 4 2 4 3 4 3 3 5% 7% 63%	2468
	11th to 20th	Day	2 5% 2 5% 3 3 3 2 4 7% 64%	2491
	21st to end	Day	2 4 1 4 3 3 4 3 4 6% 67%	2520
May	1st to 10th	Day	2 3 1 4 4 4 4 3 5% 7% 63%	5496
	11th to 20th	Day	3 3 1 4 4 3 4 3 4 6% 64%	5841
	21st to end	Day	2 2 1 3 3 3 3 3 4 5% 71%	6423
June	1st to 10th	Day	3 3 1 4 3 4 3 3 5% 7% 61%	2600
	11th to 20th	Day	2 3 1 3 3 3 4 4 4 5% 71%	2600
	21st to end	Day	5% 5% 2 6% 5% 4 5% 4 5% 6% 53%	2600
July	1st to 10th	Day	3 4 2 5% 5% 4 5% 4 5% 5% 59%	5879
	11th to 20th	Day	4 5% 2 7% 7% 7% 6% 5% 7% 9% 41%	5880
	21st to end	Day	6% 5% 2 8% 7% 7% 7% 5% 8% 9% 36%	6401
August	1st to 10th	Day	4 4 2 7% 6% 6% 7% 6% 7% 8% 43%	5779
	11th to 20th	Day	4 3 2 6% 5% 5% 6% 5% 7% 9% 47%	5462
	21st to end	Day	5% 4 2 7% 5% 6% 8% 6% 8% 10% 39%	5984
September	1st to 10th	Day	8% 5% 3 7% 5% 5% 5% 5% 6% 6% 46%	2400
	11th to 20th	Day	6% 4 2 6% 4 4 4 4 5% 6% 55%	2324
	21st to end	Day	3 2 1 5% 3 4 4 3 4 5% 65%	2140
October	1st to 10th	Day	3 3 1 3 3 3 3 3 4 5% 70%	3270
	11th to 20th	Day	4 3 1 4 3 3 3 3 5% 6% 66%	3250
	21st to end	Day	1 2 1 2 1 1 1 1 3 5% 82%	3548
November	1st to 10th	Day	2 3 1 3 2 2 2 3 4 7% 72%	2059
	11th to 20th	Day	2 2 1 4 3 3 3 4 5% 7% 67%	2100
	21st to end	Day	2 6% 2 4 4 3 4 3 4 6% 63%	1956
December	1st to 10th	Day	2 2 1 2 1 1 2 2 3 5% 78%	2700
	11th to 20th	Day	2 5% 1 6% 4 3 3 2 3 6% 65%	3465
	21st to end	Day	1 4 2 4 3 3 4 3 4 6% 66%	4609
January	1st to 10th	Night	1 8% 4 10% 5% 4 4 3 5% 7% 49%	7605
	11th to 20th	Night	1 2 1 2 2 2 2 2 3 4 79%	7650
	21st to end	Night	2 8% 3 6% 4 4 4 3 5% 7% 54%	8370
February	1st to 10th	Night	1 5% 2 5% 3 3 2 3 5% 6% 66%	2730
	11th to 20th	Night	2 6% 2 6% 3 3 3 4 6% 7% 59%	2562
	21st to end	Night	2 3 2 5% 2 2 2 3 6% 7% 65%	1957
March	1st to 10th	Night	1 5% 3 4 3 3 3 3 4 5% 66%	5050
	11th to 20th	Night	2 5% 3 6% 4 3 4 3 4 6% 60%	5110
	21st to end	Night	1 5% 2 4 3 2 3 3 5% 7% 64%	5610
April	1st to 10th	Night	1 3 1 3 3 3 3 3 5% 7% 67%	2150
	11th to 20th	Night	2 3 1 3 2 3 4 2 4 5% 69%	2077
	21st to end	Night	2 3 1 3 2 2 3 2 5% 5% 72%	2100
May	1st to 10th	Night	2 4 1 4 3 3 3 3 4 6% 67%	4490
	11th to 20th	Night	2 2 1 3 2 2 3 3 4 5% 71%	4074
	21st to end	Night	1 2 1 2 2 2 2 2 4 6% 76%	4474
June	1st to 10th	Night	3 3 1 4 3 4 3 3 4 8% 63%	1800
	11th to 20th	Night	2 2 1 2 2 2 3 2 4 5% 74%	1800
	21st to end	Night	3 5% 2 6% 4 5% 5% 4 6% 8% 53%	1800
July	1st to 10th	Night	3 3 2 6% 4 4 5% 5% 7% 8% 54%	4100
	11th to 20th	Night	3 3 2 7% 5% 7% 7% 7% 10% 12% 37%	4100
	21st to end	Night	4 4 2 8% 6% 6% 7% 6% 10% 12% 34%	4472
August	1st to 10th	Night	2 3 2 6% 5% 6% 6% 7% 9% 13% 42%	4033
	11th to 20th	Night	4 4 2 8% 6% 6% 8% 6% 9% 10% 37%	4373
	21st to end	Night	4 4 1 7% 6% 6% 8% 7% 12% 12% 31%	4993
September	1st to 10th	Night	5% 6% 2 7% 5% 4 5% 5% 7% 8% 47%	2000
	11th to 20th	Night	4 3 1 4 4 4 4 4 6% 6% 60%	2071
	21st to end	Night	4 2 1 4 3 3 3 3 4 6% 66%	2258
October	1st to 10th	Night	3 3 1 4 3 3 3 3 4 6% 67%	3920
	11th to 20th	Night	5% 3 1 4 3 3 3 2 4 6% 66%	3898
	21st to end	Night	1 1 1 2 2 2 2 2 4 5% 78%	4252
November	1st to 10th	Night	1 2 1 2 2 2 3 2 4 5% 76%	2460
	11th to 20th	Night	1 2 1 3 2 2 2 2 4 5% 74%	2520
	21st to end	Night	3 6% 2 4 3 3 3 2 4 7% 63%	2626
December	1st to 10th	Night	2 3 1 2 2 1 2 2 4 6% 75%	4710
	11th to 20th	Night	1 4 1 4 3 3 3 3 5% 8% 65%	4979
	21st to end	Night	1 5% 2 4 3 2 3 3 4 6% 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	28% 6% 4 23% 30% 9%	5321
	11th to 20th	Day	13% 1 5% 20% 32% 29%	5340
	21st to end	Day	31% 3 2 11% 23% 30%	5844
February	1st to 10th	Day	22% 3 6% 20% 27% 22%	1890
	11th to 20th	Day	24% 4 4 21% 28% 18%	1953
	21st to end	Day	22% 3 2 16% 29% 27%	1845
March	1st to 10th	Day	19% 4 3 10% 33% 30%	5064
	11th to 20th	Day	27% 4 6% 16% 31% 16%	5110
	21st to end	Day	22% 4 4 22% 37% 12%	5607
April	1st to 10th	Day	20% 4 4 18% 35% 21%	2468
	11th to 20th	Day	22% 3 2 14% 32% 27%	2494
	21st to end	Day	20% 3 3 18% 39% 18%	2520
May	1st to 10th	Day	20% 5% 4 17% 32% 21%	5496
	11th to 20th	Day	21% 4 3 10% 33% 28%	5844
	21st to end	Day	14% 4 3 8% 25% 46%	6424
June	1st to 10th	Day	21% 4 4 15% 28% 29%	2600
	11th to 20th	Day	17% 3 3 9% 23% 45%	2600
	21st to end	Day	32% 4 6% 12% 23% 22%	2600
July	1st to 10th	Day	26% 5% 7% 11% 17% 35%	5816
	11th to 20th	Day	36% 8% 17% 20% 10% 8%	5811
	21st to end	Day	40% 9% 18% 18% 11% 3	6217
August	1st to 10th	Day	40% 9% 18% 17% 12% 4	4208
	11th to 20th	Day	31% 7% 14% 22% 19% 7%	3955
	21st to end	Day	40% 9% 15% 17% 17% 4	4311
September	1st to 10th	Day	36% 6% 8% 16% 21% 13%	2400
	11th to 20th	Day	29% 5% 7% 14% 26% 19%	2324
	21st to end	Day	22% 3 5% 18% 30% 22%	2140
October	1st to 10th	Day	17% 3 5% 15% 29% 32%	3270
	11th to 20th	Day	18% 3 4 16% 38% 21%	3250
	21st to end	Day	9% 2 3 14% 36% 36%	3548
November	1st to 10th	Day	14% 2 3 22% 37% 21%	2059
	11th to 20th	Day	17% 3 4 17% 36% 22%	2100
	21st to end	Day	24% 3 4 15% 33% 21%	1956
December	1st to 10th	Day	12% 2 3 24% 41% 18%	2700
	11th to 20th	Day	23% 2 4 14% 41% 15%	3464
	21st to end	Day	19% 3 5% 23% 32% 17%	4609
January	1st to 10th	Night	32% 6% 5% 21% 28% 9%	7605
	11th to 20th	Night	10% 2 5% 19% 43% 21%	7650
	21st to end	Night	30% 4 3 11% 28% 24%	8370
February	1st to 10th	Night	20% 2 8% 21% 27% 22%	2730
	11th to 20th	Night	23% 4 5% 24% 25% 18%	2563
	21st to end	Night	18% 3 4 20% 31% 24%	1957
March	1st to 10th	Night	19% 4 3 13% 40% 22%	5050
	11th to 20th	Night	23% 4 6% 20% 30% 17%	5110
	21st to end	Night	18% 5% 3 26% 39% 10%	5610
April	1st to 10th	Night	16% 3 4 25% 35% 18%	2150
	11th to 20th	Night	18% 3 2 17% 36% 24%	2077
	21st to end	Night	15% 3 4 19% 39% 20%	2100
May	1st to 10th	Night	19% 4 6% 20% 38% 14%	4487
	11th to 20th	Night	16% 3 4 13% 41% 23%	4075
	21st to end	Night	11% 2 4 9% 24% 49%	4474
June	1st to 10th	Night	21% 3 4 17% 27% 28%	1800
	11th to 20th	Night	14% 2 3 12% 25% 44%	1800
	21st to end	Night	29% 4 10% 16% 17% 24%	1800
July	1st to 10th	Night	25% 7% 13% 12% 14% 29%	4055
	11th to 20th	Night	32% 9% 21% 19% 11% 7%	4046
	21st to end	Night	37% 10% 26% 16% 8% 2	4344
August	1st to 10th	Night	33% 10% 25% 18% 10% 4	2928
	11th to 20th	Night	38% 10% 17% 17% 11% 7%	3168
	21st to end	Night	37% 10% 22% 16% 11% 3	3597
September	1st to 10th	Night	33% 6% 13% 16% 19% 13%	2000
	11th to 20th	Night	23% 5% 9% 18% 26% 18%	2071
	21st to end	Night	19% 4 6% 17% 32% 21%	2258
October	1st to 10th	Night	18% 3 7% 15% 29% 28%	3920
	11th to 20th	Night	18% 4 6% 22% 31% 20%	3898
	21st to end	Night	11% 2 3 15% 41% 28%	4252
November	1st to 10th	Night	12% 3 2 25% 38% 20%	2460
	11th to 20th	Night	13% 2 4 21% 36% 23%	2520
	21st to end	Night	23% 2 4 18% 33% 19%	2626
December	1st to 10th	Night	13% 2 4 30% 31% 20%	4710
	11th to 20th	Night	19% 2 5% 17% 39% 18%	4979
	21st to end	Night	20% 3 6% 24% 31% 17%	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	5% 12% 39% 31% 10% 3	330
	11th to 20th	Day	5% 15% 31% 29% 16% 5%	330
	21st to end	Day	5% 12% 34% 26% 18% 6%	363
February	1st to 10th	Day	5% 10% 34% 32% 13% 6%	330
	11th to 20th	Day	8% 16% 40% 26% 10%	330
	21st to end	Day	10% 22% 34% 22% 8% 4	388
March	1st to 10th	Day	7% 23% 34% 22% 12% 2	540
	11th to 20th	Day	11% 18% 32% 21% 14% 4	540
	21st to end	Day	10% 22% 34% 20% 11% 3	594
April	1st to 10th	Day	7% 15% 35% 22% 14% 6%	540
	11th to 20th	Day	8% 26% 33% 21% 9% 4	532
	21st to end	Day	8% 18% 35% 22% 11% 5%	540
May	1st to 10th	Day	6% 14% 36% 27% 14% 3	508
	11th to 20th	Day	9% 12% 27% 29% 15% 9%	528
	21st to end	Day	4 12% 32% 29% 17% 5%	573
June	1st to 10th	Day	6% 6% 23% 30% 24% 11%	530
	11th to 20th	Day	5% 8% 19% 31% 24% 13%	530
	21st to end	Day	8% 4 18% 34% 26% 11%	530
July	1st to 10th	Day	6% 4 7% 30% 35% 18%	526
	11th to 20th	Day	9% 3 5% 21% 33% 30%	524
	21st to end	Day	8% 4 5% 18% 35% 30%	575
August	1st to 10th	Day	9% 4 4 26% 30% 27%	527
	11th to 20th	Day	7% 3 5% 27% 33% 23%	517
	21st to end	Day	7% 3 6% 27% 30% 27%	583
September	1st to 10th	Day	11% 4 8% 23% 34% 20%	530
	11th to 20th	Day	6% 4 16% 32% 30% 12%	530
	21st to end	Day	4 7% 19% 27% 29% 13%	470
October	1st to 10th	Day	5% 8% 19% 30% 23% 15%	330
	11th to 20th	Day	7% 9% 21% 31% 20% 12%	330
	21st to end	Day	5% 9% 20% 28% 28% 10%	363
November	1st to 10th	Day	3 9% 29% 31% 20% 8%	330
	11th to 20th	Day	5% 8% 20% 34% 23% 10%	330
	21st to end	Day	5% 15% 29% 30% 15% 7%	330
December	1st to 10th	Day	6% 7% 29% 32% 22% 3	330
	11th to 20th	Day	5% 13% 31% 26% 18% 7%	330
	21st to end	Day	7% 16% 34% 27% 11% 4	363
January	1st to 10th	Night	3 8% 27% 40% 18% 3	840
	11th to 20th	Night	5% 7% 31% 38% 18% 3	840
	21st to end	Night	4 8% 32% 35% 19% 2	924
February	1st to 10th	Night	3 8% 34% 36% 17% 2	840
	11th to 20th	Night	6% 8% 39% 34% 11% 1	824
	21st to end	Night	4 10% 42% 35% 9%	692
March	1st to 10th	Night	3 10% 39% 35% 12% 1	840
	11th to 20th	Night	6% 9% 32% 36% 15% 2	840
	21st to end	Night	4 11% 32% 38% 13% 1	924
April	1st to 10th	Night	4 9% 39% 36% 12% 1	680
	11th to 20th	Night	4 11% 39% 36% 9%	623
	21st to end	Night	3 11% 39% 33% 12% 1	630
May	1st to 10th	Night	5% 5% 34% 42% 12% 1	603
	11th to 20th	Night	5% 5% 26% 44% 17% 2	591
	21st to end	Night	3 4 28% 48% 16% 1	650
June	1st to 10th	Night	5% 4 16% 53% 21% 2	600
	11th to 20th	Night	3 3 21% 52% 20% 1	600
	21st to end	Night	5% 3 13% 46% 31% 2	600
July	1st to 10th	Night	6% 3 3 43% 40% 6%	585
	11th to 20th	Night	4 1 4 31% 52% 8%	582
	21st to end	Night	7% 4 4 21% 51% 13%	657
August	1st to 10th	Night	5% 2 2 25% 54% 12%	597
	11th to 20th	Night	6% 2 5% 29% 50% 8%	594
	21st to end	Night	5% 3 2 30% 52% 8%	660
September	1st to 10th	Night	7% 3 5% 37% 44% 4	600
	11th to 20th	Night	5% 1 7% 51% 34% 3	671
	21st to end	Night	4 3 14% 40% 35% 4	800
October	1st to 10th	Night	3 2 23% 44% 25% 3	800
	11th to 20th	Night	3 3 26% 42% 24% 3	800
	21st to end	Night	3 4 21% 39% 27% 6%	880
November	1st to 10th	Night	2 4 21% 42% 26% 5%	820
	11th to 20th	Night	2 4 23% 45% 23% 3	840
	21st to end	Night	7% 8% 24% 35% 20% 5%	865
December	1st to 10th	Night	6% 7% 26% 41% 18% 2	1050
	11th to 20th	Night	5% 7% 31% 36% 18% 2	865
	21st to end	Night	6% 10% 32% 37% 12% 3	924

Page generated at 2026-04-29 00:21:47. Version ac07

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