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 2025-01-07.

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 3 5% 7% 58%	108715
	Night	2 4 2 5% 3 3 4 3 5% 7% 61%	109075
Transparency	Day	25% 5% 6% 17% 28% 19%	108479
	Night	22% 4 8% 20% 29% 18%	108900
Seeing	Day	7% 11% 23% 27% 21% 11%	14649
	Night	4 6% 24% 38% 24% 4	24995

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% 4 9% 5% 3 4 3 5% 8% 53%	2912
	11th to 20th	Day	2 5% 3 4 2 2 2 3 4 7% 67%	2860
	21st to end	Day	3 12% 7% 7% 3 3 4 3 5% 6% 46%	3146
February	1st to 10th	Day	1 6% 2 5% 4 3 3 3 5% 8% 60%	1710
	11th to 20th	Day	2 6% 2 4 3 3 4 3 5% 6% 62%	1754
	21st to end	Day	1 8% 2 5% 3 3 3 3 6% 6% 61%	1674
March	1st to 10th	Day	1 5% 2 4 3 3 4 3 6% 8% 60%	3444
	11th to 20th	Day	2 8% 2 7% 4 5% 5% 4 6% 9% 46%	3490
	21st to end	Day	1 4 3 5% 3 3 4 3 6% 8% 60%	3825
April	1st to 10th	Day	1 4 2 3 3 4 3 4 5% 7% 64%	2232
	11th to 20th	Day	2 5% 2 4 3 3 3 2 4 7% 65%	2251
	21st to end	Day	2 4 1 4 3 3 4 3 4 5% 66%	2280
May	1st to 10th	Day	2 2 1 3 3 3 3 2 4 5% 72%	3830
	11th to 20th	Day	4 4 2 5% 5% 4 4 4 5% 6% 57%	4071
	21st to end	Day	2 2 1 3 2 2 3 2 3 4 76%	4477
June	1st to 10th	Day	3 3 1 4 3 4 3 3 5% 7% 62%	2470
	11th to 20th	Day	2 3 1 3 3 3 4 3 4 5% 70%	2470
	21st to end	Day	5% 5% 2 6% 5% 4 5% 4 5% 7% 52%	2470
July	1st to 10th	Day	3 4 2 5% 5% 5% 5% 4 5% 6% 56%	4110
	11th to 20th	Day	4 5% 2 7% 7% 7% 7% 5% 7% 9% 39%	4110
	21st to end	Day	6% 5% 2 7% 7% 7% 8% 6% 8% 10% 34%	4454
August	1st to 10th	Day	5% 5% 2 9% 8% 7% 7% 6% 8% 9% 35%	4092
	11th to 20th	Day	4 4 2 7% 5% 6% 6% 5% 7% 10% 45%	3796
	21st to end	Day	6% 5% 3 8% 6% 7% 7% 5% 7% 8% 38%	4180
September	1st to 10th	Day	8% 5% 3 7% 5% 5% 5% 5% 6% 6% 46%	2280
	11th to 20th	Day	5% 4 2 6% 4 4 4 4 5% 6% 55%	2208
	21st to end	Day	3 2 1 5% 3 4 4 3 4 5% 65%	2033
October	1st to 10th	Day	3 3 1 3 3 3 3 3 4 5% 70%	3170
	11th to 20th	Day	3 3 1 4 3 3 3 3 5% 6% 65%	3150
	21st to end	Day	1 2 1 2 1 1 1 1 3 5% 82%	3438
November	1st to 10th	Day	2 3 1 4 2 2 2 3 4 7% 71%	1959
	11th to 20th	Day	2 2 1 3 2 3 3 4 5% 7% 68%	2000
	21st to end	Day	2 6% 2 4 4 3 4 2 4 6% 63%	1863
December	1st to 10th	Day	2 2 1 2 1 1 2 2 3 5% 78%	2620
	11th to 20th	Day	2 5% 1 6% 4 3 3 2 3 6% 64%	3376
	21st to end	Day	1 4 2 4 3 3 4 3 4 6% 66%	4510
January	1st to 10th	Night	1 6% 4 12% 5% 4 3 3 5% 7% 51%	4175
	11th to 20th	Night	1 4 1 4 2 2 3 3 5% 6% 69%	4110
	21st to end	Night	3 12% 5% 8% 4 4 5% 3 5% 7% 45%	4521
February	1st to 10th	Night	1 5% 2 5% 3 3 2 3 5% 6% 64%	2470
	11th to 20th	Night	2 5% 3 6% 3 2 3 3 5% 7% 60%	2312
	21st to end	Night	2 4 2 5% 2 2 3 3 6% 7% 64%	1777
March	1st to 10th	Night	1 4 3 4 2 2 2 2 4 6% 70%	3430
	11th to 20th	Night	3 6% 2 6% 4 4 5% 4 5% 6% 55%	3490
	21st to end	Night	1 6% 3 5% 3 2 3 3 5% 7% 61%	3828
April	1st to 10th	Night	1 3 1 3 2 3 2 3 5% 7% 68%	1946
	11th to 20th	Night	2 3 1 3 2 4 4 2 4 5% 70%	1877
	21st to end	Night	2 4 1 3 2 2 3 2 4 5% 71%	1900
May	1st to 10th	Night	2 2 1 3 2 2 2 2 3 5% 77%	3123
	11th to 20th	Night	3 3 2 4 3 3 3 3 5% 6% 64%	2834
	21st to end	Night	1 1 1 1 1 2 1 1 3 4 83%	3110
June	1st to 10th	Night	3 3 1 4 3 4 3 3 5% 8% 64%	1710
	11th to 20th	Night	2 2 1 2 2 2 3 2 4 5% 74%	1710
	21st to end	Night	2 5% 2 6% 4 5% 5% 4 6% 8% 53%	1710
July	1st to 10th	Night	3 3 2 5% 4 4 5% 5% 6% 7% 57%	2860
	11th to 20th	Night	3 3 2 8% 5% 7% 7% 7% 10% 12% 38%	2860
	21st to end	Night	5% 4 2 7% 6% 7% 7% 6% 10% 13% 33%	3109
August	1st to 10th	Night	3 3 2 6% 6% 7% 6% 7% 10% 14% 36%	2847
	11th to 20th	Night	5% 5% 2 8% 7% 6% 8% 6% 8% 9% 35%	3042
	21st to end	Night	4 4 2 7% 6% 7% 9% 7% 12% 12% 30%	3487
September	1st to 10th	Night	5% 6% 2 7% 5% 4 5% 4 7% 8% 47%	1900
	11th to 20th	Night	4 3 1 4 4 4 4 4 5% 6% 60%	1967
	21st to end	Night	3 2 1 4 3 3 3 3 5% 7% 66%	2145
October	1st to 10th	Night	3 3 1 4 3 3 3 3 4 6% 67%	3800
	11th to 20th	Night	5% 3 1 4 3 3 3 2 4 6% 66%	3778
	21st to end	Night	1 1 1 2 2 2 2 2 4 5% 78%	4120
November	1st to 10th	Night	1 2 1 3 2 2 3 2 4 5% 75%	2340
	11th to 20th	Night	1 2 1 3 2 2 2 2 4 5% 75%	2400
	21st to end	Night	3 6% 2 4 3 3 3 2 4 7% 64%	2501
December	1st to 10th	Night	2 3 1 2 2 1 2 2 4 6% 75%	4570
	11th to 20th	Night	1 4 1 4 3 3 3 3 5% 8% 64%	4848
	21st to end	Night	1 5% 2 4 3 2 3 3 4 6% 65%	6468

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	25% 9% 4 24% 27% 11%	2913
	11th to 20th	Day	19% 2 9% 32% 30% 8%	2860
	21st to end	Day	37% 5% 3 17% 29% 9%	3146
February	1st to 10th	Day	24% 3 6% 21% 27% 19%	1710
	11th to 20th	Day	23% 4 4 21% 29% 19%	1763
	21st to end	Day	24% 3 1 17% 29% 26%	1674
March	1st to 10th	Day	21% 5% 2 14% 38% 21%	3444
	11th to 20th	Day	33% 4 7% 19% 27% 11%	3490
	21st to end	Day	21% 5% 3 22% 39% 10%	3825
April	1st to 10th	Day	19% 3 3 18% 35% 21%	2232
	11th to 20th	Day	22% 3 2 14% 32% 27%	2254
	21st to end	Day	20% 3 3 18% 39% 17%	2280
May	1st to 10th	Day	15% 4 2 11% 40% 28%	3830
	11th to 20th	Day	26% 5% 4 12% 33% 20%	4074
	21st to end	Day	12% 4 2 7% 25% 49%	4477
June	1st to 10th	Day	21% 3 4 14% 29% 28%	2470
	11th to 20th	Day	17% 3 3 9% 23% 45%	2470
	21st to end	Day	32% 4 7% 13% 24% 21%	2470
July	1st to 10th	Day	28% 5% 7% 10% 11% 38%	4046
	11th to 20th	Day	38% 8% 14% 25% 12% 4	4041
	21st to end	Day	40% 9% 21% 23% 7% 1	4314
August	1st to 10th	Day	40% 9% 18% 17% 12% 4	4078
	11th to 20th	Day	31% 7% 14% 22% 19% 7%	3833
	21st to end	Day	39% 9% 15% 16% 17% 4	4179
September	1st to 10th	Day	36% 5% 8% 16% 21% 14%	2280
	11th to 20th	Day	29% 4 7% 14% 27% 19%	2208
	21st to end	Day	22% 3 5% 17% 30% 22%	2033
October	1st to 10th	Day	17% 3 5% 15% 29% 32%	3170
	11th to 20th	Day	18% 3 4 16% 38% 21%	3150
	21st to end	Day	9% 2 3 14% 36% 36%	3438
November	1st to 10th	Day	15% 3 3 23% 35% 22%	1959
	11th to 20th	Day	17% 3 4 17% 37% 23%	2000
	21st to end	Day	24% 3 4 14% 32% 22%	1863
December	1st to 10th	Day	12% 2 3 24% 41% 18%	2620
	11th to 20th	Day	24% 2 4 14% 41% 15%	3375
	21st to end	Day	19% 3 5% 23% 32% 17%	4510
January	1st to 10th	Night	30% 8% 5% 19% 27% 12%	4175
	11th to 20th	Night	16% 3 8% 31% 34% 8%	4110
	21st to end	Night	37% 5% 4 17% 29% 7%	4521
February	1st to 10th	Night	22% 3 8% 23% 26% 19%	2470
	11th to 20th	Night	23% 3 5% 25% 25% 20%	2313
	21st to end	Night	19% 3 4 20% 32% 22%	1777
March	1st to 10th	Night	16% 4 3 18% 44% 17%	3430
	11th to 20th	Night	27% 5% 8% 22% 28% 10%	3490
	21st to end	Night	19% 6% 3 29% 35% 8%	3828
April	1st to 10th	Night	15% 2 3 25% 35% 19%	1946
	11th to 20th	Night	18% 3 2 17% 36% 25%	1877
	21st to end	Night	16% 3 4 20% 38% 18%	1900
May	1st to 10th	Night	13% 3 3 12% 50% 20%	3123
	11th to 20th	Night	21% 4 5% 16% 37% 17%	2835
	21st to end	Night	8% 1 3 9% 25% 53%	3110
June	1st to 10th	Night	20% 3 4 17% 27% 28%	1710
	11th to 20th	Night	14% 2 3 12% 25% 43%	1710
	21st to end	Night	28% 4 10% 17% 17% 23%	1710
July	1st to 10th	Night	25% 6% 10% 13% 11% 35%	2815
	11th to 20th	Night	32% 9% 17% 27% 9% 6%	2806
	21st to end	Night	36% 10% 29% 20% 4 1	3011
August	1st to 10th	Night	33% 10% 26% 18% 10% 4	2838
	11th to 20th	Night	38% 10% 16% 17% 11% 7%	3071
	21st to end	Night	37% 10% 22% 16% 11% 4	3487
September	1st to 10th	Night	33% 6% 13% 16% 19% 13%	1900
	11th to 20th	Night	23% 5% 9% 17% 27% 18%	1967
	21st to end	Night	19% 4 6% 17% 33% 21%	2145
October	1st to 10th	Night	18% 3 7% 15% 28% 28%	3800
	11th to 20th	Night	18% 4 5% 22% 31% 20%	3778
	21st to end	Night	11% 2 3 16% 41% 28%	4120
November	1st to 10th	Night	12% 3 2 26% 37% 20%	2340
	11th to 20th	Night	13% 2 4 21% 36% 24%	2400
	21st to end	Night	23% 2 4 18% 32% 20%	2501
December	1st to 10th	Night	13% 2 4 30% 30% 20%	4570
	11th to 20th	Night	20% 3 5% 17% 39% 17%	4848
	21st to end	Night	20% 3 6% 24% 31% 17%	6468

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	4 12% 38% 32% 10% 3	288
	11th to 20th	Day	6% 15% 30% 30% 14% 6%	270
	21st to end	Day	5% 10% 34% 25% 20% 5%	297
February	1st to 10th	Day	5% 10% 35% 31% 14% 5%	270
	11th to 20th	Day	6% 14% 42% 27% 11%	270
	21st to end	Day	12% 24% 33% 21% 7% 3	329
March	1st to 10th	Day	7% 23% 34% 23% 12% 2	460
	11th to 20th	Day	12% 17% 32% 22% 15% 3	460
	21st to end	Day	11% 23% 36% 17% 9% 3	506
April	1st to 10th	Day	7% 16% 37% 22% 13% 6%	460
	11th to 20th	Day	8% 26% 32% 21% 10% 4	452
	21st to end	Day	8% 17% 35% 21% 12% 6%	460
May	1st to 10th	Day	6% 14% 36% 27% 14% 3	463
	11th to 20th	Day	8% 11% 26% 30% 16% 9%	488
	21st to end	Day	4 13% 33% 28% 16% 5%	529
June	1st to 10th	Day	5% 6% 22% 31% 24% 11%	490
	11th to 20th	Day	5% 8% 19% 31% 24% 13%	490
	21st to end	Day	8% 4 17% 34% 25% 11%	490
July	1st to 10th	Day	7% 3 7% 30% 35% 18%	486
	11th to 20th	Day	9% 2 4 20% 34% 30%	484
	21st to end	Day	8% 4 5% 16% 35% 32%	531
August	1st to 10th	Day	9% 4 4 25% 30% 28%	487
	11th to 20th	Day	8% 3 5% 29% 32% 23%	477
	21st to end	Day	7% 4 7% 27% 29% 26%	539
September	1st to 10th	Day	12% 4 8% 22% 33% 21%	490
	11th to 20th	Day	7% 3 15% 32% 30% 12%	490
	21st to end	Day	5% 7% 19% 27% 29% 14%	433
October	1st to 10th	Day	4 8% 19% 28% 25% 16%	300
	11th to 20th	Day	7% 8% 20% 31% 22% 13%	300
	21st to end	Day	6% 10% 18% 29% 27% 10%	330
November	1st to 10th	Day	4 10% 31% 32% 19% 5%	300
	11th to 20th	Day	4 9% 19% 35% 23% 10%	300
	21st to end	Day	5% 15% 29% 29% 16% 7%	300
December	1st to 10th	Day	7% 7% 28% 32% 22% 4	300
	11th to 20th	Day	5% 14% 31% 26% 17% 7%	300
	21st to end	Day	8% 17% 35% 26% 10% 4	330
January	1st to 10th	Night	3 8% 27% 40% 19% 4	784
	11th to 20th	Night	5% 7% 30% 37% 18% 3	760
	21st to end	Night	4 9% 31% 34% 20% 2	836
February	1st to 10th	Night	3 9% 34% 34% 18% 2	760
	11th to 20th	Night	5% 8% 40% 34% 12% 1	744
	21st to end	Night	4 10% 43% 34% 9%	628
March	1st to 10th	Night	3 11% 39% 34% 13% 1	760
	11th to 20th	Night	6% 8% 31% 37% 15% 2	760
	21st to end	Night	4 12% 34% 37% 12% 1	836
April	1st to 10th	Night	4 9% 39% 36% 12% 1	616
	11th to 20th	Night	4 11% 38% 36% 10% 1	563
	21st to end	Night	4 11% 37% 33% 14% 1	570
May	1st to 10th	Night	4 5% 35% 42% 12% 1	570
	11th to 20th	Night	6% 5% 24% 45% 18% 2	561
	21st to end	Night	3 4 28% 47% 16% 1	617
June	1st to 10th	Night	5% 4 15% 53% 21% 2	570
	11th to 20th	Night	3 3 21% 52% 20% 1	570
	21st to end	Night	5% 2 12% 45% 32% 2	570
July	1st to 10th	Night	5% 2 3 42% 41% 6%	555
	11th to 20th	Night	5% 1 3 30% 53% 8%	552
	21st to end	Night	7% 4 4 19% 53% 13%	624
August	1st to 10th	Night	5% 2 2 23% 55% 13%	567
	11th to 20th	Night	6% 2 4 29% 50% 9%	564
	21st to end	Night	5% 3 3 30% 52% 8%	627
September	1st to 10th	Night	7% 2 5% 37% 44% 5%	570
	11th to 20th	Night	5% 1 7% 49% 35% 3	637
	21st to end	Night	4 2 14% 40% 35% 4	760
October	1st to 10th	Night	3 2 23% 43% 25% 4	760
	11th to 20th	Night	3 2 25% 42% 25% 3	760
	21st to end	Night	3 4 22% 38% 27% 6%	836
November	1st to 10th	Night	2 4 22% 41% 25% 5%	780
	11th to 20th	Night	2 4 24% 45% 22% 4	800
	21st to end	Night	7% 8% 24% 35% 21% 6%	824
December	1st to 10th	Night	6% 7% 25% 41% 19% 3	1000
	11th to 20th	Night	5% 8% 31% 36% 17% 2	824
	21st to end	Night	6% 10% 33% 36% 13% 3	880

Page generated at 2025-01-04 01:24:01. Version ac07

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