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 2024-01-17.

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	4 4 2 4 4 3 4 4 5% 7% 58%	74794
	Night	3 4 2 4 4 2 4 4 5% 7% 62%	75047
Transparency	Day	27% 4 7% 17% 27% 19%	74520
	Night	22% 4 8% 20% 29% 18%	74842
Seeing	Day	7% 11% 23% 27% 21% 11%	13364
	Night	4 6% 24% 38% 24% 4	23729

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	2 6% 3 5% 3 2 3 4 5% 9% 59%	1708
	11th to 20th	Day	3 4 3 4 2 1 3 3 4 6% 67%	1674
	21st to end	Day	3 6% 3 5% 3 2 3 3 5% 6% 61%	1782
February	1st to 10th	Day	1 5% 2 4 4 2 3 3 4 8% 62%	1620
	11th to 20th	Day	2 6% 2 4 4 2 3 4 5% 7% 61%	1660
	21st to end	Day	1 7% 2 4 3 2 4 4 6% 6% 61%	1578
March	1st to 10th	Day	2 5% 2 3 4 2 3 5% 7% 8% 57%	1980
	11th to 20th	Day	2 6% 2 4 4 3 4 4 5% 7% 59%	1980
	21st to end	Day	2 4 1 4 3 2 4 3 6% 8% 62%	2178
April	1st to 10th	Day	2 3 2 3 3 3 4 4 5% 8% 64%	2114
	11th to 20th	Day	3 5% 2 4 3 2 3 3 4 7% 65%	2131
	21st to end	Day	2 4 1 3 4 2 4 4 4 5% 67%	2160
May	1st to 10th	Day	3 3 1 3 4 2 3 4 4 6% 66%	2177
	11th to 20th	Day	5% 3 2 4 5% 3 4 4 5% 7% 58%	2301
	21st to end	Day	3 3 1 4 3 2 4 3 3 5% 70%	2530
June	1st to 10th	Day	2 3 2 4 3 3 4 4 5% 7% 63%	2340
	11th to 20th	Day	2 2 1 3 3 2 4 4 3 5% 70%	2340
	21st to end	Day	4 4 2 5% 5% 3 5% 5% 5% 7% 55%	2340
July	1st to 10th	Day	6% 5% 3 6% 7% 4 7% 7% 6% 9% 38%	2340
	11th to 20th	Day	5% 6% 3 6% 8% 6% 7% 8% 8% 9% 34%	2340
	21st to end	Day	8% 6% 4 7% 9% 6% 7% 8% 7% 9% 29%	2545
August	1st to 10th	Day	7% 6% 3 7% 9% 5% 8% 7% 8% 8% 33%	2328
	11th to 20th	Day	6% 5% 3 7% 6% 5% 7% 7% 7% 10% 36%	2169
	21st to end	Day	7% 5% 2 6% 6% 4 7% 6% 6% 8% 43%	2376
September	1st to 10th	Day	8% 5% 3 6% 6% 4 5% 6% 6% 6% 45%	2160
	11th to 20th	Day	6% 4 3 5% 5% 3 4 5% 5% 7% 54%	2092
	21st to end	Day	4 2 1 4 4 3 4 4 4 6% 64%	1926
October	1st to 10th	Day	5% 4 2 4 4 3 4 4 4 6% 62%	1800
	11th to 20th	Day	3 2 1 3 3 2 2 2 2 4 76%	1800
	21st to end	Day	2 3 2 2 3 1 2 3 3 5% 74%	1980
November	1st to 10th	Day	2 2 1 3 2 1 2 3 4 7% 72%	1859
	11th to 20th	Day	2 2 1 3 3 2 3 4 5% 7% 67%	1900
	21st to end	Day	2 5% 2 4 4 3 4 3 3 6% 64%	1770
December	1st to 10th	Day	4 4 2 3 3 2 3 4 5% 6% 66%	1520
	11th to 20th	Day	3 7% 3 7% 6% 3 5% 4 4 7% 53%	2150
	21st to end	Day	1 5% 2 4 4 3 5% 5% 4 6% 59%	3146
January	1st to 10th	Night	1 5% 2 6% 4 2 3 4 5% 8% 60%	2457
	11th to 20th	Night	2 5% 2 4 4 2 3 3 4 6% 64%	2430
	21st to end	Night	1 5% 3 5% 4 2 4 4 5% 7% 61%	2574
February	1st to 10th	Night	1 5% 2 4 4 2 3 3 4 6% 67%	2340
	11th to 20th	Night	3 5% 3 5% 4 2 3 4 5% 7% 59%	2187
	21st to end	Night	2 3 2 5% 2 2 2 3 6% 7% 65%	1675
March	1st to 10th	Night	1 4 2 4 3 2 2 4 5% 8% 66%	1980
	11th to 20th	Night	3 4 2 3 3 2 4 3 4 5% 67%	1980
	21st to end	Night	2 5% 3 4 4 2 3 3 5% 6% 64%	2178
April	1st to 10th	Night	1 2 1 2 3 2 3 4 5% 7% 69%	1844
	11th to 20th	Night	2 3 1 3 3 3 4 3 4 5% 69%	1777
	21st to end	Night	2 3 1 3 3 1 3 3 4 5% 72%	1800
May	1st to 10th	Night	2 3 2 3 3 1 3 3 3 6% 71%	1779
	11th to 20th	Night	4 3 2 4 4 2 4 4 4 7% 63%	1594
	21st to end	Night	2 2 1 2 2 2 2 2 4 5% 75%	1746
June	1st to 10th	Night	3 3 1 3 3 3 4 3 4 8% 64%	1620
	11th to 20th	Night	3 2 1 2 2 2 3 3 4 5% 73%	1620
	21st to end	Night	2 4 2 5% 4 3 6% 4 6% 8% 55%	1620
July	1st to 10th	Night	4 4 2 6% 5% 3 6% 7% 8% 10% 45%	1620
	11th to 20th	Night	3 2 2 6% 6% 4 8% 10% 13% 12% 34%	1620
	21st to end	Night	6% 4 3 7% 7% 5% 7% 8% 9% 12% 31%	1773
August	1st to 10th	Night	4 4 2 5% 5% 4 6% 7% 8% 14% 42%	1611
	11th to 20th	Night	5% 5% 3 6% 7% 5% 7% 7% 8% 9% 37%	1731
	21st to end	Night	5% 4 2 5% 5% 4 6% 7% 9% 10% 43%	1980
September	1st to 10th	Night	6% 6% 2 7% 6% 3 6% 5% 6% 8% 46%	1800
	11th to 20th	Night	4 3 1 4 4 3 4 5% 6% 6% 60%	1863
	21st to end	Night	3 2 1 3 3 2 3 4 5% 7% 65%	2032
October	1st to 10th	Night	3 3 1 3 3 2 3 3 5% 7% 67%	2160
	11th to 20th	Night	1 2 1 2 2 1 2 2 3 6% 77%	2160
	21st to end	Night	2 2 1 2 2 1 2 3 3 6% 75%	2376
November	1st to 10th	Night	1 1 1 2 2 1 3 3 4 6% 76%	2220
	11th to 20th	Night	1 2 1 3 3 2 2 3 4 5% 74%	2280
	21st to end	Night	3 5% 2 3 3 2 3 3 4 7% 64%	2375
December	1st to 10th	Night	2 5% 2 3 3 1 3 3 4 7% 66%	2660
	11th to 20th	Night	3 5% 2 5% 4 3 4 5% 6% 9% 54%	3064
	21st to end	Night	2 7% 2 4 4 2 3 4 4 7% 60%	4521

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	23% 4 7% 23% 28% 15%	1709
	11th to 20th	Day	21% 2 5% 22% 37% 13%	1674
	21st to end	Day	26% 3 4 18% 33% 16%	1782
February	1st to 10th	Day	23% 3 6% 22% 26% 20%	1620
	11th to 20th	Day	24% 3 4 21% 29% 19%	1669
	21st to end	Day	23% 3 1 17% 29% 26%	1578
March	1st to 10th	Day	23% 4 3 17% 34% 18%	1980
	11th to 20th	Day	25% 3 4 15% 34% 19%	1980
	21st to end	Day	20% 4 5% 20% 34% 16%	2178
April	1st to 10th	Day	19% 3 3 19% 35% 20%	2114
	11th to 20th	Day	22% 2 2 15% 32% 26%	2134
	21st to end	Day	20% 3 3 19% 38% 17%	2160
May	1st to 10th	Day	21% 3 3 13% 37% 23%	2177
	11th to 20th	Day	28% 4 5% 14% 30% 19%	2304
	21st to end	Day	19% 3 4 12% 34% 28%	2530
June	1st to 10th	Day	20% 3 4 14% 30% 28%	2340
	11th to 20th	Day	17% 3 3 9% 24% 44%	2340
	21st to end	Day	30% 4 6% 13% 25% 22%	2340
July	1st to 10th	Day	40% 7% 10% 14% 15% 14%	2276
	11th to 20th	Day	43% 7% 18% 17% 9% 6%	2271
	21st to end	Day	48% 8% 21% 17% 5% 1	2404
August	1st to 10th	Day	45% 8% 16% 12% 13% 5%	2313
	11th to 20th	Day	41% 7% 14% 17% 15% 6%	2169
	21st to end	Day	38% 7% 11% 19% 20% 4	2376
September	1st to 10th	Day	38% 5% 8% 17% 19% 13%	2160
	11th to 20th	Day	30% 4 7% 14% 26% 19%	2092
	21st to end	Day	23% 3 5% 18% 29% 21%	1926
October	1st to 10th	Day	26% 3 6% 18% 25% 23%	1800
	11th to 20th	Day	16% 2 2 12% 33% 35%	1800
	21st to end	Day	15% 3 5% 17% 40% 21%	1980
November	1st to 10th	Day	14% 3 3 24% 35% 22%	1859
	11th to 20th	Day	17% 3 4 18% 36% 22%	1900
	21st to end	Day	24% 3 4 14% 33% 22%	1770
December	1st to 10th	Day	20% 3 5% 27% 29% 17%	1520
	11th to 20th	Day	33% 3 7% 19% 24% 14%	2149
	21st to end	Day	25% 5% 6% 17% 27% 21%	3146
January	1st to 10th	Night	23% 4 6% 21% 31% 14%	2457
	11th to 20th	Night	23% 3 4 23% 35% 11%	2430
	21st to end	Night	24% 3 5% 20% 35% 13%	2574
February	1st to 10th	Night	20% 2 8% 23% 26% 20%	2340
	11th to 20th	Night	23% 3 5% 25% 23% 20%	2188
	21st to end	Night	19% 2 4 20% 32% 23%	1675
March	1st to 10th	Night	17% 4 4 22% 35% 17%	1980
	11th to 20th	Night	21% 3 3 18% 37% 17%	1980
	21st to end	Night	21% 3 4 23% 34% 14%	2178
April	1st to 10th	Night	15% 2 4 26% 35% 18%	1844
	11th to 20th	Night	18% 3 2 18% 36% 23%	1777
	21st to end	Night	16% 3 4 20% 37% 19%	1800
May	1st to 10th	Night	18% 2 5% 14% 40% 22%	1779
	11th to 20th	Night	23% 4 8% 17% 32% 17%	1595
	21st to end	Night	13% 1 6% 16% 36% 28%	1746
June	1st to 10th	Night	20% 3 5% 17% 28% 28%	1620
	11th to 20th	Night	15% 2 3 13% 26% 42%	1620
	21st to end	Night	26% 4 9% 17% 18% 25%	1620
July	1st to 10th	Night	30% 8% 13% 17% 16% 16%	1575
	11th to 20th	Night	33% 12% 23% 19% 7% 7%	1566
	21st to end	Night	41% 9% 28% 16% 5% 2	1674
August	1st to 10th	Night	31% 9% 24% 15% 13% 7%	1602
	11th to 20th	Night	39% 9% 16% 17% 12% 7%	1731
	21st to end	Night	31% 8% 18% 22% 17% 5%	1980
September	1st to 10th	Night	35% 6% 14% 15% 18% 13%	1800
	11th to 20th	Night	24% 5% 9% 18% 26% 18%	1863
	21st to end	Night	19% 4 6% 18% 33% 20%	2032
October	1st to 10th	Night	19% 3 9% 23% 23% 22%	2160
	11th to 20th	Night	12% 2 4 17% 32% 34%	2160
	21st to end	Night	14% 2 4 20% 41% 19%	2376
November	1st to 10th	Night	11% 3 2 26% 37% 20%	2220
	11th to 20th	Night	13% 2 4 22% 35% 23%	2280
	21st to end	Night	23% 2 4 18% 34% 19%	2375
December	1st to 10th	Night	20% 2 6% 26% 31% 14%	2660
	11th to 20th	Night	27% 4 8% 20% 28% 13%	3064
	21st to end	Night	25% 4 6% 19% 26% 20%	4521

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% 40% 30% 10% 3	270
	11th to 20th	Day	6% 15% 30% 29% 14% 6%	258
	21st to end	Day	4 11% 36% 27% 17% 5%	264
February	1st to 10th	Day	4 9% 33% 34% 15% 6%	240
	11th to 20th	Day	7% 15% 40% 27% 11%	240
	21st to end	Day	10% 25% 34% 20% 7% 3	296
March	1st to 10th	Day	7% 23% 34% 22% 12% 2	420
	11th to 20th	Day	12% 16% 31% 22% 16% 3	420
	21st to end	Day	12% 24% 35% 17% 10% 3	462
April	1st to 10th	Day	6% 16% 37% 22% 12% 6%	420
	11th to 20th	Day	8% 27% 31% 21% 9% 3	412
	21st to end	Day	9% 17% 37% 20% 11% 5%	420
May	1st to 10th	Day	6% 13% 36% 27% 14% 4	423
	11th to 20th	Day	8% 11% 25% 31% 15% 9%	448
	21st to end	Day	5% 13% 34% 28% 16% 5%	485
June	1st to 10th	Day	4 6% 24% 31% 23% 11%	450
	11th to 20th	Day	5% 8% 18% 32% 24% 14%	450
	21st to end	Day	7% 4 18% 34% 26% 10%	450
July	1st to 10th	Day	7% 4 6% 29% 34% 20%	446
	11th to 20th	Day	10% 2 4 20% 32% 30%	444
	21st to end	Day	9% 3 4 15% 36% 33%	487
August	1st to 10th	Day	10% 4 4 26% 29% 27%	447
	11th to 20th	Day	8% 3 5% 30% 32% 22%	437
	21st to end	Day	7% 4 7% 26% 29% 27%	495
September	1st to 10th	Day	13% 4 8% 22% 32% 21%	450
	11th to 20th	Day	7% 3 14% 33% 31% 13%	450
	21st to end	Day	5% 7% 19% 26% 30% 12%	396
October	1st to 10th	Day	5% 9% 20% 29% 23% 14%	270
	11th to 20th	Day	6% 7% 19% 33% 23% 13%	270
	21st to end	Day	5% 10% 18% 30% 27% 10%	297
November	1st to 10th	Day	3 10% 31% 32% 20% 4	270
	11th to 20th	Day	5% 7% 18% 37% 23% 11%	270
	21st to end	Day	5% 14% 31% 29% 15% 6%	270
December	1st to 10th	Day	7% 7% 29% 33% 21% 3	270
	11th to 20th	Day	6% 15% 31% 27% 16% 5%	270
	21st to end	Day	9% 16% 35% 27% 10% 3	297
January	1st to 10th	Night	3 8% 27% 40% 18% 4	760
	11th to 20th	Night	5% 7% 30% 37% 18% 3	748
	21st to end	Night	4 9% 32% 34% 19% 3	792
February	1st to 10th	Night	3 8% 33% 35% 18% 2	720
	11th to 20th	Night	5% 8% 39% 34% 12% 1	704
	21st to end	Night	4 10% 42% 34% 9% 1	592
March	1st to 10th	Night	3 11% 39% 33% 13% 1	720
	11th to 20th	Night	6% 8% 30% 37% 16% 3	720
	21st to end	Night	4 11% 33% 37% 13% 1	792
April	1st to 10th	Night	3 9% 39% 35% 12% 1	584
	11th to 20th	Night	5% 11% 38% 35% 10% 1	533
	21st to end	Night	4 11% 38% 32% 14% 1	540
May	1st to 10th	Night	5% 5% 35% 42% 13% 1	540
	11th to 20th	Night	6% 5% 24% 45% 18% 2	531
	21st to end	Night	3 4 28% 47% 17% 1	584
June	1st to 10th	Night	4 4 16% 53% 21% 2	540
	11th to 20th	Night	3 3 21% 52% 21% 1	540
	21st to end	Night	4 2 13% 46% 33% 2	540
July	1st to 10th	Night	6% 2 3 41% 42% 6%	525
	11th to 20th	Night	5% 1 3 28% 54% 9%	522
	21st to end	Night	7% 3 4 18% 54% 14%	591
August	1st to 10th	Night	5% 2 2 22% 56% 13%	537
	11th to 20th	Night	6% 2 4 29% 49% 9%	534
	21st to end	Night	5% 3 3 29% 52% 9%	594
September	1st to 10th	Night	7% 3 5% 36% 45% 5%	540
	11th to 20th	Night	5% 1 5% 49% 36% 4	603
	21st to end	Night	4 2 14% 40% 35% 5%	720
October	1st to 10th	Night	3 2 24% 43% 24% 3	720
	11th to 20th	Night	2 2 24% 43% 25% 4	720
	21st to end	Night	3 4 21% 38% 27% 6%	792
November	1st to 10th	Night	2 4 22% 41% 26% 5%	740
	11th to 20th	Night	2 4 22% 46% 23% 4	760
	21st to end	Night	7% 8% 24% 34% 21% 6%	782
December	1st to 10th	Night	6% 7% 25% 40% 19% 3	950
	11th to 20th	Night	6% 8% 32% 35% 17% 2	783
	21st to end	Night	6% 10% 32% 36% 12% 3	836

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