Trubble Terrestrial Telescope
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Trubble Terrestrial Telescope clear sky chart from 2005-11-06 to 2025-10-04.

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	5% 5% 2 5% 4 4 5% 4 6% 8% 52%	135323
	Night	5% 6% 2 5% 3 3 4 3 5% 7% 57%	131529
Transparency	Day	27% 5% 10% 25% 23% 10%	130259
	Night	26% 4 10% 26% 25% 9%	127597
Seeing	Day	9% 8% 28% 34% 17% 5%	16928
	Night	7% 6% 27% 43% 15% 2	26860

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 8% 5% 3 3 3 4 7% 57%	3638
	11th to 20th	Day	1 5% 3 10% 4 4 4 4 5% 7% 52%	3686
	21st to end	Day	1 4 3 5% 3 3 3 4 5% 8% 61%	4510
February	1st to 10th	Day	3 6% 3 7% 3 4 4 4 5% 8% 53%	1800
	11th to 20th	Day	1 5% 2 5% 5% 3 5% 4 6% 8% 56%	1898
	21st to end	Day	1 7% 2 7% 4 3 3 3 5% 7% 58%	1650
March	1st to 10th	Day	3 9% 2 4 3 3 3 3 6% 9% 54%	4500
	11th to 20th	Day	4 8% 3 7% 4 5% 5% 4 7% 8% 46%	4945
	21st to end	Day	4 8% 3 5% 4 3 5% 4 7% 10% 47%	5823
April	1st to 10th	Day	5% 7% 2 4 3 3 4 3 7% 8% 54%	2400
	11th to 20th	Day	10% 9% 2 6% 4 3 4 4 6% 8% 45%	2371
	21st to end	Day	10% 8% 3 5% 3 4 5% 4 6% 8% 43%	2400
May	1st to 10th	Day	11% 8% 3 5% 4 4 5% 3 5% 7% 44%	5862
	11th to 20th	Day	8% 5% 2 5% 5% 5% 6% 6% 7% 8% 43%	6298
	21st to end	Day	8% 5% 2 5% 4 5% 6% 5% 7% 9% 44%	6925
June	1st to 10th	Day	5% 4 2 4 4 5% 5% 5% 8% 11% 47%	2800
	11th to 20th	Day	6% 3 1 3 4 5% 5% 5% 7% 9% 52%	2800
	21st to end	Day	3 3 1 4 4 6% 6% 6% 7% 9% 50%	2800
July	1st to 10th	Day	3 3 1 5% 4 5% 6% 5% 8% 9% 52%	6339
	11th to 20th	Day	4 3 1 5% 5% 5% 7% 7% 9% 10% 44%	6340
	21st to end	Day	5% 3 2 6% 5% 5% 6% 5% 7% 8% 47%	6859
August	1st to 10th	Day	8% 4 1 6% 5% 5% 7% 6% 9% 10% 39%	5779
	11th to 20th	Day	2 2 1 4 3 5% 6% 6% 9% 9% 53%	5417
	21st to end	Day	7% 5% 2 6% 4 4 5% 4 7% 8% 47%	5984
September	1st to 10th	Day	5% 4 1 4 2 4 4 4 6% 7% 59%	2400
	11th to 20th	Day	6% 3 1 3 3 3 4 4 7% 7% 58%	2384
	21st to end	Day	7% 4 2 3 2 2 3 3 4 6% 64%	2200
October	1st to 10th	Day	5% 3 1 3 2 2 3 2 4 4 71%	3204
	11th to 20th	Day	5% 4 1 3 3 4 4 3 5% 7% 60%	3150
	21st to end	Day	5% 6% 1 4 3 2 3 2 4 6% 63%	3293
November	1st to 10th	Day	3 5% 1 3 4 3 3 3 5% 8% 63%	1763
	11th to 20th	Day	2 6% 1 3 3 3 3 3 5% 8% 63%	1800
	21st to end	Day	2 5% 2 5% 3 2 3 4 7% 8% 60%	1655
December	1st to 10th	Day	1 3 2 4 3 2 2 2 4 6% 71%	2620
	11th to 20th	Day	2 4 2 4 2 2 3 3 4 7% 68%	3026
	21st to end	Day	3 6% 2 7% 4 3 4 4 6% 8% 53%	4004
January	1st to 10th	Night	2 8% 3 7% 4 3 3 2 5% 7% 55%	6327
	11th to 20th	Night	1 4 3 8% 5% 4 4 3 5% 7% 56%	6322
	21st to end	Night	2 6% 3 7% 4 3 3 3 4 5% 60%	6468
February	1st to 10th	Night	2 8% 3 7% 3 3 4 3 5% 7% 55%	2600
	11th to 20th	Night	2 7% 3 7% 5% 4 4 3 5% 6% 54%	2388
	21st to end	Night	2 9% 3 6% 3 3 2 2 4 5% 60%	1980
March	1st to 10th	Night	3 10% 3 6% 3 3 3 3 4 6% 55%	5374
	11th to 20th	Night	6% 9% 2 4 3 3 3 3 4 7% 55%	5049
	21st to end	Night	7% 9% 3 6% 4 3 4 3 5% 7% 50%	5135
April	1st to 10th	Night	5% 8% 2 4 3 3 3 3 6% 6% 56%	2000
	11th to 20th	Night	10% 7% 1 6% 3 3 3 3 5% 7% 52%	1977
	21st to end	Night	15% 7% 2 5% 3 4 3 3 4 7% 48%	2000
May	1st to 10th	Night	14% 7% 2 5% 4 4 4 3 6% 7% 45%	4114
	11th to 20th	Night	9% 6% 2 7% 4 4 5% 4 6% 8% 46%	3617
	21st to end	Night	12% 6% 2 4 3 3 3 3 5% 6% 53%	3972
June	1st to 10th	Night	7% 4 1 4 3 3 4 3 5% 9% 57%	1600
	11th to 20th	Night	5% 3 1 4 3 2 4 4 5% 8% 62%	1600
	21st to end	Night	4 2 1 3 3 3 4 4 6% 8% 61%	1600
July	1st to 10th	Night	4 2 1 3 3 4 4 4 7% 8% 61%	3640
	11th to 20th	Night	4 2 1 3 3 3 4 4 8% 11% 55%	3640
	21st to end	Night	5% 3 1 4 4 3 5% 5% 7% 8% 54%	4010
August	1st to 10th	Night	8% 4 2 5% 4 4 4 4 7% 8% 49%	4033
	11th to 20th	Night	3 3 2 4 3 3 5% 4 7% 9% 57%	4432
	21st to end	Night	8% 5% 2 5% 3 4 5% 3 7% 8% 51%	4993
September	1st to 10th	Night	6% 4 1 3 3 3 5% 3 5% 6% 62%	2000
	11th to 20th	Night	7% 3 1 3 2 2 3 3 4 6% 66%	2015
	21st to end	Night	7% 3 1 3 2 3 4 3 4 6% 64%	2200
October	1st to 10th	Night	6% 3 1 2 2 2 2 2 3 4 73%	3829
	11th to 20th	Night	6% 5% 2 5% 3 3 3 3 4 6% 60%	3778
	21st to end	Night	6% 6% 2 4 3 2 3 2 4 6% 64%	4232
November	1st to 10th	Night	3 5% 1 4 3 3 3 3 4 7% 64%	2535
	11th to 20th	Night	2 5% 1 4 4 2 3 2 5% 7% 66%	2600
	21st to end	Night	3 6% 2 4 3 3 3 3 6% 7% 60%	2744
December	1st to 10th	Night	1 4 2 4 3 2 2 2 3 5% 72%	4570
	11th to 20th	Night	2 5% 2 4 3 3 3 2 5% 10% 63%	5181
	21st to end	Night	4 8% 3 6% 4 4 4 3 5% 7% 52%	6974

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	26% 3 8% 35% 20% 7%	3639
	11th to 20th	Day	31% 3 11% 23% 22% 10%	3686
	21st to end	Day	22% 3 9% 28% 26% 12%	4510
February	1st to 10th	Day	29% 3 13% 28% 21% 7%	1800
	11th to 20th	Day	24% 4 10% 31% 26% 5%	1908
	21st to end	Day	25% 3 8% 26% 26% 10%	1650
March	1st to 10th	Day	26% 3 6% 24% 31% 9%	4500
	11th to 20th	Day	34% 3 12% 23% 24% 4	4945
	21st to end	Day	31% 5% 11% 37% 15% 2	5823
April	1st to 10th	Day	26% 5% 12% 27% 22% 8%	2400
	11th to 20th	Day	37% 5% 9% 23% 20% 7%	2374
	21st to end	Day	37% 5% 11% 26% 19% 3	2400
May	1st to 10th	Day	37% 5% 9% 28% 15% 5%	5862
	11th to 20th	Day	36% 6% 11% 21% 19% 9%	6302
	21st to end	Day	32% 7% 12% 28% 17% 4	6926
June	1st to 10th	Day	27% 7% 17% 25% 17% 6%	2800
	11th to 20th	Day	25% 6% 11% 20% 23% 14%	2800
	21st to end	Day	25% 8% 10% 22% 23% 13%	2800
July	1st to 10th	Day	24% 5% 9% 16% 28% 18%	6271
	11th to 20th	Day	29% 8% 12% 22% 17% 11%	6265
	21st to end	Day	30% 6% 10% 17% 20% 17%	6661
August	1st to 10th	Day	35% 9% 14% 18% 16% 8%	4208
	11th to 20th	Day	24% 9% 12% 19% 22% 13%	3921
	21st to end	Day	24% 6% 10% 21% 25% 14%	4311
September	1st to 10th	Day	23% 6% 7% 21% 27% 16%	2400
	11th to 20th	Day	23% 4 8% 23% 28% 14%	2384
	21st to end	Day	23% 3 6% 21% 35% 12%	2200
October	1st to 10th	Day	18% 3 6% 17% 34% 23%	3204
	11th to 20th	Day	23% 4 11% 27% 28% 8%	3150
	21st to end	Day	22% 2 8% 27% 32% 9%	3293
November	1st to 10th	Day	19% 3 12% 29% 26% 12%	1763
	11th to 20th	Day	20% 3 9% 27% 32% 9%	1800
	21st to end	Day	22% 3 9% 26% 30% 10%	1655
December	1st to 10th	Day	16% 3 7% 35% 29% 11%	2620
	11th to 20th	Day	17% 3 8% 35% 31% 6%	3024
	21st to end	Day	28% 3 9% 28% 23% 9%	4004
January	1st to 10th	Night	30% 3 9% 28% 22% 8%	6327
	11th to 20th	Night	27% 3 12% 23% 24% 12%	6322
	21st to end	Night	27% 2 9% 25% 26% 10%	6468
February	1st to 10th	Night	29% 4 10% 28% 23% 6%	2600
	11th to 20th	Night	31% 3 9% 29% 22% 6%	2388
	21st to end	Night	27% 2 6% 24% 27% 13%	1980
March	1st to 10th	Night	30% 4 5% 18% 36% 8%	5374
	11th to 20th	Night	29% 2 10% 17% 29% 12%	5049
	21st to end	Night	34% 4 10% 30% 19% 3	5135
April	1st to 10th	Night	28% 5% 9% 27% 22% 9%	2000
	11th to 20th	Night	33% 4 11% 25% 21% 7%	1977
	21st to end	Night	37% 4 7% 27% 21% 3	2000
May	1st to 10th	Night	37% 5% 12% 27% 13% 6%	4111
	11th to 20th	Night	36% 4 14% 23% 16% 8%	3618
	21st to end	Night	31% 5% 14% 27% 17% 5%	3972
June	1st to 10th	Night	25% 4 19% 29% 15% 8%	1600
	11th to 20th	Night	21% 5% 11% 24% 21% 18%	1600
	21st to end	Night	19% 6% 10% 23% 29% 13%	1600
July	1st to 10th	Night	19% 4 10% 23% 30% 13%	3600
	11th to 20th	Night	20% 5% 20% 27% 19% 9%	3592
	21st to end	Night	25% 6% 14% 23% 20% 12%	3896
August	1st to 10th	Night	32% 6% 16% 23% 18% 6%	2928
	11th to 20th	Night	23% 8% 16% 22% 20% 11%	3204
	21st to end	Night	21% 6% 12% 24% 26% 11%	3597
September	1st to 10th	Night	22% 5% 9% 22% 22% 19%	2000
	11th to 20th	Night	20% 3 9% 27% 25% 17%	2015
	21st to end	Night	22% 4 6% 24% 30% 14%	2200
October	1st to 10th	Night	16% 2 7% 18% 32% 23%	3829
	11th to 20th	Night	25% 3 8% 26% 28% 9%	3778
	21st to end	Night	23% 3 6% 26% 32% 11%	4232
November	1st to 10th	Night	20% 4 10% 26% 27% 13%	2535
	11th to 20th	Night	20% 3 7% 29% 31% 10%	2600
	21st to end	Night	23% 3 9% 25% 31% 10%	2744
December	1st to 10th	Night	17% 2 8% 36% 30% 7%	4570
	11th to 20th	Night	19% 3 6% 38% 30% 5%	5182
	21st to end	Night	31% 3 9% 24% 26% 7%	6974

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% 10% 27% 42% 12% 4	300
	11th to 20th	Day	6% 10% 38% 35% 10% 1	300
	21st to end	Day	4 12% 37% 37% 9% 1	330
February	1st to 10th	Day	5% 16% 44% 32% 2 1	300
	11th to 20th	Day	4 11% 46% 31% 7%	300
	21st to end	Day	6% 14% 42% 32% 6%	249
March	1st to 10th	Day	6% 16% 29% 38% 9% 2	300
	11th to 20th	Day	13% 13% 31% 32% 10% 1	471
	21st to end	Day	13% 16% 39% 24% 7% 2	550
April	1st to 10th	Day	12% 17% 38% 25% 8% 1	500
	11th to 20th	Day	16% 20% 38% 18% 5% 2	492
	21st to end	Day	17% 16% 32% 25% 9% 1	500
May	1st to 10th	Day	15% 13% 31% 28% 11% 2	648
	11th to 20th	Day	19% 8% 24% 30% 14% 5%	725
	21st to end	Day	11% 8% 27% 29% 21% 4	789
June	1st to 10th	Day	7% 5% 23% 35% 23% 7%	730
	11th to 20th	Day	8% 6% 27% 35% 21% 3	730
	21st to end	Day	4 5% 25% 37% 22% 6%	730
July	1st to 10th	Day	7% 2 17% 36% 29% 9%	721
	11th to 20th	Day	6% 2 16% 38% 28% 10%	718
	21st to end	Day	5% 3 18% 34% 32% 8%	775
August	1st to 10th	Day	8% 2 24% 39% 22% 6%	527
	11th to 20th	Day	4 3 24% 40% 22% 7%	517
	21st to end	Day	8% 2 26% 36% 20% 8%	583
September	1st to 10th	Day	8% 3 24% 37% 21% 7%	530
	11th to 20th	Day	6% 3 28% 33% 25% 4	514
	21st to end	Day	9% 3 24% 33% 21% 10%	330
October	1st to 10th	Day	11% 5% 25% 29% 20% 10%	309
	11th to 20th	Day	3 11% 34% 34% 14% 4	300
	21st to end	Day	7% 10% 34% 35% 13% 1	330
November	1st to 10th	Day	7% 10% 31% 32% 14% 6%	300
	11th to 20th	Day	6% 9% 31% 44% 6% 4	300
	21st to end	Day	6% 11% 30% 44% 8% 1	300
December	1st to 10th	Day	6% 13% 31% 36% 12% 3	300
	11th to 20th	Day	5% 10% 35% 43% 7% 1	300
	21st to end	Day	7% 11% 34% 39% 7% 2	330
January	1st to 10th	Night	4 12% 31% 40% 11% 3	1000
	11th to 20th	Night	5% 14% 38% 34% 8% 1	995
	21st to end	Night	5% 9% 34% 44% 7% 1	880
February	1st to 10th	Night	6% 8% 46% 34% 5% 1	800
	11th to 20th	Night	4 8% 42% 40% 6%	784
	21st to end	Night	5% 10% 39% 43% 3	660
March	1st to 10th	Night	5% 7% 38% 43% 7% 1	800
	11th to 20th	Night	9% 7% 31% 42% 10% 1	800
	21st to end	Night	10% 9% 33% 40% 7% 1	733
April	1st to 10th	Night	8% 10% 36% 41% 5%	600
	11th to 20th	Night	15% 8% 33% 36% 8% 1	593
	21st to end	Night	18% 7% 27% 37% 11% 1	600
May	1st to 10th	Night	17% 8% 25% 35% 14% 2	600
	11th to 20th	Night	20% 2 21% 37% 17% 3	591
	21st to end	Night	11% 4 26% 36% 22% 2	650
June	1st to 10th	Night	8% 2 16% 50% 23% 3	600
	11th to 20th	Night	6% 3 23% 51% 16% 1	600
	21st to end	Night	6% 3 14% 49% 27% 3	600
July	1st to 10th	Night	8% 2 10% 43% 33% 4	585
	11th to 20th	Night	3 1 9% 46% 36% 4	582
	21st to end	Night	7% 1 8% 40% 40% 4	657
August	1st to 10th	Night	7% 2 10% 44% 32% 4	597
	11th to 20th	Night	6% 1 8% 49% 32% 4	594
	21st to end	Night	5% 1 10% 47% 33% 4	660
September	1st to 10th	Night	8% 2 13% 47% 25% 6%	600
	11th to 20th	Night	9% 1 16% 48% 22% 5%	600
	21st to end	Night	8% 2 21% 45% 22% 3	600
October	1st to 10th	Night	11% 4 21% 44% 17% 4	765
	11th to 20th	Night	8% 4 24% 48% 13% 2	760
	21st to end	Night	11% 6% 29% 41% 10% 3	899
November	1st to 10th	Night	5% 5% 30% 46% 12% 2	975
	11th to 20th	Night	5% 10% 34% 43% 6% 2	1000
	21st to end	Night	7% 7% 31% 45% 10% 1	1000
December	1st to 10th	Night	4 11% 32% 46% 8%	1000
	11th to 20th	Night	6% 8% 37% 40% 8% 1	1000
	21st to end	Night	6% 9% 35% 42% 7% 1	1100

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