Indian Tree Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Indian Tree Observatory clear sky chart from 2005-11-06 to 2025-09-19.

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 5% 2 5% 4 4 4 4 6% 8% 54%	134956
	Night	4 6% 2 6% 4 4 4 3 5% 7% 55%	131519
Transparency	Day	26% 6% 10% 23% 24% 12%	129927
	Night	27% 5% 10% 24% 24% 10%	127553
Seeing	Day	7% 9% 27% 30% 19% 7%	16807
	Night	7% 5% 33% 40% 14% 1	26730

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 7% 3 2 3 3 5% 9% 58%	3638
	11th to 20th	Day	1 5% 3 9% 5% 3 4 3 5% 7% 56%	3709
	21st to end	Day	2 5% 2 6% 3 3 4 3 4 6% 61%	4510
February	1st to 10th	Day	2 8% 3 6% 4 3 3 4 5% 7% 55%	1818
	11th to 20th	Day	1 4 2 6% 5% 5% 4 4 6% 6% 57%	1949
	21st to end	Day	2 7% 2 5% 4 3 4 3 5% 6% 58%	1650
March	1st to 10th	Day	2 9% 3 4 3 3 3 3 5% 6% 59%	4500
	11th to 20th	Day	4 9% 3 6% 4 4 4 3 5% 8% 51%	4857
	21st to end	Day	4 7% 2 5% 3 4 5% 4 6% 8% 53%	5873
April	1st to 10th	Day	6% 8% 2 6% 4 3 3 4 7% 6% 52%	2400
	11th to 20th	Day	7% 9% 3 6% 3 4 5% 4 6% 7% 46%	2371
	21st to end	Day	8% 8% 2 5% 5% 4 5% 4 7% 8% 45%	2400
May	1st to 10th	Day	9% 7% 2 6% 4 4 4 4 6% 7% 48%	5780
	11th to 20th	Day	8% 6% 2 5% 4 4 5% 4 6% 8% 47%	6159
	21st to end	Day	6% 6% 2 6% 4 5% 5% 5% 7% 9% 47%	6925
June	1st to 10th	Day	6% 4 1 5% 4 4 6% 5% 8% 9% 50%	2800
	11th to 20th	Day	5% 4 1 4 3 4 5% 4 8% 9% 51%	2800
	21st to end	Day	4 4 1 5% 5% 5% 6% 5% 8% 10% 50%	2800
July	1st to 10th	Day	3 3 1 3 3 4 5% 4 7% 10% 57%	6339
	11th to 20th	Day	2 2 2 5% 4 5% 6% 5% 8% 11% 49%	6340
	21st to end	Day	4 4 2 6% 5% 5% 6% 5% 8% 9% 46%	6826
August	1st to 10th	Day	6% 4 2 5% 4 5% 6% 6% 9% 10% 42%	5680
	11th to 20th	Day	3 2 1 4 4 5% 6% 5% 7% 9% 54%	5366
	21st to end	Day	5% 4 2 6% 5% 5% 6% 5% 7% 9% 45%	5984
September	1st to 10th	Day	5% 3 1 4 4 4 3 3 6% 8% 60%	2400
	11th to 20th	Day	6% 3 1 4 2 5% 4 4 6% 7% 59%	2330
	21st to end	Day	5% 3 2 4 2 3 3 2 4 5% 66%	2090
October	1st to 10th	Day	4 3 1 2 2 2 3 2 3 4 76%	3263
	11th to 20th	Day	4 4 2 5% 2 2 4 3 5% 7% 63%	3150
	21st to end	Day	4 6% 2 3 2 2 2 2 4 5% 68%	3285
November	1st to 10th	Day	2 5% 1 4 3 3 3 3 6% 6% 63%	1763
	11th to 20th	Day	2 6% 2 4 3 3 3 2 4 7% 63%	1800
	21st to end	Day	3 7% 2 5% 3 3 3 3 4 6% 62%	1751
December	1st to 10th	Day	1 6% 2 5% 3 2 3 2 4 6% 68%	2620
	11th to 20th	Day	1 4 2 4 3 3 3 3 5% 6% 65%	3026
	21st to end	Day	2 7% 3 7% 4 4 4 4 5% 7% 53%	4004
January	1st to 10th	Night	1 8% 4 8% 3 3 3 3 5% 8% 54%	6327
	11th to 20th	Night	1 5% 2 7% 5% 4 4 3 4 7% 57%	6312
	21st to end	Night	2 5% 2 6% 4 3 4 3 4 6% 60%	6468
February	1st to 10th	Night	2 7% 3 7% 4 3 4 2 5% 8% 55%	2580
	11th to 20th	Night	2 9% 3 7% 5% 4 4 4 6% 7% 50%	2338
	21st to end	Night	3 8% 2 5% 3 3 3 2 4 6% 61%	1980
March	1st to 10th	Night	3 8% 4 6% 3 3 3 3 4 5% 58%	5374
	11th to 20th	Night	5% 9% 3 6% 3 3 3 3 5% 7% 53%	5132
	21st to end	Night	6% 8% 3 6% 4 3 4 4 5% 8% 49%	5083
April	1st to 10th	Night	5% 8% 2 6% 4 4 4 3 4 6% 54%	2000
	11th to 20th	Night	9% 9% 3 5% 3 3 3 3 6% 7% 47%	1977
	21st to end	Night	10% 8% 2 6% 4 3 4 3 5% 7% 48%	2000
May	1st to 10th	Night	11% 9% 3 6% 4 4 4 3 5% 6% 46%	4184
	11th to 20th	Night	8% 6% 2 6% 4 4 4 4 7% 9% 47%	3752
	21st to end	Night	9% 6% 2 5% 4 4 4 3 6% 8% 50%	3972
June	1st to 10th	Night	5% 3 2 4 3 3 4 3 8% 11% 53%	1600
	11th to 20th	Night	4 3 1 4 2 3 4 4 5% 9% 60%	1600
	21st to end	Night	4 4 2 5% 4 4 4 5% 8% 9% 52%	1600
July	1st to 10th	Night	3 3 1 4 4 5% 6% 4 7% 9% 53%	3640
	11th to 20th	Night	3 4 2 5% 4 5% 6% 5% 7% 11% 48%	3640
	21st to end	Night	4 4 2 6% 4 5% 5% 5% 9% 10% 46%	4029
August	1st to 10th	Night	6% 5% 2 6% 4 5% 5% 5% 8% 10% 45%	4118
	11th to 20th	Night	4 3 2 5% 5% 5% 6% 5% 7% 11% 47%	4483
	21st to end	Night	7% 5% 2 6% 4 4 6% 5% 8% 9% 43%	4993
September	1st to 10th	Night	6% 5% 2 5% 4 3 3 3 8% 7% 56%	2000
	11th to 20th	Night	6% 4 1 4 3 3 4 3 5% 7% 60%	2024
	21st to end	Night	6% 3 1 4 4 3 3 3 4 6% 63%	2090
October	1st to 10th	Night	4 4 1 3 2 2 3 3 4 5% 68%	3700
	11th to 20th	Night	5% 4 2 5% 3 3 4 4 6% 8% 56%	3778
	21st to end	Night	4 6% 2 4 3 2 3 2 4 5% 65%	4240
November	1st to 10th	Night	2 3 2 4 3 3 3 3 5% 7% 65%	2535
	11th to 20th	Night	2 5% 2 4 3 3 3 3 4 5% 66%	2600
	21st to end	Night	4 6% 2 5% 4 3 3 2 5% 6% 59%	2645
December	1st to 10th	Night	1 4 2 4 2 2 2 2 3 6% 72%	4570
	11th to 20th	Night	1 4 2 5% 3 3 3 3 5% 7% 63%	5181
	21st to end	Night	3 8% 3 6% 4 4 4 3 5% 7% 53%	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	24% 3 8% 31% 24% 11%	3639
	11th to 20th	Day	26% 5% 10% 21% 28% 10%	3709
	21st to end	Day	24% 4 7% 28% 24% 12%	4510
February	1st to 10th	Day	27% 4 11% 28% 20% 10%	1818
	11th to 20th	Day	25% 5% 9% 28% 26% 8%	1959
	21st to end	Day	26% 4 6% 21% 27% 16%	1650
March	1st to 10th	Day	23% 4 6% 17% 30% 20%	4500
	11th to 20th	Day	30% 6% 7% 23% 26% 8%	4857
	21st to end	Day	27% 6% 11% 35% 19% 3	5873
April	1st to 10th	Day	29% 5% 12% 23% 22% 9%	2400
	11th to 20th	Day	34% 6% 10% 21% 19% 9%	2374
	21st to end	Day	33% 8% 11% 25% 20% 3	2400
May	1st to 10th	Day	33% 6% 10% 28% 17% 7%	5780
	11th to 20th	Day	30% 7% 9% 21% 22% 10%	6163
	21st to end	Day	30% 8% 12% 24% 19% 7%	6926
June	1st to 10th	Day	27% 8% 14% 23% 18% 9%	2800
	11th to 20th	Day	25% 7% 12% 17% 23% 15%	2800
	21st to end	Day	26% 7% 11% 19% 24% 13%	2800
July	1st to 10th	Day	20% 7% 8% 17% 27% 21%	6271
	11th to 20th	Day	25% 7% 13% 25% 19% 11%	6265
	21st to end	Day	31% 7% 13% 17% 17% 16%	6629
August	1st to 10th	Day	35% 9% 16% 21% 14% 5%	4131
	11th to 20th	Day	27% 7% 13% 18% 22% 13%	3882
	21st to end	Day	27% 7% 13% 19% 25% 10%	4311
September	1st to 10th	Day	23% 5% 8% 22% 26% 16%	2400
	11th to 20th	Day	22% 5% 9% 18% 27% 17%	2330
	21st to end	Day	20% 4 7% 21% 35% 13%	2090
October	1st to 10th	Day	14% 4 5% 15% 38% 24%	3263
	11th to 20th	Day	21% 3 7% 25% 35% 9%	3150
	21st to end	Day	20% 3 4 22% 37% 14%	3285
November	1st to 10th	Day	19% 3 10% 23% 28% 17%	1763
	11th to 20th	Day	21% 4 6% 24% 34% 11%	1800
	21st to end	Day	23% 3 8% 22% 29% 16%	1751
December	1st to 10th	Day	19% 4 7% 31% 30% 10%	2620
	11th to 20th	Day	19% 3 7% 29% 31% 11%	3024
	21st to end	Day	27% 4 9% 26% 25% 10%	4004
January	1st to 10th	Night	28% 3 6% 28% 27% 7%	6327
	11th to 20th	Night	24% 5% 10% 22% 24% 14%	6312
	21st to end	Night	24% 4 9% 26% 23% 14%	6468
February	1st to 10th	Night	28% 4 10% 27% 23% 9%	2580
	11th to 20th	Night	32% 5% 10% 26% 21% 7%	2338
	21st to end	Night	25% 3 5% 26% 26% 15%	1980
March	1st to 10th	Night	27% 3 6% 16% 35% 13%	5374
	11th to 20th	Night	30% 5% 9% 16% 28% 13%	5132
	21st to end	Night	32% 5% 10% 32% 19% 2	5083
April	1st to 10th	Night	30% 6% 10% 23% 23% 9%	2000
	11th to 20th	Night	34% 6% 11% 24% 19% 6%	1977
	21st to end	Night	36% 5% 11% 27% 18% 4	2000
May	1st to 10th	Night	37% 6% 12% 29% 9% 7%	4182
	11th to 20th	Night	31% 9% 13% 23% 17% 8%	3753
	21st to end	Night	30% 7% 18% 26% 15% 5%	3972
June	1st to 10th	Night	23% 8% 20% 28% 15% 7%	1600
	11th to 20th	Night	21% 6% 14% 24% 21% 14%	1600
	21st to end	Night	25% 8% 10% 25% 23% 10%	1600
July	1st to 10th	Night	25% 7% 12% 18% 31% 8%	3600
	11th to 20th	Night	28% 7% 19% 27% 16% 4	3592
	21st to end	Night	29% 7% 17% 18% 20% 9%	3914
August	1st to 10th	Night	38% 8% 17% 22% 12% 3	2991
	11th to 20th	Night	31% 7% 18% 20% 17% 7%	3243
	21st to end	Night	25% 7% 17% 27% 20% 5%	3597
September	1st to 10th	Night	25% 6% 10% 22% 25% 11%	2000
	11th to 20th	Night	21% 5% 11% 23% 25% 14%	2024
	21st to end	Night	22% 5% 8% 23% 31% 11%	2090
October	1st to 10th	Night	18% 6% 6% 20% 33% 18%	3700
	11th to 20th	Night	23% 6% 9% 26% 26% 9%	3778
	21st to end	Night	22% 3 5% 25% 30% 14%	4240
November	1st to 10th	Night	19% 3 10% 23% 30% 15%	2535
	11th to 20th	Night	21% 3 7% 26% 31% 12%	2600
	21st to end	Night	26% 3 7% 22% 30% 11%	2645
December	1st to 10th	Night	16% 3 7% 33% 33% 8%	4570
	11th to 20th	Night	21% 3 6% 27% 36% 8%	5182
	21st to end	Night	29% 4 8% 23% 25% 10%	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% 5% 30% 39% 17% 3	300
	11th to 20th	Day	7% 11% 29% 38% 14% 1	300
	21st to end	Day	4 9% 36% 35% 14% 2	330
February	1st to 10th	Day	4 14% 47% 27% 7% 2	300
	11th to 20th	Day	2 11% 43% 30% 12% 1	300
	21st to end	Day	3 15% 43% 29% 8% 2	249
March	1st to 10th	Day	4 11% 35% 31% 14% 5%	300
	11th to 20th	Day	9% 17% 33% 24% 13% 3	435
	21st to end	Day	13% 19% 33% 23% 9% 2	550
April	1st to 10th	Day	11% 16% 35% 25% 11% 2	500
	11th to 20th	Day	13% 24% 31% 22% 9% 2	492
	21st to end	Day	14% 18% 31% 24% 11% 2	500
May	1st to 10th	Day	14% 15% 32% 24% 13% 3	648
	11th to 20th	Day	14% 9% 27% 29% 17% 4	725
	21st to end	Day	11% 11% 26% 29% 17% 6%	789
June	1st to 10th	Day	7% 6% 24% 30% 23% 10%	730
	11th to 20th	Day	7% 8% 26% 32% 22% 6%	730
	21st to end	Day	6% 6% 24% 35% 23% 6%	730
July	1st to 10th	Day	5% 4 18% 31% 32% 10%	721
	11th to 20th	Day	5% 3 14% 31% 35% 11%	718
	21st to end	Day	7% 2 15% 29% 34% 14%	767
August	1st to 10th	Day	9% 3 20% 31% 23% 14%	527
	11th to 20th	Day	5% 3 19% 32% 27% 13%	517
	21st to end	Day	6% 4 22% 33% 25% 10%	583
September	1st to 10th	Day	6% 4 22% 32% 24% 13%	530
	11th to 20th	Day	4 5% 27% 30% 22% 12%	476
	21st to end	Day	7% 6% 19% 30% 23% 15%	300
October	1st to 10th	Day	8% 10% 19% 24% 24% 15%	300
	11th to 20th	Day	3 8% 27% 34% 19% 10%	300
	21st to end	Day	9% 9% 30% 30% 16% 6%	330
November	1st to 10th	Day	4 12% 29% 31% 18% 6%	300
	11th to 20th	Day	4 9% 32% 35% 17% 3	300
	21st to end	Day	5% 9% 35% 34% 13% 5%	300
December	1st to 10th	Day	4 10% 39% 29% 15% 4	300
	11th to 20th	Day	4 9% 40% 36% 11% 1	300
	21st to end	Day	7% 8% 38% 34% 12% 2	330
January	1st to 10th	Night	4 5% 39% 39% 12% 2	1000
	11th to 20th	Night	4 6% 39% 41% 9% 1	985
	21st to end	Night	4 4 36% 44% 11% 1	880
February	1st to 10th	Night	4 7% 45% 39% 5% 1	800
	11th to 20th	Night	5% 7% 45% 36% 7%	784
	21st to end	Night	6% 6% 47% 35% 5%	660
March	1st to 10th	Night	5% 7% 42% 39% 7% 1	800
	11th to 20th	Night	9% 7% 37% 37% 10% 1	800
	21st to end	Night	8% 8% 40% 35% 8%	707
April	1st to 10th	Night	8% 8% 45% 33% 5%	600
	11th to 20th	Night	13% 11% 36% 33% 7% 1	593
	21st to end	Night	15% 9% 34% 34% 8%	600
May	1st to 10th	Night	12% 9% 31% 37% 10% 1	600
	11th to 20th	Night	16% 3 28% 39% 12% 1	591
	21st to end	Night	11% 5% 33% 36% 13% 1	650
June	1st to 10th	Night	6% 3 26% 44% 19% 3	600
	11th to 20th	Night	6% 3 31% 44% 16% 1	600
	21st to end	Night	6% 3 26% 42% 22% 1	600
July	1st to 10th	Night	7% 2 20% 44% 24% 3	585
	11th to 20th	Night	4 1 19% 41% 31% 3	582
	21st to end	Night	8% 2 14% 41% 32% 2	657
August	1st to 10th	Night	7% 2 17% 45% 27% 2	597
	11th to 20th	Night	5% 2 21% 48% 22% 2	594
	21st to end	Night	7% 1 21% 45% 22% 3	660
September	1st to 10th	Night	7% 3 21% 46% 21% 3	600
	11th to 20th	Night	7% 2 23% 45% 20% 2	594
	21st to end	Night	7% 3 27% 42% 19% 2	570
October	1st to 10th	Night	9% 5% 30% 37% 16% 3	699
	11th to 20th	Night	4 5% 31% 41% 17% 2	760
	21st to end	Night	8% 5% 32% 37% 17% 3	907
November	1st to 10th	Night	4 5% 35% 40% 14% 1	975
	11th to 20th	Night	4 6% 36% 41% 11% 1	1000
	21st to end	Night	7% 7% 33% 38% 14% 2	1000
December	1st to 10th	Night	4 5% 38% 42% 11% 1	1000
	11th to 20th	Night	4 5% 41% 37% 12% 1	1000
	21st to end	Night	6% 7% 40% 38% 8% 1	1100

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