GradyLand Heaven Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the GradyLand Heaven Observatory clear sky chart from 2005-11-06 to 2026-02-01.

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	7% 5% 2 6% 5% 5% 6% 5% 8% 11% 40%	127102
	Night	7% 6% 2 7% 5% 5% 5% 4 7% 10% 42%	124953
Transparency	Day	20% 42% 15% 14% 7% 2	130770
	Night	23% 41% 13% 14% 8% 2	129354
Seeing	Day	7% 3 13% 26% 31% 20%	17462
	Night	9% 3 15% 28% 32% 14%	27600

By Month

Percentage Cloud Cover SE (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	7% 6% 2 6% 4 3 4 4 5% 8% 52%	4146
	11th to 20th	Day	16% 9% 4 8% 6% 5% 4 4 6% 8% 31%	4150
	21st to end	Day	15% 6% 2 6% 4 4 5% 4 7% 8% 39%	4565
February	1st to 10th	Day	10% 6% 2 6% 5% 5% 4 4 7% 8% 44%	1978
	11th to 20th	Day	7% 4 2 5% 4 4 5% 4 6% 8% 51%	1993
	21st to end	Day	9% 4 2 5% 4 4 4 3 6% 9% 50%	1815
March	1st to 10th	Day	6% 5% 2 8% 6% 5% 6% 4 6% 9% 43%	4935
	11th to 20th	Day	3 2 1 4 3 3 4 3 5% 9% 62%	4980
	21st to end	Day	8% 5% 2 6% 5% 5% 5% 4 7% 9% 45%	5465
April	1st to 10th	Day	5% 3 2 4 5% 4 5% 5% 7% 9% 52%	2200
	11th to 20th	Day	7% 5% 2 5% 5% 5% 5% 5% 8% 9% 44%	2285
	21st to end	Day	5% 4 2 5% 5% 4 5% 5% 7% 9% 51%	2400
May	1st to 10th	Day	2 2 1 3 3 4 6% 5% 8% 13% 52%	5405
	11th to 20th	Day	7% 4 2 6% 5% 5% 5% 5% 6% 9% 45%	5848
	21st to end	Day	4 4 2 5% 4 5% 6% 5% 8% 11% 43%	6433
June	1st to 10th	Day	11% 8% 3 8% 6% 6% 7% 6% 9% 10% 25%	2600
	11th to 20th	Day	6% 5% 2 7% 6% 8% 7% 7% 10% 13% 29%	2600
	21st to end	Day	7% 6% 2 8% 7% 6% 8% 6% 9% 12% 29%	2600
July	1st to 10th	Day	5% 5% 3 8% 7% 6% 7% 7% 10% 13% 29%	5900
	11th to 20th	Day	4 5% 2 6% 5% 6% 6% 6% 10% 14% 36%	5900
	21st to end	Day	4 4 2 6% 6% 5% 8% 6% 9% 13% 37%	6424
August	1st to 10th	Day	3 4 2 8% 5% 7% 9% 7% 10% 16% 28%	4234
	11th to 20th	Day	4 3 2 6% 5% 6% 7% 7% 11% 14% 36%	3917
	21st to end	Day	6% 5% 2 7% 6% 7% 7% 7% 11% 16% 26%	4085
September	1st to 10th	Day	7% 5% 3 9% 7% 6% 8% 7% 11% 12% 24%	2200
	11th to 20th	Day	9% 6% 3 8% 6% 7% 8% 6% 9% 12% 27%	2200
	21st to end	Day	9% 6% 3 8% 7% 7% 9% 6% 10% 10% 25%	2200
October	1st to 10th	Day	12% 6% 3 7% 6% 6% 6% 5% 8% 11% 29%	2105
	11th to 20th	Day	7% 5% 2 6% 5% 6% 6% 5% 6% 10% 43%	2000
	21st to end	Day	8% 5% 2 6% 4 4 4 5% 6% 10% 46%	2200
November	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 10% 39%	2057
	11th to 20th	Day	9% 7% 2 7% 4 5% 4 5% 6% 10% 41%	2100
	21st to end	Day	9% 6% 2 6% 5% 4 6% 5% 6% 10% 41%	2100
December	1st to 10th	Day	6% 4 2 5% 4 4 5% 4 6% 8% 53%	3054
	11th to 20th	Day	9% 6% 3 7% 5% 6% 6% 6% 7% 11% 33%	3463
	21st to end	Day	9% 7% 3 8% 7% 6% 7% 5% 7% 8% 34%	4565
January	1st to 10th	Night	7% 5% 2 5% 4 4 4 4 7% 9% 50%	6019
	11th to 20th	Night	13% 10% 4 10% 5% 5% 5% 4 6% 7% 31%	6030
	21st to end	Night	14% 7% 3 7% 4 4 4 3 6% 8% 41%	6633
February	1st to 10th	Night	10% 7% 2 6% 4 4 4 4 5% 8% 46%	2419
	11th to 20th	Night	9% 5% 2 5% 3 4 4 4 6% 7% 51%	2292
	21st to end	Night	8% 5% 2 4 3 3 4 4 7% 8% 52%	1815
March	1st to 10th	Night	7% 6% 3 7% 5% 5% 5% 4 6% 8% 46%	4918
	11th to 20th	Night	4 3 1 4 3 3 4 3 7% 10% 59%	4980
	21st to end	Night	7% 5% 2 6% 5% 4 5% 4 7% 9% 46%	5467
April	1st to 10th	Night	6% 3 1 5% 4 4 4 4 7% 8% 55%	2200
	11th to 20th	Night	6% 4 2 6% 4 4 6% 4 7% 9% 49%	2061
	21st to end	Night	5% 4 1 5% 5% 4 4 5% 6% 9% 51%	2000
May	1st to 10th	Night	3 3 1 4 5% 4 5% 4 7% 12% 53%	4490
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 4 7% 9% 46%	4086
	21st to end	Night	5% 4 2 6% 5% 6% 5% 5% 8% 11% 44%	4421
June	1st to 10th	Night	11% 7% 3 7% 6% 7% 7% 5% 8% 11% 29%	1800
	11th to 20th	Night	6% 6% 2 8% 6% 6% 7% 5% 9% 14% 31%	1800
	21st to end	Night	6% 6% 3 7% 6% 5% 7% 6% 8% 11% 34%	1800
July	1st to 10th	Night	8% 8% 3 9% 6% 6% 6% 5% 9% 12% 29%	4058
	11th to 20th	Night	4 4 2 6% 5% 4 6% 6% 9% 13% 41%	4057
	21st to end	Night	3 5% 2 6% 6% 6% 6% 6% 8% 11% 42%	4427
August	1st to 10th	Night	3 6% 2 9% 7% 6% 6% 5% 9% 13% 35%	2915
	11th to 20th	Night	3 5% 3 7% 5% 6% 5% 5% 9% 13% 39%	3098
	21st to end	Night	6% 6% 3 7% 6% 6% 7% 7% 10% 14% 29%	3807
September	1st to 10th	Night	6% 8% 3 9% 7% 7% 8% 6% 9% 12% 26%	2200
	11th to 20th	Night	8% 8% 4 11% 6% 7% 6% 5% 8% 11% 27%	2200
	21st to end	Night	10% 7% 3 10% 6% 7% 5% 6% 8% 11% 28%	2200
October	1st to 10th	Night	12% 7% 3 8% 5% 6% 5% 5% 7% 10% 32%	2293
	11th to 20th	Night	6% 6% 3 8% 5% 5% 5% 5% 6% 9% 43%	2400
	21st to end	Night	9% 6% 2 6% 4 4 5% 5% 6% 9% 45%	2640
November	1st to 10th	Night	7% 5% 3 7% 6% 6% 6% 4 5% 8% 42%	2461
	11th to 20th	Night	9% 6% 2 7% 4 5% 4 4 6% 8% 44%	2520
	21st to end	Night	11% 6% 2 6% 4 5% 5% 5% 6% 9% 40%	2520
December	1st to 10th	Night	4 4 1 5% 4 4 4 4 6% 9% 54%	4341
	11th to 20th	Night	11% 8% 3 8% 6% 5% 5% 4 7% 8% 36%	4952
	21st to end	Night	10% 7% 3 9% 6% 5% 6% 4 6% 8% 36%	6633

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% 28% 7% 16% 22% 3	4147
	11th to 20th	Day	43% 28% 7% 12% 8% 1	4150
	21st to end	Day	32% 31% 6% 19% 10% 2	4565
February	1st to 10th	Day	13% 63% 6% 8% 8% 2	1978
	11th to 20th	Day	8% 61% 5% 12% 11% 3	2003
	21st to end	Day	13% 62% 6% 8% 8% 3	1815
March	1st to 10th	Day	28% 33% 11% 12% 14% 2	4935
	11th to 20th	Day	11% 31% 8% 23% 15% 12%	4980
	21st to end	Day	26% 31% 9% 20% 11% 4	5465
April	1st to 10th	Day	8% 63% 7% 12% 7% 2	2200
	11th to 20th	Day	10% 64% 5% 8% 10% 3	2285
	21st to end	Day	8% 63% 5% 12% 11% 1	2400
May	1st to 10th	Day	14% 33% 21% 23% 7% 2	5437
	11th to 20th	Day	23% 32% 16% 21% 8%	5852
	21st to end	Day	16% 33% 17% 27% 8%	6447
June	1st to 10th	Day	16% 65% 11% 7% 1	2600
	11th to 20th	Day	14% 66% 11% 8% 1	2600
	21st to end	Day	11% 66% 16% 7% 1	2600
July	1st to 10th	Day	26% 33% 29% 10% 2	5835
	11th to 20th	Day	19% 33% 37% 10% 1	5831
	21st to end	Day	20% 35% 26% 20%	6240
August	1st to 10th	Day	23% 46% 24% 6% 1	4221
	11th to 20th	Day	14% 43% 23% 13% 7%	3955
	21st to end	Day	20% 46% 26% 7% 1	4085
September	1st to 10th	Day	20% 65% 10% 5% 1	2200
	11th to 20th	Day	18% 62% 12% 7% 1	2200
	21st to end	Day	16% 62% 11% 8% 3	2200
October	1st to 10th	Day	35% 42% 15% 6% 2	3430
	11th to 20th	Day	16% 39% 6% 18% 19% 2	3240
	21st to end	Day	14% 37% 8% 21% 17% 3	3538
November	1st to 10th	Day	15% 57% 10% 9% 7% 1	2057
	11th to 20th	Day	18% 56% 5% 9% 8% 3	2100
	21st to end	Day	17% 56% 6% 12% 6% 4	2100
December	1st to 10th	Day	15% 38% 15% 21% 10% 2	3054
	11th to 20th	Day	30% 34% 9% 12% 12% 3	3460
	21st to end	Day	35% 30% 9% 16% 7% 2	4565
January	1st to 10th	Night	23% 28% 7% 19% 17% 5%	6019
	11th to 20th	Night	44% 28% 10% 12% 6% 1	6030
	21st to end	Night	30% 30% 8% 18% 11% 3	6633
February	1st to 10th	Night	13% 63% 6% 9% 7% 2	2419
	11th to 20th	Night	10% 61% 5% 10% 11% 3	2293
	21st to end	Night	12% 62% 6% 9% 8% 3	1815
March	1st to 10th	Night	29% 33% 9% 14% 12% 3	4918
	11th to 20th	Night	12% 31% 15% 19% 14% 10%	4980
	21st to end	Night	25% 31% 9% 20% 13% 2	5467
April	1st to 10th	Night	9% 63% 7% 12% 6% 2	2200
	11th to 20th	Night	10% 64% 5% 9% 9% 3	2061
	21st to end	Night	9% 63% 6% 11% 10% 1	2000
May	1st to 10th	Night	17% 34% 19% 21% 7% 2	4520
	11th to 20th	Night	22% 32% 16% 24% 5%	4087
	21st to end	Night	20% 32% 17% 25% 6%	4429
June	1st to 10th	Night	17% 64% 12% 7% 1	1800
	11th to 20th	Night	17% 67% 9% 7% 1	1800
	21st to end	Night	14% 64% 14% 7% 1	1800
July	1st to 10th	Night	32% 33% 21% 12% 1	4015
	11th to 20th	Night	19% 33% 39% 9%	4006
	21st to end	Night	24% 33% 22% 20% 1	4297
August	1st to 10th	Night	27% 42% 23% 7% 2	2904
	11th to 20th	Night	20% 41% 22% 10% 7%	3126
	21st to end	Night	23% 44% 26% 6% 1	3807
September	1st to 10th	Night	21% 64% 9% 5% 1	2200
	11th to 20th	Night	19% 61% 12% 7% 1	2200
	21st to end	Night	18% 61% 8% 8% 4	2200
October	1st to 10th	Night	41% 40% 11% 5% 2	3748
	11th to 20th	Night	20% 39% 6% 17% 16% 2	3899
	21st to end	Night	19% 37% 6% 17% 17% 3	4252
November	1st to 10th	Night	18% 56% 9% 8% 7% 2	2461
	11th to 20th	Night	17% 57% 5% 9% 9% 3	2520
	21st to end	Night	19% 56% 4 11% 7% 3	2520
December	1st to 10th	Night	16% 37% 13% 23% 8% 3	4341
	11th to 20th	Night	28% 34% 13% 9% 12% 5%	4954
	21st to end	Night	35% 30% 12% 14% 6% 3	6633

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	10% 4 28% 36% 17% 4	330
	11th to 20th	Day	8% 8% 28% 36% 17% 4	330
	21st to end	Day	14% 9% 28% 38% 10%	363
February	1st to 10th	Day	11% 5% 21% 32% 25% 6%	300
	11th to 20th	Day	7% 4 23% 38% 23% 5%	345
	21st to end	Day	8% 2 23% 38% 24% 5%	414
March	1st to 10th	Day	5% 4 31% 41% 17% 2	500
	11th to 20th	Day	8% 4 24% 38% 23% 3	500
	21st to end	Day	5% 4 22% 37% 27% 5%	550
April	1st to 10th	Day	4 4 26% 38% 23% 5%	500
	11th to 20th	Day	7% 4 21% 34% 30% 3	492
	21st to end	Day	5% 3 14% 35% 34% 9%	500
May	1st to 10th	Day	4 1 12% 39% 33% 10%	503
	11th to 20th	Day	9% 2 12% 27% 36% 14%	527
	21st to end	Day	7% 1 7% 21% 41% 23%	573
June	1st to 10th	Day	11% 1 6% 21% 40% 21%	530
	11th to 20th	Day	5% 1 4 16% 37% 38%	530
	21st to end	Day	5% 1 3 11% 29% 51%	530
July	1st to 10th	Day	5% 1 3 7% 29% 55%	525
	11th to 20th	Day	5% 1 2 9% 32% 52%	524
	21st to end	Day	6% 1 2 9% 35% 48%	576
August	1st to 10th	Day	4 1 5% 7% 35% 48%	526
	11th to 20th	Day	3 1 2 7% 33% 55%	519
	21st to end	Day	7% 1 5% 13% 33% 40%	583
September	1st to 10th	Day	7% 2 4 7% 40% 41%	530
	11th to 20th	Day	7% 2 6% 16% 39% 31%	530
	21st to end	Day	10% 1 6% 16% 36% 31%	530
October	1st to 10th	Day	17% 2 10% 25% 38% 9%	530
	11th to 20th	Day	5% 4 12% 33% 38% 9%	530
	21st to end	Day	7% 2 13% 37% 32% 8%	583
November	1st to 10th	Day	10% 7% 13% 32% 29% 10%	535
	11th to 20th	Day	11% 4 19% 30% 31% 4	540
	21st to end	Day	8% 2 19% 36% 28% 6%	540
December	1st to 10th	Day	7% 4 19% 38% 26% 6%	351
	11th to 20th	Day	8% 4 21% 36% 26% 5%	330
	21st to end	Day	9% 5% 14% 39% 27% 6%	363
January	1st to 10th	Night	13% 6% 24% 34% 20% 3	840
	11th to 20th	Night	10% 7% 27% 34% 20% 3	840
	21st to end	Night	11% 8% 28% 37% 15% 1	924
February	1st to 10th	Night	13% 7% 19% 35% 22% 3	801
	11th to 20th	Night	10% 6% 24% 37% 20% 3	783
	21st to end	Night	10% 2 21% 42% 23% 1	660
March	1st to 10th	Night	5% 5% 28% 43% 17% 2	800
	11th to 20th	Night	7% 5% 22% 37% 25% 4	800
	21st to end	Night	6% 4 23% 38% 27% 2	880
April	1st to 10th	Night	6% 3 21% 40% 26% 4	800
	11th to 20th	Night	6% 3 20% 38% 30% 3	791
	21st to end	Night	6% 2 15% 43% 31% 4	800
May	1st to 10th	Night	5% 2 11% 40% 39% 3	800
	11th to 20th	Night	9% 2 10% 32% 38% 10%	619
	21st to end	Night	8% 1 5% 22% 46% 17%	650
June	1st to 10th	Night	12% 1 5% 21% 46% 15%	600
	11th to 20th	Night	6% 1 5% 14% 42% 32%	600
	21st to end	Night	7% 1 4 10% 41% 39%	600
July	1st to 10th	Night	8% 1 4 8% 40% 40%	585
	11th to 20th	Night	6% 1 3 7% 37% 47%	582
	21st to end	Night	4 1 2 7% 44% 41%	657
August	1st to 10th	Night	5% 1 3 7% 42% 42%	597
	11th to 20th	Night	5% 3 5% 40% 46%	727
	21st to end	Night	10% 1 5% 11% 39% 34%	880
September	1st to 10th	Night	8% 2 5% 11% 41% 33%	800
	11th to 20th	Night	12% 2 6% 14% 41% 25%	800
	21st to end	Night	13% 2 5% 16% 41% 23%	800
October	1st to 10th	Night	15% 2 8% 28% 38% 9%	800
	11th to 20th	Night	8% 2 11% 26% 43% 10%	800
	21st to end	Night	11% 2 16% 29% 35% 8%	880
November	1st to 10th	Night	9% 3 18% 30% 31% 9%	820
	11th to 20th	Night	10% 4 17% 36% 27% 6%	840
	21st to end	Night	12% 3 19% 37% 25% 3	840
December	1st to 10th	Night	7% 5% 21% 33% 30% 5%	840
	11th to 20th	Night	10% 5% 21% 32% 26% 6%	840
	21st to end	Night	9% 5% 22% 33% 28% 4	924

Page generated at 2026-01-29 01:21:42. Version ac07

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