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 2024-11-28.

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 7% 5% 5% 6% 5% 8% 11% 38%	101042
	Night	8% 6% 2 7% 5% 5% 5% 5% 7% 10% 40%	98459
Transparency	Day	18% 50% 13% 12% 6% 1	104754
	Night	21% 49% 11% 11% 6% 1	102886
Seeing	Day	7% 3 13% 26% 30% 20%	15881
	Night	9% 3 15% 28% 32% 14%	25974

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	10% 9% 2 8% 5% 5% 4 4 5% 6% 42%	2836
	11th to 20th	Day	17% 9% 4 7% 5% 5% 4 3 6% 9% 32%	2840
	21st to end	Day	9% 7% 3 7% 6% 5% 6% 5% 7% 7% 39%	3124
February	1st to 10th	Day	10% 6% 2 5% 5% 5% 4 4 6% 8% 44%	1879
	11th to 20th	Day	7% 4 2 4 4 4 5% 4 6% 8% 52%	1890
	21st to end	Day	9% 4 2 5% 4 4 4 3 6% 9% 51%	1727
March	1st to 10th	Day	7% 6% 3 9% 6% 6% 7% 5% 8% 9% 35%	3435
	11th to 20th	Day	4 3 2 4 3 4 5% 4 7% 11% 53%	3480
	21st to end	Day	9% 5% 2 5% 4 3 4 4 7% 9% 48%	3815
April	1st to 10th	Day	4 3 2 4 5% 4 5% 6% 8% 9% 51%	2090
	11th to 20th	Day	8% 5% 2 5% 5% 6% 5% 5% 8% 9% 41%	2169
	21st to end	Day	5% 4 2 5% 5% 4 5% 5% 7% 8% 50%	2280
May	1st to 10th	Day	3 2 1 3 3 3 4 3 7% 11% 61%	3765
	11th to 20th	Day	7% 5% 3 7% 6% 6% 6% 6% 7% 10% 37%	4068
	21st to end	Day	5% 4 2 5% 4 4 5% 5% 7% 9% 51%	4476
June	1st to 10th	Day	11% 8% 3 8% 7% 6% 7% 6% 9% 10% 25%	2470
	11th to 20th	Day	6% 5% 2 7% 7% 8% 7% 7% 10% 12% 29%	2470
	21st to end	Day	7% 6% 2 8% 7% 6% 7% 6% 9% 12% 29%	2470
July	1st to 10th	Day	5% 4 2 8% 6% 5% 6% 6% 9% 12% 36%	4120
	11th to 20th	Day	6% 6% 2 6% 5% 6% 7% 6% 10% 14% 33%	4120
	21st to end	Day	5% 4 2 7% 7% 6% 9% 7% 10% 13% 30%	4466
August	1st to 10th	Day	3 4 2 8% 6% 7% 9% 7% 10% 16% 28%	4104
	11th to 20th	Day	4 3 2 6% 5% 6% 7% 7% 11% 14% 35%	3795
	21st to end	Day	6% 5% 2 7% 6% 7% 7% 7% 11% 16% 26%	3960
September	1st to 10th	Day	7% 5% 3 9% 6% 6% 8% 7% 11% 12% 25%	2090
	11th to 20th	Day	9% 6% 3 8% 6% 8% 8% 6% 10% 12% 27%	2090
	21st to end	Day	10% 6% 3 8% 7% 7% 8% 6% 10% 10% 26%	2090
October	1st to 10th	Day	13% 6% 3 7% 5% 6% 6% 5% 7% 11% 30%	2000
	11th to 20th	Day	7% 5% 2 7% 5% 6% 6% 5% 6% 10% 42%	1900
	21st to end	Day	8% 5% 2 6% 4 4 4 5% 6% 10% 45%	2090
November	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 5% 8% 10% 39%	1957
	11th to 20th	Day	9% 7% 3 7% 4 5% 4 5% 6% 10% 38%	2000
	21st to end	Day	8% 6% 2 6% 5% 4 7% 5% 7% 10% 41%	1970
December	1st to 10th	Day	9% 5% 2 6% 5% 5% 6% 5% 6% 9% 44%	1729
	11th to 20th	Day	11% 7% 3 7% 4 5% 5% 5% 5% 8% 40%	2153
	21st to end	Day	10% 7% 3 8% 7% 6% 7% 6% 7% 7% 31%	3124
January	1st to 10th	Night	10% 7% 3 7% 5% 5% 5% 4 7% 8% 39%	4109
	11th to 20th	Night	15% 12% 5% 9% 5% 4 4 4 5% 7% 30%	4120
	21st to end	Night	9% 6% 3 8% 5% 5% 5% 4 7% 7% 40%	4532
February	1st to 10th	Night	10% 7% 2 5% 4 4 4 4 5% 8% 46%	2298
	11th to 20th	Night	9% 4 2 4 3 4 4 4 6% 7% 52%	2175
	21st to end	Night	8% 4 2 4 3 3 4 4 7% 8% 53%	1727
March	1st to 10th	Night	9% 8% 3 9% 6% 5% 5% 4 6% 8% 37%	3418
	11th to 20th	Night	4 3 1 4 3 3 4 4 9% 13% 53%	3480
	21st to end	Night	7% 5% 2 6% 5% 5% 4 4 7% 8% 46%	3817
April	1st to 10th	Night	5% 3 1 5% 4 4 4 4 6% 8% 56%	2090
	11th to 20th	Night	6% 4 2 6% 4 4 6% 4 7% 10% 47%	1957
	21st to end	Night	5% 4 1 5% 5% 5% 4 5% 6% 9% 51%	1900
May	1st to 10th	Night	4 2 1 3 3 3 3 3 6% 9% 61%	3130
	11th to 20th	Night	7% 5% 2 5% 5% 4 6% 5% 8% 11% 43%	2852
	21st to end	Night	5% 4 2 5% 4 4 4 4 7% 9% 52%	3079
June	1st to 10th	Night	11% 7% 3 7% 6% 7% 7% 5% 8% 11% 29%	1710
	11th to 20th	Night	6% 7% 2 8% 6% 6% 7% 5% 8% 14% 32%	1710
	21st to end	Night	7% 6% 3 7% 6% 6% 7% 6% 9% 11% 34%	1710
July	1st to 10th	Night	7% 5% 3 7% 4 5% 5% 5% 8% 13% 38%	2840
	11th to 20th	Night	5% 4 2 7% 6% 4 7% 6% 9% 13% 37%	2840
	21st to end	Night	4 5% 2 7% 7% 7% 6% 7% 9% 13% 32%	3087
August	1st to 10th	Night	3 6% 2 9% 6% 6% 6% 5% 9% 13% 35%	2825
	11th to 20th	Night	4 5% 3 7% 5% 6% 5% 5% 8% 13% 38%	3001
	21st to end	Night	6% 5% 3 7% 6% 6% 7% 7% 10% 14% 29%	3690
September	1st to 10th	Night	6% 7% 3 9% 6% 7% 8% 6% 9% 12% 27%	2090
	11th to 20th	Night	8% 8% 3 10% 6% 7% 6% 5% 8% 11% 27%	2090
	21st to end	Night	10% 7% 3 10% 6% 7% 5% 6% 8% 10% 28%	2090
October	1st to 10th	Night	12% 7% 3 8% 5% 6% 5% 5% 7% 10% 31%	2178
	11th to 20th	Night	6% 6% 3 8% 5% 5% 5% 5% 6% 9% 42%	2280
	21st to end	Night	9% 6% 2 6% 4 4 5% 5% 6% 10% 44%	2508
November	1st to 10th	Night	7% 6% 3 7% 7% 6% 6% 4 5% 8% 40%	2341
	11th to 20th	Night	9% 6% 3 7% 5% 5% 5% 5% 6% 9% 41%	2400
	21st to end	Night	12% 7% 2 6% 4 4 5% 5% 6% 9% 39%	2365
December	1st to 10th	Night	6% 5% 2 6% 6% 4 5% 4 6% 9% 46%	2446
	11th to 20th	Night	13% 8% 3 7% 5% 4 4 3 6% 8% 40%	3042
	21st to end	Night	10% 7% 4 9% 6% 5% 6% 4 6% 9% 33%	4532

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	30% 39% 4 9% 16% 2	2837
	11th to 20th	Day	38% 39% 5% 11% 7% 1	2840
	21st to end	Day	25% 44% 6% 11% 11% 3	3124
February	1st to 10th	Day	13% 66% 5% 8% 7% 1	1879
	11th to 20th	Day	7% 64% 5% 10% 11% 3	1900
	21st to end	Day	12% 65% 5% 7% 8% 2	1727
March	1st to 10th	Day	30% 47% 12% 8% 3 1	3435
	11th to 20th	Day	12% 44% 10% 24% 9% 1	3480
	21st to end	Day	20% 42% 8% 19% 7% 4	3815
April	1st to 10th	Day	8% 67% 7% 11% 6% 2	2090
	11th to 20th	Day	10% 68% 5% 8% 8% 2	2169
	21st to end	Day	8% 66% 5% 11% 10% 1	2280
May	1st to 10th	Day	7% 42% 13% 27% 8% 3	3800
	11th to 20th	Day	23% 45% 13% 13% 5%	4072
	21st to end	Day	8% 42% 10% 29% 11%	4489
June	1st to 10th	Day	14% 68% 10% 6% 1	2470
	11th to 20th	Day	14% 69% 9% 7%	2470
	21st to end	Day	9% 69% 15% 6% 1	2470
July	1st to 10th	Day	15% 41% 27% 14% 4	4055
	11th to 20th	Day	18% 43% 34% 6%	4051
	21st to end	Day	21% 48% 23% 8%	4324
August	1st to 10th	Day	23% 47% 23% 6% 1	4091
	11th to 20th	Day	14% 44% 23% 12% 7%	3833
	21st to end	Day	19% 47% 26% 7% 1	3960
September	1st to 10th	Day	18% 67% 9% 5% 1	2090
	11th to 20th	Day	17% 65% 12% 6% 1	2090
	21st to end	Day	15% 65% 10% 8% 3	2090
October	1st to 10th	Day	35% 42% 15% 6% 2	3325
	11th to 20th	Day	16% 40% 6% 18% 18% 2	3140
	21st to end	Day	14% 38% 8% 21% 17% 2	3428
November	1st to 10th	Day	14% 60% 10% 9% 6% 1	1957
	11th to 20th	Day	19% 59% 5% 8% 6% 3	2000
	21st to end	Day	16% 59% 5% 10% 5% 4	1970
December	1st to 10th	Day	13% 62% 7% 13% 4 1	1729
	11th to 20th	Day	19% 49% 5% 9% 13% 4	2150
	21st to end	Day	34% 42% 7% 11% 4 3	3124
January	1st to 10th	Night	30% 39% 7% 14% 8% 1	4109
	11th to 20th	Night	41% 38% 5% 10% 5% 1	4120
	21st to end	Night	25% 42% 7% 11% 10% 5%	4532
February	1st to 10th	Night	12% 66% 6% 9% 6% 1	2298
	11th to 20th	Night	8% 64% 5% 9% 11% 3	2176
	21st to end	Night	11% 65% 6% 8% 8% 3	1727
March	1st to 10th	Night	34% 46% 10% 7% 3 1	3418
	11th to 20th	Night	10% 44% 20% 19% 6% 1	3480
	21st to end	Night	21% 43% 8% 14% 11% 2	3817
April	1st to 10th	Night	8% 67% 7% 11% 5% 2	2090
	11th to 20th	Night	10% 67% 5% 9% 6% 2	1957
	21st to end	Night	8% 66% 6% 10% 9% 1	1900
May	1st to 10th	Night	9% 43% 14% 23% 7% 3	3160
	11th to 20th	Night	18% 45% 13% 19% 4	2853
	21st to end	Night	10% 42% 10% 28% 9%	3087
June	1st to 10th	Night	15% 68% 11% 6%	1710
	11th to 20th	Night	16% 70% 7% 6%	1710
	21st to end	Night	13% 66% 13% 6% 2	1710
July	1st to 10th	Night	17% 42% 23% 17% 2	2795
	11th to 20th	Night	17% 43% 35% 5%	2786
	21st to end	Night	26% 45% 21% 8%	2988
August	1st to 10th	Night	26% 43% 23% 7% 2	2814
	11th to 20th	Night	20% 42% 21% 10% 7%	3029
	21st to end	Night	22% 45% 26% 6% 1	3690
September	1st to 10th	Night	18% 67% 9% 6% 1	2090
	11th to 20th	Night	17% 64% 12% 7% 1	2090
	21st to end	Night	17% 63% 7% 8% 4	2090
October	1st to 10th	Night	42% 41% 10% 5% 2	3633
	11th to 20th	Night	20% 40% 6% 17% 15% 1	3779
	21st to end	Night	19% 39% 6% 17% 17% 3	4120
November	1st to 10th	Night	18% 59% 8% 7% 6% 2	2341
	11th to 20th	Night	18% 59% 5% 8% 7% 2	2400
	21st to end	Night	18% 59% 4 9% 7% 3	2365
December	1st to 10th	Night	13% 62% 7% 13% 4	2446
	11th to 20th	Night	21% 50% 6% 7% 9% 7%	3044
	21st to end	Night	34% 41% 9% 9% 4 2	4532

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	13% 5% 27% 37% 15% 3	270
	11th to 20th	Day	7% 7% 28% 36% 17% 5%	270
	21st to end	Day	14% 10% 30% 37% 9%	297
February	1st to 10th	Day	12% 5% 23% 34% 23% 3	270
	11th to 20th	Day	6% 4 24% 37% 22% 5%	312
	21st to end	Day	7% 2 23% 37% 25% 5%	382
March	1st to 10th	Day	5% 4 30% 40% 18% 2	460
	11th to 20th	Day	8% 4 24% 37% 24% 3	460
	21st to end	Day	5% 3 24% 36% 26% 5%	506
April	1st to 10th	Day	3 5% 27% 40% 21% 4	460
	11th to 20th	Day	7% 4 22% 35% 29% 3	452
	21st to end	Day	6% 3 15% 38% 31% 8%	460
May	1st to 10th	Day	5% 2 12% 39% 33% 10%	463
	11th to 20th	Day	9% 2 12% 27% 35% 14%	487
	21st to end	Day	8% 1 7% 21% 41% 22%	529
June	1st to 10th	Day	11% 1 6% 21% 39% 22%	490
	11th to 20th	Day	6% 1 4 17% 38% 34%	490
	21st to end	Day	5% 1 3 11% 28% 51%	490
July	1st to 10th	Day	5% 1 3 6% 30% 55%	485
	11th to 20th	Day	4 1 2 8% 32% 52%	484
	21st to end	Day	6% 1 3 10% 33% 48%	532
August	1st to 10th	Day	4 1 5% 7% 35% 47%	486
	11th to 20th	Day	3 1 2 7% 33% 54%	479
	21st to end	Day	7% 1 5% 14% 33% 41%	539
September	1st to 10th	Day	7% 2 4 7% 39% 41%	490
	11th to 20th	Day	7% 2 6% 16% 37% 33%	490
	21st to end	Day	10% 1 6% 17% 36% 31%	490
October	1st to 10th	Day	18% 2 10% 25% 35% 10%	490
	11th to 20th	Day	6% 3 12% 33% 37% 8%	490
	21st to end	Day	8% 2 13% 36% 33% 8%	539
November	1st to 10th	Day	9% 7% 13% 32% 29% 10%	495
	11th to 20th	Day	12% 4 20% 30% 29% 4	500
	21st to end	Day	8% 3 20% 37% 27% 5%	488
December	1st to 10th	Day	7% 4 18% 38% 26% 7%	289
	11th to 20th	Day	8% 5% 23% 35% 24% 5%	270
	21st to end	Day	8% 6% 17% 37% 27% 4	297
January	1st to 10th	Night	14% 6% 24% 33% 20% 3	760
	11th to 20th	Night	11% 7% 25% 33% 21% 3	760
	21st to end	Night	10% 8% 29% 37% 15% 1	836
February	1st to 10th	Night	13% 7% 20% 36% 21% 3	760
	11th to 20th	Night	10% 6% 25% 36% 20% 3	743
	21st to end	Night	10% 2 21% 42% 23% 1	628
March	1st to 10th	Night	5% 5% 28% 43% 17% 2	760
	11th to 20th	Night	7% 4 22% 37% 26% 4	760
	21st to end	Night	6% 4 24% 38% 27% 2	836
April	1st to 10th	Night	6% 3 22% 40% 25% 4	760
	11th to 20th	Night	6% 3 20% 39% 28% 3	751
	21st to end	Night	6% 2 15% 45% 29% 3	760
May	1st to 10th	Night	6% 1 12% 39% 39% 3	760
	11th to 20th	Night	8% 2 10% 32% 38% 10%	588
	21st to end	Night	8% 1 5% 22% 47% 17%	617
June	1st to 10th	Night	12% 1 5% 21% 46% 14%	570
	11th to 20th	Night	6% 1 6% 14% 44% 29%	570
	21st to end	Night	7% 1 4 10% 40% 38%	570
July	1st to 10th	Night	7% 1 3 7% 40% 41%	555
	11th to 20th	Night	6% 3 7% 37% 47%	552
	21st to end	Night	4 1 2 7% 44% 42%	624
August	1st to 10th	Night	5% 1 3 7% 43% 41%	567
	11th to 20th	Night	6% 3 6% 40% 46%	690
	21st to end	Night	10% 1 5% 11% 38% 34%	836
September	1st to 10th	Night	8% 2 5% 10% 41% 34%	760
	11th to 20th	Night	12% 1 6% 14% 41% 26%	760
	21st to end	Night	13% 2 5% 16% 41% 23%	760
October	1st to 10th	Night	16% 2 8% 28% 37% 9%	760
	11th to 20th	Night	8% 2 11% 26% 43% 9%	760
	21st to end	Night	12% 2 15% 29% 35% 7%	836
November	1st to 10th	Night	9% 3 18% 30% 31% 9%	780
	11th to 20th	Night	11% 4 17% 36% 26% 6%	800
	21st to end	Night	13% 3 20% 37% 24% 3	789
December	1st to 10th	Night	7% 4 20% 32% 31% 6%	760
	11th to 20th	Night	11% 5% 22% 31% 25% 6%	760
	21st to end	Night	9% 5% 22% 33% 28% 3	836

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