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 2024-01-14.

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	8% 5% 3 6% 6% 4 6% 7% 8% 10% 38%	74912
	Night	8% 5% 3 6% 6% 4 6% 6% 7% 10% 41%	74854
Transparency	Day	26% 41% 11% 11% 8% 2	74635
	Night	25% 40% 11% 13% 9% 3	74650
Seeing	Day	8% 3 13% 26% 31% 20%	14641
	Night	9% 3 15% 28% 32% 14%	24795

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	11% 6% 2 5% 5% 4 4 6% 6% 9% 42%	1706
	11th to 20th	Day	7% 5% 3 3 5% 3 5% 5% 7% 11% 46%	1647
	21st to end	Day	11% 5% 3 5% 5% 3 6% 5% 7% 7% 43%	1782
February	1st to 10th	Day	11% 6% 2 5% 5% 3 5% 5% 6% 9% 44%	1780
	11th to 20th	Day	6% 4 2 4 4 3 5% 5% 6% 8% 53%	1788
	21st to end	Day	10% 4 2 5% 4 2 5% 4 5% 9% 49%	1628
March	1st to 10th	Day	5% 3 1 4 5% 3 6% 5% 6% 9% 53%	1980
	11th to 20th	Day	6% 3 2 4 4 3 5% 5% 6% 10% 51%	1980
	21st to end	Day	6% 4 2 4 5% 3 5% 5% 6% 9% 52%	2178
April	1st to 10th	Day	5% 2 2 4 5% 3 5% 6% 7% 9% 51%	1980
	11th to 20th	Day	8% 5% 2 4 6% 4 6% 7% 8% 10% 40%	2053
	21st to end	Day	5% 4 2 4 5% 3 6% 5% 6% 9% 50%	2160
May	1st to 10th	Day	5% 3 1 3 4 3 5% 4 7% 10% 55%	2160
	11th to 20th	Day	9% 4 3 6% 6% 5% 7% 6% 7% 9% 39%	2288
	21st to end	Day	9% 5% 3 7% 6% 5% 6% 7% 9% 10% 34%	2531
June	1st to 10th	Day	12% 8% 3 8% 8% 4 7% 8% 9% 11% 23%	2340
	11th to 20th	Day	6% 4 3 6% 7% 6% 7% 9% 10% 13% 30%	2340
	21st to end	Day	7% 6% 3 7% 8% 4 7% 8% 9% 13% 29%	2340
July	1st to 10th	Day	7% 5% 3 7% 8% 4 8% 9% 9% 12% 29%	2340
	11th to 20th	Day	8% 6% 3 7% 6% 4 7% 8% 9% 13% 29%	2340
	21st to end	Day	8% 5% 2 6% 8% 4 8% 9% 8% 12% 28%	2547
August	1st to 10th	Day	5% 4 3 6% 6% 5% 6% 10% 9% 15% 32%	2327
	11th to 20th	Day	6% 4 2 6% 7% 5% 7% 8% 12% 15% 29%	2165
	21st to end	Day	8% 7% 3 6% 9% 5% 6% 7% 9% 13% 26%	2254
September	1st to 10th	Day	7% 5% 3 7% 7% 5% 8% 9% 11% 13% 27%	1980
	11th to 20th	Day	8% 5% 3 7% 7% 5% 8% 8% 9% 12% 27%	1980
	21st to end	Day	10% 6% 3 7% 8% 5% 8% 8% 10% 10% 26%	1980
October	1st to 10th	Day	12% 6% 3 6% 6% 4 7% 7% 7% 12% 31%	1895
	11th to 20th	Day	8% 5% 2 5% 7% 5% 5% 7% 6% 9% 41%	1800
	21st to end	Day	9% 5% 2 6% 5% 3 4 6% 7% 10% 45%	1980
November	1st to 10th	Day	8% 5% 2 5% 5% 3 6% 7% 8% 10% 40%	1857
	11th to 20th	Day	9% 7% 3 6% 6% 4 4 6% 6% 10% 39%	1900
	21st to end	Day	9% 6% 3 5% 6% 3 6% 6% 6% 10% 39%	1900
December	1st to 10th	Day	9% 4 3 5% 5% 3 5% 6% 6% 9% 44%	1729
	11th to 20th	Day	11% 6% 3 6% 5% 4 6% 6% 5% 8% 41%	2153
	21st to end	Day	10% 7% 3 7% 8% 5% 7% 8% 6% 7% 32%	3124
January	1st to 10th	Night	11% 5% 2 4 4 3 5% 4 7% 9% 45%	2459
	11th to 20th	Night	8% 5% 3 6% 5% 3 5% 5% 6% 9% 45%	2391
	21st to end	Night	10% 6% 3 6% 5% 3 5% 5% 7% 8% 42%	2574
February	1st to 10th	Night	10% 6% 2 5% 5% 3 4 5% 5% 8% 47%	2177
	11th to 20th	Night	9% 4 2 4 3 3 4 5% 6% 7% 54%	2057
	21st to end	Night	8% 4 2 4 4 2 4 5% 7% 9% 51%	1628
March	1st to 10th	Night	4 3 2 5% 4 3 4 4 6% 8% 57%	1980
	11th to 20th	Night	7% 4 2 5% 4 2 4 3 6% 9% 55%	1980
	21st to end	Night	6% 3 2 4 5% 3 5% 5% 7% 9% 51%	2178
April	1st to 10th	Night	6% 2 2 4 4 3 5% 5% 5% 8% 57%	1980
	11th to 20th	Night	6% 4 3 5% 5% 3 6% 6% 7% 10% 45%	1853
	21st to end	Night	6% 4 2 5% 5% 3 5% 6% 6% 9% 51%	1800
May	1st to 10th	Night	6% 2 2 3 5% 3 4 5% 6% 8% 59%	1800
	11th to 20th	Night	8% 5% 2 4 6% 4 6% 7% 7% 10% 42%	1618
	21st to end	Night	7% 5% 3 5% 6% 4 6% 6% 8% 12% 37%	1744
June	1st to 10th	Night	11% 7% 4 6% 7% 5% 8% 7% 8% 11% 27%	1620
	11th to 20th	Night	6% 6% 3 7% 7% 4 8% 6% 8% 14% 33%	1620
	21st to end	Night	7% 5% 3 6% 6% 4 7% 7% 8% 12% 35%	1620
July	1st to 10th	Night	9% 5% 4 7% 6% 5% 6% 8% 10% 11% 31%	1620
	11th to 20th	Night	7% 5% 3 6% 7% 4 7% 9% 8% 10% 35%	1620
	21st to end	Night	4 5% 2 6% 7% 4 7% 8% 8% 13% 36%	1771
August	1st to 10th	Night	5% 5% 2 5% 6% 4 6% 7% 9% 14% 36%	1611
	11th to 20th	Night	4 4 3 6% 5% 5% 6% 8% 10% 13% 36%	1717
	21st to end	Night	10% 6% 3 6% 8% 5% 7% 7% 9% 12% 28%	2096
September	1st to 10th	Night	6% 6% 3 7% 7% 5% 8% 8% 9% 13% 28%	1980
	11th to 20th	Night	8% 7% 3 9% 8% 5% 6% 7% 8% 11% 28%	1980
	21st to end	Night	10% 7% 3 9% 7% 4 6% 7% 8% 10% 27%	1980
October	1st to 10th	Night	11% 7% 3 7% 5% 4 5% 6% 7% 11% 33%	2063
	11th to 20th	Night	7% 6% 3 6% 6% 4 5% 6% 6% 9% 43%	2160
	21st to end	Night	10% 6% 2 5% 4 3 5% 6% 6% 10% 43%	2376
November	1st to 10th	Night	7% 5% 4 6% 7% 4 7% 5% 5% 8% 41%	2221
	11th to 20th	Night	9% 5% 3 7% 5% 4 5% 5% 6% 9% 42%	2280
	21st to end	Night	12% 6% 2 5% 5% 3 6% 6% 6% 10% 38%	2280
December	1st to 10th	Night	6% 4 3 6% 6% 4 4 6% 6% 9% 46%	2446
	11th to 20th	Night	13% 8% 3 6% 6% 3 4 4 6% 8% 40%	3042
	21st to end	Night	10% 7% 4 8% 7% 4 7% 5% 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	22% 43% 6% 11% 14% 5%	1707
	11th to 20th	Day	20% 43% 5% 15% 12% 4	1647
	21st to end	Day	25% 39% 7% 12% 12% 5%	1782
February	1st to 10th	Day	24% 37% 7% 11% 15% 6%	1780
	11th to 20th	Day	21% 40% 10% 14% 10% 5%	1798
	21st to end	Day	19% 38% 8% 12% 19% 4	1628
March	1st to 10th	Day	18% 38% 9% 15% 16% 4	1980
	11th to 20th	Day	19% 38% 11% 19% 9% 4	1980
	21st to end	Day	18% 38% 11% 16% 13% 4	2178
April	1st to 10th	Day	20% 39% 11% 16% 11% 3	1980
	11th to 20th	Day	24% 39% 10% 13% 11% 2	2053
	21st to end	Day	21% 38% 9% 14% 14% 4	2160
May	1st to 10th	Day	14% 39% 11% 18% 16% 1	2160
	11th to 20th	Day	26% 41% 11% 14% 8%	2292
	21st to end	Day	28% 43% 11% 12% 5%	2531
June	1st to 10th	Day	34% 42% 11% 9% 3	2340
	11th to 20th	Day	26% 44% 15% 14% 2	2340
	21st to end	Day	34% 42% 13% 8% 3	2340
July	1st to 10th	Day	32% 45% 15% 6% 1	2275
	11th to 20th	Day	32% 46% 14% 7%	2271
	21st to end	Day	28% 47% 13% 12% 1	2405
August	1st to 10th	Day	23% 47% 21% 9% 1	2314
	11th to 20th	Day	26% 48% 18% 7% 1	2165
	21st to end	Day	34% 45% 12% 6% 2	2254
September	1st to 10th	Day	27% 46% 16% 9% 2	1980
	11th to 20th	Day	30% 46% 14% 7% 3	1980
	21st to end	Day	34% 42% 9% 8% 6% 1	1980
October	1st to 10th	Day	27% 40% 10% 11% 8% 4	1895
	11th to 20th	Day	27% 39% 9% 13% 10% 2	1800
	21st to end	Day	24% 38% 9% 12% 14% 3	1980
November	1st to 10th	Day	23% 41% 6% 11% 14% 5%	1857
	11th to 20th	Day	26% 42% 7% 13% 10% 3	1900
	21st to end	Day	23% 43% 10% 11% 11% 2	1900
December	1st to 10th	Day	23% 41% 10% 12% 9% 3	1729
	11th to 20th	Day	32% 35% 7% 9% 13% 4	2150
	21st to end	Day	36% 29% 8% 10% 11% 5%	3124
January	1st to 10th	Night	19% 42% 8% 13% 13% 5%	2459
	11th to 20th	Night	23% 42% 6% 15% 11% 4	2391
	21st to end	Night	26% 39% 8% 13% 7% 6%	2574
February	1st to 10th	Night	24% 38% 8% 10% 13% 7%	2177
	11th to 20th	Night	22% 38% 9% 15% 10% 5%	2058
	21st to end	Night	17% 38% 11% 14% 17% 3	1628
March	1st to 10th	Night	17% 37% 7% 19% 16% 5%	1980
	11th to 20th	Night	17% 38% 10% 21% 9% 4	1980
	21st to end	Night	17% 39% 10% 17% 13% 5%	2178
April	1st to 10th	Night	17% 39% 13% 16% 11% 4	1980
	11th to 20th	Night	21% 39% 10% 15% 14% 1	1853
	21st to end	Night	22% 38% 9% 13% 14% 4	1800
May	1st to 10th	Night	15% 39% 9% 21% 15% 2	1800
	11th to 20th	Night	21% 44% 13% 16% 6% 1	1619
	21st to end	Night	27% 41% 11% 13% 7%	1745
June	1st to 10th	Night	33% 41% 12% 11% 3	1620
	11th to 20th	Night	25% 43% 14% 15% 4	1620
	21st to end	Night	29% 44% 13% 11% 3	1620
July	1st to 10th	Night	33% 44% 15% 7% 1	1575
	11th to 20th	Night	31% 48% 12% 7% 1	1566
	21st to end	Night	23% 46% 21% 10% 1	1672
August	1st to 10th	Night	23% 45% 23% 10%	1600
	11th to 20th	Night	26% 44% 20% 8% 1	1717
	21st to end	Night	32% 44% 14% 8% 2	2096
September	1st to 10th	Night	30% 45% 14% 9% 2	1980
	11th to 20th	Night	33% 45% 12% 8% 1	1980
	21st to end	Night	34% 42% 10% 8% 6% 1	1980
October	1st to 10th	Night	27% 39% 10% 14% 8% 2	2063
	11th to 20th	Night	26% 39% 10% 14% 9% 3	2160
	21st to end	Night	23% 40% 9% 11% 14% 4	2376
November	1st to 10th	Night	23% 41% 6% 10% 14% 5%	2221
	11th to 20th	Night	24% 42% 8% 14% 8% 3	2280
	21st to end	Night	26% 42% 8% 11% 12% 2	2280
December	1st to 10th	Night	22% 41% 9% 16% 7% 4	2446
	11th to 20th	Night	33% 35% 8% 10% 9% 5%	3044
	21st to end	Night	36% 29% 8% 10% 11% 5%	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	4 7% 26% 39% 18% 5%	249
	21st to end	Day	15% 10% 29% 37% 10%	264
February	1st to 10th	Day	12% 4 24% 33% 25% 3	240
	11th to 20th	Day	5% 4 21% 39% 25% 6%	280
	21st to end	Day	8% 2 22% 37% 26% 5%	346
March	1st to 10th	Day	5% 4 30% 41% 19% 2	420
	11th to 20th	Day	9% 4 23% 35% 26% 4	420
	21st to end	Day	5% 3 21% 37% 28% 6%	462
April	1st to 10th	Day	4 4 25% 40% 23% 5%	420
	11th to 20th	Day	8% 5% 20% 34% 30% 3	412
	21st to end	Day	6% 2 15% 37% 32% 8%	420
May	1st to 10th	Day	5% 2 12% 39% 34% 9%	423
	11th to 20th	Day	10% 2 10% 26% 37% 15%	447
	21st to end	Day	8% 1 8% 19% 40% 24%	485
June	1st to 10th	Day	12% 1 6% 19% 38% 22%	450
	11th to 20th	Day	4 1 4 15% 40% 36%	450
	21st to end	Day	5% 1 4 12% 28% 51%	450
July	1st to 10th	Day	5% 3 6% 29% 56%	445
	11th to 20th	Day	4 1 2 9% 33% 51%	444
	21st to end	Day	7% 1 3 10% 33% 46%	488
August	1st to 10th	Day	4 1 4 6% 36% 48%	446
	11th to 20th	Day	4 1 2 7% 33% 54%	439
	21st to end	Day	7% 1 5% 14% 33% 40%	495
September	1st to 10th	Day	7% 2 4 7% 38% 42%	450
	11th to 20th	Day	6% 1 6% 16% 36% 35%	450
	21st to end	Day	10% 5% 16% 36% 32%	450
October	1st to 10th	Day	16% 2 10% 27% 36% 10%	450
	11th to 20th	Day	6% 4 12% 34% 37% 9%	450
	21st to end	Day	8% 2 14% 35% 33% 8%	495
November	1st to 10th	Day	9% 7% 14% 33% 28% 10%	455
	11th to 20th	Day	13% 4 21% 30% 28% 3	460
	21st to end	Day	8% 3 20% 38% 26% 5%	460
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	10% 6% 25% 34% 22% 3	736
	21st to end	Night	11% 8% 29% 36% 16% 1	792
February	1st to 10th	Night	14% 6% 20% 36% 22% 3	720
	11th to 20th	Night	9% 6% 24% 37% 21% 3	703
	21st to end	Night	11% 2 20% 42% 24% 1	592
March	1st to 10th	Night	5% 4 28% 43% 18% 2	720
	11th to 20th	Night	8% 4 22% 35% 27% 4	720
	21st to end	Night	6% 3 23% 38% 28% 2	792
April	1st to 10th	Night	6% 3 20% 41% 27% 4	720
	11th to 20th	Night	7% 3 20% 39% 28% 3	711
	21st to end	Night	7% 2 15% 44% 29% 3	720
May	1st to 10th	Night	6% 1 12% 39% 39% 3	720
	11th to 20th	Night	9% 1 9% 31% 39% 11%	557
	21st to end	Night	8% 1 5% 22% 46% 18%	584
June	1st to 10th	Night	12% 1 6% 21% 45% 15%	540
	11th to 20th	Night	6% 1 5% 13% 44% 30%	540
	21st to end	Night	7% 1 4 10% 40% 38%	540
July	1st to 10th	Night	8% 1 3 7% 39% 41%	525
	11th to 20th	Night	6% 3 6% 38% 46%	522
	21st to end	Night	4 1 2 7% 44% 41%	591
August	1st to 10th	Night	5% 2 7% 44% 41%	537
	11th to 20th	Night	6% 4 6% 40% 45%	653
	21st to end	Night	10% 1 5% 12% 38% 34%	792
September	1st to 10th	Night	8% 2 5% 9% 41% 35%	720
	11th to 20th	Night	12% 1 6% 13% 41% 27%	720
	21st to end	Night	14% 1 5% 16% 41% 23%	720
October	1st to 10th	Night	15% 2 9% 29% 37% 9%	720
	11th to 20th	Night	9% 2 10% 25% 44% 10%	720
	21st to end	Night	12% 2 15% 29% 35% 8%	792
November	1st to 10th	Night	10% 3 19% 31% 30% 8%	740
	11th to 20th	Night	11% 4 18% 35% 26% 5%	760
	21st to end	Night	13% 3 19% 38% 24% 3	760
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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