Heaven Sent Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Heaven Sent Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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 7% 8% 5% 7% 7% 8% 10% 32%	74881
	Night	7% 4 2 5% 5% 3 6% 6% 7% 10% 44%	74928
Transparency	Day	40% 13% 16% 17% 11% 2	74604
	Night	33% 11% 20% 22% 12% 3	74724
Seeing	Day	8% 3 14% 29% 27% 20%	14872
	Night	8% 2 11% 31% 34% 14%	25046

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	8% 5% 3 5% 5% 4 5% 7% 7% 11% 40%	1706
	11th to 20th	Day	11% 6% 3 6% 6% 4 4 5% 6% 10% 38%	1671
	21st to end	Day	9% 5% 3 7% 6% 3 7% 6% 8% 9% 38%	1858
February	1st to 10th	Day	8% 6% 2 5% 6% 4 6% 5% 7% 9% 42%	1800
	11th to 20th	Day	6% 4 2 4 5% 3 4 6% 7% 11% 48%	1853
	21st to end	Day	6% 4 2 5% 6% 4 5% 5% 7% 9% 48%	1628
March	1st to 10th	Day	6% 5% 2 4 5% 4 6% 6% 6% 9% 48%	1980
	11th to 20th	Day	7% 4 2 5% 6% 3 6% 5% 8% 10% 44%	1980
	21st to end	Day	5% 4 2 5% 5% 4 6% 5% 6% 8% 50%	2178
April	1st to 10th	Day	5% 3 1 5% 5% 4 7% 7% 8% 11% 44%	1980
	11th to 20th	Day	5% 4 2 5% 6% 5% 6% 7% 9% 11% 40%	2012
	21st to end	Day	8% 4 2 5% 5% 3 6% 6% 6% 11% 45%	2160
May	1st to 10th	Day	4 3 2 5% 6% 4 6% 6% 7% 11% 46%	2160
	11th to 20th	Day	11% 8% 3 7% 8% 5% 7% 8% 8% 9% 28%	2128
	21st to end	Day	10% 7% 3 7% 9% 6% 8% 9% 7% 9% 25%	2531
June	1st to 10th	Day	14% 8% 5% 10% 11% 5% 8% 8% 7% 9% 15%	2340
	11th to 20th	Day	8% 7% 3 9% 11% 7% 11% 10% 8% 9% 18%	2340
	21st to end	Day	8% 6% 3 9% 11% 6% 10% 9% 8% 10% 17%	2340
July	1st to 10th	Day	6% 5% 3 9% 9% 6% 9% 8% 9% 12% 24%	2340
	11th to 20th	Day	7% 5% 4 8% 10% 6% 9% 9% 10% 13% 19%	2340
	21st to end	Day	5% 4 3 7% 9% 6% 10% 10% 12% 12% 22%	2547
August	1st to 10th	Day	6% 7% 3 9% 9% 6% 10% 9% 10% 11% 21%	2281
	11th to 20th	Day	4 5% 3 7% 9% 7% 9% 10% 11% 14% 22%	2097
	21st to end	Day	9% 7% 3 9% 10% 6% 8% 9% 9% 12% 18%	2178
September	1st to 10th	Day	7% 6% 3 8% 10% 7% 10% 10% 9% 11% 19%	1980
	11th to 20th	Day	9% 8% 4 9% 10% 7% 9% 8% 7% 10% 19%	1980
	21st to end	Day	11% 10% 4 9% 11% 5% 8% 8% 7% 9% 19%	1980
October	1st to 10th	Day	9% 7% 3 8% 8% 6% 8% 9% 9% 12% 22%	1913
	11th to 20th	Day	11% 5% 2 7% 8% 5% 8% 8% 8% 10% 27%	1800
	21st to end	Day	11% 6% 3 7% 8% 4 6% 5% 6% 9% 35%	1980
November	1st to 10th	Day	9% 6% 2 5% 7% 4 7% 5% 7% 9% 41%	1857
	11th to 20th	Day	10% 6% 3 8% 6% 5% 7% 7% 5% 9% 35%	1900
	21st to end	Day	9% 5% 2 6% 6% 4 7% 7% 5% 10% 40%	1900
December	1st to 10th	Day	8% 5% 3 6% 5% 4 6% 5% 6% 10% 42%	1886
	11th to 20th	Day	9% 4 3 5% 6% 4 5% 6% 7% 10% 41%	2153
	21st to end	Day	14% 5% 3 7% 7% 4 6% 5% 7% 10% 32%	3124
January	1st to 10th	Night	6% 3 2 4 4 3 5% 6% 8% 11% 48%	2459
	11th to 20th	Night	8% 6% 3 6% 5% 3 5% 5% 7% 9% 42%	2417
	21st to end	Night	8% 4 2 6% 6% 3 4 5% 6% 10% 47%	2494
February	1st to 10th	Night	8% 3 2 5% 5% 3 4 5% 5% 10% 49%	2160
	11th to 20th	Night	5% 3 1 4 4 2 4 5% 5% 8% 59%	1989
	21st to end	Night	5% 2 1 4 3 2 4 4 5% 9% 59%	1628
March	1st to 10th	Night	5% 2 2 4 4 2 4 5% 6% 7% 58%	1980
	11th to 20th	Night	5% 2 2 3 3 2 5% 6% 5% 8% 59%	1980
	21st to end	Night	4 3 2 3 4 2 4 5% 5% 8% 60%	2178
April	1st to 10th	Night	4 2 1 3 3 2 4 5% 6% 9% 60%	1980
	11th to 20th	Night	6% 3 2 3 4 3 5% 5% 6% 11% 54%	1896
	21st to end	Night	6% 3 2 4 4 3 5% 6% 6% 10% 53%	1800
May	1st to 10th	Night	3 2 1 2 3 2 3 5% 6% 10% 63%	1800
	11th to 20th	Night	8% 3 2 5% 5% 4 6% 5% 6% 10% 44%	1770
	21st to end	Night	7% 5% 2 5% 6% 3 6% 7% 8% 12% 40%	1752
June	1st to 10th	Night	13% 7% 4 7% 6% 5% 7% 6% 9% 11% 25%	1620
	11th to 20th	Night	6% 4 3 5% 5% 5% 8% 7% 8% 13% 35%	1620
	21st to end	Night	5% 4 1 5% 6% 5% 7% 9% 10% 13% 33%	1620
July	1st to 10th	Night	4 4 2 5% 5% 3 6% 7% 9% 16% 39%	1620
	11th to 20th	Night	4 4 2 5% 5% 3 7% 7% 9% 13% 40%	1620
	21st to end	Night	2 3 2 5% 6% 4 7% 7% 8% 14% 43%	1771
August	1st to 10th	Night	4 4 3 5% 7% 4 6% 9% 10% 14% 35%	1637
	11th to 20th	Night	3 3 2 5% 6% 4 7% 7% 8% 15% 40%	1796
	21st to end	Night	7% 6% 2 6% 8% 5% 7% 7% 8% 12% 32%	2178
September	1st to 10th	Night	5% 3 2 5% 7% 4 6% 8% 10% 15% 34%	1980
	11th to 20th	Night	6% 6% 3 7% 8% 6% 7% 7% 9% 12% 30%	1980
	21st to end	Night	8% 7% 3 8% 8% 4 7% 8% 9% 10% 28%	1980
October	1st to 10th	Night	8% 5% 2 6% 7% 5% 9% 7% 9% 12% 31%	2044
	11th to 20th	Night	7% 5% 2 6% 7% 4 6% 7% 7% 12% 36%	2160
	21st to end	Night	11% 5% 3 6% 5% 3 5% 5% 7% 9% 41%	2376
November	1st to 10th	Night	8% 4 2 6% 4 3 5% 5% 6% 9% 47%	2221
	11th to 20th	Night	10% 6% 3 6% 5% 4 6% 6% 6% 8% 39%	2280
	21st to end	Night	8% 4 2 4 5% 3 5% 5% 6% 9% 49%	2280
December	1st to 10th	Night	7% 3 2 4 5% 3 5% 6% 6% 9% 48%	2288
	11th to 20th	Night	10% 5% 2 5% 4 3 5% 4 5% 8% 51%	3042
	21st to end	Night	13% 7% 3 6% 7% 4 6% 6% 6% 8% 35%	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	35% 9% 12% 22% 15% 7%	1707
	11th to 20th	Day	40% 7% 14% 21% 13% 5%	1671
	21st to end	Day	38% 8% 12% 18% 18% 6%	1858
February	1st to 10th	Day	35% 8% 11% 26% 18% 3	1800
	11th to 20th	Day	27% 7% 12% 27% 22% 5%	1863
	21st to end	Day	29% 7% 10% 24% 24% 6%	1628
March	1st to 10th	Day	28% 9% 12% 27% 20% 4	1980
	11th to 20th	Day	30% 9% 12% 28% 17% 5%	1980
	21st to end	Day	28% 7% 13% 27% 19% 5%	2178
April	1st to 10th	Day	26% 9% 17% 32% 14% 3	1980
	11th to 20th	Day	31% 11% 18% 21% 16% 3	2012
	21st to end	Day	29% 9% 14% 25% 18% 4	2160
May	1st to 10th	Day	26% 10% 14% 26% 18% 6%	2160
	11th to 20th	Day	45% 12% 18% 17% 7%	2132
	21st to end	Day	47% 14% 15% 15% 8% 1	2531
June	1st to 10th	Day	59% 15% 15% 9% 2	2340
	11th to 20th	Day	53% 17% 18% 10% 1	2340
	21st to end	Day	52% 18% 21% 8% 1	2340
July	1st to 10th	Day	42% 20% 26% 12%	2275
	11th to 20th	Day	43% 19% 26% 10%	2271
	21st to end	Day	39% 23% 26% 11% 1	2405
August	1st to 10th	Day	45% 18% 24% 11% 2	2268
	11th to 20th	Day	40% 21% 26% 12%	2097
	21st to end	Day	50% 19% 21% 10% 1	2178
September	1st to 10th	Day	47% 21% 20% 11% 1	1980
	11th to 20th	Day	53% 19% 17% 9% 1	1980
	21st to end	Day	55% 17% 14% 9% 5%	1980
October	1st to 10th	Day	48% 14% 17% 13% 6% 1	1913
	11th to 20th	Day	44% 15% 15% 14% 11% 1	1800
	21st to end	Day	43% 9% 10% 18% 14% 5%	1980
November	1st to 10th	Day	36% 9% 13% 18% 18% 5%	1857
	11th to 20th	Day	42% 10% 10% 19% 15% 4	1900
	21st to end	Day	37% 9% 12% 19% 20% 3	1900
December	1st to 10th	Day	33% 9% 13% 23% 19% 1	1886
	11th to 20th	Day	35% 8% 12% 21% 18% 7%	2150
	21st to end	Day	44% 9% 15% 16% 14% 2	3124
January	1st to 10th	Night	28% 8% 16% 26% 17% 5%	2459
	11th to 20th	Night	37% 8% 16% 24% 13% 3	2417
	21st to end	Night	33% 8% 13% 26% 15% 5%	2494
February	1st to 10th	Night	31% 7% 14% 27% 19% 3	2160
	11th to 20th	Night	23% 7% 12% 28% 22% 8%	1990
	21st to end	Night	22% 6% 14% 25% 27% 6%	1628
March	1st to 10th	Night	24% 6% 12% 31% 21% 6%	1980
	11th to 20th	Night	22% 7% 18% 33% 17% 4	1980
	21st to end	Night	22% 6% 14% 29% 25% 4	2178
April	1st to 10th	Night	20% 7% 19% 36% 15% 3	1980
	11th to 20th	Night	25% 8% 20% 26% 18% 3	1896
	21st to end	Night	25% 8% 16% 28% 20% 3	1800
May	1st to 10th	Night	17% 7% 19% 34% 19% 5%	1800
	11th to 20th	Night	33% 11% 18% 27% 11%	1771
	21st to end	Night	35% 11% 20% 22% 11% 1	1753
June	1st to 10th	Night	50% 13% 19% 15% 3	1620
	11th to 20th	Night	36% 18% 26% 17% 3	1620
	21st to end	Night	36% 18% 36% 10%	1620
July	1st to 10th	Night	30% 17% 37% 15%	1575
	11th to 20th	Night	31% 17% 36% 15% 1	1566
	21st to end	Night	27% 19% 35% 17% 2	1672
August	1st to 10th	Night	32% 20% 29% 17% 2	1626
	11th to 20th	Night	29% 18% 37% 17%	1796
	21st to end	Night	41% 17% 31% 11% 1	2178
September	1st to 10th	Night	33% 22% 29% 14% 1	1980
	11th to 20th	Night	43% 20% 25% 10% 1	1980
	21st to end	Night	46% 17% 20% 12% 5%	1980
October	1st to 10th	Night	42% 14% 21% 16% 5% 1	2044
	11th to 20th	Night	39% 12% 22% 16% 10% 2	2160
	21st to end	Night	39% 8% 12% 21% 15% 5%	2376
November	1st to 10th	Night	32% 9% 15% 24% 16% 5%	2221
	11th to 20th	Night	41% 8% 12% 23% 13% 3	2280
	21st to end	Night	31% 8% 16% 24% 18% 3	2280
December	1st to 10th	Night	29% 10% 15% 27% 17% 2	2288
	11th to 20th	Night	32% 6% 12% 23% 19% 8%	3044
	21st to end	Night	44% 10% 14% 16% 14% 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	9% 5% 33% 39% 13% 1	270
	11th to 20th	Day	15% 8% 30% 37% 10% 1	257
	21st to end	Day	10% 8% 38% 34% 9%	264
February	1st to 10th	Day	7% 7% 33% 39% 14%	240
	11th to 20th	Day	6% 3 22% 42% 23% 3	346
	21st to end	Day	5% 4 19% 42% 27% 3	346
March	1st to 10th	Day	6% 5% 30% 40% 16% 3	420
	11th to 20th	Day	6% 4 28% 35% 23% 4	420
	21st to end	Day	5% 3 23% 39% 24% 6%	462
April	1st to 10th	Day	5% 3 28% 39% 19% 7%	420
	11th to 20th	Day	4 2 27% 38% 19% 10%	412
	21st to end	Day	6% 1 22% 36% 23% 10%	420
May	1st to 10th	Day	4 1 16% 36% 28% 16%	423
	11th to 20th	Day	11% 2 14% 32% 25% 16%	447
	21st to end	Day	10% 3 6% 29% 29% 24%	485
June	1st to 10th	Day	14% 2 6% 25% 31% 21%	450
	11th to 20th	Day	8% 2 2 21% 30% 36%	450
	21st to end	Day	6% 1 1 13% 32% 48%	450
July	1st to 10th	Day	5% 2 12% 29% 52%	445
	11th to 20th	Day	5% 1 1 13% 37% 43%	444
	21st to end	Day	5% 1 1 12% 34% 47%	488
August	1st to 10th	Day	6% 2 15% 33% 43%	446
	11th to 20th	Day	5% 1 1 15% 33% 46%	439
	21st to end	Day	7% 2 3 21% 32% 35%	495
September	1st to 10th	Day	6% 3 3 13% 30% 44%	450
	11th to 20th	Day	7% 2 3 13% 31% 44%	450
	21st to end	Day	10% 2 3 18% 32% 34%	450
October	1st to 10th	Day	8% 1 6% 35% 35% 15%	450
	11th to 20th	Day	11% 2 8% 31% 32% 16%	450
	21st to end	Day	9% 3 15% 34% 29% 10%	495
November	1st to 10th	Day	8% 4 21% 35% 25% 8%	455
	11th to 20th	Day	12% 1 23% 34% 23% 6%	460
	21st to end	Day	9% 3 18% 39% 27% 4	460
December	1st to 10th	Day	9% 2 18% 39% 28% 4	446
	11th to 20th	Day	10% 3 24% 38% 21% 5%	270
	21st to end	Day	9% 5% 26% 38% 18% 3	297
January	1st to 10th	Night	8% 3 18% 41% 26% 3	760
	11th to 20th	Night	10% 3 22% 43% 22% 1	744
	21st to end	Night	8% 5% 25% 43% 19% 2	792
February	1st to 10th	Night	9% 2 19% 43% 26% 1	720
	11th to 20th	Night	6% 1 22% 44% 24% 2	703
	21st to end	Night	5% 3 16% 47% 28% 1	592
March	1st to 10th	Night	6% 3 24% 43% 22% 3	720
	11th to 20th	Night	5% 2 17% 46% 28% 2	720
	21st to end	Night	6% 2 21% 44% 26% 2	792
April	1st to 10th	Night	5% 1 15% 50% 26% 2	720
	11th to 20th	Night	6% 2 17% 47% 26% 2	711
	21st to end	Night	7% 1 14% 48% 27% 2	720
May	1st to 10th	Night	4 1 12% 44% 35% 4	720
	11th to 20th	Night	12% 1 10% 39% 34% 5%	709
	21st to end	Night	8% 1 5% 26% 46% 14%	592
June	1st to 10th	Night	16% 1 2 22% 42% 17%	540
	11th to 20th	Night	8% 9% 47% 35%	540
	21st to end	Night	4 1 8% 43% 43%	540
July	1st to 10th	Night	4 1 5% 46% 44%	525
	11th to 20th	Night	5% 1 8% 43% 43%	522
	21st to end	Night	4 1 7% 40% 49%	591
August	1st to 10th	Night	6% 1 1 6% 48% 38%	563
	11th to 20th	Night	3 2 1 12% 44% 38%	710
	21st to end	Night	8% 1 3 15% 43% 30%	792
September	1st to 10th	Night	5% 1 3 10% 41% 40%	720
	11th to 20th	Night	8% 1 2 12% 41% 35%	720
	21st to end	Night	10% 2 2 13% 42% 31%	720
October	1st to 10th	Night	11% 1 5% 30% 39% 13%	720
	11th to 20th	Night	8% 1 7% 30% 41% 13%	720
	21st to end	Night	13% 12% 35% 32% 8%	792
November	1st to 10th	Night	9% 1 19% 33% 35% 3	740
	11th to 20th	Night	13% 2 14% 32% 34% 5%	760
	21st to end	Night	9% 2 15% 41% 30% 4	760
December	1st to 10th	Night	8% 2 12% 41% 34% 3	760
	11th to 20th	Night	10% 3 14% 38% 30% 5%	760
	21st to end	Night	8% 2 19% 36% 31% 4	836

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