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 2026-04-02.

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	6% 5% 2 7% 6% 6% 6% 6% 8% 11% 37%	131031
	Night	6% 4 2 6% 4 5% 5% 5% 7% 10% 45%	130801
Transparency	Day	35% 12% 18% 22% 11% 2	134710
	Night	31% 10% 20% 23% 12% 3	135192
Seeing	Day	7% 3 16% 30% 25% 19%	17935
	Night	7% 2 12% 32% 33% 13%	28111

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	6% 4 2 5% 3 4 4 5% 6% 10% 52%	5258
	11th to 20th	Day	12% 7% 3 9% 6% 5% 6% 5% 7% 9% 30%	5280
	21st to end	Day	8% 6% 3 8% 6% 5% 5% 4 6% 7% 41%	6038
February	1st to 10th	Day	7% 6% 2 5% 5% 5% 5% 4 7% 10% 45%	2100
	11th to 20th	Day	6% 4 1 4 4 4 4 5% 7% 11% 50%	2172
	21st to end	Day	5% 4 2 5% 5% 5% 5% 4 7% 8% 50%	1903
March	1st to 10th	Day	4 3 2 5% 5% 5% 6% 6% 8% 10% 47%	5045
	11th to 20th	Day	4 2 1 4 4 3 4 3 6% 8% 60%	5090
	21st to end	Day	6% 4 2 6% 5% 5% 6% 4 7% 9% 47%	5586
April	1st to 10th	Day	5% 3 2 5% 5% 6% 6% 5% 8% 11% 46%	2211
	11th to 20th	Day	5% 3 2 6% 5% 5% 6% 5% 9% 11% 44%	2239
	21st to end	Day	7% 4 1 6% 4 6% 4 5% 7% 10% 47%	2400
May	1st to 10th	Day	2 2 1 4 3 4 5% 4 8% 12% 56%	5405
	11th to 20th	Day	7% 4 2 5% 4 4 4 4 6% 9% 52%	5422
	21st to end	Day	4 4 2 6% 6% 7% 8% 7% 10% 13% 35%	6433
June	1st to 10th	Day	13% 9% 4 11% 10% 8% 8% 6% 7% 8% 16%	2600
	11th to 20th	Day	8% 7% 3 10% 9% 10% 9% 7% 8% 10% 19%	2600
	21st to end	Day	7% 7% 3 10% 10% 8% 9% 8% 9% 11% 17%	2600
July	1st to 10th	Day	4 5% 3 9% 7% 8% 7% 7% 10% 16% 25%	5900
	11th to 20th	Day	5% 5% 3 9% 8% 8% 8% 7% 10% 13% 25%	5900
	21st to end	Day	3 3 2 5% 5% 6% 8% 7% 11% 15% 36%	6424
August	1st to 10th	Day	4 5% 2 9% 8% 9% 9% 8% 12% 13% 21%	4144
	11th to 20th	Day	5% 5% 2 8% 7% 8% 8% 8% 11% 14% 23%	3791
	21st to end	Day	5% 4 2 8% 7% 8% 9% 9% 12% 16% 19%	3949
September	1st to 10th	Day	7% 6% 3 10% 8% 9% 9% 8% 10% 11% 19%	2200
	11th to 20th	Day	9% 8% 4 11% 9% 9% 8% 6% 8% 10% 18%	2200
	21st to end	Day	10% 9% 4 11% 8% 7% 8% 6% 7% 9% 21%	2200
October	1st to 10th	Day	9% 7% 3 9% 7% 8% 8% 7% 9% 11% 22%	2125
	11th to 20th	Day	11% 5% 2 9% 6% 6% 8% 7% 8% 11% 27%	2000
	21st to end	Day	10% 6% 3 8% 6% 5% 6% 5% 7% 9% 36%	2200
November	1st to 10th	Day	8% 6% 2 6% 6% 6% 6% 4 7% 9% 39%	2057
	11th to 20th	Day	9% 6% 2 8% 5% 7% 6% 5% 6% 9% 37%	2100
	21st to end	Day	8% 6% 1 6% 5% 6% 6% 5% 6% 10% 42%	2100
December	1st to 10th	Day	5% 4 2 5% 3 4 4 4 6% 9% 55%	3331
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 4 8% 12% 38%	3463
	21st to end	Day	11% 6% 3 9% 7% 7% 6% 5% 8% 9% 30%	4565
January	1st to 10th	Night	5% 4 2 6% 4 4 4 4 7% 9% 52%	7635
	11th to 20th	Night	13% 7% 3 8% 5% 5% 5% 5% 7% 8% 34%	7680
	21st to end	Night	10% 6% 3 8% 5% 4 4 3 5% 7% 44%	8126
February	1st to 10th	Night	7% 4 2 5% 4 4 4 4 5% 9% 51%	2520
	11th to 20th	Night	5% 3 1 4 4 3 4 3 5% 8% 58%	2330
	21st to end	Night	5% 2 1 4 3 3 4 4 6% 9% 60%	1903
March	1st to 10th	Night	4 3 2 5% 4 4 5% 4 6% 9% 54%	5028
	11th to 20th	Night	4 2 1 3 3 3 4 3 5% 8% 64%	5090
	21st to end	Night	5% 3 1 5% 4 5% 5% 4 6% 8% 54%	5588
April	1st to 10th	Night	4 2 1 4 3 3 4 4 6% 9% 60%	2212
	11th to 20th	Night	5% 3 1 3 3 4 4 3 6% 10% 57%	2108
	21st to end	Night	6% 3 1 5% 3 4 5% 4 6% 10% 53%	2000
May	1st to 10th	Night	2 1 1 2 2 3 3 3 6% 10% 68%	4490
	11th to 20th	Night	5% 2 1 5% 3 4 4 3 5% 8% 58%	4490
	21st to end	Night	4 3 1 3 3 3 4 5% 8% 13% 52%	4429
June	1st to 10th	Night	13% 7% 3 8% 5% 7% 6% 5% 9% 11% 26%	1800
	11th to 20th	Night	7% 5% 2 6% 5% 7% 7% 6% 8% 13% 35%	1800
	21st to end	Night	4 4 1 6% 6% 6% 8% 8% 11% 13% 33%	1800
July	1st to 10th	Night	3 4 2 8% 5% 5% 7% 5% 10% 16% 36%	4058
	11th to 20th	Night	4 5% 2 6% 5% 5% 6% 5% 9% 15% 39%	4057
	21st to end	Night	2 3 1 4 4 4 5% 5% 7% 14% 51%	4427
August	1st to 10th	Night	3 3 2 7% 6% 7% 8% 7% 10% 14% 34%	2971
	11th to 20th	Night	4 4 2 6% 5% 5% 7% 5% 10% 14% 39%	3249
	21st to end	Night	4 4 2 6% 6% 7% 8% 7% 12% 16% 29%	3949
September	1st to 10th	Night	5% 4 2 6% 6% 6% 7% 6% 10% 14% 34%	2200
	11th to 20th	Night	5% 7% 3 8% 7% 8% 7% 6% 9% 12% 29%	2200
	21st to end	Night	7% 7% 3 9% 7% 6% 7% 6% 9% 11% 30%	2200
October	1st to 10th	Night	8% 5% 2 7% 6% 7% 8% 6% 9% 12% 30%	2272
	11th to 20th	Night	8% 5% 2 8% 5% 6% 6% 5% 7% 11% 35%	2400
	21st to end	Night	10% 5% 2 7% 4 5% 5% 4 7% 8% 42%	2640
November	1st to 10th	Night	8% 4 2 7% 4 5% 5% 5% 7% 8% 46%	2461
	11th to 20th	Night	9% 6% 2 7% 5% 5% 5% 5% 6% 8% 41%	2520
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 8% 51%	2520
December	1st to 10th	Night	6% 3 1 4 3 3 4 4 6% 9% 56%	4063
	11th to 20th	Night	9% 7% 2 7% 4 4 5% 4 6% 8% 44%	4952
	21st to end	Night	11% 7% 3 9% 6% 6% 6% 5% 6% 8% 32%	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	24% 8% 11% 34% 17% 6%	5259
	11th to 20th	Day	45% 9% 16% 19% 7% 3	5280
	21st to end	Day	38% 7% 9% 26% 16% 4	6038
February	1st to 10th	Day	31% 8% 11% 26% 19% 5%	2100
	11th to 20th	Day	26% 7% 12% 29% 22% 5%	2182
	21st to end	Day	27% 7% 10% 25% 24% 8%	1903
March	1st to 10th	Day	24% 11% 14% 26% 23% 3	5045
	11th to 20th	Day	17% 9% 14% 30% 18% 12%	5090
	21st to end	Day	28% 8% 12% 28% 19% 4	5586
April	1st to 10th	Day	25% 9% 15% 32% 16% 3	2211
	11th to 20th	Day	28% 10% 17% 22% 18% 5%	2239
	21st to end	Day	27% 9% 14% 26% 21% 4	2400
May	1st to 10th	Day	15% 9% 17% 38% 19% 2	5437
	11th to 20th	Day	26% 7% 15% 42% 9%	5426
	21st to end	Day	28% 15% 19% 32% 6%	6447
June	1st to 10th	Day	57% 15% 15% 10% 2	2600
	11th to 20th	Day	52% 16% 19% 12% 2	2600
	21st to end	Day	51% 18% 22% 9% 1	2600
July	1st to 10th	Day	39% 15% 33% 12%	5835
	11th to 20th	Day	39% 15% 33% 12% 1	5831
	21st to end	Day	25% 15% 36% 23% 2	6240
August	1st to 10th	Day	42% 18% 28% 10% 2	4131
	11th to 20th	Day	39% 17% 29% 14% 1	3827
	21st to end	Day	42% 18% 33% 6% 1	3949
September	1st to 10th	Day	47% 21% 21% 10% 1	2200
	11th to 20th	Day	53% 19% 17% 9% 1	2200
	21st to end	Day	52% 17% 14% 11% 6%	2200
October	1st to 10th	Day	46% 13% 20% 16% 4 1	3463
	11th to 20th	Day	48% 13% 16% 14% 8% 1	3240
	21st to end	Day	37% 8% 10% 27% 15% 3	3538
November	1st to 10th	Day	36% 10% 13% 18% 19% 5%	2057
	11th to 20th	Day	40% 10% 9% 19% 18% 4	2100
	21st to end	Day	35% 8% 12% 19% 21% 4	2100
December	1st to 10th	Day	24% 7% 11% 31% 23% 3	3331
	11th to 20th	Day	34% 9% 15% 22% 16% 4	3460
	21st to end	Day	45% 9% 14% 15% 15% 1	4565
January	1st to 10th	Night	26% 8% 13% 31% 19% 5%	7636
	11th to 20th	Night	45% 8% 18% 18% 8% 2	7680
	21st to end	Night	37% 7% 11% 29% 14% 3	8126
February	1st to 10th	Night	29% 7% 13% 26% 20% 5%	2520
	11th to 20th	Night	23% 7% 12% 28% 23% 7%	2331
	21st to end	Night	21% 6% 13% 25% 27% 8%	1903
March	1st to 10th	Night	24% 8% 13% 26% 23% 6%	5028
	11th to 20th	Night	17% 7% 19% 28% 18% 11%	5090
	21st to end	Night	25% 7% 14% 27% 22% 4	5588
April	1st to 10th	Night	20% 7% 17% 35% 18% 3	2212
	11th to 20th	Night	23% 7% 19% 25% 20% 5%	2108
	21st to end	Night	24% 8% 16% 29% 21% 3	2000
May	1st to 10th	Night	11% 6% 22% 44% 14% 2	4520
	11th to 20th	Night	23% 8% 15% 40% 12% 1	4491
	21st to end	Night	20% 10% 29% 33% 8%	4437
June	1st to 10th	Night	48% 13% 21% 16% 3	1800
	11th to 20th	Night	36% 17% 26% 19% 3	1800
	21st to end	Night	35% 18% 36% 11%	1800
July	1st to 10th	Night	33% 14% 40% 13%	4015
	11th to 20th	Night	32% 16% 41% 11%	4006
	21st to end	Night	22% 13% 40% 21% 3	4297
August	1st to 10th	Night	35% 17% 32% 14% 2	2960
	11th to 20th	Night	31% 15% 38% 16% 1	3279
	21st to end	Night	36% 20% 37% 7%	3949
September	1st to 10th	Night	35% 21% 30% 13% 1	2200
	11th to 20th	Night	44% 19% 25% 10% 1	2200
	21st to end	Night	45% 16% 20% 13% 5%	2200
October	1st to 10th	Night	43% 14% 21% 17% 3 1	3714
	11th to 20th	Night	48% 12% 19% 12% 9% 1	3899
	21st to end	Night	37% 8% 13% 25% 14% 3	4252
November	1st to 10th	Night	33% 9% 14% 23% 16% 5%	2461
	11th to 20th	Night	38% 8% 12% 24% 14% 4	2520
	21st to end	Night	30% 8% 15% 25% 19% 4	2520
December	1st to 10th	Night	23% 8% 13% 36% 16% 4	4063
	11th to 20th	Night	36% 8% 13% 22% 17% 5%	4954
	21st to end	Night	45% 10% 14% 16% 13% 2	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	8% 5% 34% 36% 15% 2	330
	11th to 20th	Day	15% 10% 32% 32% 9% 1	330
	21st to end	Day	11% 8% 40% 34% 7%	363
February	1st to 10th	Day	6% 7% 34% 39% 12% 2	330
	11th to 20th	Day	7% 3 25% 41% 21% 2	455
	21st to end	Day	4 7% 22% 40% 24% 3	446
March	1st to 10th	Day	6% 4 31% 41% 15% 2	540
	11th to 20th	Day	6% 5% 31% 35% 20% 3	540
	21st to end	Day	5% 4 26% 38% 22% 5%	594
April	1st to 10th	Day	4 3 31% 38% 18% 6%	504
	11th to 20th	Day	4 2 30% 37% 17% 9%	492
	21st to end	Day	5% 1 23% 38% 22% 10%	500
May	1st to 10th	Day	4 1 18% 37% 27% 14%	503
	11th to 20th	Day	9% 2 17% 34% 23% 15%	527
	21st to end	Day	9% 3 6% 30% 28% 24%	573
June	1st to 10th	Day	14% 3 6% 26% 32% 19%	530
	11th to 20th	Day	10% 2 5% 23% 28% 33%	530
	21st to end	Day	6% 2 2 15% 32% 44%	530
July	1st to 10th	Day	5% 2 1 16% 27% 48%	525
	11th to 20th	Day	4 2 2 15% 36% 40%	524
	21st to end	Day	5% 1 2 14% 34% 45%	576
August	1st to 10th	Day	6% 1 2 17% 32% 42%	526
	11th to 20th	Day	4 1 1 17% 33% 44%	519
	21st to end	Day	7% 2 4 21% 31% 36%	583
September	1st to 10th	Day	6% 4 3 16% 30% 42%	530
	11th to 20th	Day	8% 2 3 13% 32% 41%	530
	21st to end	Day	8% 3 4 21% 32% 32%	530
October	1st to 10th	Day	9% 2 7% 33% 33% 15%	530
	11th to 20th	Day	11% 3 8% 33% 31% 14%	530
	21st to end	Day	8% 3 18% 37% 27% 8%	583
November	1st to 10th	Day	8% 4 20% 38% 23% 7%	535
	11th to 20th	Day	11% 2 21% 36% 25% 6%	540
	21st to end	Day	8% 3 20% 39% 26% 4	540
December	1st to 10th	Day	8% 4 19% 38% 27% 4	524
	11th to 20th	Day	8% 4 27% 38% 19% 4	330
	21st to end	Day	9% 6% 27% 37% 17% 4	363
January	1st to 10th	Night	7% 3 19% 42% 26% 3	840
	11th to 20th	Night	10% 3 23% 42% 21% 1	840
	21st to end	Night	8% 5% 26% 43% 17% 1	924
February	1st to 10th	Night	8% 4 20% 43% 25% 1	840
	11th to 20th	Night	6% 1 22% 44% 24% 2	823
	21st to end	Night	5% 3 19% 46% 27% 1	692
March	1st to 10th	Night	5% 3 24% 45% 21% 2	840
	11th to 20th	Night	6% 2 19% 45% 26% 2	840
	21st to end	Night	6% 2 23% 43% 24% 2	924
April	1st to 10th	Night	4 2 16% 51% 25% 2	805
	11th to 20th	Night	6% 2 19% 47% 25% 1	791
	21st to end	Night	6% 2 15% 49% 27% 2	800
May	1st to 10th	Night	4 1 12% 46% 34% 4	800
	11th to 20th	Night	10% 1 12% 40% 33% 4	789
	21st to end	Night	7% 1 4 25% 48% 14%	658
June	1st to 10th	Night	15% 1 2 24% 42% 16%	600
	11th to 20th	Night	9% 1 1 10% 48% 32%	600
	21st to end	Night	4 1 8% 46% 41%	600
July	1st to 10th	Night	4 2 1 5% 47% 42%	585
	11th to 20th	Night	4 1 8% 43% 43%	582
	21st to end	Night	4 1 7% 40% 47%	657
August	1st to 10th	Night	6% 1 1 7% 48% 37%	627
	11th to 20th	Night	3 2 1 11% 45% 38%	790
	21st to end	Night	7% 1 3 14% 44% 30%	880
September	1st to 10th	Night	5% 2 3 10% 42% 39%	800
	11th to 20th	Night	8% 2 2 13% 41% 34%	800
	21st to end	Night	9% 2 4 15% 40% 29%	800
October	1st to 10th	Night	11% 1 5% 29% 40% 14%	800
	11th to 20th	Night	9% 1 7% 30% 40% 12%	800
	21st to end	Night	12% 1 13% 36% 31% 7%	880
November	1st to 10th	Night	9% 3 18% 34% 33% 3	820
	11th to 20th	Night	12% 3 13% 32% 35% 5%	840
	21st to end	Night	8% 3 15% 40% 30% 3	840
December	1st to 10th	Night	8% 3 13% 41% 33% 3	840
	11th to 20th	Night	10% 3 15% 39% 29% 4	840
	21st to end	Night	8% 2 20% 36% 30% 4	924

Page generated at 2026-03-30 02:18:41. Version ac07

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