Heavens View Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Heavens View Observatory clear sky chart from 2005-11-06 to 2025-10-04.

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	13% 11% 4 9% 5% 5% 5% 4 6% 7% 33%	125702
	Night	13% 12% 4 9% 5% 4 4 3 5% 6% 33%	123769
Transparency	Day	49% 4 12% 21% 12% 1	129270
	Night	49% 4 11% 22% 12% 1	128270
Seeing	Day	19% 12% 29% 23% 12% 5%	16672
	Night	20% 10% 30% 28% 11% 1	26330

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% 17% 7% 18% 5% 4 4 3 4 5% 23%	3617
	11th to 20th	Day	13% 18% 6% 18% 10% 10% 5% 3 4 5% 9%	3620
	21st to end	Day	20% 19% 6% 14% 6% 5% 6% 3 4 4 15%	4351
February	1st to 10th	Day	13% 14% 6% 14% 9% 5% 5% 3 4 5% 22%	1800
	11th to 20th	Day	13% 14% 6% 13% 8% 6% 5% 3 5% 5% 22%	1884
	21st to end	Day	12% 17% 5% 13% 7% 5% 5% 4 5% 4 24%	1650
March	1st to 10th	Day	14% 12% 5% 10% 6% 4 4 3 4 5% 31%	4586
	11th to 20th	Day	11% 13% 5% 10% 5% 4 4 3 5% 8% 32%	4980
	21st to end	Day	17% 16% 4 10% 5% 4 4 3 4 5% 28%	5465
April	1st to 10th	Day	16% 14% 4 8% 5% 5% 4 4 5% 6% 30%	2330
	11th to 20th	Day	16% 10% 3 8% 4 4 4 4 5% 7% 34%	2542
	21st to end	Day	16% 9% 4 8% 5% 5% 4 4 5% 7% 34%	2600
May	1st to 10th	Day	18% 10% 3 7% 4 4 4 3 5% 7% 34%	5862
	11th to 20th	Day	15% 9% 3 8% 5% 5% 5% 4 6% 8% 34%	5861
	21st to end	Day	14% 8% 3 6% 4 4 4 3 5% 8% 41%	6474
June	1st to 10th	Day	10% 5% 2 7% 5% 4 4 5% 7% 9% 40%	2920
	11th to 20th	Day	10% 6% 3 7% 5% 4 5% 4 6% 8% 41%	3000
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 5% 8% 10% 37%	2887
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 5% 5% 8% 10% 39%	6360
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 5% 6% 9% 46%	6360
	21st to end	Day	8% 6% 2 6% 5% 4 5% 4 7% 10% 44%	6570
August	1st to 10th	Day	12% 5% 3 6% 5% 6% 5% 4 6% 8% 41%	4234
	11th to 20th	Day	10% 7% 2 7% 5% 4 6% 5% 7% 8% 40%	4174
	21st to end	Day	8% 6% 3 7% 5% 5% 6% 5% 7% 9% 39%	4642
September	1st to 10th	Day	10% 7% 2 7% 4 6% 5% 5% 7% 8% 40%	2265
	11th to 20th	Day	10% 4 2 5% 4 4 3 4 7% 8% 48%	2200
	21st to end	Day	16% 7% 3 6% 4 5% 5% 4 7% 8% 35%	2200
October	1st to 10th	Day	13% 7% 3 7% 5% 4 5% 4 5% 6% 41%	2068
	11th to 20th	Day	17% 10% 3 7% 6% 5% 5% 3 5% 6% 32%	1900
	21st to end	Day	19% 12% 4 8% 5% 4 5% 3 5% 7% 28%	1929
November	1st to 10th	Day	12% 9% 4 9% 5% 4 4 4 6% 7% 35%	1762
	11th to 20th	Day	14% 14% 5% 9% 6% 5% 4 4 6% 6% 28%	1800
	21st to end	Day	16% 16% 6% 8% 5% 5% 5% 3 5% 6% 25%	1665
December	1st to 10th	Day	13% 17% 7% 13% 6% 5% 5% 4 5% 7% 20%	2600
	11th to 20th	Day	16% 22% 7% 10% 5% 4 4 4 5% 4 19%	2704
	21st to end	Day	22% 22% 7% 14% 5% 4 3 2 4 4 13%	3840
January	1st to 10th	Night	13% 13% 6% 13% 7% 5% 5% 4 4 5% 25%	6348
	11th to 20th	Night	10% 16% 7% 16% 10% 8% 6% 3 5% 5% 14%	6360
	21st to end	Night	16% 16% 9% 11% 6% 5% 5% 3 4 5% 20%	6608
February	1st to 10th	Night	15% 16% 5% 11% 7% 7% 5% 4 4 5% 22%	2600
	11th to 20th	Night	13% 13% 5% 12% 7% 5% 5% 4 6% 5% 26%	2402
	21st to end	Night	16% 12% 3 10% 7% 5% 5% 3 4 6% 28%	1980
March	1st to 10th	Night	14% 12% 4 10% 5% 4 4 3 5% 6% 34%	5266
	11th to 20th	Night	10% 9% 3 7% 4 5% 5% 4 6% 8% 39%	4980
	21st to end	Night	15% 11% 4 8% 5% 4 4 3 4 5% 37%	5467
April	1st to 10th	Night	17% 12% 3 7% 4 4 4 4 5% 6% 34%	2086
	11th to 20th	Night	17% 9% 2 7% 4 4 4 3 5% 7% 37%	1807
	21st to end	Night	16% 8% 2 7% 5% 5% 4 4 6% 7% 38%	1800
May	1st to 10th	Night	19% 11% 4 8% 4 4 4 3 5% 6% 33%	4033
	11th to 20th	Night	16% 9% 3 7% 4 4 3 3 4 6% 40%	4032
	21st to end	Night	14% 7% 2 5% 4 4 4 4 5% 7% 44%	4377
June	1st to 10th	Night	11% 5% 2 6% 4 3 4 3 7% 9% 46%	1470
	11th to 20th	Night	10% 6% 3 5% 4 5% 5% 4 7% 9% 43%	1400
	21st to end	Night	9% 6% 2 6% 4 4 4 3 5% 8% 48%	1505
July	1st to 10th	Night	7% 6% 2 5% 4 4 4 4 7% 11% 46%	3618
	11th to 20th	Night	9% 5% 2 5% 4 4 4 3 6% 9% 49%	3617
	21st to end	Night	8% 6% 2 6% 4 4 4 4 5% 9% 48%	4283
August	1st to 10th	Night	11% 5% 2 5% 3 4 4 3 5% 8% 48%	2915
	11th to 20th	Night	9% 7% 2 5% 4 4 4 4 5% 8% 47%	2875
	21st to end	Night	8% 6% 2 7% 4 4 4 4 5% 8% 49%	3251
September	1st to 10th	Night	10% 5% 3 7% 4 4 4 3 6% 7% 48%	2115
	11th to 20th	Night	9% 4 2 5% 3 3 4 3 5% 7% 55%	2200
	21st to end	Night	13% 9% 2 6% 5% 4 4 3 5% 7% 42%	2200
October	1st to 10th	Night	13% 7% 3 7% 5% 3 3 4 4 6% 45%	2178
	11th to 20th	Night	18% 11% 3 8% 4 5% 4 3 4 5% 34%	2280
	21st to end	Night	20% 10% 3 8% 6% 5% 4 3 5% 6% 30%	2668
November	1st to 10th	Night	13% 10% 3 6% 4 4 4 3 5% 6% 41%	2536
	11th to 20th	Night	15% 12% 4 8% 6% 5% 5% 5% 6% 6% 29%	2600
	21st to end	Night	18% 16% 4 9% 5% 3 4 3 5% 7% 25%	2731
December	1st to 10th	Night	11% 16% 8% 14% 6% 5% 5% 4 6% 6% 21%	4580
	11th to 20th	Night	14% 21% 7% 13% 7% 5% 3 3 3 5% 19%	5483
	21st to end	Night	21% 23% 8% 12% 5% 4 4 2 3 4 15%	7118

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	64% 4 10% 15% 7% 1	3618
	11th to 20th	Day	76% 4 8% 7% 4	3620
	21st to end	Day	73% 3 10% 9% 5% 1	4351
February	1st to 10th	Day	64% 5% 8% 15% 8%	1800
	11th to 20th	Day	64% 4 8% 15% 9% 1	1893
	21st to end	Day	61% 4 9% 16% 8% 2	1650
March	1st to 10th	Day	56% 3 12% 18% 9% 2	4586
	11th to 20th	Day	50% 4 14% 21% 9% 2	4980
	21st to end	Day	58% 4 11% 15% 10% 2	5465
April	1st to 10th	Day	54% 5% 10% 20% 10% 1	2330
	11th to 20th	Day	49% 4 10% 22% 13% 2	2542
	21st to end	Day	49% 5% 13% 19% 13% 1	2600
May	1st to 10th	Day	50% 4 11% 19% 13% 4	5897
	11th to 20th	Day	48% 5% 12% 22% 12% 1	5865
	21st to end	Day	42% 4 10% 21% 19% 4	6488
June	1st to 10th	Day	38% 5% 12% 27% 15% 2	2920
	11th to 20th	Day	40% 5% 13% 25% 16% 2	3000
	21st to end	Day	39% 7% 12% 24% 16% 1	2887
July	1st to 10th	Day	38% 5% 15% 26% 16% 1	6291
	11th to 20th	Day	33% 5% 14% 30% 17% 1	6285
	21st to end	Day	34% 4 15% 24% 22% 1	6383
August	1st to 10th	Day	39% 5% 13% 25% 17% 1	4221
	11th to 20th	Day	39% 6% 11% 32% 11% 1	4213
	21st to end	Day	39% 5% 14% 28% 13% 1	4641
September	1st to 10th	Day	40% 6% 13% 28% 12% 2	2265
	11th to 20th	Day	33% 4 13% 35% 15% 1	2200
	21st to end	Day	45% 5% 12% 25% 13% 1	2200
October	1st to 10th	Day	32% 4 10% 33% 19% 2	3419
	11th to 20th	Day	41% 4 11% 26% 18%	3140
	21st to end	Day	47% 4 13% 24% 11% 1	3153
November	1st to 10th	Day	46% 5% 11% 25% 11% 1	1762
	11th to 20th	Day	56% 4 12% 18% 9% 1	1800
	21st to end	Day	59% 4 11% 18% 7% 1	1665
December	1st to 10th	Day	62% 5% 16% 14% 3	2600
	11th to 20th	Day	67% 3 10% 12% 7%	2700
	21st to end	Day	76% 3 7% 8% 5%	3840
January	1st to 10th	Night	58% 4 9% 19% 7% 2	6348
	11th to 20th	Night	69% 5% 9% 11% 5%	6360
	21st to end	Night	66% 3 9% 15% 6%	6608
February	1st to 10th	Night	64% 5% 8% 14% 8% 1	2600
	11th to 20th	Night	58% 5% 8% 18% 10% 1	2403
	21st to end	Night	57% 3 11% 18% 10% 1	1980
March	1st to 10th	Night	50% 3 13% 23% 10% 1	5266
	11th to 20th	Night	41% 5% 14% 24% 15% 1	4980
	21st to end	Night	50% 3 10% 21% 14% 2	5467
April	1st to 10th	Night	51% 3 9% 22% 13% 2	2086
	11th to 20th	Night	46% 4 10% 20% 18% 2	1807
	21st to end	Night	44% 5% 11% 22% 16% 3	1800
May	1st to 10th	Night	52% 3 10% 17% 13% 5%	4060
	11th to 20th	Night	46% 4 10% 26% 11% 3	4033
	21st to end	Night	38% 4 11% 25% 17% 5%	4385
June	1st to 10th	Night	34% 5% 10% 30% 20% 2	1470
	11th to 20th	Night	36% 5% 14% 26% 18% 2	1400
	21st to end	Night	34% 5% 13% 26% 20% 1	1505
July	1st to 10th	Night	32% 4 14% 28% 17% 4	3580
	11th to 20th	Night	32% 4 14% 31% 18% 1	3572
	21st to end	Night	33% 6% 14% 23% 23% 2	4157
August	1st to 10th	Night	34% 5% 15% 29% 17% 1	2904
	11th to 20th	Night	34% 5% 13% 35% 11% 2	2902
	21st to end	Night	34% 5% 12% 33% 15% 1	3251
September	1st to 10th	Night	35% 4 12% 30% 16% 2	2115
	11th to 20th	Night	28% 5% 12% 32% 21% 2	2200
	21st to end	Night	41% 3 12% 29% 14% 1	2200
October	1st to 10th	Night	27% 3 10% 33% 26% 1	3607
	11th to 20th	Night	45% 3 11% 23% 18%	3779
	21st to end	Night	44% 3 11% 29% 12% 1	4394
November	1st to 10th	Night	42% 5% 11% 26% 15% 1	2536
	11th to 20th	Night	53% 5% 13% 20% 9% 1	2600
	21st to end	Night	58% 4 13% 19% 6% 1	2731
December	1st to 10th	Night	61% 4 16% 15% 3	4580
	11th to 20th	Night	67% 5% 9% 13% 5% 1	5486
	21st to end	Night	73% 4 8% 8% 6%	7118

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	24% 19% 38% 14% 5%	300
	11th to 20th	Day	24% 15% 46% 12% 2 1	300
	21st to end	Day	32% 14% 35% 14% 5% 1	330
February	1st to 10th	Day	28% 22% 34% 13% 2 1	300
	11th to 20th	Day	26% 23% 37% 13% 1	300
	21st to end	Day	29% 28% 31% 10% 2	249
March	1st to 10th	Day	28% 21% 34% 12% 4	341
	11th to 20th	Day	25% 13% 33% 22% 6% 1	500
	21st to end	Day	30% 13% 34% 18% 4 1	550
April	1st to 10th	Day	22% 16% 36% 18% 7% 1	500
	11th to 20th	Day	27% 17% 31% 15% 5% 4	492
	21st to end	Day	25% 14% 28% 23% 9% 2	500
May	1st to 10th	Day	24% 16% 27% 21% 9% 2	503
	11th to 20th	Day	20% 10% 29% 25% 11% 6%	527
	21st to end	Day	17% 10% 23% 27% 15% 7%	588
June	1st to 10th	Day	14% 8% 31% 29% 15% 3	730
	11th to 20th	Day	15% 8% 30% 27% 16% 3	730
	21st to end	Day	13% 7% 30% 30% 15% 5%	730
July	1st to 10th	Day	10% 5% 23% 33% 19% 10%	721
	11th to 20th	Day	11% 6% 27% 31% 18% 7%	718
	21st to end	Day	11% 4 23% 33% 21% 9%	644
August	1st to 10th	Day	10% 3 21% 28% 24% 14%	526
	11th to 20th	Day	15% 3 21% 27% 23% 11%	519
	21st to end	Day	12% 5% 22% 28% 21% 12%	583
September	1st to 10th	Day	14% 6% 24% 23% 19% 13%	530
	11th to 20th	Day	11% 3 19% 25% 27% 15%	530
	21st to end	Day	20% 10% 22% 21% 19% 8%	530
October	1st to 10th	Day	19% 9% 27% 22% 17% 6%	502
	11th to 20th	Day	20% 16% 30% 26% 7% 1	490
	21st to end	Day	29% 15% 36% 15% 5% 1	378
November	1st to 10th	Day	21% 19% 35% 18% 6% 1	300
	11th to 20th	Day	25% 22% 33% 15% 4 2	300
	21st to end	Day	27% 18% 39% 13% 3	233
December	1st to 10th	Day	25% 23% 32% 14% 7% 1	200
	11th to 20th	Day	28% 27% 36% 8% 2 1	200
	21st to end	Day	29% 24% 34% 12% 1	298
January	1st to 10th	Night	22% 17% 36% 20% 5% 1	1000
	11th to 20th	Night	24% 15% 36% 21% 4 1	1000
	21st to end	Night	21% 12% 38% 25% 5%	933
February	1st to 10th	Night	25% 16% 35% 22% 3	800
	11th to 20th	Night	23% 16% 33% 23% 5%	783
	21st to end	Night	26% 17% 36% 19% 2	660
March	1st to 10th	Night	22% 14% 34% 23% 6%	800
	11th to 20th	Night	19% 15% 36% 24% 6% 1	800
	21st to end	Night	24% 11% 30% 27% 7% 1	880
April	1st to 10th	Night	23% 14% 35% 23% 4 1	686
	11th to 20th	Night	22% 12% 34% 22% 7% 2	593
	21st to end	Night	21% 8% 33% 30% 7%	600
May	1st to 10th	Night	21% 5% 33% 32% 7% 1	600
	11th to 20th	Night	21% 5% 29% 32% 12% 1	591
	21st to end	Night	13% 4 26% 39% 16% 2	650
June	1st to 10th	Night	12% 7% 24% 43% 13%	470
	11th to 20th	Night	12% 4 29% 42% 13% 1	400
	21st to end	Night	11% 5% 22% 43% 17% 1	505
July	1st to 10th	Night	11% 2 18% 40% 26% 3	585
	11th to 20th	Night	11% 3 21% 41% 21% 4	582
	21st to end	Night	7% 3 18% 50% 21% 2	657
August	1st to 10th	Night	10% 2 15% 44% 27% 3	597
	11th to 20th	Night	11% 3 19% 34% 29% 4	594
	21st to end	Night	11% 3 19% 35% 29% 3	660
September	1st to 10th	Night	12% 6% 20% 33% 24% 5%	715
	11th to 20th	Night	12% 4 17% 34% 30% 4	800
	21st to end	Night	17% 7% 20% 31% 20% 4	800
October	1st to 10th	Night	19% 8% 24% 29% 18% 1	773
	11th to 20th	Night	26% 11% 28% 26% 8% 1	760
	21st to end	Night	27% 12% 32% 24% 5%	836
November	1st to 10th	Night	21% 12% 29% 26% 11% 1	780
	11th to 20th	Night	23% 15% 34% 20% 6% 1	800
	21st to end	Night	27% 14% 35% 19% 5%	800
December	1st to 10th	Night	22% 16% 36% 21% 5% 1	800
	11th to 20th	Night	23% 16% 38% 20% 3	940
	21st to end	Night	31% 13% 37% 17% 2	1100

Page generated at 2025-10-01 01:39:04. Version ac07

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