Hepcat Sky Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hepcat Sky Observatory clear sky chart from 2005-11-06 to 2025-08-05.

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% 6% 2 6% 5% 4 5% 4 6% 8% 41%	125588
	Night	14% 6% 2 5% 4 3 4 3 5% 7% 46%	122720
Transparency	Day	39% 6% 12% 19% 16% 7%	129250
	Night	37% 5% 12% 20% 17% 9%	127121
Seeing	Day	12% 7% 18% 25% 25% 12%	15873
	Night	15% 6% 21% 34% 22% 3	25685

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	20% 10% 3 7% 3 3 3 2 3 4 42%	4077
	11th to 20th	Day	16% 6% 2 5% 4 3 3 2 3 5% 51%	4080
	21st to end	Day	31% 8% 2 5% 3 3 2 2 4 6% 35%	4488
February	1st to 10th	Day	22% 8% 3 7% 4 3 3 3 4 4 38%	1915
	11th to 20th	Day	21% 10% 3 6% 4 3 3 3 4 5% 38%	1949
	21st to end	Day	20% 8% 3 6% 4 3 4 4 4 5% 40%	1813
March	1st to 10th	Day	15% 7% 2 7% 5% 5% 5% 4 5% 7% 39%	4936
	11th to 20th	Day	14% 6% 3 7% 4 4 3 3 5% 6% 47%	4980
	21st to end	Day	18% 7% 3 7% 4 4 4 4 4 5% 40%	5462
April	1st to 10th	Day	14% 8% 3 9% 5% 5% 5% 3 4 6% 38%	2200
	11th to 20th	Day	18% 6% 2 6% 5% 4 4 4 5% 7% 39%	2314
	21st to end	Day	12% 7% 2 7% 6% 4 4 4 7% 7% 40%	2400
May	1st to 10th	Day	18% 7% 3 7% 6% 5% 5% 4 6% 7% 32%	5588
	11th to 20th	Day	11% 6% 2 6% 4 5% 5% 4 6% 8% 44%	5862
	21st to end	Day	9% 6% 3 7% 5% 5% 6% 5% 7% 10% 36%	6434
June	1st to 10th	Day	5% 4 2 7% 4 5% 6% 5% 7% 10% 44%	2600
	11th to 20th	Day	5% 3 1 5% 5% 7% 6% 6% 8% 11% 43%	2600
	21st to end	Day	6% 6% 3 8% 7% 7% 8% 7% 9% 9% 30%	2600
July	1st to 10th	Day	8% 5% 2 7% 6% 6% 6% 6% 9% 11% 33%	5900
	11th to 20th	Day	6% 6% 2 7% 5% 6% 6% 6% 9% 14% 32%	5900
	21st to end	Day	10% 7% 2 8% 5% 4 5% 5% 7% 10% 36%	6422
August	1st to 10th	Day	2 2 1 3 4 4 5% 4 7% 10% 60%	4157
	11th to 20th	Day	3 2 1 4 4 5% 6% 5% 10% 13% 46%	3916
	21st to end	Day	7% 4 2 8% 6% 7% 7% 5% 7% 9% 38%	4053
September	1st to 10th	Day	7% 6% 2 7% 6% 6% 6% 4 6% 10% 40%	2090
	11th to 20th	Day	11% 6% 2 7% 6% 6% 6% 5% 6% 7% 37%	2090
	21st to end	Day	7% 4 3 6% 5% 4 6% 4 6% 8% 47%	2090
October	1st to 10th	Day	7% 4 1 4 4 4 4 4 5% 7% 58%	1990
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 5% 6% 52%	1900
	21st to end	Day	13% 5% 2 5% 3 3 4 3 4 5% 53%	2090
November	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 3 5% 7% 45%	1958
	11th to 20th	Day	12% 6% 3 5% 4 4 4 3 5% 6% 49%	2000
	21st to end	Day	15% 7% 2 6% 6% 4 4 3 4 6% 43%	1915
December	1st to 10th	Day	23% 8% 2 6% 4 2 3 2 4 7% 37%	2940
	11th to 20th	Day	15% 8% 2 6% 4 4 4 3 5% 6% 43%	3391
	21st to end	Day	16% 6% 2 6% 4 4 3 3 4 5% 47%	4488
January	1st to 10th	Night	20% 7% 2 5% 3 3 3 2 4 6% 47%	5888
	11th to 20th	Night	16% 7% 2 4 3 3 3 2 3 5% 52%	5900
	21st to end	Night	30% 8% 2 5% 3 3 3 2 4 6% 35%	6490
February	1st to 10th	Night	21% 8% 3 6% 3 4 3 2 4 5% 40%	2500
	11th to 20th	Night	22% 8% 3 5% 3 3 2 2 4 5% 43%	2338
	21st to end	Night	22% 8% 3 5% 4 3 4 3 4 5% 41%	1815
March	1st to 10th	Night	20% 10% 2 6% 4 3 4 3 5% 6% 38%	4919
	11th to 20th	Night	18% 7% 3 6% 4 2 3 3 4 5% 45%	4980
	21st to end	Night	20% 8% 3 6% 4 4 3 2 4 5% 41%	5467
April	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 6% 6% 41%	2200
	11th to 20th	Night	17% 6% 2 6% 3 3 3 3 5% 7% 45%	2035
	21st to end	Night	14% 7% 3 6% 4 4 4 3 4 7% 44%	2000
May	1st to 10th	Night	24% 9% 3 6% 4 4 4 3 5% 8% 30%	4297
	11th to 20th	Night	12% 6% 2 6% 4 4 4 4 6% 8% 42%	4053
	21st to end	Night	13% 8% 3 7% 4 4 5% 5% 7% 10% 33%	4443
June	1st to 10th	Night	6% 3 1 4 3 3 4 4 6% 11% 55%	1800
	11th to 20th	Night	4 4 2 2 4 4 4 4 6% 12% 55%	1800
	21st to end	Night	5% 4 2 5% 5% 4 5% 5% 9% 11% 45%	1800
July	1st to 10th	Night	6% 5% 2 6% 5% 5% 5% 5% 10% 13% 40%	4078
	11th to 20th	Night	4 3 2 6% 4 5% 5% 4 6% 12% 49%	4077
	21st to end	Night	7% 6% 2 6% 4 4 5% 4 7% 8% 47%	4449
August	1st to 10th	Night	1 1 1 3 2 2 3 2 6% 10% 69%	2872
	11th to 20th	Night	2 2 1 3 2 2 4 4 8% 13% 59%	2917
	21st to end	Night	5% 3 1 4 4 5% 5% 5% 8% 12% 48%	3596
September	1st to 10th	Night	5% 5% 2 6% 5% 4 4 5% 8% 10% 47%	2090
	11th to 20th	Night	11% 6% 2 6% 5% 4 4 4 6% 7% 45%	2090
	21st to end	Night	7% 4 2 4 4 3 3 3 5% 9% 55%	2090
October	1st to 10th	Night	4 4 2 4 3 3 4 3 4 7% 62%	2171
	11th to 20th	Night	10% 4 1 4 3 2 3 2 4 6% 60%	2280
	21st to end	Night	11% 5% 2 4 3 2 3 2 4 4 60%	2508
November	1st to 10th	Night	11% 6% 2 5% 4 4 3 3 5% 6% 52%	2340
	11th to 20th	Night	13% 5% 2 4 4 3 3 3 5% 5% 52%	2400
	21st to end	Night	16% 7% 3 6% 3 3 3 3 5% 6% 45%	2481
December	1st to 10th	Night	21% 10% 4 8% 4 3 3 2 4 6% 35%	4250
	11th to 20th	Night	17% 9% 3 6% 4 3 3 3 4 5% 45%	4816
	21st to end	Night	15% 7% 2 5% 3 3 3 2 4 5% 50%	6490

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	49% 3 4 19% 18% 7%	4078
	11th to 20th	Day	38% 3 4 18% 24% 13%	4080
	21st to end	Day	53% 2 3 12% 15% 14%	4488
February	1st to 10th	Day	49% 4 5% 13% 16% 12%	1915
	11th to 20th	Day	50% 4 7% 13% 17% 10%	1959
	21st to end	Day	47% 4 8% 16% 14% 11%	1813
March	1st to 10th	Day	44% 4 9% 14% 17% 12%	4936
	11th to 20th	Day	39% 5% 7% 10% 19% 20%	4980
	21st to end	Day	46% 4 7% 19% 18% 6%	5464
April	1st to 10th	Day	48% 5% 7% 16% 15% 9%	2200
	11th to 20th	Day	45% 4 9% 15% 17% 10%	2314
	21st to end	Day	40% 7% 8% 20% 19% 6%	2400
May	1st to 10th	Day	49% 6% 10% 20% 11% 3	5623
	11th to 20th	Day	38% 5% 12% 22% 20% 3	5866
	21st to end	Day	40% 7% 12% 29% 11% 1	6447
June	1st to 10th	Day	31% 8% 19% 25% 16% 1	2600
	11th to 20th	Day	32% 9% 18% 24% 15% 2	2600
	21st to end	Day	44% 11% 17% 20% 8%	2600
July	1st to 10th	Day	41% 8% 30% 18% 2	5836
	11th to 20th	Day	39% 8% 26% 22% 5%	5831
	21st to end	Day	40% 9% 19% 24% 8% 1	6240
August	1st to 10th	Day	19% 7% 16% 40% 19%	4143
	11th to 20th	Day	26% 9% 31% 27% 6% 1	3955
	21st to end	Day	40% 11% 15% 20% 13% 1	4052
September	1st to 10th	Day	39% 10% 14% 19% 14% 5%	2090
	11th to 20th	Day	43% 8% 12% 17% 16% 4	2090
	21st to end	Day	34% 6% 12% 19% 17% 12%	2090
October	1st to 10th	Day	18% 4 6% 15% 18% 39%	3302
	11th to 20th	Day	22% 3 6% 16% 34% 18%	3140
	21st to end	Day	28% 4 4 16% 35% 13%	3428
November	1st to 10th	Day	37% 5% 10% 19% 20% 10%	1958
	11th to 20th	Day	37% 4 9% 13% 23% 14%	2000
	21st to end	Day	44% 5% 8% 16% 17% 11%	1915
December	1st to 10th	Day	49% 3 6% 16% 21% 5%	2940
	11th to 20th	Day	41% 5% 7% 15% 21% 11%	3389
	21st to end	Day	40% 4 5% 15% 24% 11%	4488
January	1st to 10th	Night	40% 3 6% 22% 19% 9%	5888
	11th to 20th	Night	37% 3 4 19% 25% 12%	5900
	21st to end	Night	52% 3 3 13% 17% 12%	6490
February	1st to 10th	Night	48% 4 6% 12% 17% 14%	2500
	11th to 20th	Night	45% 3 7% 15% 18% 12%	2338
	21st to end	Night	47% 5% 6% 13% 16% 13%	1815
March	1st to 10th	Night	47% 5% 7% 14% 18% 9%	4919
	11th to 20th	Night	42% 4 7% 14% 16% 18%	4980
	21st to end	Night	46% 4 8% 15% 19% 8%	5467
April	1st to 10th	Night	42% 6% 9% 18% 15% 10%	2200
	11th to 20th	Night	39% 5% 11% 17% 16% 13%	2035
	21st to end	Night	39% 6% 10% 20% 18% 7%	2000
May	1st to 10th	Night	53% 6% 10% 20% 9% 3	4327
	11th to 20th	Night	37% 5% 14% 22% 17% 4	4054
	21st to end	Night	44% 7% 17% 25% 7% 1	4452
June	1st to 10th	Night	22% 8% 19% 33% 17% 1	1800
	11th to 20th	Night	22% 6% 19% 31% 17% 4	1800
	21st to end	Night	29% 10% 22% 27% 11% 1	1800
July	1st to 10th	Night	34% 7% 35% 22% 3	4035
	11th to 20th	Night	28% 7% 28% 30% 6%	4026
	21st to end	Night	34% 9% 20% 29% 8%	4320
August	1st to 10th	Night	12% 6% 18% 46% 18% 1	2862
	11th to 20th	Night	15% 8% 35% 35% 7% 1	2944
	21st to end	Night	27% 14% 21% 27% 9% 1	3596
September	1st to 10th	Night	30% 10% 22% 21% 14% 5%	2090
	11th to 20th	Night	36% 7% 14% 22% 16% 5%	2090
	21st to end	Night	27% 6% 15% 22% 18% 12%	2090
October	1st to 10th	Night	16% 4 8% 18% 16% 39%	3626
	11th to 20th	Night	16% 3 5% 23% 32% 21%	3779
	21st to end	Night	21% 3 5% 19% 36% 16%	4120
November	1st to 10th	Night	33% 5% 11% 21% 21% 10%	2340
	11th to 20th	Night	34% 5% 9% 17% 21% 14%	2400
	21st to end	Night	40% 4 9% 18% 18% 11%	2481
December	1st to 10th	Night	53% 3 5% 15% 18% 6%	4250
	11th to 20th	Night	43% 3 10% 10% 20% 14%	4817
	21st to end	Night	38% 4 6% 14% 30% 9%	6490

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% 12% 33% 23% 6% 1	300
	11th to 20th	Day	25% 13% 27% 26% 8% 1	300
	21st to end	Day	24% 13% 33% 22% 6% 1	330
February	1st to 10th	Day	25% 15% 27% 24% 8%	300
	11th to 20th	Day	27% 19% 30% 19% 4 1	300
	21st to end	Day	24% 18% 35% 18% 5%	249
March	1st to 10th	Day	20% 18% 34% 21% 7%	300
	11th to 20th	Day	14% 20% 41% 18% 4 2	300
	21st to end	Day	22% 14% 32% 21% 11%	330
April	1st to 10th	Day	16% 17% 35% 25% 7%	300
	11th to 20th	Day	23% 13% 27% 24% 11% 1	432
	21st to end	Day	14% 9% 35% 28% 13% 1	500
May	1st to 10th	Day	14% 10% 32% 28% 15% 1	582
	11th to 20th	Day	14% 6% 25% 33% 20% 2	725
	21st to end	Day	9% 5% 22% 39% 21% 4	789
June	1st to 10th	Day	6% 2 11% 32% 36% 12%	730
	11th to 20th	Day	7% 2 8% 34% 36% 13%	730
	21st to end	Day	8% 2 8% 25% 35% 22%	730
July	1st to 10th	Day	8% 3 6% 23% 36% 25%	721
	11th to 20th	Day	3 1 3 19% 44% 29%	718
	21st to end	Day	5% 2 3 17% 46% 26%	794
August	1st to 10th	Day	3 1 1 16% 45% 33%	697
	11th to 20th	Day	6% 1 2 15% 44% 33%	592
	21st to end	Day	10% 2 4 16% 33% 36%	464
September	1st to 10th	Day	7% 1 6% 23% 38% 25%	300
	11th to 20th	Day	9% 2 6% 23% 39% 22%	300
	21st to end	Day	8% 2 5% 28% 34% 23%	300
October	1st to 10th	Day	6% 1 10% 41% 32% 10%	300
	11th to 20th	Day	9% 5% 17% 36% 22% 10%	300
	21st to end	Day	18% 9% 20% 23% 20% 9%	330
November	1st to 10th	Day	14% 5% 17% 39% 20% 4	300
	11th to 20th	Day	14% 7% 28% 33% 15% 3	300
	21st to end	Day	14% 18% 31% 22% 11% 4	300
December	1st to 10th	Day	17% 8% 36% 24% 11% 4	300
	11th to 20th	Day	18% 14% 31% 28% 7% 2	300
	21st to end	Day	19% 10% 34% 26% 8% 2	330
January	1st to 10th	Night	22% 9% 29% 31% 9%	1000
	11th to 20th	Night	22% 10% 28% 33% 7%	1000
	21st to end	Night	23% 9% 28% 34% 6%	1100
February	1st to 10th	Night	25% 10% 29% 31% 5%	900
	11th to 20th	Night	25% 9% 30% 30% 6%	784
	21st to end	Night	21% 15% 36% 25% 3	495
March	1st to 10th	Night	25% 9% 33% 30% 4	600
	11th to 20th	Night	18% 9% 33% 32% 7%	600
	21st to end	Night	17% 9% 35% 32% 7% 1	660
April	1st to 10th	Night	18% 9% 32% 35% 7%	600
	11th to 20th	Night	21% 7% 30% 31% 11%	593
	21st to end	Night	18% 7% 30% 34% 11%	600
May	1st to 10th	Night	14% 9% 31% 38% 8%	600
	11th to 20th	Night	13% 4 27% 41% 14% 2	591
	21st to end	Night	8% 5% 22% 41% 22% 2	650
June	1st to 10th	Night	6% 2 11% 44% 34% 3	600
	11th to 20th	Night	4 2 10% 49% 33% 2	600
	21st to end	Night	7% 2 8% 32% 44% 7%	600
July	1st to 10th	Night	5% 3 4 35% 45% 9%	585
	11th to 20th	Night	4 1 3 32% 52% 9%	582
	21st to end	Night	2 1 4 34% 51% 8%	657
August	1st to 10th	Night	1 1 2 33% 56% 7%	579
	11th to 20th	Night	4 2 30% 56% 8%	564
	21st to end	Night	4 2 3 26% 55% 11%	627
September	1st to 10th	Night	5% 1 5% 27% 52% 9%	570
	11th to 20th	Night	12% 1 5% 29% 48% 5%	570
	21st to end	Night	8% 1 8% 34% 44% 6%	570
October	1st to 10th	Night	5% 1 10% 37% 41% 6%	651
	11th to 20th	Night	11% 2 19% 40% 25% 2	760
	21st to end	Night	12% 7% 22% 35% 22% 2	836
November	1st to 10th	Night	12% 4 20% 40% 23% 2	780
	11th to 20th	Night	15% 7% 23% 37% 17% 1	800
	21st to end	Night	18% 8% 27% 32% 14% 1	881
December	1st to 10th	Night	21% 6% 25% 33% 14% 1	1000
	11th to 20th	Night	21% 11% 26% 33% 9% 1	1000
	21st to end	Night	21% 9% 29% 32% 9% 1	1100

Page generated at 2025-08-02 00:32:22. Version ac07

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