HG Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the HG Observatory clear sky chart from 2005-11-06 to 2026-03-18.

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% 10% 4 9% 5% 5% 5% 4 5% 7% 33%	131448
	Night	12% 11% 4 10% 6% 5% 5% 4 5% 6% 32%	130373
Transparency	Day	49% 5% 12% 22% 11% 1	134886
	Night	51% 5% 11% 21% 11% 1	135009
Seeing	Day	19% 13% 31% 22% 11% 3	17850
	Night	18% 10% 32% 29% 10% 1	27906

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	9% 14% 6% 19% 10% 6% 5% 3 5% 6% 17%	4691
	11th to 20th	Day	6% 17% 7% 14% 9% 7% 9% 5% 6% 6% 13%	4710
	21st to end	Day	7% 12% 8% 13% 5% 6% 8% 4 6% 8% 25%	5174
February	1st to 10th	Day	8% 14% 5% 12% 6% 7% 7% 4 6% 6% 27%	1890
	11th to 20th	Day	9% 11% 5% 12% 7% 6% 5% 4 6% 6% 28%	1902
	21st to end	Day	11% 15% 6% 12% 7% 6% 4 4 5% 6% 24%	1730
March	1st to 10th	Day	14% 9% 3 7% 5% 5% 4 3 4 6% 40%	4580
	11th to 20th	Day	11% 11% 4 11% 6% 5% 5% 5% 6% 7% 29%	4905
	21st to end	Day	22% 13% 5% 11% 4 4 4 3 3 4 27%	5874
April	1st to 10th	Day	16% 14% 5% 10% 5% 5% 5% 4 4 5% 28%	2400
	11th to 20th	Day	20% 13% 5% 9% 5% 4 4 3 4 5% 29%	2374
	21st to end	Day	19% 12% 3 9% 5% 5% 5% 3 5% 6% 30%	2526
May	1st to 10th	Day	16% 7% 2 6% 3 3 3 3 4 6% 46%	6319
	11th to 20th	Day	13% 8% 3 8% 5% 5% 5% 4 6% 7% 38%	6319
	21st to end	Day	15% 9% 3 7% 5% 4 5% 4 5% 7% 36%	6935
June	1st to 10th	Day	14% 7% 2 7% 4 4 4 4 6% 8% 39%	2800
	11th to 20th	Day	16% 7% 3 6% 5% 5% 5% 5% 6% 7% 36%	2800
	21st to end	Day	13% 8% 3 7% 5% 5% 5% 4 7% 9% 35%	2800
July	1st to 10th	Day	12% 5% 2 6% 4 4 5% 5% 7% 10% 41%	6360
	11th to 20th	Day	9% 5% 2 5% 3 4 4 4 6% 9% 48%	6360
	21st to end	Day	9% 7% 3 6% 5% 5% 5% 4 7% 8% 43%	6924
August	1st to 10th	Day	13% 7% 3 7% 4 5% 5% 4 6% 7% 38%	4563
	11th to 20th	Day	14% 5% 2 5% 3 4 4 3 5% 7% 47%	4143
	21st to end	Day	14% 9% 3 7% 4 5% 5% 3 5% 7% 38%	4312
September	1st to 10th	Day	13% 8% 2 7% 4 3 4 3 5% 8% 42%	2400
	11th to 20th	Day	14% 7% 2 7% 5% 5% 4 4 6% 8% 39%	2355
	21st to end	Day	16% 9% 3 6% 4 4 4 5% 5% 7% 37%	2200
October	1st to 10th	Day	17% 9% 3 6% 4 4 4 4 6% 7% 37%	2003
	11th to 20th	Day	20% 15% 3 8% 5% 4 4 4 5% 6% 25%	1997
	21st to end	Day	21% 15% 4 8% 4 3 5% 4 4 5% 27%	1980
November	1st to 10th	Day	15% 13% 4 9% 4 4 4 4 6% 6% 31%	1852
	11th to 20th	Day	15% 16% 5% 13% 6% 4 4 4 5% 5% 23%	1742
	21st to end	Day	15% 14% 5% 15% 7% 5% 4 4 5% 5% 20%	1680
December	1st to 10th	Day	8% 13% 8% 28% 10% 6% 4 3 4 3 12%	2690
	11th to 20th	Day	10% 21% 7% 15% 8% 5% 5% 3 4 4 18%	3088
	21st to end	Day	20% 23% 6% 11% 6% 4 4 3 4 4 15%	4070
January	1st to 10th	Night	10% 14% 6% 18% 10% 8% 5% 3 4 4 18%	8232
	11th to 20th	Night	6% 15% 7% 17% 9% 7% 7% 6% 6% 5% 14%	8280
	21st to end	Night	6% 13% 7% 16% 8% 6% 6% 4 5% 6% 25%	9026
February	1st to 10th	Night	8% 12% 7% 12% 7% 6% 6% 4 5% 6% 27%	2730
	11th to 20th	Night	8% 11% 4 12% 7% 6% 5% 4 6% 6% 31%	2604
	21st to end	Night	11% 14% 5% 10% 7% 6% 5% 4 5% 5% 28%	2076
March	1st to 10th	Night	11% 7% 3 8% 4 4 4 4 6% 8% 42%	5493
	11th to 20th	Night	10% 11% 5% 12% 6% 5% 5% 4 6% 8% 29%	5254
	21st to end	Night	20% 13% 4 9% 5% 4 4 3 5% 5% 28%	5054
April	1st to 10th	Night	19% 12% 4 8% 4 3 4 4 5% 6% 30%	2000
	11th to 20th	Night	23% 10% 3 6% 3 3 3 3 5% 6% 35%	1975
	21st to end	Night	18% 11% 4 7% 4 4 4 3 6% 6% 35%	1870
May	1st to 10th	Night	14% 9% 2 6% 4 3 4 3 5% 7% 45%	3596
	11th to 20th	Night	10% 6% 3 7% 5% 4 4 4 5% 6% 45%	3596
	21st to end	Night	14% 8% 3 8% 5% 5% 4 3 4 7% 40%	3942
June	1st to 10th	Night	13% 6% 2 6% 4 5% 4 4 6% 8% 42%	1600
	11th to 20th	Night	13% 7% 2 8% 6% 5% 4 4 6% 7% 38%	1600
	21st to end	Night	11% 6% 2 7% 4 4 5% 4 6% 9% 42%	1600
July	1st to 10th	Night	12% 6% 2 5% 3 3 5% 4 6% 9% 45%	3618
	11th to 20th	Night	10% 5% 2 4 4 3 3 3 5% 8% 53%	3617
	21st to end	Night	9% 5% 2 6% 4 4 4 3 5% 8% 49%	3947
August	1st to 10th	Night	10% 7% 3 7% 5% 5% 5% 4 6% 9% 41%	2597
	11th to 20th	Night	12% 6% 2 5% 4 3 4 3 5% 6% 50%	2907
	21st to end	Night	15% 9% 3 6% 4 3 4 3 4 6% 44%	3586
September	1st to 10th	Night	12% 6% 2 5% 4 3 3 3 5% 7% 49%	2000
	11th to 20th	Night	11% 7% 2 5% 4 4 4 4 5% 7% 47%	2025
	21st to end	Night	14% 9% 3 6% 4 4 4 3 5% 6% 42%	2200
October	1st to 10th	Night	15% 8% 2 6% 4 4 4 4 4 7% 43%	2395
	11th to 20th	Night	17% 13% 4 7% 4 4 4 4 4 8% 32%	2402
	21st to end	Night	19% 13% 3 8% 6% 4 5% 4 5% 6% 28%	2860
November	1st to 10th	Night	16% 13% 4 8% 4 4 4 3 4 7% 34%	2666
	11th to 20th	Night	14% 15% 6% 12% 5% 4 4 3 4 5% 28%	2876
	21st to end	Night	13% 15% 6% 13% 7% 5% 4 3 4 6% 23%	2940
December	1st to 10th	Night	10% 13% 7% 17% 10% 6% 5% 4 5% 6% 17%	4720
	11th to 20th	Night	11% 19% 8% 15% 7% 5% 5% 4 4 4 19%	5339
	21st to end	Night	22% 19% 5% 12% 6% 5% 4 3 4 4 17%	7150

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	67% 3 9% 15% 5%	4692
	11th to 20th	Day	70% 4 7% 13% 7%	4710
	21st to end	Day	56% 4 9% 20% 10%	5174
February	1st to 10th	Day	56% 5% 10% 17% 12%	1890
	11th to 20th	Day	53% 6% 10% 22% 8% 1	1911
	21st to end	Day	60% 5% 9% 17% 8% 1	1730
March	1st to 10th	Day	45% 3 14% 19% 15% 3	4580
	11th to 20th	Day	51% 5% 14% 17% 11% 1	4905
	21st to end	Day	61% 4 8% 15% 12%	5876
April	1st to 10th	Day	57% 5% 10% 17% 11% 1	2400
	11th to 20th	Day	56% 5% 9% 20% 9% 1	2374
	21st to end	Day	55% 5% 9% 17% 12% 1	2526
May	1st to 10th	Day	40% 4 10% 28% 17% 1	6357
	11th to 20th	Day	43% 6% 12% 25% 12% 2	6323
	21st to end	Day	46% 6% 9% 22% 16% 1	6949
June	1st to 10th	Day	41% 7% 13% 26% 11% 1	2800
	11th to 20th	Day	45% 8% 13% 21% 13% 1	2800
	21st to end	Day	42% 9% 17% 22% 10% 1	2800
July	1st to 10th	Day	37% 6% 16% 32% 8%	6291
	11th to 20th	Day	31% 6% 17% 33% 11% 1	6285
	21st to end	Day	38% 6% 13% 29% 15%	6728
August	1st to 10th	Day	42% 6% 16% 22% 12% 1	4548
	11th to 20th	Day	36% 7% 13% 33% 12%	4184
	21st to end	Day	45% 5% 14% 25% 10% 1	4311
September	1st to 10th	Day	40% 6% 14% 28% 12%	2400
	11th to 20th	Day	41% 8% 17% 27% 7%	2355
	21st to end	Day	45% 7% 12% 26% 10%	2200
October	1st to 10th	Day	34% 5% 13% 29% 18%	3263
	11th to 20th	Day	52% 4 11% 19% 14%	3225
	21st to end	Day	52% 4 17% 21% 6%	3180
November	1st to 10th	Day	52% 6% 11% 21% 10% 1	1852
	11th to 20th	Day	62% 5% 10% 16% 7%	1742
	21st to end	Day	63% 6% 13% 14% 3 1	1680
December	1st to 10th	Day	76% 4 7% 10% 3	2690
	11th to 20th	Day	69% 5% 8% 11% 7%	3085
	21st to end	Day	73% 4 5% 12% 7%	4070
January	1st to 10th	Night	68% 4 9% 14% 5%	8233
	11th to 20th	Night	69% 5% 8% 11% 7%	8280
	21st to end	Night	59% 4 7% 18% 11%	9026
February	1st to 10th	Night	55% 5% 8% 20% 11% 1	2730
	11th to 20th	Night	51% 5% 11% 21% 10% 1	2605
	21st to end	Night	56% 4 13% 18% 8% 1	2076
March	1st to 10th	Night	39% 4 17% 22% 15% 2	5493
	11th to 20th	Night	52% 5% 14% 20% 9% 1	5254
	21st to end	Night	58% 4 9% 17% 11% 1	5054
April	1st to 10th	Night	53% 5% 9% 17% 15% 1	2000
	11th to 20th	Night	51% 4 9% 20% 13% 2	1975
	21st to end	Night	49% 5% 11% 20% 14% 1	1870
May	1st to 10th	Night	39% 5% 13% 24% 16% 2	3620
	11th to 20th	Night	37% 7% 13% 22% 18% 2	3597
	21st to end	Night	43% 7% 12% 21% 16% 1	3950
June	1st to 10th	Night	39% 6% 14% 29% 13% 1	1600
	11th to 20th	Night	43% 7% 13% 22% 14% 2	1600
	21st to end	Night	37% 9% 15% 28% 10% 1	1600
July	1st to 10th	Night	35% 6% 19% 28% 10% 2	3580
	11th to 20th	Night	29% 6% 17% 35% 12% 1	3572
	21st to end	Night	33% 5% 16% 27% 17% 1	3832
August	1st to 10th	Night	40% 7% 15% 21% 16% 1	2588
	11th to 20th	Night	34% 7% 14% 30% 13% 2	2932
	21st to end	Night	42% 5% 14% 25% 13% 2	3586
September	1st to 10th	Night	34% 6% 13% 33% 14% 1	2000
	11th to 20th	Night	35% 7% 15% 34% 9%	2025
	21st to end	Night	40% 7% 13% 27% 12% 1	2200
October	1st to 10th	Night	32% 4 9% 32% 20% 2	3915
	11th to 20th	Night	41% 5% 10% 30% 13% 1	3913
	21st to end	Night	55% 5% 10% 20% 10%	4610
November	1st to 10th	Night	50% 5% 12% 22% 11%	2666
	11th to 20th	Night	57% 5% 13% 16% 8% 1	2876
	21st to end	Night	62% 6% 11% 16% 5% 1	2940
December	1st to 10th	Night	67% 5% 12% 13% 3	4720
	11th to 20th	Night	67% 4 8% 13% 8%	5341
	21st to end	Night	70% 4 6% 13% 7%	7150

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	16% 17% 34% 25% 7% 2	330
	11th to 20th	Day	20% 16% 37% 21% 5% 1	330
	21st to end	Day	18% 17% 39% 21% 5%	363
February	1st to 10th	Day	20% 15% 45% 15% 5% 2	330
	11th to 20th	Day	16% 22% 42% 14% 5%	330
	21st to end	Day	19% 22% 47% 11% 1	273
March	1st to 10th	Day	20% 19% 40% 16% 5%	330
	11th to 20th	Day	22% 19% 36% 15% 7% 2	321
	21st to end	Day	27% 14% 32% 18% 7% 2	509
April	1st to 10th	Day	26% 15% 32% 22% 5%	500
	11th to 20th	Day	30% 17% 25% 20% 8% 1	492
	21st to end	Day	25% 14% 29% 21% 9% 2	506
May	1st to 10th	Day	23% 16% 30% 22% 8% 2	703
	11th to 20th	Day	20% 14% 30% 23% 9% 3	725
	21st to end	Day	21% 14% 26% 24% 11% 3	789
June	1st to 10th	Day	14% 14% 32% 25% 13% 3	730
	11th to 20th	Day	19% 11% 33% 24% 9% 3	730
	21st to end	Day	15% 12% 32% 24% 13% 5%	730
July	1st to 10th	Day	9% 6% 27% 31% 20% 8%	721
	11th to 20th	Day	13% 9% 29% 27% 16% 7%	718
	21st to end	Day	8% 7% 29% 30% 18% 8%	794
August	1st to 10th	Day	13% 6% 26% 26% 23% 6%	726
	11th to 20th	Day	16% 6% 23% 26% 23% 6%	658
	21st to end	Day	16% 9% 25% 26% 17% 6%	583
September	1st to 10th	Day	19% 12% 22% 24% 18% 6%	530
	11th to 20th	Day	13% 8% 29% 30% 16% 3	530
	21st to end	Day	22% 12% 21% 27% 16% 3	530
October	1st to 10th	Day	20% 16% 29% 24% 8% 4	333
	11th to 20th	Day	24% 23% 34% 14% 6%	330
	21st to end	Day	31% 15% 36% 15% 2 1	363
November	1st to 10th	Day	25% 15% 36% 18% 5% 2	330
	11th to 20th	Day	26% 20% 38% 11% 5% 1	330
	21st to end	Day	22% 21% 41% 13% 3	330
December	1st to 10th	Day	18% 14% 44% 19% 5%	330
	11th to 20th	Day	24% 18% 40% 15% 3	330
	21st to end	Day	28% 22% 33% 14% 3 1	363
January	1st to 10th	Night	16% 12% 40% 26% 6%	1050
	11th to 20th	Night	19% 11% 38% 25% 5% 1	1050
	21st to end	Night	17% 10% 38% 29% 6%	1155
February	1st to 10th	Night	15% 13% 39% 26% 7%	1050
	11th to 20th	Night	14% 15% 40% 25% 6%	970
	21st to end	Night	19% 13% 39% 24% 4	692
March	1st to 10th	Night	16% 13% 36% 27% 7% 1	840
	11th to 20th	Night	17% 15% 35% 26% 6% 1	714
	21st to end	Night	21% 10% 30% 28% 10% 1	660
April	1st to 10th	Night	25% 9% 34% 26% 6% 1	600
	11th to 20th	Night	27% 10% 27% 28% 7%	593
	21st to end	Night	23% 9% 31% 29% 6% 1	600
May	1st to 10th	Night	21% 10% 33% 30% 7%	600
	11th to 20th	Night	18% 10% 31% 32% 8% 1	591
	21st to end	Night	16% 8% 27% 36% 12% 1	650
June	1st to 10th	Night	14% 8% 26% 40% 12%	600
	11th to 20th	Night	16% 7% 30% 36% 10%	600
	21st to end	Night	14% 5% 24% 42% 14% 1	600
July	1st to 10th	Night	10% 2 21% 41% 24% 2	585
	11th to 20th	Night	12% 4 27% 36% 20% 1	582
	21st to end	Night	8% 2 24% 44% 20% 3	657
August	1st to 10th	Night	11% 3 22% 40% 24% 2	597
	11th to 20th	Night	14% 4 19% 35% 26% 3	594
	21st to end	Night	14% 6% 20% 35% 23% 2	660
September	1st to 10th	Night	15% 7% 23% 35% 17% 4	600
	11th to 20th	Night	13% 4 28% 35% 18% 2	625
	21st to end	Night	18% 9% 26% 33% 13% 1	800
October	1st to 10th	Night	19% 9% 31% 27% 14% 1	800
	11th to 20th	Night	23% 13% 32% 25% 8%	800
	21st to end	Night	25% 9% 31% 26% 8%	880
November	1st to 10th	Night	21% 11% 37% 25% 5% 1	820
	11th to 20th	Night	20% 16% 36% 22% 6%	986
	21st to end	Night	21% 16% 35% 23% 5%	1050
December	1st to 10th	Night	19% 14% 36% 24% 6% 1	1050
	11th to 20th	Night	21% 13% 36% 22% 7% 1	1050
	21st to end	Night	28% 12% 34% 22% 4	1155

Page generated at 2026-03-15 01:23:56. Version ac07

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