Halfmoon Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Halfmoon Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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% 8% 3 7% 4 4 4 4 6% 7% 40%	126583
	Night	14% 8% 3 6% 4 4 4 3 5% 7% 42%	123961
Transparency	Day	41% 5% 10% 24% 17% 3	130227
	Night	41% 4 9% 24% 19% 4	128389
Seeing	Day	17% 12% 25% 24% 16% 6%	16792
	Night	18% 10% 25% 28% 16% 2	26537

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	15% 11% 7% 10% 4 4 4 4 5% 5% 30%	3617
	11th to 20th	Day	14% 8% 3 10% 5% 3 5% 3 4 5% 40%	3620
	21st to end	Day	19% 10% 4 6% 4 4 4 3 4 5% 37%	4658
February	1st to 10th	Day	18% 12% 4 6% 4 3 3 3 5% 6% 37%	2000
	11th to 20th	Day	20% 10% 4 8% 5% 4 3 3 5% 5% 36%	1948
	21st to end	Day	17% 11% 3 8% 4 4 4 3 4 6% 36%	1650
March	1st to 10th	Day	16% 14% 4 7% 4 3 3 3 4 5% 37%	4480
	11th to 20th	Day	13% 10% 2 6% 3 3 2 2 4 6% 48%	4652
	21st to end	Day	12% 8% 3 8% 5% 4 4 4 5% 6% 42%	5961
April	1st to 10th	Day	15% 6% 2 6% 5% 4 4 4 5% 7% 43%	2400
	11th to 20th	Day	15% 7% 3 7% 5% 5% 4 3 5% 6% 40%	2374
	21st to end	Day	16% 7% 2 6% 4 4 4 3 5% 8% 40%	2410
May	1st to 10th	Day	18% 10% 3 9% 5% 5% 5% 4 6% 8% 26%	5862
	11th to 20th	Day	17% 8% 3 8% 5% 5% 5% 4 6% 8% 32%	5861
	21st to end	Day	14% 7% 2 7% 5% 5% 5% 4 6% 7% 38%	6469
June	1st to 10th	Day	10% 6% 2 5% 5% 4 5% 4 6% 8% 46%	2800
	11th to 20th	Day	5% 4 2 5% 5% 5% 6% 5% 7% 10% 45%	2800
	21st to end	Day	5% 5% 2 6% 6% 5% 5% 5% 7% 11% 44%	2800
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 4 6% 11% 40%	6360
	11th to 20th	Day	6% 5% 2 6% 5% 5% 6% 5% 8% 12% 39%	6360
	21st to end	Day	8% 7% 3 8% 6% 6% 6% 5% 8% 11% 31%	6680
August	1st to 10th	Day	5% 4 2 5% 4 4 4 4 6% 9% 53%	3974
	11th to 20th	Day	8% 5% 2 7% 4 5% 6% 4 8% 10% 40%	3849
	21st to end	Day	5% 4 2 4 4 4 4 3 6% 9% 56%	4312
September	1st to 10th	Day	10% 6% 2 6% 5% 4 5% 5% 6% 8% 44%	2400
	11th to 20th	Day	10% 5% 2 6% 5% 5% 4 4 5% 7% 47%	2222
	21st to end	Day	12% 6% 2 7% 4 3 4 3 5% 6% 49%	2200
October	1st to 10th	Day	10% 5% 2 4 3 3 3 3 4 7% 57%	2200
	11th to 20th	Day	12% 5% 1 6% 4 3 4 2 4 6% 53%	2064
	21st to end	Day	20% 7% 2 5% 3 3 2 3 4 5% 46%	2030
November	1st to 10th	Day	10% 7% 2 6% 3 3 3 2 4 5% 54%	1852
	11th to 20th	Day	15% 7% 2 4 4 3 4 2 5% 6% 48%	1863
	21st to end	Day	17% 9% 3 7% 4 3 4 3 4 5% 43%	1800
December	1st to 10th	Day	15% 10% 2 6% 3 3 3 3 4 5% 44%	2930
	11th to 20th	Day	16% 12% 5% 8% 5% 4 4 3 4 6% 34%	3143
	21st to end	Day	19% 14% 5% 9% 5% 5% 5% 3 4 6% 27%	3982
January	1st to 10th	Night	16% 14% 4 9% 3 3 4 3 5% 5% 34%	6348
	11th to 20th	Night	16% 12% 4 9% 5% 5% 4 2 3 4 38%	6360
	21st to end	Night	20% 10% 4 6% 4 3 3 2 4 5% 39%	6306
February	1st to 10th	Night	21% 11% 4 8% 5% 4 3 3 4 5% 32%	2400
	11th to 20th	Night	21% 8% 4 8% 5% 4 4 3 3 5% 36%	2338
	21st to end	Night	21% 9% 4 7% 5% 4 4 3 4 5% 36%	1980
March	1st to 10th	Night	18% 12% 4 7% 3 3 4 3 4 5% 36%	5373
	11th to 20th	Night	12% 7% 2 5% 4 3 3 3 5% 6% 49%	5307
	21st to end	Night	14% 8% 3 8% 5% 4 4 3 5% 7% 40%	5010
April	1st to 10th	Night	14% 6% 2 5% 4 3 4 3 6% 7% 46%	2000
	11th to 20th	Night	15% 6% 3 5% 4 4 4 4 5% 6% 46%	1975
	21st to end	Night	14% 8% 3 5% 4 4 4 4 6% 8% 39%	1990
May	1st to 10th	Night	19% 10% 3 7% 4 4 4 4 6% 8% 31%	4033
	11th to 20th	Night	15% 8% 3 8% 5% 5% 4 4 7% 8% 32%	4032
	21st to end	Night	13% 7% 2 6% 5% 5% 5% 4 6% 8% 39%	4382
June	1st to 10th	Night	11% 6% 2 6% 3 4 5% 3 6% 8% 46%	1600
	11th to 20th	Night	6% 3 2 4 4 4 4 4 7% 9% 53%	1600
	21st to end	Night	3 3 1 4 5% 4 5% 4 8% 13% 51%	1600
July	1st to 10th	Night	7% 4 2 7% 5% 5% 5% 4 6% 9% 46%	3618
	11th to 20th	Night	6% 4 2 6% 4 4 5% 5% 7% 11% 47%	3617
	21st to end	Night	7% 6% 3 8% 5% 6% 6% 5% 8% 13% 33%	4178
August	1st to 10th	Night	5% 3 2 5% 3 3 3 3 5% 8% 59%	3171
	11th to 20th	Night	4 4 2 5% 4 5% 5% 4 7% 9% 51%	3200
	21st to end	Night	4 3 1 4 3 2 3 4 6% 8% 61%	3586
September	1st to 10th	Night	9% 5% 2 5% 4 4 3 4 5% 6% 53%	2000
	11th to 20th	Night	8% 5% 2 6% 4 3 5% 3 5% 6% 52%	2157
	21st to end	Night	12% 5% 2 4 4 3 3 3 5% 6% 53%	2200
October	1st to 10th	Night	7% 5% 2 5% 3 2 4 2 3 6% 61%	2200
	11th to 20th	Night	14% 5% 2 5% 2 3 3 2 4 4 56%	2332
	21st to end	Night	18% 8% 1 5% 2 4 3 3 4 6% 46%	2810
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 3 4 6% 52%	2666
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 5% 50%	2692
	21st to end	Night	17% 9% 3 4 4 3 3 2 5% 5% 46%	2600
December	1st to 10th	Night	18% 10% 2 8% 3 3 3 3 4 4 42%	4250
	11th to 20th	Night	20% 9% 2 6% 3 3 3 3 5% 5% 40%	5054
	21st to end	Night	22% 14% 5% 9% 4 4 4 3 4 5% 26%	6996

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	56% 4 9% 20% 9% 2	3618
	11th to 20th	Day	48% 3 5% 25% 17% 2	3620
	21st to end	Day	50% 3 8% 18% 16% 4	4658
February	1st to 10th	Day	49% 3 10% 23% 13% 2	2000
	11th to 20th	Day	52% 3 8% 20% 12% 4	1958
	21st to end	Day	50% 4 8% 18% 16% 5%	1650
March	1st to 10th	Day	51% 2 4 20% 18% 5%	4480
	11th to 20th	Day	39% 3 7% 27% 20% 4	4652
	21st to end	Day	44% 4 9% 27% 15% 1	5963
April	1st to 10th	Day	41% 4 11% 23% 19% 2	2400
	11th to 20th	Day	45% 4 8% 22% 18% 3	2374
	21st to end	Day	43% 5% 10% 27% 13% 2	2410
May	1st to 10th	Day	54% 7% 11% 18% 9% 1	5897
	11th to 20th	Day	49% 6% 13% 22% 10%	5865
	21st to end	Day	44% 6% 9% 20% 17% 3	6483
June	1st to 10th	Day	35% 6% 13% 24% 19% 3	2800
	11th to 20th	Day	32% 7% 13% 27% 19% 3	2800
	21st to end	Day	31% 8% 15% 24% 17% 5%	2800
July	1st to 10th	Day	38% 6% 11% 28% 14% 2	6291
	11th to 20th	Day	35% 6% 17% 31% 9% 1	6285
	21st to end	Day	44% 6% 19% 23% 7% 1	6489
August	1st to 10th	Day	26% 6% 14% 32% 21% 1	3961
	11th to 20th	Day	36% 6% 11% 28% 17% 1	3885
	21st to end	Day	25% 5% 10% 36% 20% 3	4311
September	1st to 10th	Day	36% 7% 10% 23% 20% 4	2400
	11th to 20th	Day	35% 5% 10% 23% 23% 2	2222
	21st to end	Day	36% 4 7% 24% 23% 6%	2200
October	1st to 10th	Day	19% 3 6% 18% 47% 7%	3590
	11th to 20th	Day	23% 3 6% 27% 31% 9%	3343
	21st to end	Day	31% 3 5% 27% 28% 6%	3255
November	1st to 10th	Day	34% 3 9% 25% 24% 5%	1852
	11th to 20th	Day	38% 3 8% 24% 21% 6%	1863
	21st to end	Day	45% 3 6% 26% 16% 4	1800
December	1st to 10th	Day	43% 3 7% 28% 17% 1	2930
	11th to 20th	Day	53% 3 8% 21% 13% 2	3140
	21st to end	Day	59% 5% 11% 18% 7%	3982
January	1st to 10th	Night	53% 3 9% 24% 11% 1	6348
	11th to 20th	Night	52% 2 5% 22% 16% 3	6360
	21st to end	Night	49% 3 6% 22% 17% 3	6306
February	1st to 10th	Night	56% 3 9% 18% 12% 1	2400
	11th to 20th	Night	52% 3 6% 22% 15% 3	2339
	21st to end	Night	51% 4 7% 18% 16% 4	1980
March	1st to 10th	Night	51% 3 5% 16% 20% 5%	5373
	11th to 20th	Night	37% 4 9% 22% 20% 9%	5307
	21st to end	Night	45% 4 9% 26% 14% 2	5010
April	1st to 10th	Night	37% 4 10% 26% 19% 3	2000
	11th to 20th	Night	39% 5% 9% 21% 21% 6%	1975
	21st to end	Night	42% 5% 10% 24% 18% 2	1990
May	1st to 10th	Night	50% 6% 11% 23% 9% 1	4060
	11th to 20th	Night	49% 7% 13% 20% 11% 1	4033
	21st to end	Night	42% 7% 12% 23% 14% 1	4390
June	1st to 10th	Night	35% 5% 12% 24% 21% 3	1600
	11th to 20th	Night	26% 6% 15% 24% 25% 4	1600
	21st to end	Night	23% 8% 16% 27% 19% 6%	1600
July	1st to 10th	Night	33% 6% 12% 27% 19% 2	3580
	11th to 20th	Night	30% 8% 19% 30% 11% 2	3572
	21st to end	Night	40% 7% 20% 24% 8% 1	4056
August	1st to 10th	Night	24% 5% 12% 37% 21% 1	3160
	11th to 20th	Night	28% 6% 16% 30% 19% 1	3230
	21st to end	Night	21% 5% 9% 37% 25% 3	3586
September	1st to 10th	Night	31% 6% 10% 25% 23% 5%	2000
	11th to 20th	Night	32% 5% 8% 27% 25% 4	2157
	21st to end	Night	32% 4 9% 21% 26% 7%	2200
October	1st to 10th	Night	19% 3 5% 16% 47% 11%	3590
	11th to 20th	Night	21% 2 5% 23% 39% 10%	3792
	21st to end	Night	30% 2 7% 23% 28% 10%	4535
November	1st to 10th	Night	34% 3 8% 25% 25% 5%	2666
	11th to 20th	Night	35% 4 6% 26% 21% 9%	2692
	21st to end	Night	41% 3 7% 25% 20% 4	2600
December	1st to 10th	Night	46% 3 8% 23% 19% 1	4250
	11th to 20th	Night	47% 2 9% 23% 18% 2	5056
	21st to end	Night	61% 4 8% 19% 8% 1	6996

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	27% 25% 32% 13% 3	300
	11th to 20th	Day	28% 27% 34% 9% 2	300
	21st to end	Day	22% 28% 39% 9% 1	330
February	1st to 10th	Day	28% 27% 31% 11% 2 1	300
	11th to 20th	Day	35% 27% 30% 7% 1	300
	21st to end	Day	33% 23% 32% 10% 2	249
March	1st to 10th	Day	28% 24% 33% 12% 3	300
	11th to 20th	Day	28% 24% 32% 12% 3	354
	21st to end	Day	28% 21% 33% 14% 5%	550
April	1st to 10th	Day	22% 21% 29% 21% 6% 1	500
	11th to 20th	Day	24% 19% 27% 20% 8% 1	492
	21st to end	Day	21% 16% 31% 21% 8% 2	500
May	1st to 10th	Day	21% 18% 29% 21% 7% 3	503
	11th to 20th	Day	18% 11% 25% 29% 12% 5%	527
	21st to end	Day	13% 7% 27% 27% 16% 9%	583
June	1st to 10th	Day	13% 5% 23% 28% 21% 10%	730
	11th to 20th	Day	7% 5% 22% 34% 24% 8%	730
	21st to end	Day	6% 5% 23% 35% 20% 10%	730
July	1st to 10th	Day	11% 3 17% 27% 25% 17%	721
	11th to 20th	Day	8% 4 18% 32% 28% 11%	718
	21st to end	Day	8% 2 16% 32% 28% 14%	689
August	1st to 10th	Day	13% 2 13% 34% 25% 14%	526
	11th to 20th	Day	14% 3 15% 32% 29% 8%	519
	21st to end	Day	8% 3 13% 29% 31% 15%	583
September	1st to 10th	Day	10% 4 18% 29% 29% 10%	530
	11th to 20th	Day	8% 4 15% 26% 34% 13%	530
	21st to end	Day	17% 8% 20% 26% 20% 8%	530
October	1st to 10th	Day	10% 5% 25% 32% 22% 6%	530
	11th to 20th	Day	14% 13% 33% 24% 12% 4	394
	21st to end	Day	26% 18% 30% 19% 6% 1	363
November	1st to 10th	Day	18% 12% 39% 20% 10% 1	330
	11th to 20th	Day	18% 20% 34% 23% 4 1	321
	21st to end	Day	24% 16% 31% 20% 8% 1	300
December	1st to 10th	Day	24% 22% 30% 17% 6% 1	300
	11th to 20th	Day	26% 17% 33% 20% 4	300
	21st to end	Day	27% 24% 33% 14% 2	330
January	1st to 10th	Night	25% 19% 31% 21% 3	1000
	11th to 20th	Night	27% 19% 34% 17% 3	1000
	21st to end	Night	23% 18% 36% 21% 2	1010
February	1st to 10th	Night	27% 18% 33% 20% 2	800
	11th to 20th	Night	29% 19% 33% 18% 2	783
	21st to end	Night	26% 19% 35% 19% 1	660
March	1st to 10th	Night	23% 18% 30% 25% 5%	800
	11th to 20th	Night	22% 16% 35% 21% 5%	800
	21st to end	Night	23% 13% 30% 28% 6%	682
April	1st to 10th	Night	18% 14% 33% 27% 8% 1	600
	11th to 20th	Night	19% 13% 30% 26% 12%	593
	21st to end	Night	18% 11% 32% 28% 10% 1	600
May	1st to 10th	Night	23% 7% 31% 24% 13% 2	600
	11th to 20th	Night	15% 6% 22% 35% 19% 3	591
	21st to end	Night	9% 5% 21% 34% 27% 4	650
June	1st to 10th	Night	13% 3 18% 35% 28% 4	600
	11th to 20th	Night	7% 2 15% 43% 31% 3	600
	21st to end	Night	4 2 17% 40% 34% 4	600
July	1st to 10th	Night	9% 1 10% 33% 38% 9%	585
	11th to 20th	Night	9% 2 10% 31% 41% 6%	582
	21st to end	Night	9% 2 12% 35% 34% 9%	657
August	1st to 10th	Night	8% 2 10% 33% 40% 7%	597
	11th to 20th	Night	7% 2 11% 40% 36% 4	594
	21st to end	Night	8% 1 8% 34% 40% 8%	660
September	1st to 10th	Night	12% 2 15% 32% 34% 5%	600
	11th to 20th	Night	11% 3 11% 31% 39% 6%	757
	21st to end	Night	13% 4 22% 30% 26% 5%	800
October	1st to 10th	Night	11% 6% 17% 33% 28% 5%	800
	11th to 20th	Night	17% 7% 26% 33% 16% 2	800
	21st to end	Night	23% 9% 30% 26% 11% 1	880
November	1st to 10th	Night	15% 10% 30% 32% 13% 1	820
	11th to 20th	Night	16% 14% 32% 29% 9%	829
	21st to end	Night	22% 14% 28% 25% 10% 1	800
December	1st to 10th	Night	27% 11% 32% 24% 6%	800
	11th to 20th	Night	26% 11% 31% 25% 6%	907
	21st to end	Night	29% 14% 30% 23% 3	1100

Page generated at 2025-11-14 23:55:54. Version ac07

Copyright © 2025 Attilla Danko.