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 2024-01-12.

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% 7% 3 6% 5% 3 5% 5% 5% 7% 41%	74836
	Night	14% 7% 3 5% 5% 3 4 4 5% 7% 43%	74897
Transparency	Day	42% 6% 11% 23% 16% 3	74544
	Night	41% 5% 10% 23% 18% 4	74710
Seeing	Day	16% 11% 25% 24% 16% 7%	14233
	Night	18% 10% 25% 28% 17% 2	24128

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	18% 8% 5% 8% 4 2 4 4 4 5% 37%	1517
	11th to 20th	Day	21% 9% 5% 8% 7% 3 5% 4 5% 5% 29%	1453
	21st to end	Day	15% 9% 3 6% 6% 4 4 4 4 5% 41%	1849
February	1st to 10th	Day	18% 11% 4 6% 5% 2 3 4 5% 6% 36%	1800
	11th to 20th	Day	20% 10% 4 7% 5% 3 3 4 5% 5% 35%	1748
	21st to end	Day	18% 11% 4 8% 5% 3 5% 4 4 6% 34%	1480
March	1st to 10th	Day	17% 9% 3 6% 5% 3 3 5% 4 6% 39%	1800
	11th to 20th	Day	21% 10% 4 7% 5% 2 3 3 4 5% 35%	1848
	21st to end	Day	18% 9% 4 8% 5% 3 5% 5% 5% 6% 32%	2370
April	1st to 10th	Day	13% 5% 2 6% 6% 3 5% 4 5% 8% 44%	2160
	11th to 20th	Day	16% 7% 3 6% 6% 4 4 4 5% 6% 39%	2134
	21st to end	Day	17% 6% 3 5% 5% 2 4 4 5% 8% 40%	2168
May	1st to 10th	Day	17% 6% 3 6% 5% 3 5% 5% 5% 7% 38%	2340
	11th to 20th	Day	15% 7% 3 6% 6% 3 4 4 5% 6% 39%	2301
	21st to end	Day	8% 5% 3 7% 7% 4 6% 6% 8% 9% 37%	2539
June	1st to 10th	Day	9% 5% 2 4 5% 2 5% 5% 6% 8% 48%	2520
	11th to 20th	Day	5% 3 2 4 6% 4 6% 7% 7% 9% 47%	2520
	21st to end	Day	6% 4 2 5% 6% 3 6% 6% 7% 11% 44%	2520
July	1st to 10th	Day	8% 5% 3 7% 7% 5% 7% 6% 7% 11% 34%	2520
	11th to 20th	Day	7% 6% 2 4 5% 4 6% 6% 6% 10% 44%	2520
	21st to end	Day	7% 5% 3 5% 6% 4 6% 6% 7% 11% 39%	2649
August	1st to 10th	Day	7% 4 2 5% 6% 4 5% 7% 7% 9% 43%	2187
	11th to 20th	Day	10% 5% 2 6% 6% 3 6% 6% 6% 10% 40%	2125
	21st to end	Day	8% 5% 2 5% 6% 3 6% 4 6% 9% 45%	2376
September	1st to 10th	Day	10% 5% 2 5% 6% 3 6% 6% 5% 8% 43%	2160
	11th to 20th	Day	11% 5% 2 5% 6% 4 4 5% 5% 7% 47%	2000
	21st to end	Day	10% 5% 2 5% 4 2 4 4 5% 6% 52%	1980
October	1st to 10th	Day	10% 5% 2 4 4 2 3 4 4 8% 55%	1980
	11th to 20th	Day	13% 5% 2 6% 5% 3 4 4 4 6% 50%	1858
	21st to end	Day	20% 7% 2 4 4 2 3 3 5% 5% 45%	1828
November	1st to 10th	Day	10% 6% 2 5% 3 3 3 3 4 5% 56%	1672
	11th to 20th	Day	16% 6% 2 4 4 2 3 4 4 6% 48%	1710
	21st to end	Day	17% 8% 3 6% 4 2 3 5% 4 5% 43%	1710
December	1st to 10th	Day	19% 10% 3 6% 5% 3 4 4 4 6% 37%	1710
	11th to 20th	Day	15% 12% 4 6% 6% 3 4 5% 5% 5% 35%	2001
	21st to end	Day	16% 14% 5% 7% 6% 4 5% 5% 3 6% 29%	2783
January	1st to 10th	Night	18% 10% 4 6% 5% 3 4 4 4 5% 37%	2648
	11th to 20th	Night	20% 12% 4 8% 6% 3 4 4 4 5% 30%	2547
	21st to end	Night	17% 9% 3 7% 5% 3 4 3 5% 6% 38%	2502
February	1st to 10th	Night	22% 11% 4 7% 5% 3 4 3 3 5% 32%	2160
	11th to 20th	Night	20% 8% 4 6% 6% 3 4 4 3 5% 37%	2098
	21st to end	Night	22% 9% 4 6% 5% 2 4 4 4 5% 34%	1776
March	1st to 10th	Night	16% 8% 3 5% 4 3 3 4 4 6% 43%	2160
	11th to 20th	Night	18% 9% 4 6% 5% 3 4 4 5% 5% 38%	2111
	21st to end	Night	18% 9% 4 6% 6% 3 4 4 5% 6% 35%	2000
April	1st to 10th	Night	12% 5% 2 5% 4 2 4 5% 6% 8% 49%	1800
	11th to 20th	Night	16% 6% 3 4 4 3 4 5% 4 6% 46%	1775
	21st to end	Night	15% 7% 3 5% 5% 3 4 5% 6% 8% 40%	1792
May	1st to 10th	Night	17% 5% 3 5% 4 2 5% 3 5% 6% 43%	1620
	11th to 20th	Night	12% 6% 3 6% 6% 3 4 5% 7% 9% 40%	1592
	21st to end	Night	6% 5% 1 4 6% 4 6% 6% 7% 11% 44%	1731
June	1st to 10th	Night	12% 6% 2 5% 4 2 4 4 6% 8% 47%	1440
	11th to 20th	Night	5% 2 2 4 4 3 4 5% 7% 8% 56%	1440
	21st to end	Night	2 2 2 4 5% 3 5% 6% 8% 13% 50%	1440
July	1st to 10th	Night	7% 3 2 5% 6% 3 6% 6% 6% 11% 46%	1440
	11th to 20th	Night	7% 4 3 4 4 2 5% 5% 6% 11% 48%	1440
	21st to end	Night	8% 3 2 4 5% 3 6% 6% 6% 10% 47%	1665
August	1st to 10th	Night	7% 4 2 4 5% 3 4 5% 7% 10% 50%	1747
	11th to 20th	Night	6% 5% 2 5% 5% 3 4 4 7% 9% 49%	1776
	21st to end	Night	7% 4 2 4 4 2 4 5% 6% 9% 53%	1980
September	1st to 10th	Night	10% 5% 2 4 4 2 3 4 5% 6% 53%	1800
	11th to 20th	Night	9% 5% 2 5% 5% 2 5% 4 5% 6% 52%	1941
	21st to end	Night	11% 4 2 4 4 2 3 4 4 7% 55%	1980
October	1st to 10th	Night	8% 5% 2 4 4 2 4 3 3 6% 59%	1980
	11th to 20th	Night	16% 5% 2 4 3 2 3 3 4 5% 52%	2098
	21st to end	Night	18% 7% 2 5% 3 3 4 3 4 6% 45%	2528
November	1st to 10th	Night	12% 6% 2 4 4 3 3 3 4 5% 54%	2406
	11th to 20th	Night	14% 5% 2 4 4 3 3 4 5% 6% 50%	2470
	21st to end	Night	17% 8% 3 4 4 2 3 3 5% 5% 46%	2470
December	1st to 10th	Night	20% 10% 3 5% 4 3 4 4 5% 5% 36%	2470
	11th to 20th	Night	17% 9% 3 6% 4 2 4 4 5% 5% 40%	3190
	21st to end	Night	18% 13% 5% 8% 6% 4 5% 5% 4 6% 27%	4884

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	50% 4 7% 24% 12% 3	1518
	11th to 20th	Day	57% 4 9% 20% 9% 1	1453
	21st to end	Day	47% 3 10% 26% 12% 2	1849
February	1st to 10th	Day	50% 3 9% 24% 12% 2	1800
	11th to 20th	Day	53% 3 8% 21% 11% 4	1758
	21st to end	Day	52% 4 8% 17% 14% 5%	1480
March	1st to 10th	Day	47% 3 7% 22% 17% 4	1800
	11th to 20th	Day	53% 3 10% 21% 11% 2	1848
	21st to end	Day	53% 5% 10% 21% 10% 1	2370
April	1st to 10th	Day	39% 5% 11% 24% 19% 2	2160
	11th to 20th	Day	45% 4 8% 20% 18% 3	2134
	21st to end	Day	43% 5% 10% 27% 13% 2	2168
May	1st to 10th	Day	44% 7% 12% 25% 11% 1	2340
	11th to 20th	Day	45% 6% 11% 22% 14% 1	2305
	21st to end	Day	41% 9% 16% 21% 12% 2	2539
June	1st to 10th	Day	33% 6% 13% 24% 20% 3	2520
	11th to 20th	Day	30% 7% 14% 28% 19% 3	2520
	21st to end	Day	32% 9% 15% 23% 16% 5%	2520
July	1st to 10th	Day	41% 10% 12% 21% 12% 4	2451
	11th to 20th	Day	34% 7% 16% 25% 15% 3	2445
	21st to end	Day	36% 8% 17% 26% 12% 2	2502
August	1st to 10th	Day	34% 9% 14% 28% 14% 1	2174
	11th to 20th	Day	38% 8% 11% 24% 16% 2	2125
	21st to end	Day	34% 7% 13% 25% 17% 5%	2376
September	1st to 10th	Day	37% 7% 10% 24% 18% 4	2160
	11th to 20th	Day	37% 5% 10% 23% 22% 3	2000
	21st to end	Day	33% 4 7% 26% 24% 6%	1980
October	1st to 10th	Day	29% 4 10% 24% 27% 5%	1980
	11th to 20th	Day	36% 4 6% 24% 22% 7%	1858
	21st to end	Day	42% 4 6% 24% 19% 4	1828
November	1st to 10th	Day	32% 3 9% 25% 25% 5%	1672
	11th to 20th	Day	38% 4 8% 23% 21% 6%	1710
	21st to end	Day	44% 4 6% 26% 16% 4	1710
December	1st to 10th	Day	49% 4 10% 22% 12% 2	1710
	11th to 20th	Day	51% 4 9% 22% 13% 2	1998
	21st to end	Day	56% 6% 11% 17% 10%	2783
January	1st to 10th	Night	50% 3 9% 21% 15% 2	2648
	11th to 20th	Night	57% 3 8% 19% 12% 1	2547
	21st to end	Night	47% 3 11% 24% 13% 2	2502
February	1st to 10th	Night	57% 3 9% 19% 12% 1	2160
	11th to 20th	Night	51% 3 6% 23% 14% 3	2099
	21st to end	Night	54% 4 8% 16% 14% 4	1776
March	1st to 10th	Night	44% 4 8% 17% 21% 6%	2160
	11th to 20th	Night	49% 4 11% 21% 13% 3	2111
	21st to end	Night	49% 5% 11% 22% 12% 2	2000
April	1st to 10th	Night	34% 5% 11% 27% 20% 3	1800
	11th to 20th	Night	39% 5% 10% 20% 21% 6%	1775
	21st to end	Night	41% 5% 10% 24% 18% 2	1792
May	1st to 10th	Night	42% 4 10% 25% 16% 2	1620
	11th to 20th	Night	40% 8% 13% 23% 15% 1	1593
	21st to end	Night	31% 9% 17% 23% 17% 3	1732
June	1st to 10th	Night	35% 5% 12% 24% 22% 3	1440
	11th to 20th	Night	24% 7% 16% 26% 23% 4	1440
	21st to end	Night	23% 9% 16% 27% 18% 6%	1440
July	1st to 10th	Night	30% 9% 15% 26% 16% 4	1400
	11th to 20th	Night	28% 8% 18% 26% 16% 5%	1392
	21st to end	Night	30% 9% 14% 31% 15% 2	1572
August	1st to 10th	Night	29% 8% 15% 27% 21% 1	1736
	11th to 20th	Night	30% 8% 15% 27% 20% 2	1776
	21st to end	Night	27% 8% 12% 28% 21% 5%	1980
September	1st to 10th	Night	31% 6% 10% 27% 22% 5%	1800
	11th to 20th	Night	32% 5% 8% 27% 24% 5%	1941
	21st to end	Night	30% 4 9% 23% 27% 7%	1980
October	1st to 10th	Night	28% 4 8% 23% 30% 7%	1980
	11th to 20th	Night	36% 3 7% 22% 25% 7%	2098
	21st to end	Night	42% 3 6% 23% 21% 5%	2528
November	1st to 10th	Night	33% 3 7% 25% 26% 6%	2406
	11th to 20th	Night	35% 4 6% 24% 22% 9%	2470
	21st to end	Night	40% 3 8% 25% 20% 4	2470
December	1st to 10th	Night	50% 4 10% 21% 13% 2	2470
	11th to 20th	Night	46% 3 8% 22% 18% 3	3192
	21st to end	Night	58% 5% 8% 17% 10% 2	4884

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	26% 25% 32% 14% 4	270
	11th to 20th	Day	28% 23% 34% 12% 2	245
	21st to end	Day	21% 29% 40% 9% 1	264
February	1st to 10th	Day	30% 27% 31% 10% 1 1	240
	11th to 20th	Day	35% 27% 31% 7%	240
	21st to end	Day	33% 23% 31% 11% 2	198
March	1st to 10th	Day	26% 25% 35% 12% 2	240
	11th to 20th	Day	31% 23% 31% 12% 4	288
	21st to end	Day	28% 19% 32% 15% 6%	462
April	1st to 10th	Day	18% 21% 29% 25% 7% 1	420
	11th to 20th	Day	22% 17% 28% 22% 9% 2	412
	21st to end	Day	23% 15% 29% 22% 8% 3	420
May	1st to 10th	Day	22% 16% 30% 22% 7% 3	423
	11th to 20th	Day	17% 11% 24% 30% 13% 5%	447
	21st to end	Day	11% 6% 24% 30% 18% 11%	493
June	1st to 10th	Day	12% 5% 21% 28% 23% 11%	630
	11th to 20th	Day	5% 5% 23% 34% 24% 9%	630
	21st to end	Day	6% 5% 24% 35% 20% 10%	630
July	1st to 10th	Day	10% 3 16% 26% 25% 19%	621
	11th to 20th	Day	8% 4 17% 29% 30% 12%	618
	21st to end	Day	8% 2 16% 32% 28% 15%	590
August	1st to 10th	Day	14% 2 13% 33% 25% 14%	446
	11th to 20th	Day	15% 3 13% 33% 29% 8%	439
	21st to end	Day	9% 3 12% 27% 34% 15%	495
September	1st to 10th	Day	10% 4 16% 29% 30% 12%	450
	11th to 20th	Day	9% 3 14% 27% 33% 14%	450
	21st to end	Day	15% 8% 19% 26% 22% 10%	450
October	1st to 10th	Day	10% 6% 26% 31% 21% 6%	450
	11th to 20th	Day	15% 14% 34% 24% 10% 3	328
	21st to end	Day	27% 18% 28% 19% 7% 1	297
November	1st to 10th	Day	16% 10% 39% 21% 12% 1	270
	11th to 20th	Day	18% 20% 33% 23% 4 1	270
	21st to end	Day	23% 17% 31% 21% 8% 1	270
December	1st to 10th	Day	23% 23% 29% 18% 6% 1	270
	11th to 20th	Day	24% 17% 34% 20% 4	270
	21st to end	Day	26% 25% 33% 14% 2	297
January	1st to 10th	Night	25% 19% 31% 22% 3	950
	11th to 20th	Night	27% 17% 34% 18% 4	910
	21st to end	Night	23% 18% 35% 22% 2	909
February	1st to 10th	Night	27% 18% 32% 21% 2	720
	11th to 20th	Night	27% 19% 34% 18% 2	703
	21st to end	Night	28% 19% 34% 18% 1	592
March	1st to 10th	Night	22% 18% 30% 25% 5%	720
	11th to 20th	Night	23% 15% 34% 21% 6%	720
	21st to end	Night	24% 12% 28% 30% 6%	614
April	1st to 10th	Night	15% 13% 35% 29% 8% 1	540
	11th to 20th	Night	19% 11% 30% 26% 14%	533
	21st to end	Night	19% 10% 31% 29% 10% 1	540
May	1st to 10th	Night	21% 7% 32% 24% 14% 2	540
	11th to 20th	Night	14% 6% 23% 34% 20% 3	531
	21st to end	Night	8% 3 20% 34% 30% 4	584
June	1st to 10th	Night	13% 3 16% 34% 30% 4	540
	11th to 20th	Night	5% 1 14% 45% 31% 4	540
	21st to end	Night	3 2 18% 39% 34% 4	540
July	1st to 10th	Night	9% 10% 32% 38% 10%	525
	11th to 20th	Night	9% 3 11% 28% 43% 7%	522
	21st to end	Night	9% 2 12% 34% 34% 9%	591
August	1st to 10th	Night	8% 2 8% 34% 41% 7%	537
	11th to 20th	Night	7% 2 10% 40% 36% 4	534
	21st to end	Night	9% 1 8% 33% 41% 8%	594
September	1st to 10th	Night	13% 2 13% 31% 36% 5%	540
	11th to 20th	Night	11% 2 11% 29% 40% 6%	681
	21st to end	Night	11% 4 23% 29% 28% 5%	720
October	1st to 10th	Night	12% 6% 18% 32% 27% 5%	720
	11th to 20th	Night	18% 7% 27% 32% 15% 2	720
	21st to end	Night	23% 9% 29% 27% 12% 1	792
November	1st to 10th	Night	15% 9% 28% 33% 14% 1	740
	11th to 20th	Night	16% 13% 32% 28% 10%	760
	21st to end	Night	22% 13% 28% 25% 10% 1	760
December	1st to 10th	Night	26% 11% 32% 24% 6%	760
	11th to 20th	Night	25% 11% 31% 26% 6%	861
	21st to end	Night	29% 14% 30% 24% 3	1045

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