Halfmoon Stream Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Halfmoon Stream Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	18% 9% 3 7% 4 4 4 4 5% 7% 36%	130773
	Night	17% 9% 3 7% 4 4 4 3 5% 7% 36%	131714
Transparency	Day	48% 3 11% 22% 13% 2	134294
	Night	48% 3 11% 23% 14% 2	136281
Seeing	Day	25% 19% 31% 18% 6% 1	17449
	Night	23% 16% 32% 22% 6% 1	26027

By Month

Percentage Cloud Cover NE (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	13% 8% 4 7% 6% 6% 6% 4 5% 6% 36%	4661
	11th to 20th	Day	15% 11% 3 6% 4 4 4 4 5% 5% 40%	4650
	21st to end	Day	10% 9% 3 7% 4 4 4 4 4 5% 46%	5139
February	1st to 10th	Day	16% 10% 3 6% 4 5% 5% 4 5% 6% 36%	1890
	11th to 20th	Day	12% 9% 3 7% 6% 5% 5% 4 5% 5% 37%	1984
	21st to end	Day	14% 10% 3 9% 5% 5% 4 4 5% 5% 37%	1730
March	1st to 10th	Day	22% 13% 3 9% 5% 4 4 3 4 4 29%	4576
	11th to 20th	Day	17% 12% 4 9% 5% 4 4 2 4 5% 34%	5081
	21st to end	Day	27% 9% 3 7% 3 3 4 4 6% 6% 29%	5618
April	1st to 10th	Day	20% 12% 3 7% 5% 3 3 4 4 5% 32%	2520
	11th to 20th	Day	19% 9% 2 6% 5% 3 4 4 6% 7% 35%	2653
	21st to end	Day	22% 9% 3 7% 4 4 4 3 4 6% 33%	2730
May	1st to 10th	Day	29% 11% 3 8% 4 4 4 3 5% 5% 24%	5899
	11th to 20th	Day	19% 10% 3 8% 5% 5% 4 4 5% 6% 31%	5875
	21st to end	Day	18% 8% 2 6% 5% 4 5% 4 6% 8% 32%	7013
June	1st to 10th	Day	23% 8% 3 6% 5% 5% 5% 3 5% 7% 31%	3000
	11th to 20th	Day	19% 7% 3 6% 4 4 5% 4 6% 7% 36%	3000
	21st to end	Day	24% 8% 3 6% 4 4 4 4 6% 7% 30%	3000
July	1st to 10th	Day	16% 8% 2 8% 5% 5% 5% 5% 8% 9% 30%	6372
	11th to 20th	Day	14% 6% 2 6% 4 5% 5% 4 7% 9% 37%	6360
	21st to end	Day	15% 6% 2 6% 4 4 4 4 6% 8% 41%	6886
August	1st to 10th	Day	17% 7% 3 7% 5% 5% 5% 4 6% 8% 34%	4235
	11th to 20th	Day	15% 9% 3 8% 5% 5% 4 4 6% 8% 33%	4175
	21st to end	Day	10% 6% 2 5% 4 4 5% 4 5% 9% 44%	4595
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 6% 7% 49%	2320
	11th to 20th	Day	12% 4 2 5% 4 4 4 2 5% 6% 53%	2200
	21st to end	Day	16% 6% 2 5% 4 3 4 3 5% 8% 44%	2200
October	1st to 10th	Day	16% 7% 3 7% 5% 4 4 3 5% 6% 41%	2165
	11th to 20th	Day	18% 6% 2 7% 4 4 4 2 4 7% 41%	1960
	21st to end	Day	20% 10% 3 7% 4 4 4 3 5% 5% 36%	1980
November	1st to 10th	Day	16% 8% 2 6% 4 4 5% 3 5% 7% 40%	1851
	11th to 20th	Day	18% 8% 2 5% 4 3 4 3 6% 6% 42%	1890
	21st to end	Day	19% 10% 2 5% 4 3 4 3 3 5% 41%	1696
December	1st to 10th	Day	16% 12% 2 4 3 3 3 3 5% 7% 40%	2452
	11th to 20th	Day	18% 8% 2 5% 3 4 3 3 4 6% 43%	2688
	21st to end	Day	21% 12% 4 6% 3 4 4 3 5% 4 35%	3729
January	1st to 10th	Night	11% 10% 3 9% 5% 5% 5% 3 5% 6% 38%	8234
	11th to 20th	Night	15% 13% 3 9% 6% 5% 5% 4 5% 6% 30%	8235
	21st to end	Night	10% 10% 3 7% 4 5% 5% 4 5% 7% 39%	9039
February	1st to 10th	Night	15% 9% 4 7% 5% 5% 5% 4 5% 6% 34%	2730
	11th to 20th	Night	12% 8% 4 8% 6% 5% 4 3 5% 5% 38%	2520
	21st to end	Night	13% 12% 2 8% 5% 4 4 3 5% 6% 39%	2076
March	1st to 10th	Night	26% 13% 3 7% 3 4 4 3 4 5% 29%	5484
	11th to 20th	Night	16% 11% 3 7% 5% 5% 5% 4 6% 7% 31%	5095
	21st to end	Night	24% 10% 2 7% 4 4 4 4 6% 6% 29%	5552
April	1st to 10th	Night	21% 12% 3 7% 4 3 4 3 5% 5% 33%	2100
	11th to 20th	Night	19% 8% 3 8% 4 4 4 4 6% 7% 33%	1914
	21st to end	Night	22% 10% 3 6% 4 3 4 3 5% 6% 34%	1890
May	1st to 10th	Night	28% 10% 4 7% 4 4 3 2 5% 7% 25%	4064
	11th to 20th	Night	19% 10% 3 8% 5% 5% 5% 4 6% 7% 28%	4033
	21st to end	Night	15% 8% 3 7% 5% 5% 5% 3 6% 8% 34%	3877
June	1st to 10th	Night	21% 8% 2 7% 4 5% 4 4 6% 8% 31%	1400
	11th to 20th	Night	16% 8% 3 5% 5% 4 4 3 6% 8% 39%	1400
	21st to end	Night	22% 9% 3 6% 4 4 4 4 6% 8% 30%	1400
July	1st to 10th	Night	12% 7% 3 7% 5% 5% 6% 5% 7% 9% 35%	3570
	11th to 20th	Night	14% 8% 2 7% 5% 5% 4 4 6% 8% 36%	3600
	21st to end	Night	10% 5% 2 6% 4 4 5% 4 6% 9% 44%	3926
August	1st to 10th	Night	19% 7% 3 7% 5% 5% 5% 4 5% 7% 31%	2910
	11th to 20th	Night	14% 9% 3 7% 4 5% 6% 4 6% 7% 35%	2872
	21st to end	Night	10% 6% 2 6% 4 4 4 3 6% 8% 46%	3287
September	1st to 10th	Night	13% 5% 2 5% 3 3 4 4 5% 6% 51%	2080
	11th to 20th	Night	14% 6% 2 4 3 2 3 2 4 7% 53%	2200
	21st to end	Night	15% 7% 2 5% 4 4 4 3 4 7% 45%	2200
October	1st to 10th	Night	17% 5% 2 7% 4 4 4 4 5% 6% 42%	2234
	11th to 20th	Night	17% 6% 2 5% 4 4 4 3 5% 7% 42%	2438
	21st to end	Night	21% 10% 2 6% 4 4 4 3 5% 5% 36%	2860
November	1st to 10th	Night	18% 7% 2 6% 4 3 4 3 5% 6% 42%	2667
	11th to 20th	Night	17% 6% 3 5% 4 4 5% 3 5% 7% 40%	2730
	21st to end	Night	17% 9% 3 5% 5% 4 4 3 5% 7% 39%	2921
December	1st to 10th	Night	16% 11% 3 7% 4 4 5% 4 6% 7% 36%	4956
	11th to 20th	Night	22% 9% 2 4 4 3 3 3 5% 5% 40%	5741
	21st to end	Night	20% 11% 3 7% 3 3 3 3 4 5% 37%	7479

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% 2 10% 26% 12% 1	4662
	11th to 20th	Day	46% 2 9% 28% 13% 2	4680
	21st to end	Day	42% 2 10% 22% 19% 6%	5139
February	1st to 10th	Day	49% 3 9% 22% 14% 3	1890
	11th to 20th	Day	47% 4 10% 22% 15% 2	1993
	21st to end	Day	49% 3 9% 21% 13% 5%	1730
March	1st to 10th	Day	60% 3 6% 10% 15% 6%	4579
	11th to 20th	Day	54% 2 12% 19% 11% 2	5081
	21st to end	Day	56% 2 4 18% 17% 4	5618
April	1st to 10th	Day	54% 3 7% 16% 16% 4	2520
	11th to 20th	Day	48% 3 8% 23% 16% 2	2653
	21st to end	Day	52% 3 7% 20% 14% 4	2730
May	1st to 10th	Day	64% 3 10% 15% 7% 1	5910
	11th to 20th	Day	55% 3 11% 20% 8% 3	5879
	21st to end	Day	50% 3 13% 21% 13% 2	7013
June	1st to 10th	Day	53% 3 11% 22% 10% 1	3000
	11th to 20th	Day	46% 4 11% 21% 15% 2	3000
	21st to end	Day	53% 5% 11% 20% 10% 2	3000
July	1st to 10th	Day	48% 5% 14% 23% 9%	6302
	11th to 20th	Day	43% 5% 14% 23% 14% 1	6285
	21st to end	Day	40% 4 13% 28% 14% 1	6688
August	1st to 10th	Day	48% 4 14% 27% 7%	4222
	11th to 20th	Day	49% 5% 17% 22% 8%	4214
	21st to end	Day	36% 4 12% 28% 19% 1	4595
September	1st to 10th	Day	34% 4 14% 29% 18% 1	2320
	11th to 20th	Day	34% 3 9% 30% 22% 2	2200
	21st to end	Day	40% 3 10% 28% 18% 1	2200
October	1st to 10th	Day	52% 3 8% 24% 12% 1	3529
	11th to 20th	Day	36% 2 10% 31% 20%	3176
	21st to end	Day	37% 2 13% 26% 19% 3	3180
November	1st to 10th	Day	44% 3 13% 23% 15% 2	1851
	11th to 20th	Day	43% 4 9% 23% 18% 3	1890
	21st to end	Day	46% 3 14% 24% 11% 1	1696
December	1st to 10th	Day	44% 2 14% 26% 13% 1	2452
	11th to 20th	Day	43% 2 11% 19% 17% 7%	2688
	21st to end	Day	53% 2 9% 19% 12% 4	3729
January	1st to 10th	Night	48% 1 11% 26% 13% 1	8234
	11th to 20th	Night	54% 3 10% 19% 12% 2	8280
	21st to end	Night	45% 2 6% 23% 18% 5%	9039
February	1st to 10th	Night	50% 3 10% 21% 14% 2	2730
	11th to 20th	Night	47% 3 9% 22% 17% 2	2522
	21st to end	Night	47% 3 8% 22% 16% 4	2076
March	1st to 10th	Night	58% 3 5% 13% 14% 7%	5494
	11th to 20th	Night	52% 2 16% 20% 9% 1	5096
	21st to end	Night	55% 2 5% 17% 19% 3	5552
April	1st to 10th	Night	54% 3 6% 17% 17% 3	2100
	11th to 20th	Night	50% 3 8% 22% 16% 1	1914
	21st to end	Night	51% 3 10% 18% 15% 3	1890
May	1st to 10th	Night	61% 2 12% 16% 8% 1	4070
	11th to 20th	Night	55% 2 11% 22% 8% 1	4034
	21st to end	Night	49% 3 14% 23% 10% 1	3878
June	1st to 10th	Night	52% 4 11% 23% 10% 1	1400
	11th to 20th	Night	45% 3 14% 24% 12% 2	1400
	21st to end	Night	52% 4 12% 20% 9% 2	1400
July	1st to 10th	Night	45% 5% 15% 25% 9%	3530
	11th to 20th	Night	47% 4 14% 25% 10%	3552
	21st to end	Night	36% 4 17% 32% 10% 1	3809
August	1st to 10th	Night	53% 5% 13% 22% 7%	2899
	11th to 20th	Night	48% 4 16% 22% 10%	2899
	21st to end	Night	36% 4 14% 27% 17% 1	3287
September	1st to 10th	Night	34% 4 14% 32% 17% 1	2080
	11th to 20th	Night	33% 2 12% 31% 21% 2	2200
	21st to end	Night	40% 3 9% 30% 16% 2	2200
October	1st to 10th	Night	42% 3 13% 26% 15% 1	3650
	11th to 20th	Night	34% 2 10% 31% 21% 1	3961
	21st to end	Night	39% 3 11% 29% 16% 2	4610
November	1st to 10th	Night	42% 4 11% 26% 16% 1	2667
	11th to 20th	Night	43% 4 9% 25% 17% 3	2730
	21st to end	Night	45% 4 14% 21% 15% 2	2921
December	1st to 10th	Night	47% 2 9% 29% 12%	4956
	11th to 20th	Night	47% 3 10% 19% 19% 2	5742
	21st to end	Night	51% 2 10% 19% 17% 2	7479

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	30% 26% 29% 12% 3	220
	11th to 20th	Day	30% 25% 30% 10% 3 1	220
	21st to end	Day	25% 30% 32% 11% 1	244
February	1st to 10th	Day	26% 22% 31% 17% 3	430
	11th to 20th	Day	28% 29% 32% 9% 2	503
	21st to end	Day	24% 28% 31% 13% 3	446
March	1st to 10th	Day	31% 27% 29% 11% 1	540
	11th to 20th	Day	26% 29% 31% 11% 3 1	540
	21st to end	Day	34% 24% 26% 11% 4 1	594
April	1st to 10th	Day	36% 25% 27% 9% 3 1	540
	11th to 20th	Day	34% 25% 26% 11% 4 1	532
	21st to end	Day	31% 21% 31% 11% 4 1	540
May	1st to 10th	Day	33% 20% 32% 12% 2 1	507
	11th to 20th	Day	22% 20% 34% 18% 4 2	527
	21st to end	Day	22% 21% 35% 18% 5%	598
June	1st to 10th	Day	25% 15% 33% 18% 8% 2	730
	11th to 20th	Day	23% 14% 33% 19% 10% 1	730
	21st to end	Day	28% 12% 30% 22% 7% 1	730
July	1st to 10th	Day	20% 12% 39% 21% 7% 1	537
	11th to 20th	Day	20% 10% 38% 21% 9% 2	524
	21st to end	Day	17% 10% 37% 25% 9% 2	576
August	1st to 10th	Day	16% 9% 36% 29% 9% 2	526
	11th to 20th	Day	18% 10% 30% 29% 10% 3	519
	21st to end	Day	17% 13% 33% 26% 9% 2	583
September	1st to 10th	Day	15% 12% 35% 26% 11% 2	530
	11th to 20th	Day	18% 12% 29% 22% 12% 7%	530
	21st to end	Day	19% 16% 32% 20% 9% 5%	530
October	1st to 10th	Day	21% 15% 27% 24% 9% 3	511
	11th to 20th	Day	25% 17% 31% 17% 7% 2	420
	21st to end	Day	31% 15% 35% 15% 4	462
November	1st to 10th	Day	24% 21% 28% 19% 5% 1	425
	11th to 20th	Day	27% 21% 32% 16% 3 1	430
	21st to end	Day	30% 24% 27% 16% 4	430
December	1st to 10th	Day	29% 23% 28% 16% 4	283
	11th to 20th	Day	35% 26% 27% 9% 2	220
	21st to end	Day	32% 26% 26% 14% 2	242
January	1st to 10th	Night	23% 23% 34% 17% 3	840
	11th to 20th	Night	27% 22% 30% 17% 4 1	840
	21st to end	Night	18% 26% 35% 18% 3	924
February	1st to 10th	Night	22% 22% 34% 19% 3	840
	11th to 20th	Night	22% 25% 36% 15% 2	823
	21st to end	Night	23% 23% 34% 18% 3	692
March	1st to 10th	Night	26% 21% 38% 14% 1	840
	11th to 20th	Night	23% 23% 32% 16% 5%	840
	21st to end	Night	25% 20% 32% 18% 4	924
April	1st to 10th	Night	30% 20% 33% 15% 2	840
	11th to 20th	Night	26% 19% 34% 18% 4	670
	21st to end	Night	31% 16% 30% 19% 4	630
May	1st to 10th	Night	28% 12% 32% 24% 4	604
	11th to 20th	Night	24% 12% 39% 21% 4	592
	21st to end	Night	23% 9% 34% 28% 6%	447
June	1st to 10th	Night	26% 9% 32% 28% 5%	400
	11th to 20th	Night	18% 10% 33% 31% 9%	400
	21st to end	Night	27% 5% 33% 31% 4	400
July	1st to 10th	Night	18% 5% 33% 37% 7%	390
	11th to 20th	Night	18% 3 31% 37% 10% 1	388
	21st to end	Night	17% 4 33% 34% 11% 1	438
August	1st to 10th	Night	15% 3 35% 38% 9%	593
	11th to 20th	Night	16% 6% 31% 34% 12% 1	592
	21st to end	Night	16% 7% 29% 35% 12% 1	698
September	1st to 10th	Night	16% 7% 35% 32% 10% 1	800
	11th to 20th	Night	18% 9% 28% 28% 14% 4	800
	21st to end	Night	21% 9% 32% 26% 10% 2	800
October	1st to 10th	Night	21% 11% 30% 26% 12%	800
	11th to 20th	Night	22% 14% 32% 23% 9% 1	800
	21st to end	Night	29% 16% 31% 20% 4	880
November	1st to 10th	Night	25% 18% 29% 21% 6% 1	820
	11th to 20th	Night	21% 20% 32% 21% 5% 1	840
	21st to end	Night	25% 24% 29% 18% 4	941
December	1st to 10th	Night	26% 19% 32% 17% 5%	950
	11th to 20th	Night	27% 24% 31% 15% 3	950
	21st to end	Night	29% 21% 31% 16% 2	1001

Page generated at 2026-04-29 02:21:42. Version ac07

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