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HAA Dark Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the HAA Dark Sky Site clear sky chart from 2005-11-06 to 2025-09-19.

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	14% 10% 3 9% 5% 5% 5% 4 6% 7% 32%	126404
	Night	14% 10% 4 8% 5% 5% 5% 4 5% 7% 34%	122629
Transparency	Day	49% 6% 13% 20% 11% 1	129999
	Night	49% 6% 12% 20% 12% 1	127120
Seeing	Day	19% 12% 29% 23% 12% 5%	15573
	Night	20% 10% 29% 28% 12% 1	25007

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	11% 15% 5% 13% 6% 5% 4 4 4 5% 26%	3620
	11th to 20th	Day	10% 19% 8% 18% 9% 6% 5% 3 5% 5% 13%	4026
	21st to end	Day	17% 16% 6% 16% 7% 6% 5% 4 4 4 16%	4466
February	1st to 10th	Day	16% 16% 5% 12% 7% 6% 5% 4 5% 5% 20%	1800
	11th to 20th	Day	12% 14% 6% 13% 7% 5% 5% 4 4 5% 24%	1753
	21st to end	Day	18% 13% 5% 13% 6% 5% 5% 4 4 5% 23%	1569
March	1st to 10th	Day	16% 11% 5% 11% 6% 5% 4 3 5% 6% 29%	4932
	11th to 20th	Day	10% 12% 5% 10% 6% 5% 5% 4 5% 7% 31%	4980
	21st to end	Day	17% 11% 3 9% 6% 5% 4 3 5% 5% 31%	5465
April	1st to 10th	Day	18% 10% 4 8% 6% 5% 5% 3 5% 6% 30%	2340
	11th to 20th	Day	17% 9% 3 8% 5% 4 4 4 5% 6% 35%	2459
	21st to end	Day	18% 9% 3 8% 5% 5% 4 3 5% 5% 34%	2600
May	1st to 10th	Day	19% 10% 3 8% 5% 4 5% 4 5% 6% 32%	5888
	11th to 20th	Day	15% 10% 4 9% 6% 5% 5% 3 5% 7% 31%	6150
	21st to end	Day	16% 7% 3 7% 5% 4 5% 4 6% 7% 38%	6949
June	1st to 10th	Day	12% 6% 2 6% 5% 6% 5% 4 6% 8% 39%	2800
	11th to 20th	Day	10% 6% 2 8% 5% 5% 5% 5% 6% 9% 39%	2800
	21st to end	Day	12% 5% 2 6% 4 5% 6% 4 6% 9% 41%	2800
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 6% 5% 7% 10% 42%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 5% 5% 7% 10% 41%	6234
	21st to end	Day	7% 5% 2 6% 5% 5% 4 5% 7% 9% 46%	6424
August	1st to 10th	Day	16% 6% 2 6% 4 4 5% 4 6% 8% 37%	4234
	11th to 20th	Day	11% 6% 2 7% 5% 5% 6% 4 8% 10% 35%	4174
	21st to end	Day	8% 5% 2 7% 5% 5% 5% 4 7% 10% 43%	4510
September	1st to 10th	Day	12% 6% 3 7% 3 5% 5% 4 6% 8% 42%	2400
	11th to 20th	Day	9% 6% 2 5% 5% 5% 4 3 5% 9% 47%	2228
	21st to end	Day	14% 7% 3 7% 6% 5% 5% 4 7% 7% 35%	2068
October	1st to 10th	Day	18% 8% 3 7% 5% 5% 4 4 5% 7% 35%	1900
	11th to 20th	Day	17% 9% 4 8% 5% 5% 4 3 6% 7% 32%	1900
	21st to end	Day	22% 11% 3 10% 5% 4 5% 3 5% 6% 25%	2090
November	1st to 10th	Day	12% 9% 3 8% 6% 5% 5% 4 7% 7% 35%	1739
	11th to 20th	Day	12% 13% 4 9% 7% 5% 5% 4 6% 9% 28%	1600
	21st to end	Day	18% 15% 3 8% 6% 5% 4 4 7% 7% 23%	1600
December	1st to 10th	Day	16% 16% 6% 11% 7% 6% 6% 5% 6% 6% 16%	2600
	11th to 20th	Day	19% 16% 4 10% 5% 6% 4 4 5% 4 24%	2986
	21st to end	Day	24% 18% 7% 11% 6% 5% 4 3 5% 4 14%	3960
January	1st to 10th	Night	14% 11% 4 11% 7% 6% 5% 4 5% 7% 26%	6321
	11th to 20th	Night	10% 15% 6% 14% 9% 6% 6% 5% 6% 5% 18%	5859
	21st to end	Night	18% 15% 5% 13% 5% 5% 4 4 5% 6% 22%	6490
February	1st to 10th	Night	15% 15% 5% 10% 7% 5% 4 4 6% 6% 23%	2600
	11th to 20th	Night	14% 13% 5% 10% 6% 5% 5% 4 6% 6% 27%	2533
	21st to end	Night	16% 11% 5% 10% 7% 6% 5% 4 6% 7% 24%	2060
March	1st to 10th	Night	14% 9% 4 9% 6% 5% 5% 4 5% 6% 32%	4908
	11th to 20th	Night	11% 10% 3 7% 5% 4 4 4 6% 8% 38%	4979
	21st to end	Night	17% 10% 3 7% 4 4 4 3 4 6% 37%	5467
April	1st to 10th	Night	18% 11% 3 8% 6% 4 4 4 5% 5% 32%	2060
	11th to 20th	Night	18% 8% 3 7% 5% 5% 4 3 6% 6% 36%	1907
	21st to end	Night	15% 8% 4 7% 6% 5% 4 3 6% 7% 37%	1800
May	1st to 10th	Night	20% 10% 3 8% 5% 4 4 2 4 6% 33%	4053
	11th to 20th	Night	14% 7% 3 9% 6% 5% 5% 4 5% 7% 34%	3741
	21st to end	Night	17% 6% 2 6% 4 3 4 3 5% 8% 42%	3926
June	1st to 10th	Night	10% 7% 2 6% 5% 5% 6% 4 6% 8% 42%	1600
	11th to 20th	Night	9% 5% 2 6% 5% 5% 4 6% 7% 8% 43%	1600
	21st to end	Night	10% 6% 2 6% 4 5% 5% 4 5% 8% 45%	1600
July	1st to 10th	Night	7% 5% 2 6% 5% 5% 4 4 6% 9% 48%	3600
	11th to 20th	Night	8% 5% 2 6% 4 4 4 4 6% 9% 47%	3709
	21st to end	Night	7% 5% 2 6% 3 4 4 4 6% 8% 50%	4428
August	1st to 10th	Night	11% 5% 2 6% 4 4 5% 4 6% 9% 45%	2915
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 6% 8% 46%	2875
	21st to end	Night	7% 5% 1 5% 4 5% 4 4 6% 10% 50%	3355
September	1st to 10th	Night	11% 6% 3 6% 5% 4 4 4 6% 8% 45%	2000
	11th to 20th	Night	9% 4 1 5% 3 4 4 3 5% 8% 54%	2126
	21st to end	Night	14% 7% 2 8% 5% 5% 4 3 6% 7% 38%	2111
October	1st to 10th	Night	16% 7% 3 6% 4 4 3 3 5% 7% 42%	2280
	11th to 20th	Night	17% 10% 3 7% 5% 4 4 4 6% 6% 35%	2280
	21st to end	Night	21% 9% 4 7% 5% 4 5% 4 5% 5% 31%	2508
November	1st to 10th	Night	15% 7% 2 6% 4 4 4 4 6% 7% 40%	2554
	11th to 20th	Night	14% 11% 3 8% 5% 5% 5% 4 5% 8% 31%	2800
	21st to end	Night	18% 11% 4 7% 5% 5% 5% 3 6% 7% 27%	2800
December	1st to 10th	Night	13% 16% 4 10% 6% 5% 6% 5% 6% 8% 20%	4580
	11th to 20th	Night	15% 17% 5% 10% 6% 5% 4 3 5% 5% 24%	5208
	21st to end	Night	23% 17% 6% 11% 6% 5% 4 3 4 5% 16%	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	60% 7% 11% 11% 10% 1	3621
	11th to 20th	Day	75% 5% 9% 7% 4 1	4060
	21st to end	Day	72% 5% 11% 7% 6%	4466
February	1st to 10th	Day	68% 5% 10% 13% 4 1	1800
	11th to 20th	Day	63% 5% 10% 14% 7% 1	1762
	21st to end	Day	64% 5% 10% 14% 7% 1	1569
March	1st to 10th	Day	56% 4 14% 17% 8%	4935
	11th to 20th	Day	51% 5% 17% 17% 7% 2	4980
	21st to end	Day	54% 5% 9% 21% 10% 1	5465
April	1st to 10th	Day	55% 5% 11% 17% 11% 1	2340
	11th to 20th	Day	49% 6% 11% 19% 13% 1	2459
	21st to end	Day	51% 6% 10% 18% 12% 2	2600
May	1st to 10th	Day	51% 6% 11% 16% 10% 5%	5897
	11th to 20th	Day	51% 8% 10% 19% 11% 1	6154
	21st to end	Day	45% 7% 11% 21% 14% 2	6949
June	1st to 10th	Day	41% 8% 11% 22% 16% 2	2800
	11th to 20th	Day	40% 9% 14% 23% 14% 1	2800
	21st to end	Day	38% 8% 15% 24% 15% 1	2800
July	1st to 10th	Day	36% 7% 13% 26% 17% 2	6290
	11th to 20th	Day	36% 7% 14% 27% 15% 1	6159
	21st to end	Day	33% 7% 14% 26% 19% 1	6240
August	1st to 10th	Day	42% 7% 15% 21% 14% 1	4221
	11th to 20th	Day	40% 8% 16% 25% 10% 1	4213
	21st to end	Day	35% 7% 13% 32% 12% 1	4510
September	1st to 10th	Day	39% 8% 15% 25% 13% 1	2400
	11th to 20th	Day	34% 6% 12% 31% 16%	2228
	21st to end	Day	44% 8% 15% 23% 9% 1	2068
October	1st to 10th	Day	38% 5% 16% 24% 17% 1	3160
	11th to 20th	Day	41% 6% 13% 24% 15% 1	3140
	21st to end	Day	49% 6% 15% 23% 8%	3428
November	1st to 10th	Day	46% 8% 14% 22% 9% 1	1739
	11th to 20th	Day	53% 7% 15% 15% 10% 1	1600
	21st to end	Day	58% 6% 15% 15% 5%	1600
December	1st to 10th	Day	67% 6% 14% 9% 3	2600
	11th to 20th	Day	63% 5% 12% 10% 9% 1	2986
	21st to end	Day	73% 6% 8% 9% 5%	3960
January	1st to 10th	Night	57% 6% 12% 17% 7%	6321
	11th to 20th	Night	64% 6% 13% 12% 5%	5900
	21st to end	Night	64% 5% 9% 11% 10% 1	6490
February	1st to 10th	Night	62% 6% 11% 15% 5% 1	2600
	11th to 20th	Night	58% 5% 12% 17% 8% 1	2535
	21st to end	Night	59% 6% 11% 17% 7%	2060
March	1st to 10th	Night	52% 5% 13% 17% 11% 2	4918
	11th to 20th	Night	43% 5% 18% 20% 12% 1	4980
	21st to end	Night	47% 4 11% 19% 17% 2	5467
April	1st to 10th	Night	53% 5% 10% 18% 12% 1	2060
	11th to 20th	Night	50% 5% 9% 19% 15% 2	1907
	21st to end	Night	47% 6% 10% 19% 15% 3	1800
May	1st to 10th	Night	53% 5% 10% 15% 12% 6%	4060
	11th to 20th	Night	46% 7% 13% 21% 11% 2	3742
	21st to end	Night	40% 6% 11% 24% 17% 2	3927
June	1st to 10th	Night	39% 8% 10% 24% 17% 2	1600
	11th to 20th	Night	37% 6% 13% 26% 17% 2	1600
	21st to end	Night	37% 7% 14% 23% 18% 2	1600
July	1st to 10th	Night	34% 5% 13% 23% 22% 3	3560
	11th to 20th	Night	33% 6% 13% 29% 18% 2	3661
	21st to end	Night	32% 6% 12% 24% 24% 2	4297
August	1st to 10th	Night	36% 6% 13% 25% 16% 3	2904
	11th to 20th	Night	36% 7% 16% 29% 11% 1	2902
	21st to end	Night	29% 6% 15% 33% 16% 1	3355
September	1st to 10th	Night	37% 7% 12% 27% 16% 2	2000
	11th to 20th	Night	29% 7% 11% 34% 19% 1	2126
	21st to end	Night	45% 7% 11% 24% 13% 1	2111
October	1st to 10th	Night	33% 5% 9% 27% 26% 1	3800
	11th to 20th	Night	42% 5% 13% 25% 14%	3779
	21st to end	Night	40% 5% 15% 26% 13% 1	4120
November	1st to 10th	Night	42% 6% 13% 25% 13% 1	2554
	11th to 20th	Night	50% 7% 13% 19% 10% 1	2800
	21st to end	Night	55% 7% 15% 18% 5%	2800
December	1st to 10th	Night	59% 7% 17% 13% 3	4580
	11th to 20th	Night	62% 5% 9% 13% 10% 1	5208
	21st to end	Night	69% 6% 10% 10% 5%	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	21% 23% 33% 14% 8% 1	205
	11th to 20th	Day	25% 26% 32% 13% 3 1	300
	21st to end	Day	23% 24% 30% 18% 4 1	330
February	1st to 10th	Day	28% 19% 38% 12% 3	300
	11th to 20th	Day	26% 25% 36% 12% 2	300
	21st to end	Day	30% 27% 31% 11%	259
March	1st to 10th	Day	24% 14% 35% 20% 7%	500
	11th to 20th	Day	25% 15% 39% 15% 6% 1	500
	21st to end	Day	30% 14% 30% 18% 7% 1	550
April	1st to 10th	Day	24% 16% 32% 21% 6% 1	500
	11th to 20th	Day	25% 19% 27% 18% 8% 3	492
	21st to end	Day	24% 12% 32% 22% 9% 2	500
May	1st to 10th	Day	24% 11% 30% 22% 9% 4	503
	11th to 20th	Day	18% 9% 28% 24% 16% 4	527
	21st to end	Day	14% 9% 26% 27% 17% 7%	573
June	1st to 10th	Day	13% 11% 34% 28% 12% 2	530
	11th to 20th	Day	13% 8% 28% 31% 17% 3	530
	21st to end	Day	16% 7% 29% 30% 14% 5%	530
July	1st to 10th	Day	10% 6% 20% 31% 24% 10%	525
	11th to 20th	Day	14% 7% 24% 27% 19% 9%	524
	21st to end	Day	9% 3 29% 33% 19% 7%	576
August	1st to 10th	Day	12% 4 20% 30% 23% 11%	526
	11th to 20th	Day	12% 4 24% 29% 18% 13%	519
	21st to end	Day	11% 7% 22% 32% 19% 9%	583
September	1st to 10th	Day	14% 8% 22% 25% 20% 11%	530
	11th to 20th	Day	9% 5% 17% 25% 27% 17%	522
	21st to end	Day	16% 10% 26% 26% 15% 8%	490
October	1st to 10th	Day	20% 12% 25% 21% 16% 6%	490
	11th to 20th	Day	20% 15% 33% 21% 8% 3	490
	21st to end	Day	28% 15% 30% 19% 8%	539
November	1st to 10th	Day	19% 24% 31% 17% 6% 2	310
	11th to 20th	Day	21% 26% 27% 19% 7% 2	200
	21st to end	Day	27% 20% 36% 15% 3 1	200
December	1st to 10th	Day	28% 18% 31% 18% 5% 1	200
	11th to 20th	Day	29% 23% 33% 14% 3	200
	21st to end	Day	27% 19% 35% 15% 3	220
January	1st to 10th	Night	24% 16% 36% 18% 5% 1	800
	11th to 20th	Night	27% 18% 33% 17% 5%	800
	21st to end	Night	21% 14% 38% 22% 5%	880
February	1st to 10th	Night	25% 19% 33% 22% 2	800
	11th to 20th	Night	22% 16% 34% 23% 4	783
	21st to end	Night	24% 16% 35% 23% 3	660
March	1st to 10th	Night	20% 12% 36% 26% 6%	800
	11th to 20th	Night	23% 13% 36% 21% 7% 1	800
	21st to end	Night	21% 11% 33% 26% 8% 1	880
April	1st to 10th	Night	23% 12% 33% 28% 4	800
	11th to 20th	Night	20% 12% 33% 25% 9% 1	723
	21st to end	Night	18% 7% 33% 33% 9%	600
May	1st to 10th	Night	22% 5% 28% 28% 16% 1	600
	11th to 20th	Night	17% 3 26% 40% 13% 1	469
	21st to end	Night	14% 6% 24% 36% 19% 1	433
June	1st to 10th	Night	13% 4 34% 36% 14%	400
	11th to 20th	Night	13% 3 28% 40% 16% 1	400
	21st to end	Night	13% 5% 25% 42% 16%	400
July	1st to 10th	Night	6% 4 21% 46% 23% 1	390
	11th to 20th	Night	11% 3 22% 42% 20% 2	432
	21st to end	Night	11% 2 17% 46% 22% 2	657
August	1st to 10th	Night	8% 4 14% 42% 29% 3	597
	11th to 20th	Night	10% 2 16% 36% 33% 3	594
	21st to end	Night	9% 3 19% 37% 29% 3	740
September	1st to 10th	Night	14% 6% 19% 33% 24% 5%	800
	11th to 20th	Night	11% 4 16% 33% 31% 6%	793
	21st to end	Night	18% 6% 22% 29% 21% 5%	760
October	1st to 10th	Night	18% 7% 27% 27% 19% 2	760
	11th to 20th	Night	22% 12% 28% 27% 9% 1	760
	21st to end	Night	26% 11% 31% 26% 6%	836
November	1st to 10th	Night	20% 11% 30% 26% 11% 2	780
	11th to 20th	Night	22% 14% 33% 22% 9% 1	800
	21st to end	Night	25% 13% 34% 21% 7%	800
December	1st to 10th	Night	28% 18% 30% 18% 6%	800
	11th to 20th	Night	25% 18% 34% 19% 5%	800
	21st to end	Night	31% 12% 32% 22% 2	880

Page generated at 2025-09-16 01:49:10. Version ac07

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