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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 2024-01-22.

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% 9% 4 7% 6% 4 5% 5% 6% 7% 33%	74723
	Night	15% 10% 4 7% 6% 3 5% 5% 5% 7% 34%	75208
Transparency	Day	50% 7% 13% 19% 10% 1	74440
	Night	50% 7% 12% 20% 11% 1	75025
Seeing	Day	19% 12% 28% 23% 13% 5%	13368
	Night	20% 10% 29% 28% 12% 1	22982

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	16% 13% 5% 10% 8% 4 5% 6% 5% 6% 23%	1526
	11th to 20th	Day	15% 16% 5% 11% 9% 4 6% 4 5% 5% 18%	1710
	21st to end	Day	13% 15% 6% 12% 9% 6% 7% 6% 5% 5% 17%	1791
February	1st to 10th	Day	17% 15% 6% 11% 8% 5% 5% 5% 5% 5% 19%	1620
	11th to 20th	Day	13% 13% 7% 12% 8% 4 5% 5% 4 5% 24%	1573
	21st to end	Day	19% 13% 5% 11% 7% 4 5% 4 4 6% 23%	1406
March	1st to 10th	Day	14% 11% 5% 8% 8% 5% 5% 4 5% 7% 28%	1977
	11th to 20th	Day	13% 13% 5% 8% 7% 4 4 4 4 6% 32%	1980
	21st to end	Day	18% 10% 4 7% 7% 3 5% 3 6% 6% 31%	2178
April	1st to 10th	Day	17% 9% 4 7% 7% 4 5% 5% 5% 6% 30%	2106
	11th to 20th	Day	18% 8% 3 7% 6% 3 4 4 5% 7% 35%	2209
	21st to end	Day	19% 8% 3 7% 6% 4 4 4 6% 6% 34%	2340
May	1st to 10th	Day	14% 10% 3 7% 6% 4 5% 5% 5% 6% 35%	2340
	11th to 20th	Day	14% 7% 3 7% 6% 3 5% 4 5% 6% 37%	2408
	21st to end	Day	10% 4 2 5% 5% 4 6% 6% 6% 9% 42%	2725
June	1st to 10th	Day	12% 6% 2 5% 5% 4 5% 5% 6% 8% 41%	2520
	11th to 20th	Day	11% 5% 3 6% 6% 3 5% 6% 6% 9% 40%	2520
	21st to end	Day	13% 5% 2 5% 5% 3 5% 5% 6% 9% 42%	2520
July	1st to 10th	Day	6% 5% 3 5% 5% 3 6% 6% 7% 9% 46%	2520
	11th to 20th	Day	12% 6% 2 5% 6% 4 6% 7% 7% 10% 36%	2478
	21st to end	Day	9% 5% 2 6% 6% 4 5% 7% 7% 11% 38%	2547
August	1st to 10th	Day	7% 5% 2 6% 6% 3 6% 7% 7% 10% 41%	2327
	11th to 20th	Day	9% 5% 2 5% 7% 4 7% 7% 7% 10% 37%	2303
	21st to end	Day	10% 6% 2 5% 6% 3 5% 5% 6% 9% 41%	2484
September	1st to 10th	Day	13% 6% 3 6% 4 4 5% 6% 6% 8% 40%	2160
	11th to 20th	Day	9% 5% 2 5% 5% 4 5% 4 5% 9% 46%	2026
	21st to end	Day	13% 6% 3 6% 7% 4 6% 5% 7% 7% 37%	1960
October	1st to 10th	Day	19% 8% 4 6% 6% 3 5% 4 5% 6% 34%	1800
	11th to 20th	Day	18% 10% 4 7% 6% 4 4 4 6% 7% 30%	1800
	21st to end	Day	23% 11% 3 8% 7% 3 5% 4 4 7% 25%	1980
November	1st to 10th	Day	13% 8% 3 7% 7% 4 5% 4 7% 8% 35%	1651
	11th to 20th	Day	13% 12% 4 8% 7% 4 5% 6% 6% 9% 28%	1520
	21st to end	Day	18% 14% 4 7% 7% 3 5% 4 6% 7% 24%	1520
December	1st to 10th	Day	15% 15% 7% 9% 8% 4 6% 5% 6% 6% 21%	1520
	11th to 20th	Day	18% 14% 5% 10% 5% 5% 6% 6% 5% 5% 21%	1906
	21st to end	Day	27% 18% 9% 10% 7% 3 3 4 4 4 11%	2772
January	1st to 10th	Night	15% 13% 5% 10% 7% 5% 6% 5% 4 6% 24%	2635
	11th to 20th	Night	18% 15% 6% 10% 7% 4 5% 6% 5% 5% 19%	2470
	21st to end	Night	12% 12% 6% 10% 8% 4 5% 5% 6% 7% 23%	2599
February	1st to 10th	Night	16% 15% 5% 9% 8% 3 5% 4 6% 6% 22%	2340
	11th to 20th	Night	14% 13% 5% 9% 7% 4 5% 5% 5% 6% 26%	2273
	21st to end	Night	18% 10% 6% 9% 7% 4 5% 4 5% 7% 25%	1849
March	1st to 10th	Night	13% 9% 4 7% 8% 4 6% 5% 6% 6% 33%	1970
	11th to 20th	Night	17% 12% 4 7% 5% 3 4 4 4 7% 33%	1979
	21st to end	Night	16% 10% 4 7% 5% 3 5% 5% 4 5% 36%	2178
April	1st to 10th	Night	17% 9% 3 7% 6% 3 5% 4 5% 6% 34%	1854
	11th to 20th	Night	17% 8% 3 6% 5% 4 5% 4 6% 5% 37%	1715
	21st to end	Night	15% 7% 5% 6% 6% 3 5% 4 6% 6% 36%	1620
May	1st to 10th	Night	17% 8% 3 6% 5% 3 4 5% 5% 8% 37%	1620
	11th to 20th	Night	15% 4 3 5% 6% 3 5% 5% 6% 6% 41%	1483
	21st to end	Night	10% 5% 1 4 5% 2 5% 4 6% 8% 49%	1550
June	1st to 10th	Night	10% 6% 3 5% 6% 4 6% 6% 6% 8% 42%	1440
	11th to 20th	Night	9% 5% 3 5% 5% 4 4 7% 6% 8% 43%	1440
	21st to end	Night	10% 5% 3 4 5% 3 5% 6% 5% 9% 45%	1440
July	1st to 10th	Night	6% 4 2 5% 5% 3 3 4 4 8% 56%	1440
	11th to 20th	Night	9% 4 2 4 4 3 5% 4 7% 9% 48%	1479
	21st to end	Night	8% 6% 2 4 4 3 5% 6% 6% 8% 49%	1771
August	1st to 10th	Night	8% 5% 2 4 5% 4 5% 6% 7% 9% 46%	1611
	11th to 20th	Night	8% 4 2 4 5% 3 4 5% 7% 10% 48%	1598
	21st to end	Night	8% 5% 2 5% 5% 4 5% 4 6% 9% 48%	1854
September	1st to 10th	Night	11% 6% 3 6% 6% 2 5% 5% 6% 8% 43%	1800
	11th to 20th	Night	10% 4 2 4 4 3 4 4 5% 8% 53%	1931
	21st to end	Night	14% 6% 3 6% 6% 4 5% 4 6% 7% 40%	1999
October	1st to 10th	Night	16% 8% 3 6% 5% 3 3 4 5% 7% 41%	2160
	11th to 20th	Night	17% 9% 3 6% 6% 3 4 5% 5% 6% 34%	2160
	21st to end	Night	22% 9% 4 7% 6% 3 5% 5% 5% 5% 30%	2376
November	1st to 10th	Night	15% 7% 3 5% 5% 3 4 5% 6% 8% 40%	2422
	11th to 20th	Night	14% 10% 4 7% 6% 4 5% 5% 5% 8% 31%	2660
	21st to end	Night	18% 10% 5% 7% 6% 4 6% 5% 6% 7% 28%	2660
December	1st to 10th	Night	17% 15% 5% 9% 7% 3 6% 5% 5% 7% 22%	2660
	11th to 20th	Night	15% 16% 6% 9% 7% 4 4 4 6% 6% 24%	3288
	21st to end	Night	27% 18% 7% 10% 7% 3 4 4 4 4 12%	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	62% 7% 10% 13% 7%	1527
	11th to 20th	Day	68% 7% 10% 11% 4	1710
	21st to end	Day	68% 7% 10% 10% 5%	1791
February	1st to 10th	Day	70% 5% 9% 11% 4 1	1620
	11th to 20th	Day	63% 5% 10% 14% 7% 1	1582
	21st to end	Day	64% 5% 9% 14% 7% 1	1406
March	1st to 10th	Day	56% 6% 14% 17% 6% 1	1980
	11th to 20th	Day	55% 5% 10% 18% 10% 2	1980
	21st to end	Day	56% 5% 8% 18% 11% 2	2178
April	1st to 10th	Day	55% 5% 11% 16% 12% 1	2106
	11th to 20th	Day	49% 6% 11% 19% 13% 2	2209
	21st to end	Day	52% 6% 11% 18% 12% 2	2340
May	1st to 10th	Day	49% 6% 11% 21% 12% 2	2340
	11th to 20th	Day	46% 8% 11% 21% 13% 2	2412
	21st to end	Day	37% 9% 15% 23% 15% 2	2725
June	1st to 10th	Day	40% 8% 12% 23% 15% 2	2520
	11th to 20th	Day	39% 9% 14% 23% 14% 1	2520
	21st to end	Day	37% 8% 15% 24% 15% 1	2520
July	1st to 10th	Day	32% 9% 13% 25% 18% 3	2450
	11th to 20th	Day	39% 9% 16% 24% 11% 1	2403
	21st to end	Day	38% 10% 15% 22% 13% 1	2405
August	1st to 10th	Day	35% 9% 18% 25% 13% 1	2314
	11th to 20th	Day	38% 10% 17% 24% 10% 1	2303
	21st to end	Day	38% 8% 14% 27% 13% 1	2484
September	1st to 10th	Day	41% 8% 15% 23% 12% 1	2160
	11th to 20th	Day	36% 7% 13% 28% 15% 1	2026
	21st to end	Day	43% 8% 16% 23% 10% 1	1960
October	1st to 10th	Day	50% 6% 14% 18% 11% 1	1800
	11th to 20th	Day	52% 8% 12% 19% 8% 1	1800
	21st to end	Day	60% 6% 12% 15% 6% 1	1980
November	1st to 10th	Day	46% 8% 14% 22% 9% 2	1651
	11th to 20th	Day	53% 8% 15% 14% 10% 1	1520
	21st to end	Day	57% 7% 16% 16% 5%	1520
December	1st to 10th	Day	63% 7% 13% 13% 4	1520
	11th to 20th	Day	63% 7% 16% 9% 5%	1906
	21st to end	Day	77% 6% 8% 8% 2	2772
January	1st to 10th	Night	62% 5% 11% 14% 7%	2635
	11th to 20th	Night	65% 7% 12% 10% 5% 1	2470
	21st to end	Night	59% 6% 10% 16% 7% 1	2599
February	1st to 10th	Night	63% 6% 10% 15% 5% 1	2340
	11th to 20th	Night	59% 5% 12% 16% 8% 1	2275
	21st to end	Night	60% 6% 10% 16% 7%	1849
March	1st to 10th	Night	52% 7% 11% 20% 9% 2	1980
	11th to 20th	Night	52% 5% 9% 20% 12% 2	1980
	21st to end	Night	50% 5% 8% 20% 14% 2	2178
April	1st to 10th	Night	51% 6% 11% 19% 13% 1	1854
	11th to 20th	Night	49% 5% 9% 19% 16% 3	1715
	21st to end	Night	47% 6% 10% 19% 15% 3	1620
May	1st to 10th	Night	46% 5% 9% 22% 15% 4	1620
	11th to 20th	Night	42% 5% 12% 24% 13% 4	1484
	21st to end	Night	34% 7% 13% 24% 19% 3	1551
June	1st to 10th	Night	39% 8% 10% 24% 18% 2	1440
	11th to 20th	Night	37% 6% 11% 25% 18% 2	1440
	21st to end	Night	36% 7% 14% 24% 17% 2	1440
July	1st to 10th	Night	28% 6% 10% 27% 25% 4	1400
	11th to 20th	Night	32% 7% 13% 28% 19% 2	1431
	21st to end	Night	32% 8% 15% 24% 18% 3	1672
August	1st to 10th	Night	32% 8% 15% 29% 15% 2	1600
	11th to 20th	Night	31% 9% 15% 30% 14% 1	1598
	21st to end	Night	32% 7% 15% 27% 16% 2	1854
September	1st to 10th	Night	39% 7% 13% 24% 15% 2	1800
	11th to 20th	Night	30% 7% 11% 33% 18% 1	1931
	21st to end	Night	43% 7% 11% 24% 14% 1	1999
October	1st to 10th	Night	43% 7% 11% 23% 16% 1	2160
	11th to 20th	Night	48% 7% 14% 21% 9% 1	2160
	21st to end	Night	55% 6% 10% 16% 11% 1	2376
November	1st to 10th	Night	42% 6% 13% 24% 13% 1	2422
	11th to 20th	Night	50% 7% 13% 20% 10% 1	2660
	21st to end	Night	54% 7% 15% 18% 5%	2660
December	1st to 10th	Night	61% 8% 14% 13% 4	2660
	11th to 20th	Night	61% 8% 13% 14% 5%	3288
	21st to end	Night	74% 6% 10% 7% 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	21% 22% 32% 15% 9% 2	184
	11th to 20th	Day	24% 25% 33% 13% 3 1	270
	21st to end	Day	21% 22% 31% 21% 4 1	267
February	1st to 10th	Day	27% 20% 42% 10% 1	240
	11th to 20th	Day	23% 25% 40% 11%	240
	21st to end	Day	32% 26% 31% 11%	206
March	1st to 10th	Day	23% 14% 35% 21% 7%	420
	11th to 20th	Day	25% 12% 40% 17% 6% 1	420
	21st to end	Day	29% 14% 30% 18% 8% 1	462
April	1st to 10th	Day	20% 15% 33% 23% 8% 1	420
	11th to 20th	Day	23% 17% 27% 19% 9% 4	412
	21st to end	Day	24% 11% 32% 22% 9% 2	420
May	1st to 10th	Day	24% 11% 28% 22% 10% 5%	423
	11th to 20th	Day	18% 9% 28% 25% 16% 5%	447
	21st to end	Day	13% 7% 25% 28% 19% 8%	485
June	1st to 10th	Day	12% 9% 34% 30% 13% 3	450
	11th to 20th	Day	14% 8% 25% 30% 19% 4	450
	21st to end	Day	16% 6% 28% 30% 15% 5%	450
July	1st to 10th	Day	9% 4 18% 32% 27% 11%	445
	11th to 20th	Day	13% 6% 22% 28% 20% 10%	444
	21st to end	Day	9% 3 28% 34% 18% 7%	488
August	1st to 10th	Day	10% 3 21% 30% 24% 12%	446
	11th to 20th	Day	11% 4 23% 29% 19% 14%	439
	21st to end	Day	13% 6% 22% 30% 19% 10%	495
September	1st to 10th	Day	14% 7% 22% 26% 19% 11%	450
	11th to 20th	Day	10% 6% 19% 25% 25% 15%	450
	21st to end	Day	14% 11% 26% 26% 15% 8%	450
October	1st to 10th	Day	21% 12% 24% 21% 17% 6%	450
	11th to 20th	Day	21% 15% 34% 20% 7% 3	450
	21st to end	Day	30% 14% 29% 18% 8%	495
November	1st to 10th	Day	19% 23% 31% 17% 7% 2	282
	11th to 20th	Day	22% 26% 24% 19% 7% 2	180
	21st to end	Day	26% 19% 36% 16% 3 1	180
December	1st to 10th	Day	27% 18% 31% 17% 5% 1	180
	11th to 20th	Day	26% 24% 33% 14% 3	180
	21st to end	Day	27% 20% 37% 13% 3	198
January	1st to 10th	Night	24% 16% 35% 18% 5% 1	760
	11th to 20th	Night	28% 17% 32% 18% 4	760
	21st to end	Night	19% 14% 38% 24% 5%	800
February	1st to 10th	Night	25% 19% 33% 20% 2	720
	11th to 20th	Night	22% 16% 35% 24% 4	703
	21st to end	Night	24% 15% 35% 23% 3	592
March	1st to 10th	Night	20% 11% 35% 27% 7%	720
	11th to 20th	Night	24% 12% 34% 22% 7% 1	720
	21st to end	Night	21% 9% 33% 27% 9% 1	792
April	1st to 10th	Night	23% 11% 33% 29% 4	720
	11th to 20th	Night	20% 10% 33% 26% 9% 1	651
	21st to end	Night	18% 6% 33% 33% 10%	540
May	1st to 10th	Night	22% 4 28% 28% 17% 1	540
	11th to 20th	Night	17% 3 26% 41% 13% 1	422
	21st to end	Night	14% 5% 24% 36% 20% 2	389
June	1st to 10th	Night	13% 3 33% 36% 15%	360
	11th to 20th	Night	13% 3 28% 39% 17% 1	360
	21st to end	Night	13% 4 24% 43% 16%	360
July	1st to 10th	Night	6% 3 20% 44% 25% 1	350
	11th to 20th	Night	12% 3 21% 43% 19% 2	387
	21st to end	Night	12% 2 18% 46% 21% 2	591
August	1st to 10th	Night	8% 4 14% 42% 29% 3	537
	11th to 20th	Night	10% 2 16% 35% 34% 3	534
	21st to end	Night	9% 3 19% 36% 30% 3	666
September	1st to 10th	Night	15% 6% 17% 33% 23% 6%	720
	11th to 20th	Night	12% 4 18% 33% 28% 5%	720
	21st to end	Night	16% 6% 22% 29% 22% 5%	720
October	1st to 10th	Night	19% 7% 27% 27% 18% 2	720
	11th to 20th	Night	22% 12% 29% 27% 8% 1	720
	21st to end	Night	28% 10% 31% 26% 5%	792
November	1st to 10th	Night	20% 11% 29% 26% 12% 2	740
	11th to 20th	Night	21% 14% 33% 21% 9% 1	760
	21st to end	Night	25% 13% 34% 21% 7%	760
December	1st to 10th	Night	28% 18% 30% 18% 6%	760
	11th to 20th	Night	24% 18% 35% 19% 5%	760
	21st to end	Night	31% 13% 33% 21% 2	836

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