W. Armitage Memorial Dark Sky Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the W. Armitage Memorial 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	17% 12% 4 10% 6% 5% 4 4 5% 6% 27%	126957
	Night	16% 14% 5% 11% 6% 5% 5% 3 5% 5% 25%	122050
Transparency	Day	59% 2 10% 19% 9% 1	130423
	Night	62% 2 9% 17% 8% 1	126672
Seeing	Day	27% 13% 31% 19% 7% 2	15620
	Night	27% 11% 28% 25% 9% 1	24388

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% 18% 8% 21% 11% 6% 4 2 3 4 10%	3597
	11th to 20th	Day	10% 17% 7% 20% 11% 6% 3 3 5% 5% 14%	3570
	21st to end	Day	8% 16% 9% 17% 6% 4 6% 4 5% 5% 18%	4031
February	1st to 10th	Day	9% 19% 7% 14% 10% 7% 4 4 4 5% 17%	1800
	11th to 20th	Day	12% 19% 6% 15% 8% 5% 4 3 4 5% 20%	1753
	21st to end	Day	16% 19% 6% 10% 5% 5% 5% 3 5% 5% 21%	1600
March	1st to 10th	Day	19% 18% 5% 12% 6% 4 4 3 4 4 22%	4512
	11th to 20th	Day	16% 14% 6% 13% 7% 5% 4 3 4 5% 25%	4980
	21st to end	Day	15% 14% 5% 9% 6% 5% 4 3 4 4 31%	5736
April	1st to 10th	Day	21% 17% 4 8% 4 4 3 3 4 5% 28%	2400
	11th to 20th	Day	22% 12% 4 8% 4 4 3 3 4 6% 30%	2374
	21st to end	Day	21% 11% 4 10% 5% 4 4 3 4 4 31%	2650
May	1st to 10th	Day	22% 12% 3 7% 4 4 4 4 5% 6% 29%	6346
	11th to 20th	Day	18% 9% 3 8% 5% 5% 5% 4 5% 7% 30%	6318
	21st to end	Day	19% 10% 4 8% 5% 5% 4 4 5% 6% 31%	6949
June	1st to 10th	Day	22% 9% 3 9% 5% 5% 5% 4 6% 7% 26%	2800
	11th to 20th	Day	14% 8% 3 7% 5% 5% 5% 5% 7% 7% 33%	2800
	21st to end	Day	19% 7% 3 9% 6% 5% 5% 5% 6% 7% 27%	2993
July	1st to 10th	Day	18% 8% 3 7% 5% 5% 5% 5% 7% 9% 28%	6433
	11th to 20th	Day	14% 7% 2 6% 5% 5% 5% 4 6% 9% 36%	6360
	21st to end	Day	12% 7% 3 7% 4 4 5% 4 7% 9% 38%	6926
August	1st to 10th	Day	15% 8% 3 9% 6% 6% 6% 4 6% 7% 30%	4563
	11th to 20th	Day	17% 7% 3 7% 5% 5% 6% 4 7% 7% 32%	4176
	21st to end	Day	14% 7% 3 8% 4 5% 5% 4 6% 8% 35%	4355
September	1st to 10th	Day	18% 8% 3 8% 4 4 3 3 5% 7% 37%	2400
	11th to 20th	Day	16% 8% 3 6% 4 5% 4 4 4 6% 39%	2376
	21st to end	Day	22% 10% 3 7% 3 4 3 3 5% 7% 33%	2014
October	1st to 10th	Day	21% 13% 4 8% 4 4 3 4 4 6% 30%	1900
	11th to 20th	Day	28% 17% 5% 8% 5% 4 3 2 4 5% 19%	1900
	21st to end	Day	29% 16% 4 9% 5% 4 3 3 4 5% 18%	1923
November	1st to 10th	Day	18% 20% 6% 9% 4 3 3 2 4 5% 26%	1676
	11th to 20th	Day	19% 18% 9% 13% 8% 4 4 2 4 4 17%	1600
	21st to end	Day	23% 23% 8% 14% 6% 5% 3 3 3 3 10%	1600
December	1st to 10th	Day	19% 23% 9% 17% 7% 5% 3 2 3 4 8%	2600
	11th to 20th	Day	17% 20% 5% 13% 11% 7% 4 4 3 2 14%	2986
	21st to end	Day	17% 21% 8% 13% 8% 4 4 3 4 4 14%	3960
January	1st to 10th	Night	12% 22% 9% 19% 9% 5% 3 3 2 3 12%	6349
	11th to 20th	Night	11% 17% 7% 17% 10% 7% 6% 4 5% 4 12%	6315
	21st to end	Night	7% 15% 10% 17% 7% 7% 7% 4 4 5% 16%	6922
February	1st to 10th	Night	10% 15% 6% 11% 10% 8% 5% 4 5% 5% 20%	2600
	11th to 20th	Night	12% 17% 6% 13% 8% 6% 5% 4 5% 5% 19%	2533
	21st to end	Night	12% 17% 6% 11% 7% 5% 5% 4 5% 6% 22%	2029
March	1st to 10th	Night	18% 20% 6% 9% 6% 4 5% 3 5% 4 22%	5323
	11th to 20th	Night	14% 17% 4 13% 6% 5% 5% 4 5% 6% 21%	4979
	21st to end	Night	15% 12% 4 10% 6% 6% 5% 4 5% 5% 26%	5195
April	1st to 10th	Night	19% 14% 5% 10% 7% 4 3 2 4 4 29%	2000
	11th to 20th	Night	17% 11% 3 7% 5% 4 4 3 5% 5% 35%	1975
	21st to end	Night	22% 10% 4 8% 4 3 4 3 4 5% 31%	1766
May	1st to 10th	Night	20% 12% 4 7% 4 4 4 3 5% 5% 32%	3594
	11th to 20th	Night	17% 9% 3 8% 5% 4 4 3 5% 6% 36%	3575
	21st to end	Night	18% 9% 3 8% 5% 5% 5% 3 5% 7% 31%	3926
June	1st to 10th	Night	18% 9% 3 8% 5% 4 5% 5% 7% 7% 30%	1600
	11th to 20th	Night	17% 9% 3 7% 4 3 4 4 5% 6% 37%	1593
	21st to end	Night	20% 8% 2 5% 4 5% 5% 4 5% 8% 35%	1400
July	1st to 10th	Night	15% 9% 4 9% 6% 7% 5% 5% 7% 6% 27%	3505
	11th to 20th	Night	15% 7% 3 8% 5% 5% 6% 4 6% 7% 35%	3600
	21st to end	Night	12% 7% 2 6% 4 4 5% 3 5% 8% 44%	3926
August	1st to 10th	Night	13% 6% 2 7% 4 5% 5% 4 7% 8% 38%	2586
	11th to 20th	Night	19% 7% 3 7% 4 4 4 4 5% 6% 38%	2870
	21st to end	Night	16% 7% 3 8% 5% 5% 5% 4 6% 7% 34%	3509
September	1st to 10th	Night	16% 8% 2 6% 4 4 3 3 5% 8% 42%	2000
	11th to 20th	Night	18% 6% 2 5% 4 4 4 3 5% 6% 42%	1981
	21st to end	Night	22% 8% 3 6% 4 3 3 3 4 6% 38%	2162
October	1st to 10th	Night	22% 10% 3 7% 4 4 4 3 5% 5% 32%	2280
	11th to 20th	Night	25% 17% 4 7% 4 3 4 3 4 5% 24%	2280
	21st to end	Night	26% 17% 4 9% 4 4 4 3 4 5% 20%	2674
November	1st to 10th	Night	19% 17% 6% 9% 6% 4 4 2 4 5% 25%	2619
	11th to 20th	Night	18% 18% 6% 12% 8% 5% 4 2 4 5% 18%	2800
	21st to end	Night	23% 21% 7% 14% 7% 4 4 2 3 3 12%	2800
December	1st to 10th	Night	17% 19% 9% 19% 10% 5% 4 3 3 3 8%	4580
	11th to 20th	Night	21% 19% 5% 12% 7% 5% 4 3 4 4 16%	5208
	21st to end	Night	19% 21% 6% 14% 8% 4 4 2 4 4 14%	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	81% 1 6% 7% 4	3598
	11th to 20th	Day	74% 2 4 12% 8%	3600
	21st to end	Day	68% 1 5% 13% 11%	4031
February	1st to 10th	Day	70% 2 5% 13% 8% 1	1800
	11th to 20th	Day	68% 2 7% 12% 10% 1	1762
	21st to end	Day	67% 2 6% 14% 10% 1	1600
March	1st to 10th	Day	69% 1 4 12% 12% 2	4515
	11th to 20th	Day	64% 2 9% 13% 10% 1	4980
	21st to end	Day	57% 3 5% 18% 15% 2	5736
April	1st to 10th	Day	60% 2 8% 16% 12% 2	2400
	11th to 20th	Day	57% 3 6% 22% 11% 2	2374
	21st to end	Day	58% 2 7% 17% 14% 2	2650
May	1st to 10th	Day	56% 3 6% 20% 12% 3	6357
	11th to 20th	Day	53% 3 11% 24% 9% 1	6322
	21st to end	Day	55% 3 9% 16% 16% 2	6949
June	1st to 10th	Day	57% 4 9% 19% 11% 1	2800
	11th to 20th	Day	48% 5% 12% 22% 12% 1	2800
	21st to end	Day	55% 4 11% 21% 9%	2993
July	1st to 10th	Day	53% 2 14% 26% 4	6364
	11th to 20th	Day	46% 2 17% 29% 6%	6285
	21st to end	Day	45% 2 15% 29% 8%	6728
August	1st to 10th	Day	53% 3 13% 26% 5%	4549
	11th to 20th	Day	51% 2 15% 24% 7%	4215
	21st to end	Day	48% 2 10% 28% 11% 1	4355
September	1st to 10th	Day	48% 3 13% 24% 12% 1	2400
	11th to 20th	Day	46% 4 15% 25% 9% 1	2376
	21st to end	Day	52% 3 14% 23% 7%	2014
October	1st to 10th	Day	61% 2 8% 24% 5%	3160
	11th to 20th	Day	62% 2 13% 18% 5%	3140
	21st to end	Day	69% 3 13% 11% 4	3148
November	1st to 10th	Day	63% 2 11% 17% 6%	1676
	11th to 20th	Day	73% 3 8% 11% 6%	1600
	21st to end	Day	82% 2 6% 6% 4	1600
December	1st to 10th	Day	83% 2 5% 9% 2	2600
	11th to 20th	Day	77% 2 3 13% 4	2986
	21st to end	Day	75% 1 4 15% 4	3960
January	1st to 10th	Night	81% 1 8% 8% 2	6349
	11th to 20th	Night	76% 2 6% 10% 4	6360
	21st to end	Night	71% 2 5% 12% 10%	6922
February	1st to 10th	Night	65% 3 6% 16% 8% 1	2600
	11th to 20th	Night	67% 3 7% 14% 7% 1	2535
	21st to end	Night	63% 2 8% 16% 10% 1	2029
March	1st to 10th	Night	67% 2 7% 10% 12% 3	5333
	11th to 20th	Night	65% 3 11% 13% 8% 1	4980
	21st to end	Night	58% 4 6% 19% 10% 2	5195
April	1st to 10th	Night	60% 2 6% 16% 14% 2	2000
	11th to 20th	Night	50% 3 8% 23% 14% 2	1975
	21st to end	Night	55% 3 8% 19% 14% 2	1766
May	1st to 10th	Night	54% 3 9% 18% 13% 4	3600
	11th to 20th	Night	51% 3 12% 20% 12% 1	3576
	21st to end	Night	54% 3 11% 18% 13% 2	3927
June	1st to 10th	Night	51% 5% 9% 23% 11% 1	1600
	11th to 20th	Night	48% 3 11% 22% 14% 2	1593
	21st to end	Night	49% 3 12% 25% 10% 1	1400
July	1st to 10th	Night	57% 3 12% 20% 7%	3465
	11th to 20th	Night	48% 3 20% 23% 5%	3552
	21st to end	Night	41% 2 15% 31% 11%	3809
August	1st to 10th	Night	44% 3 20% 26% 7%	2576
	11th to 20th	Night	48% 3 15% 26% 8%	2897
	21st to end	Night	49% 3 13% 25% 8% 1	3509
September	1st to 10th	Night	42% 3 18% 26% 10%	2000
	11th to 20th	Night	44% 3 17% 27% 8%	1981
	21st to end	Night	49% 4 13% 27% 7%	2162
October	1st to 10th	Night	54% 2 11% 21% 12%	3800
	11th to 20th	Night	61% 2 10% 22% 5%	3779
	21st to end	Night	63% 2 15% 12% 8% 1	4399
November	1st to 10th	Night	65% 2 11% 16% 5%	2619
	11th to 20th	Night	70% 3 9% 12% 6% 1	2800
	21st to end	Night	79% 3 7% 9% 3	2800
December	1st to 10th	Night	83% 2 6% 7% 2	4580
	11th to 20th	Night	73% 2 4 14% 6%	5208
	21st to end	Night	76% 2 3 11% 8%	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	23% 18% 45% 13% 2 1	200
	11th to 20th	Day	24% 14% 37% 18% 7% 2	200
	21st to end	Day	24% 20% 33% 19% 4	237
February	1st to 10th	Day	26% 25% 38% 8% 2 1	300
	11th to 20th	Day	29% 25% 35% 10% 1	300
	21st to end	Day	32% 21% 33% 12% 2 1	364
March	1st to 10th	Day	27% 19% 33% 17% 5%	500
	11th to 20th	Day	30% 18% 33% 15% 4 1	500
	21st to end	Day	31% 17% 32% 16% 4 1	550
April	1st to 10th	Day	36% 15% 26% 16% 7% 1	500
	11th to 20th	Day	35% 14% 25% 15% 8% 3	492
	21st to end	Day	29% 18% 33% 15% 4 1	500
May	1st to 10th	Day	32% 14% 31% 16% 5% 2	503
	11th to 20th	Day	27% 15% 29% 19% 9% 2	527
	21st to end	Day	27% 16% 29% 21% 5% 1	573
June	1st to 10th	Day	29% 10% 35% 22% 4 1	530
	11th to 20th	Day	22% 14% 35% 18% 9% 2	530
	21st to end	Day	24% 11% 32% 22% 9% 1	723
July	1st to 10th	Day	20% 7% 37% 27% 7% 2	560
	11th to 20th	Day	19% 9% 36% 25% 9% 2	524
	21st to end	Day	17% 10% 32% 27% 12% 2	576
August	1st to 10th	Day	18% 7% 34% 27% 11% 3	526
	11th to 20th	Day	20% 5% 29% 27% 13% 6%	519
	21st to end	Day	21% 9% 31% 21% 12% 6%	583
September	1st to 10th	Day	25% 8% 29% 23% 12% 4	530
	11th to 20th	Day	19% 9% 27% 24% 14% 8%	522
	21st to end	Day	26% 13% 24% 19% 16% 3	490
October	1st to 10th	Day	32% 12% 24% 21% 8% 3	490
	11th to 20th	Day	35% 14% 29% 15% 5% 2	490
	21st to end	Day	40% 13% 25% 18% 4	451
November	1st to 10th	Day	35% 15% 29% 16% 4 1	310
	11th to 20th	Day	30% 18% 35% 14% 4 1	200
	21st to end	Day	37% 22% 31% 8% 2 2	200
December	1st to 10th	Day	36% 14% 27% 19% 3 2	200
	11th to 20th	Day	30% 20% 34% 13% 4 1	200
	21st to end	Day	31% 13% 37% 13% 5%	220
January	1st to 10th	Night	30% 14% 34% 18% 4	800
	11th to 20th	Night	32% 11% 30% 21% 7%	800
	21st to end	Night	20% 10% 35% 26% 9% 1	880
February	1st to 10th	Night	18% 17% 35% 24% 5% 1	800
	11th to 20th	Night	24% 15% 34% 22% 4 1	783
	21st to end	Night	23% 17% 33% 23% 4	660
March	1st to 10th	Night	25% 15% 31% 23% 5% 1	800
	11th to 20th	Night	28% 15% 32% 20% 5% 1	800
	21st to end	Night	23% 14% 32% 24% 6%	880
April	1st to 10th	Night	31% 13% 26% 24% 6%	800
	11th to 20th	Night	26% 14% 27% 22% 10% 1	791
	21st to end	Night	29% 11% 28% 24% 7% 1	566
May	1st to 10th	Night	25% 8% 27% 31% 9%	400
	11th to 20th	Night	25% 8% 27% 27% 12% 1	394
	21st to end	Night	24% 7% 33% 28% 8%	433
June	1st to 10th	Night	24% 7% 31% 31% 7% 1	400
	11th to 20th	Night	22% 8% 27% 31% 12% 1	400
	21st to end	Night	25% 4 25% 33% 13% 1	400
July	1st to 10th	Night	16% 4 28% 41% 11%	390
	11th to 20th	Night	17% 5% 25% 40% 13% 1	388
	21st to end	Night	16% 3 26% 38% 16% 1	438
August	1st to 10th	Night	17% 3 22% 35% 22% 1	398
	11th to 20th	Night	19% 3 18% 35% 23% 3	589
	21st to end	Night	20% 5% 25% 30% 18% 2	829
September	1st to 10th	Night	21% 5% 27% 32% 13% 2	800
	11th to 20th	Night	24% 5% 25% 26% 18% 3	793
	21st to end	Night	26% 7% 25% 27% 13% 2	760
October	1st to 10th	Night	28% 8% 28% 23% 13%	760
	11th to 20th	Night	36% 10% 25% 20% 8% 1	760
	21st to end	Night	37% 11% 27% 21% 4 1	836
November	1st to 10th	Night	32% 12% 30% 19% 7% 1	780
	11th to 20th	Night	31% 15% 29% 16% 7% 1	800
	21st to end	Night	37% 15% 22% 19% 5% 1	800
December	1st to 10th	Night	33% 13% 30% 18% 6% 1	800
	11th to 20th	Night	32% 15% 26% 21% 6% 1	800
	21st to end	Night	34% 14% 26% 20% 5% 1	880

Page generated at 2025-09-16 01:49:14. Version ac07

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