Armstrong
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Armstrong clear sky chart from 2005-11-06 to 2026-07-01.

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% 11% 4 10% 6% 5% 5% 4 5% 6% 30%	138600
	Night	12% 12% 5% 11% 7% 6% 6% 4 6% 6% 26%	134001
Transparency	Day	53% 5% 11% 21% 10% 1	141955
	Night	57% 5% 11% 19% 8% 1	138732
Seeing	Day	21% 15% 33% 21% 8% 2	18100
	Night	20% 12% 32% 26% 9% 1	27604

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	7% 13% 6% 18% 9% 10% 9% 5% 6% 5% 11%	4379
	11th to 20th	Day	5% 11% 8% 18% 10% 10% 9% 7% 9% 4 9%	4679
	21st to end	Day	8% 12% 6% 16% 8% 8% 8% 5% 8% 6% 16%	5156
February	1st to 10th	Day	6% 13% 5% 12% 8% 8% 8% 7% 7% 5% 20%	1704
	11th to 20th	Day	6% 13% 5% 12% 7% 7% 7% 6% 7% 6% 22%	1844
	21st to end	Day	5% 13% 7% 14% 8% 6% 6% 4 6% 7% 25%	1721
March	1st to 10th	Day	13% 14% 5% 13% 5% 5% 4 3 4 4 29%	4578
	11th to 20th	Day	9% 14% 6% 13% 6% 6% 7% 3 4 5% 27%	4808
	21st to end	Day	13% 16% 5% 9% 6% 7% 5% 3 4 5% 28%	6090
April	1st to 10th	Day	12% 12% 5% 10% 6% 4 4 3 4 6% 34%	2520
	11th to 20th	Day	17% 12% 4 9% 5% 4 4 3 5% 6% 32%	2543
	21st to end	Day	16% 13% 4 7% 5% 4 3 4 5% 6% 34%	2860
May	1st to 10th	Day	14% 13% 4 8% 5% 4 5% 3 5% 6% 32%	8241
	11th to 20th	Day	19% 10% 3 6% 4 3 4 3 5% 6% 37%	8222
	21st to end	Day	14% 7% 2 6% 4 4 4 3 6% 7% 43%	9379
June	1st to 10th	Day	20% 8% 3 7% 5% 4 5% 4 5% 6% 33%	3172
	11th to 20th	Day	14% 7% 3 8% 6% 6% 5% 4 6% 7% 34%	3360
	21st to end	Day	15% 7% 3 7% 6% 5% 5% 4 6% 8% 34%	3360
July	1st to 10th	Day	13% 6% 2 6% 5% 4 5% 4 5% 7% 43%	7132
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 5% 6% 9% 39%	6414
	21st to end	Day	12% 7% 3 7% 5% 5% 6% 5% 7% 9% 35%	6926
August	1st to 10th	Day	18% 6% 2 6% 5% 4 4 4 5% 7% 38%	4563
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 5% 8% 49%	4496
	21st to end	Day	12% 7% 3 7% 5% 4 5% 4 7% 9% 38%	4394
September	1st to 10th	Day	18% 8% 3 8% 5% 4 5% 3 5% 6% 36%	2400
	11th to 20th	Day	22% 9% 4 7% 4 3 5% 4 5% 7% 31%	2343
	21st to end	Day	20% 12% 3 7% 5% 3 3 4 4 6% 34%	2200
October	1st to 10th	Day	18% 18% 4 8% 4 5% 3 3 5% 5% 26%	2007
	11th to 20th	Day	19% 20% 5% 10% 5% 4 4 3 5% 5% 20%	1895
	21st to end	Day	18% 22% 5% 11% 6% 3 5% 3 3 5% 19%	1980
November	1st to 10th	Day	16% 18% 6% 14% 6% 5% 3 3 4 6% 19%	1764
	11th to 20th	Day	17% 23% 7% 15% 8% 5% 4 3 3 4 11%	1680
	21st to end	Day	15% 22% 8% 18% 7% 6% 4 2 3 3 10%	1470
December	1st to 10th	Day	9% 16% 8% 18% 16% 9% 5% 3 4 4 9%	2350
	11th to 20th	Day	9% 14% 6% 15% 6% 10% 8% 5% 8% 6% 13%	2691
	21st to end	Day	13% 22% 7% 15% 7% 6% 5% 3 5% 5% 12%	3279
January	1st to 10th	Night	5% 14% 6% 17% 9% 10% 9% 4 5% 5% 16%	8505
	11th to 20th	Night	6% 12% 7% 18% 11% 10% 9% 6% 9% 4 10%	8236
	21st to end	Night	5% 10% 5% 13% 9% 10% 10% 6% 9% 7% 15%	9058
February	1st to 10th	Night	7% 12% 5% 11% 7% 8% 10% 6% 8% 7% 20%	2914
	11th to 20th	Night	6% 12% 4 9% 9% 8% 8% 6% 8% 7% 22%	2663
	21st to end	Night	6% 12% 6% 13% 7% 7% 8% 5% 6% 7% 25%	2084
March	1st to 10th	Night	10% 12% 4 11% 7% 5% 5% 4 5% 6% 33%	5482
	11th to 20th	Night	10% 14% 6% 12% 8% 7% 5% 3 5% 6% 24%	5402
	21st to end	Night	12% 17% 5% 10% 6% 5% 5% 3 5% 5% 26%	5083
April	1st to 10th	Night	13% 13% 4 10% 6% 5% 3 3 5% 6% 32%	2100
	11th to 20th	Night	16% 14% 4 8% 4 5% 4 4 5% 6% 32%	2025
	21st to end	Night	15% 11% 5% 8% 5% 4 4 3 6% 6% 35%	1758
May	1st to 10th	Night	13% 11% 4 8% 4 4 5% 3 5% 5% 39%	4688
	11th to 20th	Night	20% 8% 3 7% 5% 4 4 3 5% 6% 35%	4670
	21st to end	Night	16% 7% 2 6% 3 3 4 3 5% 6% 45%	4813
June	1st to 10th	Night	17% 6% 2 6% 5% 4 5% 3 8% 8% 35%	1443
	11th to 20th	Night	14% 8% 2 6% 5% 4 5% 4 6% 6% 41%	1260
	21st to end	Night	16% 7% 3 6% 4 4 5% 5% 6% 8% 36%	1260
July	1st to 10th	Night	10% 7% 4 7% 5% 5% 5% 3 5% 8% 41%	2845
	11th to 20th	Night	13% 9% 3 6% 3 4 4 4 6% 8% 42%	3563
	21st to end	Night	10% 7% 3 6% 5% 4 5% 4 6% 9% 41%	3948
August	1st to 10th	Night	17% 4 2 5% 4 4 3 3 6% 7% 44%	2597
	11th to 20th	Night	10% 5% 1 6% 4 4 5% 5% 6% 9% 44%	2563
	21st to end	Night	14% 6% 2 5% 3 4 4 3 5% 7% 45%	3502
September	1st to 10th	Night	15% 6% 2 5% 4 3 4 3 5% 7% 46%	2000
	11th to 20th	Night	18% 9% 3 7% 4 4 4 3 6% 7% 35%	2036
	21st to end	Night	23% 9% 4 6% 3 3 4 2 4 6% 36%	2200
October	1st to 10th	Night	18% 14% 3 7% 4 4 3 3 4 6% 33%	2390
	11th to 20th	Night	18% 18% 5% 9% 5% 4 4 3 4 5% 25%	2505
	21st to end	Night	20% 17% 5% 10% 6% 5% 4 3 5% 6% 19%	2860
November	1st to 10th	Night	17% 16% 6% 12% 7% 5% 5% 3 4 5% 20%	2750
	11th to 20th	Night	18% 18% 6% 15% 8% 6% 4 3 4 4 14%	2940
	21st to end	Night	15% 20% 7% 16% 9% 6% 5% 3 4 4 12%	3147
December	1st to 10th	Night	8% 21% 10% 19% 11% 7% 5% 3 4 4 9%	5060
	11th to 20th	Night	10% 15% 5% 16% 8% 8% 6% 4 4 5% 18%	5736
	21st to end	Night	13% 18% 7% 17% 7% 6% 5% 4 5% 4 14%	7915

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	73% 5% 6% 10% 5% 2	4380
	11th to 20th	Day	72% 5% 6% 14% 4	4710
	21st to end	Day	65% 4 6% 15% 9% 1	5156
February	1st to 10th	Day	61% 6% 10% 17% 6% 1	1704
	11th to 20th	Day	58% 5% 10% 18% 8%	1853
	21st to end	Day	56% 5% 10% 15% 14% 1	1721
March	1st to 10th	Day	58% 4 7% 15% 15% 2	4580
	11th to 20th	Day	59% 5% 7% 16% 13% 1	4808
	21st to end	Day	58% 4 6% 20% 11% 1	6090
April	1st to 10th	Day	52% 4 10% 19% 14% 1	2520
	11th to 20th	Day	52% 5% 9% 22% 11% 1	2543
	21st to end	Day	50% 5% 10% 20% 14% 2	2860
May	1st to 10th	Day	52% 5% 8% 22% 13%	8250
	11th to 20th	Day	48% 4 10% 20% 16% 2	8226
	21st to end	Day	39% 5% 11% 25% 17% 3	9379
June	1st to 10th	Day	50% 7% 10% 18% 14%	3172
	11th to 20th	Day	46% 8% 11% 22% 13% 1	3360
	21st to end	Day	45% 8% 15% 23% 9%	3360
July	1st to 10th	Day	40% 5% 13% 33% 9%	7056
	11th to 20th	Day	40% 5% 19% 29% 6%	6337
	21st to end	Day	44% 5% 16% 28% 7%	6728
August	1st to 10th	Day	44% 6% 14% 30% 7%	4548
	11th to 20th	Day	33% 5% 20% 33% 9%	4538
	21st to end	Day	40% 6% 14% 31% 8% 1	4393
September	1st to 10th	Day	47% 7% 12% 26% 8%	2400
	11th to 20th	Day	52% 7% 16% 19% 5%	2343
	21st to end	Day	51% 5% 15% 22% 7%	2200
October	1st to 10th	Day	50% 6% 15% 18% 10% 1	3267
	11th to 20th	Day	57% 3 16% 20% 3	3059
	21st to end	Day	62% 5% 14% 17% 2	3180
November	1st to 10th	Day	67% 5% 11% 12% 4	1764
	11th to 20th	Day	77% 7% 8% 6% 2	1680
	21st to end	Day	78% 6% 7% 5% 3	1470
December	1st to 10th	Day	77% 6% 8% 7% 2	2350
	11th to 20th	Day	66% 5% 9% 11% 8%	2691
	21st to end	Day	72% 5% 6% 14% 3	3279
January	1st to 10th	Night	68% 5% 8% 11% 6% 2	8505
	11th to 20th	Night	71% 5% 6% 14% 3	8280
	21st to end	Night	63% 5% 10% 17% 5%	9058
February	1st to 10th	Night	58% 7% 10% 17% 8%	2914
	11th to 20th	Night	56% 6% 12% 20% 6%	2664
	21st to end	Night	56% 6% 11% 19% 8% 1	2084
March	1st to 10th	Night	51% 4 7% 14% 19% 5%	5493
	11th to 20th	Night	60% 5% 8% 18% 8% 1	5403
	21st to end	Night	59% 4 8% 17% 12% 1	5083
April	1st to 10th	Night	52% 5% 7% 21% 13% 2	2100
	11th to 20th	Night	53% 5% 9% 19% 12% 2	2025
	21st to end	Night	50% 5% 10% 20% 12% 4	1758
May	1st to 10th	Night	46% 5% 9% 26% 13% 1	4695
	11th to 20th	Night	48% 5% 9% 21% 15% 1	4671
	21st to end	Night	41% 4 9% 26% 18% 1	4814
June	1st to 10th	Night	44% 8% 12% 22% 13% 1	1443
	11th to 20th	Night	42% 6% 10% 25% 15% 1	1260
	21st to end	Night	44% 7% 14% 26% 9% 1	1260
July	1st to 10th	Night	42% 5% 14% 29% 10%	2813
	11th to 20th	Night	40% 5% 19% 28% 7% 1	3517
	21st to end	Night	40% 5% 17% 31% 7%	3832
August	1st to 10th	Night	39% 6% 16% 31% 8%	2588
	11th to 20th	Night	35% 5% 21% 31% 7%	2587
	21st to end	Night	37% 6% 18% 27% 11%	3502
September	1st to 10th	Night	38% 7% 16% 29% 9% 1	2000
	11th to 20th	Night	48% 7% 17% 25% 4	2036
	21st to end	Night	50% 7% 13% 23% 7%	2200
October	1st to 10th	Night	43% 6% 16% 24% 11%	3910
	11th to 20th	Night	57% 5% 13% 19% 7%	4080
	21st to end	Night	61% 6% 16% 15% 2	4610
November	1st to 10th	Night	66% 7% 11% 12% 4 1	2750
	11th to 20th	Night	74% 5% 9% 9% 3	2940
	21st to end	Night	76% 7% 8% 7% 2	3147
December	1st to 10th	Night	78% 6% 7% 6% 2	5060
	11th to 20th	Night	67% 6% 9% 13% 5%	5735
	21st to end	Night	71% 5% 6% 14% 3	7915

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	18% 14% 39% 26% 4	314
	11th to 20th	Day	15% 12% 39% 24% 8% 2	330
	21st to end	Day	14% 12% 39% 25% 9% 1	363
February	1st to 10th	Day	16% 18% 41% 21% 4 1	330
	11th to 20th	Day	14% 16% 47% 16% 6% 1	330
	21st to end	Day	15% 23% 39% 16% 5%	273
March	1st to 10th	Day	22% 17% 43% 12% 5% 1	330
	11th to 20th	Day	21% 24% 33% 18% 3 1	330
	21st to end	Day	20% 16% 33% 21% 8% 2	583
April	1st to 10th	Day	18% 19% 33% 22% 6% 1	540
	11th to 20th	Day	25% 17% 29% 20% 9% 1	532
	21st to end	Day	22% 17% 30% 20% 9% 2	670
May	1st to 10th	Day	23% 16% 32% 21% 7% 2	753
	11th to 20th	Day	22% 18% 33% 18% 7% 2	775
	21st to end	Day	24% 15% 33% 20% 7% 2	844
June	1st to 10th	Day	23% 16% 32% 19% 9% 2	780
	11th to 20th	Day	19% 17% 38% 18% 6% 3	780
	21st to end	Day	19% 14% 34% 23% 7% 3	780
July	1st to 10th	Day	16% 13% 34% 23% 11% 3	722
	11th to 20th	Day	16% 14% 34% 24% 9% 3	718
	21st to end	Day	13% 13% 32% 27% 12% 4	794
August	1st to 10th	Day	18% 12% 33% 24% 10% 3	726
	11th to 20th	Day	17% 10% 29% 27% 12% 4	715
	21st to end	Day	20% 14% 29% 23% 11% 3	606
September	1st to 10th	Day	24% 13% 27% 24% 11% 2	530
	11th to 20th	Day	25% 11% 31% 23% 9% 1	530
	21st to end	Day	27% 12% 28% 21% 10% 3	530
October	1st to 10th	Day	29% 16% 31% 18% 5% 1	337
	11th to 20th	Day	24% 18% 37% 15% 5% 1	330
	21st to end	Day	34% 18% 33% 11% 4	363
November	1st to 10th	Day	25% 18% 37% 16% 4	330
	11th to 20th	Day	30% 21% 31% 13% 4 1	330
	21st to end	Day	32% 21% 31% 13% 3	220
December	1st to 10th	Day	15% 19% 39% 22% 4 2	220
	11th to 20th	Day	19% 19% 36% 20% 5%	220
	21st to end	Day	22% 16% 33% 19% 7% 3	242
January	1st to 10th	Night	15% 16% 39% 25% 5%	1050
	11th to 20th	Night	18% 11% 35% 27% 8% 1	1050
	21st to end	Night	14% 10% 40% 26% 9% 1	1155
February	1st to 10th	Night	15% 12% 39% 26% 7% 1	1050
	11th to 20th	Night	12% 14% 35% 28% 10% 1	1029
	21st to end	Night	12% 13% 37% 27% 9% 1	700
March	1st to 10th	Night	14% 15% 34% 25% 11% 1	840
	11th to 20th	Night	18% 15% 36% 22% 7% 1	774
	21st to end	Night	18% 12% 30% 27% 10% 3	693
April	1st to 10th	Night	18% 13% 30% 31% 7% 1	630
	11th to 20th	Night	24% 10% 29% 30% 8%	623
	21st to end	Night	20% 8% 31% 28% 12% 2	630
May	1st to 10th	Night	21% 11% 29% 30% 8%	630
	11th to 20th	Night	22% 10% 31% 28% 8% 1	621
	21st to end	Night	24% 10% 29% 29% 8%	569
June	1st to 10th	Night	20% 7% 32% 30% 10%	420
	11th to 20th	Night	19% 10% 35% 26% 10%	420
	21st to end	Night	18% 9% 31% 29% 12% 1	420
July	1st to 10th	Night	12% 8% 31% 35% 13% 1	435
	11th to 20th	Night	13% 8% 31% 34% 14% 1	582
	21st to end	Night	13% 6% 26% 35% 19% 2	657
August	1st to 10th	Night	13% 8% 30% 34% 13%	597
	11th to 20th	Night	15% 8% 25% 30% 20% 2	594
	21st to end	Night	15% 7% 32% 31% 13% 1	660
September	1st to 10th	Night	19% 8% 25% 36% 12% 1	600
	11th to 20th	Night	24% 10% 30% 25% 11%	636
	21st to end	Night	28% 8% 27% 26% 10% 1	800
October	1st to 10th	Night	28% 15% 26% 24% 7% 1	800
	11th to 20th	Night	29% 11% 29% 25% 6% 1	800
	21st to end	Night	28% 13% 32% 20% 6% 1	880
November	1st to 10th	Night	27% 15% 33% 19% 6% 1	904
	11th to 20th	Night	28% 13% 32% 20% 6% 1	1050
	21st to end	Night	26% 19% 32% 18% 5%	1050
December	1st to 10th	Night	16% 17% 40% 19% 6% 2	1050
	11th to 20th	Night	20% 16% 35% 20% 8% 1	1050
	21st to end	Night	24% 14% 29% 25% 7% 2	1155

Page generated at 2026-06-28 02:10:42. Version ac07

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