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 2025-02-25.

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% 11% 4 10% 6% 5% 5% 4 5% 6% 29%	108725
	Night	12% 13% 5% 12% 7% 6% 5% 4 6% 6% 26%	110085
Transparency	Day	53% 6% 12% 20% 9% 1	112128
	Night	57% 6% 11% 18% 7% 1	114844
Seeing	Day	21% 15% 33% 21% 8% 2	16187
	Night	20% 12% 32% 26% 9% 1	25905

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% 12% 5% 15% 10% 12% 9% 5% 6% 6% 13%	3368
	11th to 20th	Day	5% 10% 6% 17% 11% 10% 9% 7% 9% 5% 12%	3589
	21st to end	Day	10% 15% 7% 19% 7% 6% 7% 4 6% 5% 14%	3982
February	1st to 10th	Day	6% 13% 5% 13% 8% 8% 8% 8% 7% 5% 20%	1623
	11th to 20th	Day	6% 12% 5% 13% 8% 8% 7% 6% 8% 6% 22%	1754
	21st to end	Day	4 12% 7% 14% 8% 6% 6% 4 6% 7% 26%	1601
March	1st to 10th	Day	14% 10% 4 11% 6% 5% 4 3 4 5% 36%	3118
	11th to 20th	Day	8% 16% 7% 13% 5% 6% 7% 3 4 5% 28%	3288
	21st to end	Day	13% 17% 5% 9% 5% 6% 4 2 4 5% 29%	4155
April	1st to 10th	Day	12% 12% 5% 10% 6% 4 5% 4 4 6% 34%	2280
	11th to 20th	Day	17% 12% 4 8% 5% 3 4 4 5% 6% 33%	2298
	21st to end	Day	15% 12% 4 7% 5% 4 3 4 5% 6% 35%	2588
May	1st to 10th	Day	18% 11% 4 8% 5% 4 4 3 5% 5% 31%	4428
	11th to 20th	Day	16% 9% 3 7% 5% 4 5% 4 6% 7% 33%	4399
	21st to end	Day	21% 8% 2 6% 4 4 4 3 5% 6% 36%	5015
June	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 4 5% 6% 31%	2870
	11th to 20th	Day	15% 7% 3 7% 6% 5% 5% 4 6% 7% 34%	3040
	21st to end	Day	16% 7% 3 7% 6% 5% 5% 4 6% 7% 34%	3040
July	1st to 10th	Day	14% 7% 3 7% 6% 5% 6% 4 5% 7% 37%	4984
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 5% 6% 8% 42%	4481
	21st to end	Day	13% 7% 3 7% 5% 5% 5% 4 7% 8% 36%	4814
August	1st to 10th	Day	18% 6% 2 6% 5% 4 4 4 5% 7% 38%	4423
	11th to 20th	Day	10% 5% 2 5% 4 4 4 3 5% 9% 49%	4356
	21st to end	Day	12% 7% 3 7% 5% 4 5% 4 7% 9% 38%	4260
September	1st to 10th	Day	17% 7% 3 8% 5% 4 5% 3 5% 6% 37%	2280
	11th to 20th	Day	23% 9% 4 7% 4 3 5% 4 5% 7% 30%	2226
	21st to end	Day	21% 12% 3 7% 5% 4 3 4 4 6% 32%	2090
October	1st to 10th	Day	19% 18% 4 8% 4 5% 3 3 4 5% 26%	1907
	11th to 20th	Day	19% 21% 5% 10% 6% 4 4 3 5% 5% 20%	1800
	21st to end	Day	18% 22% 5% 11% 6% 3 5% 3 3 5% 18%	1881
November	1st to 10th	Day	16% 18% 6% 14% 6% 5% 3 3 4 5% 19%	1678
	11th to 20th	Day	17% 22% 7% 15% 8% 5% 4 3 3 4 11%	1600
	21st to end	Day	16% 23% 9% 18% 7% 6% 4 3 3 3 10%	1400
December	1st to 10th	Day	9% 17% 8% 18% 16% 8% 5% 3 4 4 9%	2280
	11th to 20th	Day	9% 14% 5% 15% 6% 10% 7% 5% 8% 6% 14%	2621
	21st to end	Day	13% 23% 7% 15% 6% 6% 5% 3 5% 5% 12%	3208
January	1st to 10th	Night	5% 14% 6% 16% 10% 11% 7% 4 6% 6% 16%	6570
	11th to 20th	Night	6% 10% 6% 18% 11% 10% 9% 6% 10% 4 10%	6316
	21st to end	Night	7% 13% 7% 15% 8% 8% 8% 5% 7% 7% 15%	6996
February	1st to 10th	Night	6% 12% 5% 11% 7% 7% 10% 6% 8% 7% 21%	2775
	11th to 20th	Night	7% 12% 4 9% 9% 8% 8% 6% 8% 7% 23%	2533
	21st to end	Night	6% 11% 6% 13% 7% 6% 7% 5% 7% 7% 26%	1947
March	1st to 10th	Night	11% 11% 3 11% 7% 5% 5% 3 5% 5% 33%	3722
	11th to 20th	Night	9% 16% 7% 12% 7% 7% 5% 3 4 5% 26%	3687
	21st to end	Night	13% 16% 5% 9% 5% 5% 5% 3 5% 5% 30%	3476
April	1st to 10th	Night	13% 13% 4 10% 6% 5% 3 3 5% 6% 32%	1900
	11th to 20th	Night	16% 13% 4 8% 4 5% 4 4 5% 6% 31%	1830
	21st to end	Night	14% 11% 4 8% 5% 4 4 3 6% 6% 36%	1591
May	1st to 10th	Night	15% 10% 3 8% 4 4 4 4 5% 6% 37%	2523
	11th to 20th	Night	15% 7% 3 7% 5% 5% 5% 4 6% 7% 36%	2506
	21st to end	Night	23% 9% 2 6% 3 3 4 3 4 5% 37%	2569
June	1st to 10th	Night	18% 6% 2 6% 6% 5% 5% 4 8% 7% 34%	1306
	11th to 20th	Night	14% 8% 2 5% 5% 4 5% 4 6% 6% 43%	1140
	21st to end	Night	16% 8% 3 6% 4 4 4 4 5% 8% 36%	1140
July	1st to 10th	Night	12% 8% 4 7% 5% 5% 5% 4 6% 8% 37%	1983
	11th to 20th	Night	10% 7% 3 6% 3 4 4 4 6% 8% 46%	2486
	21st to end	Night	9% 6% 2 6% 4 4 5% 5% 6% 8% 45%	2750
August	1st to 10th	Night	18% 5% 2 5% 4 4 3 3 6% 7% 44%	2517
	11th to 20th	Night	10% 5% 1 6% 4 4 5% 5% 7% 9% 44%	2483
	21st to end	Night	14% 6% 2 5% 3 4 4 3 5% 7% 45%	3395
September	1st to 10th	Night	14% 6% 2 5% 3 3 4 4 5% 7% 47%	1900
	11th to 20th	Night	19% 9% 3 7% 4 4 4 3 6% 7% 34%	1934
	21st to end	Night	24% 9% 4 6% 3 4 4 2 4 6% 34%	2090
October	1st to 10th	Night	19% 14% 3 7% 4 4 4 3 4 6% 33%	2270
	11th to 20th	Night	18% 18% 5% 9% 5% 5% 4 3 4 6% 24%	2380
	21st to end	Night	20% 17% 5% 10% 6% 5% 5% 3 5% 6% 18%	2717
November	1st to 10th	Night	18% 17% 6% 11% 7% 5% 5% 3 4 5% 20%	2616
	11th to 20th	Night	18% 18% 6% 14% 8% 6% 4 3 4 4 14%	2800
	21st to end	Night	15% 20% 7% 16% 9% 6% 5% 3 4 3 12%	2997
December	1st to 10th	Night	9% 22% 10% 19% 11% 7% 5% 3 3 4 9%	4910
	11th to 20th	Night	10% 14% 5% 16% 8% 8% 6% 4 5% 5% 18%	5586
	21st to end	Night	13% 18% 7% 17% 7% 6% 5% 4 5% 5% 14%	7744

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	69% 6% 7% 12% 6% 1	3369
	11th to 20th	Day	67% 5% 7% 16% 5%	3620
	21st to end	Day	70% 5% 5% 11% 8% 1	3982
February	1st to 10th	Day	60% 6% 10% 17% 6% 1	1623
	11th to 20th	Day	58% 5% 10% 18% 9%	1763
	21st to end	Day	55% 5% 10% 15% 14% 1	1601
March	1st to 10th	Day	50% 4 9% 20% 15% 1	3120
	11th to 20th	Day	59% 5% 8% 19% 8% 1	3288
	21st to end	Day	56% 4 8% 18% 13% 1	4155
April	1st to 10th	Day	52% 4 10% 19% 14% 2	2280
	11th to 20th	Day	51% 5% 9% 22% 11% 1	2298
	21st to end	Day	48% 5% 10% 20% 14% 2	2588
May	1st to 10th	Day	52% 6% 9% 21% 11%	4440
	11th to 20th	Day	47% 7% 11% 20% 13% 3	4403
	21st to end	Day	47% 7% 11% 20% 14% 1	5015
June	1st to 10th	Day	52% 7% 10% 17% 13%	2870
	11th to 20th	Day	46% 8% 10% 21% 13% 1	3040
	21st to end	Day	45% 9% 15% 22% 9%	3040
July	1st to 10th	Day	44% 7% 15% 25% 9% 1	4907
	11th to 20th	Day	37% 7% 20% 30% 7%	4404
	21st to end	Day	44% 6% 15% 29% 6%	4662
August	1st to 10th	Day	44% 6% 13% 30% 7%	4408
	11th to 20th	Day	33% 5% 20% 33% 9%	4398
	21st to end	Day	40% 6% 14% 31% 8% 1	4259
September	1st to 10th	Day	46% 7% 12% 26% 8%	2280
	11th to 20th	Day	54% 8% 16% 18% 5%	2226
	21st to end	Day	52% 5% 15% 20% 7%	2090
October	1st to 10th	Day	51% 6% 14% 18% 11% 1	3167
	11th to 20th	Day	57% 4 17% 20% 3	2964
	21st to end	Day	62% 6% 15% 16% 2	3081
November	1st to 10th	Day	68% 5% 11% 11% 4	1678
	11th to 20th	Day	77% 7% 8% 7% 2	1600
	21st to end	Day	79% 6% 7% 5% 3	1400
December	1st to 10th	Day	78% 6% 8% 7% 2	2280
	11th to 20th	Day	66% 5% 9% 11% 8%	2621
	21st to end	Day	72% 5% 6% 14% 3	3208
January	1st to 10th	Night	67% 5% 9% 12% 5% 1	6570
	11th to 20th	Night	69% 6% 7% 14% 3	6360
	21st to end	Night	65% 5% 9% 15% 6%	6996
February	1st to 10th	Night	57% 8% 10% 17% 8%	2775
	11th to 20th	Night	56% 6% 12% 20% 6%	2534
	21st to end	Night	54% 6% 11% 19% 9% 1	1947
March	1st to 10th	Night	52% 5% 9% 17% 17% 1	3733
	11th to 20th	Night	60% 5% 9% 20% 6% 1	3688
	21st to end	Night	54% 4 10% 16% 15% 2	3476
April	1st to 10th	Night	52% 5% 8% 21% 12% 2	1900
	11th to 20th	Night	53% 6% 8% 19% 12% 2	1830
	21st to end	Night	48% 5% 10% 21% 12% 4	1591
May	1st to 10th	Night	44% 7% 10% 24% 13% 1	2530
	11th to 20th	Night	42% 9% 11% 25% 11% 2	2507
	21st to end	Night	49% 7% 9% 18% 15% 1	2570
June	1st to 10th	Night	44% 9% 12% 21% 12% 1	1306
	11th to 20th	Night	41% 6% 11% 25% 16% 1	1140
	21st to end	Night	44% 8% 14% 25% 9% 1	1140
July	1st to 10th	Night	45% 6% 16% 22% 11%	1951
	11th to 20th	Night	35% 6% 19% 31% 8% 1	2440
	21st to end	Night	36% 6% 18% 35% 5%	2662
August	1st to 10th	Night	39% 6% 16% 31% 8%	2508
	11th to 20th	Night	35% 5% 21% 31% 8%	2507
	21st to end	Night	37% 6% 18% 27% 12%	3395
September	1st to 10th	Night	36% 7% 16% 31% 10% 1	1900
	11th to 20th	Night	49% 7% 17% 24% 4	1934
	21st to end	Night	51% 7% 13% 21% 7%	2090
October	1st to 10th	Night	43% 6% 15% 25% 11%	3790
	11th to 20th	Night	57% 5% 13% 19% 6%	3955
	21st to end	Night	61% 7% 16% 14% 2	4467
November	1st to 10th	Night	66% 7% 11% 12% 4 1	2616
	11th to 20th	Night	74% 5% 9% 10% 3	2800
	21st to end	Night	76% 7% 8% 7% 2	2997
December	1st to 10th	Night	79% 6% 7% 6% 2	4910
	11th to 20th	Night	66% 6% 9% 13% 5%	5585
	21st to end	Night	71% 5% 6% 14% 3	7744

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	17% 13% 39% 27% 4	285
	11th to 20th	Day	14% 11% 39% 26% 8% 3	300
	21st to end	Day	16% 13% 38% 22% 10% 1	330
February	1st to 10th	Day	15% 17% 41% 22% 4 1	300
	11th to 20th	Day	14% 15% 47% 16% 6% 1	300
	21st to end	Day	15% 24% 40% 16% 4	237
March	1st to 10th	Day	17% 16% 44% 15% 6% 1	270
	11th to 20th	Day	21% 23% 32% 20% 3 1	270
	21st to end	Day	20% 14% 33% 22% 9% 2	496
April	1st to 10th	Day	18% 18% 32% 23% 7% 2	460
	11th to 20th	Day	26% 15% 28% 20% 9% 1	452
	21st to end	Day	21% 16% 31% 21% 8% 3	578
May	1st to 10th	Day	24% 17% 32% 19% 7% 2	653
	11th to 20th	Day	21% 18% 32% 19% 7% 3	675
	21st to end	Day	25% 14% 33% 20% 7% 1	734
June	1st to 10th	Day	24% 16% 30% 19% 9% 2	680
	11th to 20th	Day	19% 16% 38% 18% 7% 3	680
	21st to end	Day	18% 13% 34% 24% 8% 3	680
July	1st to 10th	Day	16% 13% 34% 24% 11% 3	671
	11th to 20th	Day	16% 14% 32% 25% 10% 3	668
	21st to end	Day	13% 12% 30% 28% 12% 4	739
August	1st to 10th	Day	18% 12% 33% 24% 11% 3	676
	11th to 20th	Day	17% 10% 29% 27% 13% 5%	665
	21st to end	Day	21% 14% 29% 22% 12% 3	561
September	1st to 10th	Day	23% 14% 27% 23% 10% 2	490
	11th to 20th	Day	27% 11% 31% 22% 8% 1	490
	21st to end	Day	28% 11% 28% 22% 9% 2	490
October	1st to 10th	Day	30% 15% 30% 19% 6% 1	307
	11th to 20th	Day	24% 17% 37% 16% 5% 1	300
	21st to end	Day	35% 20% 33% 9% 3	330
November	1st to 10th	Day	25% 16% 36% 18% 4	300
	11th to 20th	Day	29% 19% 33% 14% 4 1	300
	21st to end	Day	33% 22% 28% 14% 3	200
December	1st to 10th	Day	16% 18% 38% 23% 4 3	200
	11th to 20th	Day	20% 18% 36% 21% 6%	200
	21st to end	Day	21% 17% 32% 20% 7% 3	220
January	1st to 10th	Night	15% 14% 40% 26% 5%	1000
	11th to 20th	Night	17% 11% 35% 27% 8% 1	1000
	21st to end	Night	14% 11% 39% 26% 9% 1	1100
February	1st to 10th	Night	14% 12% 39% 27% 8% 1	1000
	11th to 20th	Night	12% 14% 35% 29% 10% 1	979
	21st to end	Night	12% 12% 37% 27% 10% 1	655
March	1st to 10th	Night	13% 15% 34% 25% 12% 1	760
	11th to 20th	Night	19% 14% 35% 22% 7% 1	700
	21st to end	Night	18% 11% 30% 28% 11% 3	627
April	1st to 10th	Night	18% 13% 27% 32% 8% 1	570
	11th to 20th	Night	24% 10% 29% 29% 8%	563
	21st to end	Night	19% 8% 31% 27% 13% 2	570
May	1st to 10th	Night	21% 12% 29% 29% 8%	570
	11th to 20th	Night	21% 10% 31% 29% 8% 1	561
	21st to end	Night	26% 10% 28% 28% 8%	514
June	1st to 10th	Night	21% 8% 31% 29% 11% 1	380
	11th to 20th	Night	18% 9% 36% 26% 11%	380
	21st to end	Night	19% 8% 31% 30% 11% 1	380
July	1st to 10th	Night	13% 8% 30% 34% 14% 1	412
	11th to 20th	Night	12% 8% 30% 35% 14% 1	552
	21st to end	Night	13% 6% 25% 35% 19% 2	624
August	1st to 10th	Night	14% 9% 29% 35% 14%	567
	11th to 20th	Night	15% 7% 24% 31% 20% 2	564
	21st to end	Night	15% 8% 32% 31% 13% 1	627
September	1st to 10th	Night	18% 8% 26% 36% 12% 1	570
	11th to 20th	Night	25% 10% 30% 25% 10%	604
	21st to end	Night	29% 7% 27% 26% 9% 1	760
October	1st to 10th	Night	28% 15% 25% 24% 7% 1	760
	11th to 20th	Night	28% 12% 28% 25% 6% 1	760
	21st to end	Night	28% 13% 32% 21% 6% 1	836
November	1st to 10th	Night	27% 14% 33% 19% 6% 1	860
	11th to 20th	Night	28% 13% 32% 20% 6% 1	1000
	21st to end	Night	27% 19% 31% 19% 5%	1000
December	1st to 10th	Night	17% 16% 40% 19% 6% 2	1000
	11th to 20th	Night	19% 16% 35% 21% 8% 1	1000
	21st to end	Night	23% 13% 29% 25% 8% 2	1100

Page generated at 2025-02-22 06:51:37. Version ac07

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