Armour Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Armour Observatory clear sky chart from 2005-11-06 to 2026-03-03.

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% 13% 4 9% 5% 5% 4 3 5% 6% 28%	131277
	Night	17% 15% 5% 11% 6% 5% 4 3 4 5% 25%	129855
Transparency	Day	55% 7% 11% 17% 9% 1	134870
	Night	59% 7% 10% 15% 8% 1	134348
Seeing	Day	26% 17% 28% 18% 8% 2	16128
	Night	26% 16% 27% 22% 8% 1	25674

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% 26% 11% 19% 6% 5% 3 2 2 2 8%	4661
	11th to 20th	Day	10% 26% 12% 22% 12% 6% 3 2 2 2 4	5108
	21st to end	Day	14% 15% 8% 18% 11% 11% 6% 3 3 3 8%	5780
February	1st to 10th	Day	16% 21% 7% 16% 8% 6% 5% 3 2 4 12%	1890
	11th to 20th	Day	13% 20% 8% 16% 8% 4 5% 2 3 5% 15%	1843
	21st to end	Day	19% 17% 6% 13% 7% 5% 4 4 4 6% 16%	1638
March	1st to 10th	Day	16% 16% 7% 12% 5% 4 4 2 3 5% 26%	4954
	11th to 20th	Day	11% 18% 6% 8% 4 4 4 4 4 6% 31%	4980
	21st to end	Day	21% 16% 5% 9% 4 3 4 3 4 5% 26%	5465
April	1st to 10th	Day	20% 15% 4 7% 5% 3 4 3 4 6% 29%	2385
	11th to 20th	Day	19% 13% 3 7% 4 4 3 3 5% 6% 34%	2377
	21st to end	Day	22% 11% 3 6% 4 4 4 3 4 5% 34%	2600
May	1st to 10th	Day	26% 10% 3 7% 5% 5% 4 3 4 5% 28%	5888
	11th to 20th	Day	19% 9% 3 8% 5% 5% 4 4 5% 7% 31%	6290
	21st to end	Day	24% 7% 2 5% 4 4 3 4 5% 6% 35%	6949
June	1st to 10th	Day	17% 7% 3 5% 4 5% 5% 4 6% 7% 37%	2800
	11th to 20th	Day	15% 6% 2 6% 5% 4 6% 5% 7% 9% 37%	2800
	21st to end	Day	16% 6% 2 5% 3 4 4 4 6% 9% 40%	2800
July	1st to 10th	Day	11% 5% 2 6% 4 4 5% 4 6% 10% 43%	6360
	11th to 20th	Day	13% 6% 2 7% 4 4 4 4 6% 8% 40%	6360
	21st to end	Day	11% 5% 2 6% 4 4 4 5% 6% 8% 44%	6473
August	1st to 10th	Day	17% 6% 3 6% 4 4 4 4 5% 8% 38%	4234
	11th to 20th	Day	15% 6% 2 8% 5% 6% 6% 5% 8% 9% 30%	4174
	21st to end	Day	14% 5% 2 6% 4 4 4 3 6% 9% 43%	4412
September	1st to 10th	Day	16% 7% 2 6% 4 4 4 4 6% 6% 40%	2400
	11th to 20th	Day	14% 5% 2 5% 4 4 4 4 6% 8% 44%	2326
	21st to end	Day	17% 8% 3 8% 6% 5% 5% 4 5% 7% 32%	2130
October	1st to 10th	Day	23% 8% 3 7% 4 4 4 3 6% 7% 34%	2000
	11th to 20th	Day	24% 10% 3 9% 5% 4 3 3 5% 5% 28%	2000
	21st to end	Day	29% 15% 3 8% 4 4 3 3 4 5% 21%	2200
November	1st to 10th	Day	19% 14% 4 7% 7% 4 4 3 5% 6% 28%	1846
	11th to 20th	Day	16% 22% 5% 8% 6% 4 3 3 5% 5% 23%	1680
	21st to end	Day	22% 24% 6% 10% 5% 4 3 3 3 4 16%	1680
December	1st to 10th	Day	20% 27% 11% 13% 5% 4 4 2 3 3 8%	2680
	11th to 20th	Day	21% 23% 6% 16% 7% 5% 3 2 3 3 12%	3066
	21st to end	Day	21% 25% 7% 12% 7% 4 4 3 3 3 11%	4048
January	1st to 10th	Night	17% 26% 9% 19% 8% 5% 3 2 2 3 8%	8234
	11th to 20th	Night	11% 24% 11% 24% 11% 5% 4 2 2 2 4	7759
	21st to end	Night	14% 18% 8% 18% 9% 10% 4 2 3 3 9%	8401
February	1st to 10th	Night	17% 22% 7% 15% 8% 6% 4 3 2 3 14%	2730
	11th to 20th	Night	15% 17% 8% 16% 7% 5% 4 3 4 5% 16%	2663
	21st to end	Night	18% 18% 6% 15% 6% 5% 4 3 4 4 16%	2161
March	1st to 10th	Night	19% 15% 5% 9% 7% 6% 3 2 4 4 26%	4931
	11th to 20th	Night	16% 17% 4 8% 4 4 3 3 4 6% 32%	4979
	21st to end	Night	16% 16% 5% 9% 4 4 3 3 4 5% 31%	5467
April	1st to 10th	Night	21% 16% 4 8% 4 4 3 3 4 5% 28%	2013
	11th to 20th	Night	20% 11% 2 7% 5% 3 3 4 4 6% 33%	1975
	21st to end	Night	19% 9% 4 6% 4 4 5% 3 4 7% 34%	1814
May	1st to 10th	Night	27% 11% 3 7% 4 4 4 3 4 5% 27%	4053
	11th to 20th	Night	16% 8% 2 8% 5% 5% 5% 4 6% 7% 34%	3589
	21st to end	Night	23% 8% 2 6% 3 3 3 3 5% 7% 35%	3926
June	1st to 10th	Night	15% 7% 2 7% 6% 4 6% 4 6% 7% 36%	1600
	11th to 20th	Night	13% 7% 2 6% 4 5% 5% 4 7% 7% 40%	1600
	21st to end	Night	16% 6% 2 6% 5% 4 4 4 7% 7% 40%	1600
July	1st to 10th	Night	13% 6% 2 7% 4 4 4 3 6% 9% 41%	3600
	11th to 20th	Night	13% 5% 2 7% 4 4 5% 3 6% 8% 42%	3600
	21st to end	Night	11% 5% 2 6% 4 4 4 4 5% 7% 47%	4375
August	1st to 10th	Night	12% 8% 3 7% 5% 4 4 3 7% 8% 38%	2915
	11th to 20th	Night	19% 8% 3 7% 4 4 5% 4 6% 7% 35%	2875
	21st to end	Night	11% 6% 2 6% 5% 4 5% 4 6% 9% 43%	3451
September	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 5% 7% 44%	2000
	11th to 20th	Night	14% 5% 2 5% 3 3 4 3 4 7% 50%	2070
	21st to end	Night	19% 8% 2 6% 5% 4 4 4 6% 7% 34%	2270
October	1st to 10th	Night	19% 7% 3 7% 4 4 4 4 5% 6% 38%	2400
	11th to 20th	Night	25% 11% 3 7% 4 5% 4 4 4 6% 29%	2400
	21st to end	Night	30% 10% 3 7% 4 5% 3 3 5% 5% 24%	2640
November	1st to 10th	Night	19% 14% 4 8% 4 4 3 2 4 5% 33%	2666
	11th to 20th	Night	17% 18% 4 9% 6% 5% 4 3 5% 5% 23%	2940
	21st to end	Night	20% 21% 6% 10% 5% 5% 4 3 4 4 17%	2940
December	1st to 10th	Night	19% 28% 10% 13% 5% 4 3 3 3 3 8%	4720
	11th to 20th	Night	19% 27% 8% 14% 6% 4 4 2 3 3 10%	5348
	21st to end	Night	22% 24% 6% 12% 6% 5% 4 2 3 4 12%	7150

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% 6% 4 6% 3	4662
	11th to 20th	Day	85% 6% 4 2 2	5142
	21st to end	Day	81% 6% 7% 5% 1	5780
February	1st to 10th	Day	77% 6% 8% 7% 1	1890
	11th to 20th	Day	72% 7% 8% 10% 3	1852
	21st to end	Day	68% 7% 11% 10% 4	1638
March	1st to 10th	Day	60% 6% 13% 15% 5%	4957
	11th to 20th	Day	52% 8% 17% 15% 6% 1	4980
	21st to end	Day	58% 7% 9% 18% 7% 1	5465
April	1st to 10th	Day	56% 6% 12% 16% 9% 1	2385
	11th to 20th	Day	50% 6% 12% 19% 11% 2	2377
	21st to end	Day	53% 6% 11% 16% 12% 2	2600
May	1st to 10th	Day	56% 7% 11% 15% 8% 3	5897
	11th to 20th	Day	50% 10% 10% 17% 11% 1	6294
	21st to end	Day	47% 7% 13% 20% 11% 2	6949
June	1st to 10th	Day	43% 10% 13% 18% 15% 1	2800
	11th to 20th	Day	41% 11% 14% 21% 12% 1	2800
	21st to end	Day	39% 10% 15% 23% 13% 1	2800
July	1st to 10th	Day	36% 7% 13% 27% 16% 1	6290
	11th to 20th	Day	40% 7% 13% 23% 16% 1	6285
	21st to end	Day	36% 8% 14% 26% 17%	6287
August	1st to 10th	Day	43% 8% 14% 22% 13% 1	4221
	11th to 20th	Day	46% 10% 16% 21% 7%	4213
	21st to end	Day	37% 7% 12% 32% 11% 1	4412
September	1st to 10th	Day	43% 8% 13% 24% 12% 1	2400
	11th to 20th	Day	37% 8% 13% 28% 13% 1	2326
	21st to end	Day	50% 8% 13% 18% 9% 1	2130
October	1st to 10th	Day	43% 7% 12% 24% 13% 1	3260
	11th to 20th	Day	49% 6% 15% 18% 12% 1	3240
	21st to end	Day	52% 4 14% 21% 8% 1	3538
November	1st to 10th	Day	57% 7% 11% 17% 8% 2	1846
	11th to 20th	Day	61% 8% 12% 13% 5% 2	1680
	21st to end	Day	71% 7% 10% 9% 3	1680
December	1st to 10th	Day	80% 7% 7% 6% 1	2680
	11th to 20th	Day	75% 8% 8% 8% 1	3066
	21st to end	Day	77% 6% 8% 7% 2	4048
January	1st to 10th	Night	78% 8% 7% 5% 3	8234
	11th to 20th	Night	84% 7% 4 4 1	7800
	21st to end	Night	80% 6% 6% 5% 2	8401
February	1st to 10th	Night	75% 6% 8% 9% 2	2730
	11th to 20th	Night	70% 7% 11% 10% 3	2665
	21st to end	Night	69% 7% 9% 10% 5%	2161
March	1st to 10th	Night	61% 6% 10% 16% 6% 1	4941
	11th to 20th	Night	51% 8% 14% 18% 7% 2	4980
	21st to end	Night	53% 7% 11% 17% 11% 1	5467
April	1st to 10th	Night	58% 6% 10% 17% 9% 1	2013
	11th to 20th	Night	51% 6% 11% 18% 13% 2	1975
	21st to end	Night	50% 7% 10% 18% 13% 2	1814
May	1st to 10th	Night	58% 7% 8% 13% 9% 5%	4060
	11th to 20th	Night	45% 11% 14% 19% 10% 1	3590
	21st to end	Night	47% 6% 12% 20% 13% 2	3927
June	1st to 10th	Night	43% 10% 12% 20% 14% 2	1600
	11th to 20th	Night	40% 9% 13% 20% 16% 1	1600
	21st to end	Night	41% 9% 15% 20% 14% 2	1600
July	1st to 10th	Night	39% 5% 12% 25% 16% 2	3560
	11th to 20th	Night	39% 6% 14% 25% 16% 1	3552
	21st to end	Night	34% 6% 14% 22% 20% 2	4246
August	1st to 10th	Night	41% 7% 15% 25% 11% 1	2904
	11th to 20th	Night	47% 8% 15% 23% 8%	2902
	21st to end	Night	35% 7% 14% 31% 12% 1	3451
September	1st to 10th	Night	37% 8% 14% 26% 13% 2	2000
	11th to 20th	Night	34% 5% 14% 29% 16% 1	2070
	21st to end	Night	46% 9% 13% 20% 11% 1	2270
October	1st to 10th	Night	43% 6% 10% 23% 17%	3920
	11th to 20th	Night	51% 6% 12% 17% 13% 1	3899
	21st to end	Night	48% 6% 15% 21% 9% 1	4252
November	1st to 10th	Night	52% 6% 11% 20% 9% 2	2666
	11th to 20th	Night	60% 8% 11% 13% 6% 2	2940
	21st to end	Night	68% 9% 11% 9% 3	2940
December	1st to 10th	Night	79% 7% 9% 4 1	4720
	11th to 20th	Night	77% 7% 6% 8% 2	5348
	21st to end	Night	76% 7% 7% 8% 2	7150

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	34% 28% 25% 10% 3 1	220
	11th to 20th	Day	37% 30% 24% 8% 2	305
	21st to end	Day	33% 28% 26% 11% 3	363
February	1st to 10th	Day	36% 25% 27% 10% 2	330
	11th to 20th	Day	35% 32% 28% 5%	330
	21st to end	Day	39% 24% 29% 7%	343
March	1st to 10th	Day	32% 23% 29% 13% 4	508
	11th to 20th	Day	30% 23% 32% 10% 3 1	500
	21st to end	Day	34% 20% 27% 13% 5%	550
April	1st to 10th	Day	34% 21% 28% 12% 4	500
	11th to 20th	Day	36% 21% 22% 13% 6% 3	492
	21st to end	Day	32% 17% 31% 15% 4 1	500
May	1st to 10th	Day	31% 15% 30% 15% 6% 2	503
	11th to 20th	Day	22% 16% 30% 21% 9% 2	527
	21st to end	Day	21% 15% 31% 21% 10% 4	573
June	1st to 10th	Day	23% 16% 34% 21% 6% 1	530
	11th to 20th	Day	17% 14% 31% 27% 10% 2	530
	21st to end	Day	20% 14% 37% 20% 8% 2	530
July	1st to 10th	Day	14% 9% 28% 30% 15% 3	525
	11th to 20th	Day	15% 11% 29% 27% 13% 4	524
	21st to end	Day	13% 10% 35% 28% 11% 4	576
August	1st to 10th	Day	13% 10% 25% 29% 17% 6%	526
	11th to 20th	Day	16% 9% 27% 27% 16% 5%	519
	21st to end	Day	16% 9% 32% 24% 14% 5%	583
September	1st to 10th	Day	18% 12% 26% 22% 17% 6%	530
	11th to 20th	Day	13% 8% 21% 27% 22% 10%	530
	21st to end	Day	18% 19% 25% 22% 12% 4	530
October	1st to 10th	Day	23% 13% 26% 19% 15% 4	530
	11th to 20th	Day	30% 19% 28% 17% 5% 1	530
	21st to end	Day	38% 18% 24% 16% 3	583
November	1st to 10th	Day	30% 21% 27% 16% 5% 2	386
	11th to 20th	Day	43% 18% 25% 10% 3 1	220
	21st to end	Day	41% 26% 23% 8% 1	220
December	1st to 10th	Day	42% 22% 25% 8% 3	220
	11th to 20th	Day	41% 23% 23% 12%	220
	21st to end	Day	40% 26% 23% 7% 3	242
January	1st to 10th	Night	34% 26% 26% 11% 3	840
	11th to 20th	Night	35% 27% 26% 11% 1	840
	21st to end	Night	26% 26% 31% 14% 3	924
February	1st to 10th	Night	30% 27% 27% 15% 1	840
	11th to 20th	Night	31% 25% 31% 12% 1	823
	21st to end	Night	33% 23% 29% 14% 1	692
March	1st to 10th	Night	29% 19% 32% 16% 4	809
	11th to 20th	Night	31% 19% 28% 17% 4 1	800
	21st to end	Night	27% 16% 33% 18% 6% 1	880
April	1st to 10th	Night	30% 18% 30% 20% 2	800
	11th to 20th	Night	28% 16% 29% 20% 6% 1	791
	21st to end	Night	24% 11% 32% 26% 6%	614
May	1st to 10th	Night	26% 11% 24% 31% 9% 1	600
	11th to 20th	Night	15% 9% 33% 37% 7%	408
	21st to end	Night	18% 10% 26% 33% 13% 1	433
June	1st to 10th	Night	15% 9% 33% 31% 12% 1	400
	11th to 20th	Night	15% 12% 27% 37% 9%	400
	21st to end	Night	18% 7% 33% 34% 9%	400
July	1st to 10th	Night	10% 6% 25% 41% 16% 1	390
	11th to 20th	Night	12% 5% 29% 37% 16% 1	388
	21st to end	Night	14% 5% 26% 36% 18% 2	630
August	1st to 10th	Night	12% 5% 20% 41% 20% 2	597
	11th to 20th	Night	12% 6% 23% 34% 23% 2	594
	21st to end	Night	14% 7% 26% 32% 21% 1	797
September	1st to 10th	Night	18% 8% 23% 32% 16% 3	800
	11th to 20th	Night	15% 6% 20% 29% 27% 3	800
	21st to end	Night	21% 11% 22% 27% 15% 3	800
October	1st to 10th	Night	20% 10% 28% 24% 16% 1	800
	11th to 20th	Night	29% 17% 25% 22% 6% 1	800
	21st to end	Night	35% 15% 28% 19% 3	880
November	1st to 10th	Night	29% 15% 28% 21% 6% 1	820
	11th to 20th	Night	31% 20% 29% 14% 5% 1	840
	21st to end	Night	34% 24% 26% 13% 4	840
December	1st to 10th	Night	38% 25% 20% 13% 4	840
	11th to 20th	Night	37% 25% 24% 13% 2	840
	21st to end	Night	37% 23% 26% 13% 1	924

Page generated at 2026-02-28 01:40:19. Version ac07

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