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 2024-12-02.

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 8% 5% 4 4 4 5% 6% 30%	101659
	Night	18% 15% 5% 9% 5% 4 4 3 4 5% 27%	97963
Transparency	Day	52% 8% 12% 18% 9% 1	105263
	Night	56% 8% 11% 16% 8% 1	102447
Seeing	Day	26% 17% 28% 18% 8% 2	14612
	Night	26% 16% 27% 22% 8% 1	24051

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	17% 30% 11% 14% 6% 5% 2 2 2 2 9%	2517
	11th to 20th	Day	12% 25% 10% 21% 9% 5% 4 2 3 3 6%	2758
	21st to end	Day	21% 21% 9% 14% 7% 6% 4 3 3 3 8%	3124
February	1st to 10th	Day	17% 22% 7% 17% 9% 5% 5% 3 2 3 11%	1710
	11th to 20th	Day	13% 20% 8% 16% 8% 4 5% 2 3 5% 15%	1663
	21st to end	Day	20% 15% 6% 14% 7% 4 4 4 3 6% 17%	1485
March	1st to 10th	Day	20% 16% 5% 8% 4 4 4 3 4 5% 28%	3432
	11th to 20th	Day	11% 22% 6% 9% 4 4 3 4 4 5% 28%	3480
	21st to end	Day	18% 15% 5% 10% 5% 3 4 3 5% 5% 27%	3815
April	1st to 10th	Day	20% 15% 3 7% 5% 3 4 3 4 6% 30%	2266
	11th to 20th	Day	20% 12% 3 7% 4 4 3 3 5% 6% 34%	2257
	21st to end	Day	22% 11% 3 7% 4 4 4 3 4 5% 33%	2470
May	1st to 10th	Day	23% 9% 3 6% 4 5% 4 4 5% 6% 31%	4108
	11th to 20th	Day	18% 10% 3 8% 5% 5% 5% 5% 5% 7% 30%	4384
	21st to end	Day	15% 6% 2 5% 4 4 4 4 5% 7% 44%	4837
June	1st to 10th	Day	17% 7% 3 5% 4 5% 5% 4 6% 7% 36%	2660
	11th to 20th	Day	15% 5% 2 6% 5% 4 5% 5% 7% 9% 38%	2660
	21st to end	Day	16% 6% 2 5% 3 4 4 4 6% 9% 40%	2660
July	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 6% 10% 42%	4440
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 7% 8% 40%	4440
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 6% 7% 9% 40%	4501
August	1st to 10th	Day	18% 6% 3 6% 5% 4 4 4 5% 8% 38%	4104
	11th to 20th	Day	15% 6% 2 8% 5% 6% 6% 5% 8% 9% 29%	4044
	21st to end	Day	14% 5% 2 5% 4 4 4 3 6% 9% 43%	4277
September	1st to 10th	Day	16% 7% 3 7% 5% 4 4 4 6% 6% 39%	2280
	11th to 20th	Day	15% 5% 2 5% 4 4 4 4 6% 8% 43%	2210
	21st to end	Day	18% 8% 3 8% 6% 5% 4 4 5% 7% 32%	2024
October	1st to 10th	Day	23% 8% 3 7% 4 4 4 3 6% 7% 31%	1900
	11th to 20th	Day	25% 11% 3 8% 5% 4 3 3 5% 5% 28%	1900
	21st to end	Day	30% 16% 3 8% 4 4 3 3 4 5% 20%	2090
November	1st to 10th	Day	19% 14% 4 7% 6% 4 4 3 5% 6% 28%	1757
	11th to 20th	Day	16% 22% 5% 7% 6% 4 3 4 5% 5% 22%	1600
	21st to end	Day	22% 23% 6% 10% 5% 4 3 3 3 5% 16%	1600
December	1st to 10th	Day	19% 28% 8% 12% 6% 5% 5% 2 3 3 10%	1528
	11th to 20th	Day	21% 23% 7% 16% 7% 4 3 2 3 3 11%	1906
	21st to end	Day	25% 28% 7% 11% 5% 4 3 2 2 2 10%	2772
January	1st to 10th	Night	18% 30% 10% 13% 6% 4 3 2 2 3 9%	4429
	11th to 20th	Night	13% 26% 10% 20% 9% 4 3 3 3 2 7%	4184
	21st to end	Night	21% 24% 9% 16% 6% 5% 3 2 2 3 9%	4532
February	1st to 10th	Night	18% 22% 7% 14% 7% 5% 4 3 3 3 14%	2470
	11th to 20th	Night	15% 17% 8% 16% 7% 6% 4 3 4 5% 16%	2403
	21st to end	Night	19% 17% 5% 15% 7% 5% 4 3 4 4 17%	1962
March	1st to 10th	Night	21% 15% 3 9% 5% 5% 3 2 3 4 30%	3408
	11th to 20th	Night	20% 21% 5% 9% 4 4 3 3 3 4 24%	3479
	21st to end	Night	16% 13% 4 8% 4 3 3 3 4 5% 37%	3817
April	1st to 10th	Night	21% 16% 4 7% 4 4 3 3 4 5% 29%	1913
	11th to 20th	Night	20% 11% 2 7% 5% 4 3 4 4 6% 34%	1875
	21st to end	Night	20% 9% 4 6% 4 4 5% 3 4 7% 34%	1724
May	1st to 10th	Night	26% 9% 3 6% 4 4 4 3 5% 6% 29%	2833
	11th to 20th	Night	15% 8% 3 8% 5% 6% 5% 4 6% 7% 33%	2509
	21st to end	Night	16% 6% 2 5% 4 4 3 3 6% 8% 44%	2738
June	1st to 10th	Night	15% 7% 2 6% 6% 4 6% 4 7% 7% 35%	1520
	11th to 20th	Night	13% 7% 2 6% 5% 5% 5% 4 6% 7% 40%	1520
	21st to end	Night	17% 6% 2 6% 4 4 4 4 7% 7% 39%	1520
July	1st to 10th	Night	13% 7% 2 7% 4 4 4 3 6% 8% 41%	2520
	11th to 20th	Night	14% 5% 3 7% 4 4 5% 3 5% 7% 43%	2520
	21st to end	Night	11% 5% 2 6% 4 5% 5% 5% 6% 7% 44%	3048
August	1st to 10th	Night	13% 8% 3 6% 5% 4 4 3 7% 8% 38%	2825
	11th to 20th	Night	19% 8% 3 7% 4 4 5% 4 6% 6% 35%	2785
	21st to end	Night	11% 6% 2 6% 5% 4 5% 4 7% 9% 43%	3345
September	1st to 10th	Night	17% 6% 2 5% 4 4 4 3 5% 7% 44%	1900
	11th to 20th	Night	14% 6% 2 5% 3 3 4 3 4 7% 49%	1966
	21st to end	Night	19% 8% 2 6% 5% 4 5% 4 6% 7% 33%	2156
October	1st to 10th	Night	19% 7% 3 8% 4 4 4 4 5% 6% 36%	2280
	11th to 20th	Night	25% 11% 3 7% 4 5% 4 4 4 6% 28%	2280
	21st to end	Night	31% 10% 3 7% 4 5% 3 3 5% 5% 24%	2508
November	1st to 10th	Night	19% 13% 4 7% 4 4 4 2 4 5% 34%	2536
	11th to 20th	Night	18% 18% 4 9% 6% 5% 4 3 5% 5% 24%	2800
	21st to end	Night	20% 21% 6% 10% 5% 5% 4 3 4 4 17%	2800
December	1st to 10th	Night	22% 27% 7% 10% 6% 4 3 3 4 4 11%	2686
	11th to 20th	Night	18% 24% 10% 12% 6% 4 4 2 3 3 12%	3288
	21st to end	Night	27% 25% 6% 10% 4 4 4 3 4 4 9%	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	79% 9% 4 5% 2	2518
	11th to 20th	Day	80% 10% 6% 3 1	2758
	21st to end	Day	80% 8% 7% 3 1	3124
February	1st to 10th	Day	78% 6% 9% 6% 1	1710
	11th to 20th	Day	72% 7% 7% 11% 3	1672
	21st to end	Day	67% 7% 11% 9% 4	1485
March	1st to 10th	Day	56% 8% 11% 19% 6%	3435
	11th to 20th	Day	55% 10% 16% 13% 5% 1	3480
	21st to end	Day	54% 9% 11% 18% 6% 1	3815
April	1st to 10th	Day	56% 6% 12% 16% 9% 1	2266
	11th to 20th	Day	50% 7% 12% 18% 11% 2	2257
	21st to end	Day	54% 6% 11% 16% 12% 2	2470
May	1st to 10th	Day	50% 8% 13% 18% 10% 2	4120
	11th to 20th	Day	49% 13% 13% 18% 7% 1	4388
	21st to end	Day	36% 9% 15% 23% 14% 3	4837
June	1st to 10th	Day	43% 10% 13% 18% 14% 1	2660
	11th to 20th	Day	40% 11% 14% 22% 12% 1	2660
	21st to end	Day	40% 9% 15% 23% 13% 1	2660
July	1st to 10th	Day	37% 7% 13% 28% 14% 2	4370
	11th to 20th	Day	38% 8% 14% 23% 16%	4365
	21st to end	Day	38% 10% 16% 21% 15%	4357
August	1st to 10th	Day	44% 8% 14% 22% 12% 1	4091
	11th to 20th	Day	46% 10% 16% 21% 6%	4083
	21st to end	Day	37% 7% 12% 31% 11% 1	4277
September	1st to 10th	Day	44% 8% 13% 23% 11% 1	2280
	11th to 20th	Day	38% 8% 13% 26% 13% 1	2210
	21st to end	Day	50% 8% 13% 19% 9% 1	2024
October	1st to 10th	Day	44% 7% 13% 23% 12% 1	3160
	11th to 20th	Day	49% 6% 15% 18% 12% 1	3140
	21st to end	Day	52% 4 15% 21% 7%	3428
November	1st to 10th	Day	57% 7% 11% 16% 8% 2	1757
	11th to 20th	Day	61% 8% 12% 13% 5% 2	1600
	21st to end	Day	71% 7% 10% 9% 3	1600
December	1st to 10th	Day	76% 8% 8% 7% 1	1528
	11th to 20th	Day	76% 9% 8% 6% 1	1906
	21st to end	Day	81% 6% 7% 5%	2772
January	1st to 10th	Night	76% 11% 7% 4 1	4429
	11th to 20th	Night	79% 11% 5% 3 1	4184
	21st to end	Night	81% 8% 6% 4 1	4532
February	1st to 10th	Night	75% 6% 8% 8% 2	2470
	11th to 20th	Night	70% 7% 10% 10% 3	2405
	21st to end	Night	68% 7% 9% 10% 5%	1962
March	1st to 10th	Night	57% 8% 10% 20% 6%	3418
	11th to 20th	Night	59% 10% 12% 13% 5% 1	3480
	21st to end	Night	46% 8% 13% 20% 12% 2	3817
April	1st to 10th	Night	57% 6% 10% 18% 9%	1913
	11th to 20th	Night	51% 6% 11% 17% 13% 2	1875
	21st to end	Night	50% 7% 11% 17% 13% 2	1724
May	1st to 10th	Night	53% 8% 10% 17% 9% 3	2840
	11th to 20th	Night	44% 12% 16% 19% 7% 1	2510
	21st to end	Night	36% 7% 14% 23% 18% 2	2739
June	1st to 10th	Night	43% 10% 12% 19% 14% 1	1520
	11th to 20th	Night	40% 9% 13% 21% 17% 1	1520
	21st to end	Night	42% 8% 15% 21% 13% 2	1520
July	1st to 10th	Night	41% 6% 12% 23% 16% 2	2480
	11th to 20th	Night	39% 6% 14% 25% 14% 1	2472
	21st to end	Night	36% 8% 17% 22% 15% 2	2951
August	1st to 10th	Night	41% 7% 15% 26% 10% 1	2814
	11th to 20th	Night	47% 8% 15% 22% 8%	2812
	21st to end	Night	35% 7% 13% 31% 13% 1	3345
September	1st to 10th	Night	38% 8% 14% 24% 14% 2	1900
	11th to 20th	Night	35% 6% 14% 27% 17% 2	1966
	21st to end	Night	47% 9% 13% 20% 11% 1	2156
October	1st to 10th	Night	44% 6% 10% 23% 16%	3800
	11th to 20th	Night	51% 6% 12% 17% 13% 1	3779
	21st to end	Night	48% 6% 16% 21% 8% 1	4120
November	1st to 10th	Night	52% 7% 11% 20% 9% 2	2536
	11th to 20th	Night	60% 8% 11% 12% 6% 2	2800
	21st to end	Night	67% 9% 12% 10% 3	2800
December	1st to 10th	Night	76% 7% 10% 6% 1	2686
	11th to 20th	Night	74% 10% 9% 6% 1	3288
	21st to end	Night	78% 8% 8% 6%	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	31% 28% 26% 12% 3 1	180
	11th to 20th	Day	34% 32% 23% 8% 2	249
	21st to end	Day	29% 28% 26% 12% 4	297
February	1st to 10th	Day	37% 27% 26% 9% 1	270
	11th to 20th	Day	33% 33% 27% 6%	270
	21st to end	Day	40% 24% 28% 8%	288
March	1st to 10th	Day	32% 22% 29% 13% 4	460
	11th to 20th	Day	30% 23% 33% 10% 3 1	460
	21st to end	Day	33% 20% 28% 13% 5%	506
April	1st to 10th	Day	33% 21% 29% 12% 5%	460
	11th to 20th	Day	35% 22% 22% 13% 6% 3	452
	21st to end	Day	32% 17% 31% 16% 4 1	460
May	1st to 10th	Day	30% 15% 29% 16% 6% 3	463
	11th to 20th	Day	23% 15% 30% 22% 10% 2	487
	21st to end	Day	17% 15% 31% 22% 10% 4	529
June	1st to 10th	Day	23% 15% 34% 22% 6% 1	490
	11th to 20th	Day	18% 13% 30% 28% 11% 1	490
	21st to end	Day	21% 15% 36% 19% 8% 1	490
July	1st to 10th	Day	14% 9% 28% 30% 16% 4	485
	11th to 20th	Day	16% 11% 29% 26% 13% 4	484
	21st to end	Day	13% 11% 34% 28% 10% 4	532
August	1st to 10th	Day	14% 10% 26% 28% 16% 5%	486
	11th to 20th	Day	15% 10% 28% 26% 16% 5%	479
	21st to end	Day	15% 9% 32% 25% 14% 6%	539
September	1st to 10th	Day	18% 12% 27% 23% 15% 6%	490
	11th to 20th	Day	13% 8% 21% 27% 22% 9%	490
	21st to end	Day	19% 19% 25% 21% 11% 4	490
October	1st to 10th	Day	24% 14% 26% 18% 15% 3	490
	11th to 20th	Day	30% 19% 27% 17% 5% 1	490
	21st to end	Day	39% 18% 24% 16% 3	539
November	1st to 10th	Day	30% 20% 27% 16% 5% 2	357
	11th to 20th	Day	41% 20% 26% 10% 3 1	200
	21st to end	Day	41% 28% 22% 9% 2	200
December	1st to 10th	Day	41% 23% 26% 8% 3	182
	11th to 20th	Day	41% 23% 21% 14% 1	180
	21st to end	Day	41% 27% 24% 7% 2	198
January	1st to 10th	Night	34% 25% 26% 11% 4	760
	11th to 20th	Night	35% 26% 27% 11% 1	760
	21st to end	Night	26% 25% 31% 14% 4	836
February	1st to 10th	Night	31% 28% 27% 14% 1	760
	11th to 20th	Night	29% 25% 33% 11% 2	743
	21st to end	Night	33% 23% 29% 14% 1	628
March	1st to 10th	Night	28% 19% 33% 16% 4	760
	11th to 20th	Night	31% 19% 28% 17% 4 1	760
	21st to end	Night	27% 16% 33% 18% 6% 1	836
April	1st to 10th	Night	29% 18% 31% 20% 3	760
	11th to 20th	Night	28% 16% 29% 20% 6% 1	751
	21st to end	Night	24% 11% 31% 27% 7%	584
May	1st to 10th	Night	25% 11% 23% 31% 9% 1	570
	11th to 20th	Night	15% 9% 32% 37% 7%	388
	21st to end	Night	16% 9% 26% 34% 14% 1	411
June	1st to 10th	Night	15% 9% 33% 30% 12% 1	380
	11th to 20th	Night	15% 12% 27% 37% 9%	380
	21st to end	Night	19% 6% 34% 32% 9%	380
July	1st to 10th	Night	10% 7% 26% 40% 17% 1	370
	11th to 20th	Night	12% 5% 30% 35% 17% 1	368
	21st to end	Night	14% 5% 27% 35% 18% 2	598
August	1st to 10th	Night	13% 5% 21% 40% 19% 2	567
	11th to 20th	Night	13% 6% 24% 33% 23% 2	564
	21st to end	Night	13% 7% 26% 31% 21% 1	757
September	1st to 10th	Night	19% 8% 24% 31% 16% 3	760
	11th to 20th	Night	16% 6% 21% 28% 26% 3	760
	21st to end	Night	21% 11% 23% 27% 15% 3	760
October	1st to 10th	Night	21% 11% 29% 22% 16% 1	760
	11th to 20th	Night	29% 18% 24% 21% 7% 1	760
	21st to end	Night	35% 16% 28% 18% 3	836
November	1st to 10th	Night	27% 15% 28% 22% 7% 1	780
	11th to 20th	Night	31% 20% 29% 14% 5% 1	800
	21st to end	Night	33% 24% 26% 13% 4	800
December	1st to 10th	Night	39% 25% 20% 12% 4	768
	11th to 20th	Night	37% 24% 25% 12% 2	760
	21st to end	Night	37% 24% 26% 12% 1	836

Page generated at 2024-11-29 10:43:08. Version ac07

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