APUS Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the APUS Observatory clear sky chart from 2005-11-06 to 2026-05-02.

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	13% 6% 2 6% 4 5% 5% 4 6% 8% 40%	131805
	Night	13% 7% 2 6% 4 4 5% 4 6% 7% 41%	130648
Transparency	Day	39% 6% 12% 23% 16% 3	135402
	Night	39% 6% 12% 24% 16% 3	135121
Seeing	Day	18% 11% 25% 24% 16% 6%	16790
	Night	17% 9% 29% 32% 13% 1	26466

By Month

Percentage Cloud Cover SE (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% 5% 2 7% 5% 5% 5% 4 6% 7% 41%	4919
	11th to 20th	Day	12% 9% 3 6% 4 4 4 3 5% 6% 45%	5280
	21st to end	Day	12% 6% 2 6% 5% 4 6% 5% 7% 7% 38%	5780
February	1st to 10th	Day	13% 9% 3 7% 5% 4 5% 4 6% 6% 39%	1890
	11th to 20th	Day	16% 8% 3 8% 5% 4 3 3 5% 6% 38%	1853
	21st to end	Day	16% 9% 3 7% 4 4 4 3 4 6% 39%	1735
March	1st to 10th	Day	17% 7% 3 6% 4 5% 4 3 4 6% 41%	5045
	11th to 20th	Day	10% 6% 2 6% 4 4 4 3 6% 7% 48%	5090
	21st to end	Day	15% 7% 2 7% 5% 6% 4 4 5% 7% 39%	5603
April	1st to 10th	Day	14% 7% 2 6% 5% 5% 4 4 6% 7% 39%	2520
	11th to 20th	Day	13% 7% 2 5% 4 4 5% 4 5% 6% 46%	2494
	21st to end	Day	17% 7% 3 5% 4 4 3 4 6% 6% 41%	2541
May	1st to 10th	Day	18% 7% 3 7% 4 4 5% 5% 7% 9% 30%	5875
	11th to 20th	Day	21% 7% 2 6% 4 4 5% 4 7% 9% 30%	6043
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 4 6% 9% 37%	6934
June	1st to 10th	Day	11% 6% 2 5% 4 4 5% 5% 7% 8% 43%	2800
	11th to 20th	Day	11% 6% 3 6% 4 5% 5% 5% 7% 10% 39%	2800
	21st to end	Day	9% 4 2 5% 4 5% 6% 4 7% 10% 45%	2800
July	1st to 10th	Day	6% 4 2 6% 5% 5% 5% 5% 7% 10% 45%	6360
	11th to 20th	Day	8% 6% 2 7% 6% 6% 7% 6% 8% 12% 33%	6046
	21st to end	Day	8% 6% 2 7% 5% 5% 6% 4 7% 11% 40%	6424
August	1st to 10th	Day	15% 6% 2 6% 5% 5% 5% 5% 7% 9% 35%	4234
	11th to 20th	Day	8% 5% 2 7% 4 5% 6% 5% 6% 9% 42%	4076
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 6% 8% 48%	4312
September	1st to 10th	Day	15% 5% 2 6% 3 4 4 3 6% 6% 46%	2400
	11th to 20th	Day	12% 6% 2 5% 3 3 4 3 7% 7% 48%	2383
	21st to end	Day	20% 7% 2 7% 3 4 4 3 5% 7% 36%	2095
October	1st to 10th	Day	16% 5% 2 5% 3 3 4 3 5% 8% 46%	2000
	11th to 20th	Day	15% 4 2 5% 4 4 4 4 5% 6% 47%	2000
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 6% 49%	2200
November	1st to 10th	Day	10% 6% 2 4 4 3 4 3 5% 7% 51%	1991
	11th to 20th	Day	15% 6% 2 4 3 4 4 4 5% 6% 46%	1804
	21st to end	Day	14% 7% 3 5% 4 4 5% 3 5% 7% 44%	1680
December	1st to 10th	Day	12% 6% 2 6% 3 5% 5% 4 6% 7% 44%	2680
	11th to 20th	Day	18% 8% 2 6% 4 4 4 3 5% 6% 39%	3070
	21st to end	Day	15% 8% 3 8% 5% 5% 5% 5% 7% 8% 32%	4048
January	1st to 10th	Night	9% 8% 3 8% 5% 5% 5% 4 6% 8% 40%	7957
	11th to 20th	Night	13% 8% 2 6% 4 3 4 3 5% 5% 47%	7680
	21st to end	Night	12% 7% 2 6% 4 4 4 4 5% 6% 45%	8401
February	1st to 10th	Night	13% 7% 3 7% 4 4 5% 3 6% 7% 41%	2730
	11th to 20th	Night	17% 8% 3 6% 4 4 5% 3 5% 5% 39%	2652
	21st to end	Night	15% 9% 2 7% 5% 4 4 3 6% 6% 39%	2067
March	1st to 10th	Night	16% 7% 2 6% 4 3 3 3 4 6% 46%	5028
	11th to 20th	Night	9% 5% 2 6% 5% 4 5% 4 6% 7% 48%	5090
	21st to end	Night	11% 7% 3 7% 5% 4 5% 4 5% 6% 44%	5557
April	1st to 10th	Night	15% 8% 3 7% 5% 4 5% 4 5% 6% 39%	2100
	11th to 20th	Night	13% 6% 3 7% 5% 3 4 3 7% 8% 42%	2075
	21st to end	Night	16% 5% 2 5% 3 5% 5% 4 5% 8% 43%	2095
May	1st to 10th	Night	20% 8% 3 8% 5% 5% 5% 4 6% 8% 28%	4042
	11th to 20th	Night	23% 8% 3 6% 4 4 4 4 6% 8% 30%	3832
	21st to end	Night	16% 7% 3 7% 5% 5% 5% 4 7% 8% 34%	3920
June	1st to 10th	Night	10% 6% 2 5% 4 4 5% 4 7% 9% 44%	1600
	11th to 20th	Night	12% 6% 2 6% 5% 3 5% 5% 8% 9% 41%	1600
	21st to end	Night	8% 5% 2 6% 5% 5% 5% 4 7% 11% 44%	1600
July	1st to 10th	Night	6% 5% 2 8% 5% 6% 6% 5% 7% 10% 39%	3598
	11th to 20th	Night	6% 6% 3 10% 7% 7% 7% 6% 8% 10% 29%	3907
	21st to end	Night	7% 5% 3 7% 4 5% 6% 5% 7% 10% 41%	4427
August	1st to 10th	Night	14% 6% 2 8% 5% 5% 6% 5% 6% 10% 33%	2915
	11th to 20th	Night	9% 6% 3 7% 5% 5% 6% 5% 7% 9% 39%	2938
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 46%	3586
September	1st to 10th	Night	14% 6% 2 5% 5% 4 4 4 6% 7% 45%	2000
	11th to 20th	Night	10% 6% 3 6% 4 3 4 3 4 6% 51%	2013
	21st to end	Night	18% 8% 2 6% 4 5% 4 3 5% 6% 39%	2305
October	1st to 10th	Night	18% 5% 2 5% 3 3 3 3 4 6% 47%	2400
	11th to 20th	Night	14% 4 2 5% 3 3 3 3 5% 6% 51%	2400
	21st to end	Night	15% 5% 2 5% 3 3 3 3 5% 7% 50%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 3 5% 6% 54%	2524
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 7% 46%	2814
	21st to end	Night	14% 5% 2 6% 4 4 4 4 6% 8% 44%	2940
December	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 6% 8% 48%	4720
	11th to 20th	Night	17% 7% 2 6% 4 4 5% 4 5% 6% 38%	5345
	21st to end	Night	14% 8% 3 7% 5% 5% 6% 5% 7% 8% 31%	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	42% 4 11% 24% 14% 5%	4920
	11th to 20th	Day	40% 3 8% 21% 20% 8%	5280
	21st to end	Day	42% 4 11% 28% 12% 3	5780
February	1st to 10th	Day	43% 6% 12% 23% 13% 2	1890
	11th to 20th	Day	47% 4 10% 23% 14% 2	1862
	21st to end	Day	46% 5% 11% 21% 14% 3	1735
March	1st to 10th	Day	43% 5% 9% 22% 14% 7%	5045
	11th to 20th	Day	33% 5% 11% 26% 19% 5%	5090
	21st to end	Day	44% 5% 11% 18% 17% 5%	5603
April	1st to 10th	Day	43% 5% 12% 18% 18% 4	2520
	11th to 20th	Day	38% 5% 11% 22% 19% 5%	2494
	21st to end	Day	43% 6% 9% 24% 15% 3	2541
May	1st to 10th	Day	47% 8% 13% 19% 10% 2	5910
	11th to 20th	Day	48% 8% 10% 24% 9% 1	6047
	21st to end	Day	40% 8% 13% 21% 15% 2	6949
June	1st to 10th	Day	36% 8% 14% 25% 17% 1	2800
	11th to 20th	Day	38% 9% 15% 22% 14% 2	2800
	21st to end	Day	32% 9% 14% 26% 17% 2	2800
July	1st to 10th	Day	33% 10% 15% 23% 15% 4	6290
	11th to 20th	Day	40% 9% 16% 24% 8% 1	5973
	21st to end	Day	37% 7% 10% 22% 22% 1	6240
August	1st to 10th	Day	43% 9% 14% 25% 9%	4221
	11th to 20th	Day	37% 7% 12% 28% 15% 1	4115
	21st to end	Day	33% 7% 14% 27% 17% 2	4312
September	1st to 10th	Day	38% 7% 10% 20% 21% 4	2400
	11th to 20th	Day	34% 5% 10% 27% 21% 2	2383
	21st to end	Day	47% 5% 9% 19% 18% 2	2095
October	1st to 10th	Day	36% 3 9% 21% 27% 5%	3260
	11th to 20th	Day	28% 4 10% 29% 24% 6%	3240
	21st to end	Day	25% 3 11% 30% 28% 4	3538
November	1st to 10th	Day	32% 4 13% 25% 21% 4	1991
	11th to 20th	Day	37% 5% 11% 22% 22% 4	1804
	21st to end	Day	39% 5% 12% 28% 15% 1	1680
December	1st to 10th	Day	37% 6% 14% 29% 11% 4	2680
	11th to 20th	Day	45% 5% 14% 24% 10% 2	3066
	21st to end	Day	45% 9% 16% 17% 13%	4048
January	1st to 10th	Night	41% 5% 11% 25% 17% 2	7958
	11th to 20th	Night	38% 3 9% 22% 21% 7%	7680
	21st to end	Night	38% 4 10% 29% 16% 2	8401
February	1st to 10th	Night	42% 6% 13% 24% 14% 2	2730
	11th to 20th	Night	46% 4 11% 21% 14% 4	2654
	21st to end	Night	45% 4 12% 21% 13% 5%	2067
March	1st to 10th	Night	40% 4 10% 26% 13% 8%	5028
	11th to 20th	Night	32% 6% 11% 29% 16% 5%	5090
	21st to end	Night	40% 4 12% 22% 19% 4	5557
April	1st to 10th	Night	44% 6% 10% 20% 17% 4	2100
	11th to 20th	Night	39% 7% 11% 21% 18% 5%	2075
	21st to end	Night	38% 6% 12% 26% 14% 4	2095
May	1st to 10th	Night	52% 7% 11% 16% 9% 4	4069
	11th to 20th	Night	49% 7% 12% 22% 10% 1	3833
	21st to end	Night	44% 8% 13% 24% 8% 2	3927
June	1st to 10th	Night	35% 9% 10% 26% 18% 3	1600
	11th to 20th	Night	37% 10% 13% 25% 14% 2	1600
	21st to end	Night	33% 8% 15% 24% 18% 2	1600
July	1st to 10th	Night	38% 10% 15% 18% 15% 4	3560
	11th to 20th	Night	46% 11% 16% 15% 10% 1	3860
	21st to end	Night	35% 8% 14% 23% 20% 1	4297
August	1st to 10th	Night	44% 10% 15% 22% 9% 1	2904
	11th to 20th	Night	38% 8% 13% 25% 15% 1	2965
	21st to end	Night	34% 7% 14% 28% 16% 2	3586
September	1st to 10th	Night	37% 6% 12% 22% 19% 4	2000
	11th to 20th	Night	35% 4 9% 27% 23% 2	2013
	21st to end	Night	45% 6% 11% 19% 17% 2	2305
October	1st to 10th	Night	39% 4 11% 22% 21% 3	3920
	11th to 20th	Night	25% 5% 11% 23% 31% 5%	3899
	21st to end	Night	24% 3 10% 32% 26% 4	4252
November	1st to 10th	Night	31% 4 11% 26% 23% 5%	2524
	11th to 20th	Night	37% 4 9% 24% 21% 5%	2814
	21st to end	Night	38% 5% 12% 25% 17% 3	2940
December	1st to 10th	Night	34% 5% 14% 30% 16% 1	4720
	11th to 20th	Night	46% 5% 12% 20% 15% 3	5348
	21st to end	Night	45% 9% 15% 18% 12% 1	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	28% 24% 26% 17% 4 1	267
	11th to 20th	Day	28% 26% 30% 13% 3	330
	21st to end	Day	23% 25% 34% 13% 5% 1	363
February	1st to 10th	Day	24% 21% 36% 15% 4	330
	11th to 20th	Day	35% 22% 33% 8% 2	340
	21st to end	Day	28% 15% 35% 18% 4	446
March	1st to 10th	Day	24% 20% 27% 24% 4	540
	11th to 20th	Day	24% 20% 31% 18% 7% 1	540
	21st to end	Day	22% 17% 33% 21% 6% 1	594
April	1st to 10th	Day	24% 20% 26% 21% 8% 1	540
	11th to 20th	Day	18% 16% 32% 18% 12% 5%	532
	21st to end	Day	23% 13% 28% 23% 11% 2	540
May	1st to 10th	Day	21% 14% 28% 22% 12% 4	507
	11th to 20th	Day	20% 12% 26% 20% 16% 5%	527
	21st to end	Day	18% 6% 24% 23% 21% 9%	573
June	1st to 10th	Day	14% 7% 26% 25% 19% 9%	530
	11th to 20th	Day	14% 6% 24% 28% 20% 8%	530
	21st to end	Day	9% 6% 23% 29% 23% 10%	530
July	1st to 10th	Day	10% 3 16% 27% 29% 14%	525
	11th to 20th	Day	6% 3 14% 31% 31% 14%	524
	21st to end	Day	11% 3 17% 30% 28% 10%	576
August	1st to 10th	Day	11% 3 17% 27% 28% 14%	526
	11th to 20th	Day	12% 4 17% 28% 26% 13%	519
	21st to end	Day	9% 2 19% 26% 27% 16%	583
September	1st to 10th	Day	16% 5% 18% 23% 26% 13%	530
	11th to 20th	Day	14% 3 15% 29% 25% 14%	530
	21st to end	Day	26% 7% 20% 25% 17% 5%	530
October	1st to 10th	Day	18% 6% 17% 27% 23% 9%	530
	11th to 20th	Day	15% 11% 28% 26% 16% 4	530
	21st to end	Day	15% 10% 33% 26% 13% 3	583
November	1st to 10th	Day	15% 13% 28% 28% 13% 3	499
	11th to 20th	Day	23% 11% 31% 24% 9% 2	344
	21st to end	Day	22% 16% 31% 22% 7% 1	220
December	1st to 10th	Day	21% 19% 33% 21% 4 1	220
	11th to 20th	Day	23% 17% 29% 23% 8%	220
	21st to end	Day	22% 13% 34% 22% 8%	242
January	1st to 10th	Night	17% 17% 40% 23% 3	840
	11th to 20th	Night	22% 13% 35% 27% 4	840
	21st to end	Night	18% 13% 43% 24% 2	924
February	1st to 10th	Night	18% 15% 36% 30% 2	840
	11th to 20th	Night	23% 18% 36% 21% 1	823
	21st to end	Night	22% 17% 37% 21% 2	692
March	1st to 10th	Night	20% 15% 37% 25% 3	840
	11th to 20th	Night	23% 15% 32% 24% 6%	840
	21st to end	Night	18% 14% 39% 25% 4 1	924
April	1st to 10th	Night	19% 17% 34% 26% 5%	840
	11th to 20th	Night	17% 11% 34% 29% 8%	831
	21st to end	Night	18% 9% 32% 33% 7%	835
May	1st to 10th	Night	21% 5% 28% 35% 12%	603
	11th to 20th	Night	19% 5% 25% 39% 12% 1	508
	21st to end	Night	14% 3 23% 39% 20% 1	433
June	1st to 10th	Night	11% 3 23% 40% 23% 1	400
	11th to 20th	Night	13% 3 18% 44% 22% 1	400
	21st to end	Night	8% 1 20% 49% 21% 1	400
July	1st to 10th	Night	11% 3 13% 42% 27% 4	390
	11th to 20th	Night	6% 2 13% 40% 37% 3	514
	21st to end	Night	7% 2 13% 44% 32% 3	657
August	1st to 10th	Night	10% 1 11% 44% 32% 2	597
	11th to 20th	Night	11% 3 12% 37% 33% 3	631
	21st to end	Night	12% 3 14% 40% 27% 4	880
September	1st to 10th	Night	15% 1 14% 38% 27% 4	800
	11th to 20th	Night	12% 3 16% 36% 30% 3	800
	21st to end	Night	20% 5% 21% 33% 21% 1	800
October	1st to 10th	Night	20% 5% 20% 30% 23% 3	800
	11th to 20th	Night	16% 6% 27% 36% 14% 1	800
	21st to end	Night	17% 8% 31% 33% 11%	880
November	1st to 10th	Night	13% 8% 30% 37% 11% 1	820
	11th to 20th	Night	18% 10% 32% 32% 7% 1	840
	21st to end	Night	17% 12% 33% 30% 7%	840
December	1st to 10th	Night	19% 11% 37% 26% 7%	840
	11th to 20th	Night	19% 8% 39% 29% 4	840
	21st to end	Night	18% 11% 34% 35% 3	924

Page generated at 2026-04-29 02:21:44. Version ac07

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