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 2025-01-22.

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% 6% 2 6% 4 4 5% 4 6% 8% 40%	104974
	Night	14% 7% 2 6% 4 4 5% 4 6% 7% 41%	104323
Transparency	Day	39% 7% 12% 24% 16% 3	108615
	Night	39% 6% 12% 24% 16% 3	108821
Seeing	Day	18% 11% 25% 24% 16% 6%	15114
	Night	17% 9% 28% 32% 13% 1	24776

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	16% 5% 2 5% 5% 5% 5% 4 5% 6% 42%	2734
	11th to 20th	Day	13% 11% 3 7% 5% 5% 5% 4 5% 6% 37%	2930
	21st to end	Day	16% 11% 3 7% 6% 4 5% 4 5% 6% 34%	3133
February	1st to 10th	Day	14% 9% 3 7% 5% 5% 5% 4 6% 6% 37%	1710
	11th to 20th	Day	15% 8% 3 7% 5% 4 3 3 5% 7% 39%	1671
	21st to end	Day	17% 9% 3 7% 5% 4 4 3 4 6% 38%	1575
March	1st to 10th	Day	20% 9% 3 6% 5% 4 4 3 5% 7% 34%	3435
	11th to 20th	Day	10% 6% 2 5% 4 5% 5% 4 6% 7% 47%	3480
	21st to end	Day	19% 7% 2 6% 4 5% 3 3 5% 6% 39%	3832
April	1st to 10th	Day	13% 7% 2 7% 5% 5% 4 4 7% 7% 39%	2280
	11th to 20th	Day	13% 7% 2 5% 4 4 5% 4 5% 6% 46%	2254
	21st to end	Day	18% 7% 3 5% 4 5% 3 4 5% 6% 40%	2299
May	1st to 10th	Day	22% 7% 3 7% 5% 4 4 4 6% 8% 30%	4082
	11th to 20th	Day	23% 7% 2 6% 5% 4 4 4 5% 8% 32%	4207
	21st to end	Day	10% 5% 2 6% 4 5% 5% 4 7% 9% 44%	4823
June	1st to 10th	Day	11% 6% 2 5% 4 4 5% 5% 7% 8% 43%	2660
	11th to 20th	Day	11% 6% 3 6% 4 5% 5% 5% 7% 10% 40%	2660
	21st to end	Day	9% 4 2 5% 4 5% 6% 4 7% 10% 44%	2660
July	1st to 10th	Day	5% 4 2 6% 5% 5% 5% 5% 8% 10% 44%	4440
	11th to 20th	Day	8% 5% 2 6% 5% 5% 6% 5% 6% 11% 41%	4224
	21st to end	Day	10% 7% 3 7% 6% 5% 5% 4 6% 9% 38%	4466
August	1st to 10th	Day	15% 5% 2 6% 5% 6% 5% 5% 7% 9% 34%	4104
	11th to 20th	Day	8% 5% 2 7% 4 5% 6% 5% 6% 8% 42%	3949
	21st to end	Day	8% 5% 2 6% 4 4 5% 5% 6% 8% 47%	4180
September	1st to 10th	Day	16% 5% 2 6% 4 4 4 3 6% 6% 45%	2280
	11th to 20th	Day	12% 6% 2 5% 3 3 4 3 7% 7% 48%	2264
	21st to end	Day	20% 6% 2 7% 3 4 4 3 5% 7% 38%	1990
October	1st to 10th	Day	16% 5% 2 5% 3 3 4 3 5% 9% 45%	1900
	11th to 20th	Day	15% 4 2 5% 4 4 5% 4 5% 6% 47%	1900
	21st to end	Day	13% 5% 2 6% 4 4 4 3 4 6% 49%	2090
November	1st to 10th	Day	11% 6% 2 4 4 3 4 3 5% 7% 51%	1894
	11th to 20th	Day	15% 6% 2 4 3 4 4 4 5% 6% 46%	1718
	21st to end	Day	15% 7% 2 5% 4 4 5% 3 5% 7% 44%	1600
December	1st to 10th	Day	12% 6% 2 6% 3 5% 4 4 6% 8% 44%	2600
	11th to 20th	Day	19% 8% 2 6% 4 4 4 3 5% 6% 39%	2990
	21st to end	Day	15% 8% 3 8% 5% 5% 5% 5% 7% 8% 32%	3960
January	1st to 10th	Night	11% 8% 3 6% 5% 4 5% 4 6% 7% 41%	4428
	11th to 20th	Night	17% 8% 2 6% 4 3 4 3 5% 6% 43%	4250
	21st to end	Night	18% 10% 3 7% 4 4 4 4 5% 5% 37%	4546
February	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 6% 7% 40%	2470
	11th to 20th	Night	17% 8% 3 6% 4 4 5% 3 5% 6% 40%	2394
	21st to end	Night	16% 9% 2 7% 5% 4 4 3 6% 6% 38%	1876
March	1st to 10th	Night	20% 8% 2 7% 4 4 4 4 4 6% 37%	3418
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 6% 6% 47%	3480
	21st to end	Night	13% 8% 3 6% 4 3 3 3 4 6% 47%	3800
April	1st to 10th	Night	15% 8% 3 6% 5% 4 5% 4 5% 6% 38%	1900
	11th to 20th	Night	12% 6% 2 7% 5% 3 4 3 7% 8% 43%	1875
	21st to end	Night	17% 5% 2 5% 3 4 5% 4 5% 8% 43%	1895
May	1st to 10th	Night	22% 8% 3 8% 5% 4 4 4 6% 7% 30%	2813
	11th to 20th	Night	23% 8% 3 7% 4 4 3 4 5% 6% 32%	2668
	21st to end	Night	10% 5% 2 6% 4 5% 5% 5% 8% 10% 40%	2732
June	1st to 10th	Night	10% 5% 2 5% 4 4 5% 4 7% 10% 44%	1520
	11th to 20th	Night	11% 5% 2 6% 5% 3 5% 5% 8% 9% 41%	1520
	21st to end	Night	8% 5% 2 6% 4 5% 4 4 7% 11% 44%	1520
July	1st to 10th	Night	6% 4 2 7% 5% 6% 8% 5% 8% 10% 39%	2520
	11th to 20th	Night	5% 6% 2 8% 6% 6% 6% 5% 7% 10% 38%	2732
	21st to end	Night	10% 7% 3 7% 4 5% 6% 5% 7% 8% 38%	3087
August	1st to 10th	Night	14% 5% 2 8% 5% 5% 6% 5% 6% 10% 33%	2825
	11th to 20th	Night	9% 6% 3 7% 5% 5% 6% 5% 7% 9% 39%	2846
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 6% 8% 47%	3476
September	1st to 10th	Night	14% 6% 2 5% 5% 4 4 4 5% 7% 45%	1900
	11th to 20th	Night	10% 7% 3 6% 4 3 4 3 4 6% 52%	1912
	21st to end	Night	18% 7% 2 6% 4 5% 4 3 4 6% 41%	2190
October	1st to 10th	Night	18% 5% 2 5% 3 3 4 3 4 6% 46%	2280
	11th to 20th	Night	13% 4 2 5% 3 4 3 3 5% 6% 51%	2280
	21st to end	Night	15% 5% 2 5% 3 3 3 3 5% 7% 50%	2508
November	1st to 10th	Night	10% 5% 2 5% 3 3 3 3 5% 6% 54%	2401
	11th to 20th	Night	14% 6% 2 5% 3 3 3 4 6% 7% 46%	2680
	21st to end	Night	14% 5% 2 6% 4 3 4 4 5% 8% 44%	2800
December	1st to 10th	Night	10% 5% 2 5% 4 4 4 3 6% 8% 49%	4580
	11th to 20th	Night	18% 7% 2 7% 4 5% 5% 4 5% 6% 37%	5205
	21st to end	Night	15% 8% 3 7% 5% 5% 6% 5% 7% 8% 31%	6996

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	41% 5% 10% 28% 12% 4	2735
	11th to 20th	Day	45% 5% 10% 20% 16% 4	2930
	21st to end	Day	50% 6% 11% 22% 11% 2	3133
February	1st to 10th	Day	45% 7% 13% 23% 11% 2	1710
	11th to 20th	Day	45% 5% 10% 24% 14% 3	1680
	21st to end	Day	46% 5% 12% 19% 14% 3	1575
March	1st to 10th	Day	48% 6% 10% 21% 12% 2	3435
	11th to 20th	Day	33% 6% 10% 28% 20% 3	3480
	21st to end	Day	45% 6% 12% 20% 15% 3	3832
April	1st to 10th	Day	42% 6% 13% 18% 18% 4	2280
	11th to 20th	Day	38% 6% 11% 22% 19% 5%	2254
	21st to end	Day	44% 6% 9% 22% 16% 4	2299
May	1st to 10th	Day	50% 8% 10% 20% 11% 1	4120
	11th to 20th	Day	48% 9% 11% 22% 8% 2	4211
	21st to end	Day	33% 9% 14% 25% 16% 2	4837
June	1st to 10th	Day	36% 8% 14% 25% 16% 1	2660
	11th to 20th	Day	37% 10% 15% 23% 14% 2	2660
	21st to end	Day	32% 9% 14% 27% 16% 2	2660
July	1st to 10th	Day	33% 9% 17% 25% 11% 5%	4370
	11th to 20th	Day	35% 8% 15% 29% 12% 2	4151
	21st to end	Day	42% 7% 10% 21% 18% 2	4324
August	1st to 10th	Day	43% 9% 14% 24% 8% 1	4091
	11th to 20th	Day	36% 7% 12% 28% 15% 1	3988
	21st to end	Day	34% 7% 14% 27% 17% 2	4180
September	1st to 10th	Day	39% 7% 10% 19% 21% 4	2280
	11th to 20th	Day	34% 6% 10% 27% 21% 2	2264
	21st to end	Day	45% 5% 9% 20% 18% 2	1990
October	1st to 10th	Day	36% 3 9% 20% 27% 5%	3160
	11th to 20th	Day	28% 4 10% 29% 24% 6%	3140
	21st to end	Day	25% 3 11% 30% 27% 4	3428
November	1st to 10th	Day	33% 5% 12% 26% 21% 4	1894
	11th to 20th	Day	37% 5% 11% 22% 22% 4	1718
	21st to end	Day	39% 5% 12% 28% 15% 1	1600
December	1st to 10th	Day	36% 6% 13% 29% 11% 4	2600
	11th to 20th	Day	46% 5% 14% 24% 10% 2	2986
	21st to end	Day	45% 9% 16% 16% 13%	3960
January	1st to 10th	Night	39% 6% 11% 26% 15% 3	4428
	11th to 20th	Night	41% 5% 12% 26% 14% 3	4250
	21st to end	Night	47% 5% 10% 25% 11% 1	4546
February	1st to 10th	Night	42% 6% 12% 25% 13% 2	2470
	11th to 20th	Night	45% 5% 10% 21% 15% 4	2396
	21st to end	Night	46% 4 12% 20% 13% 5%	1876
March	1st to 10th	Night	46% 5% 11% 26% 10% 1	3418
	11th to 20th	Night	33% 7% 11% 31% 16% 2	3480
	21st to end	Night	39% 4 10% 24% 19% 4	3800
April	1st to 10th	Night	44% 6% 10% 19% 17% 4	1900
	11th to 20th	Night	38% 7% 10% 21% 18% 6%	1875
	21st to end	Night	39% 7% 12% 25% 13% 4	1895
May	1st to 10th	Night	52% 7% 10% 18% 11% 3	2840
	11th to 20th	Night	50% 9% 11% 20% 10% 1	2669
	21st to end	Night	34% 10% 17% 26% 11% 3	2739
June	1st to 10th	Night	34% 9% 11% 26% 17% 3	1520
	11th to 20th	Night	36% 10% 13% 25% 14% 2	1520
	21st to end	Night	33% 8% 15% 25% 17% 2	1520
July	1st to 10th	Night	36% 10% 18% 18% 13% 5%	2480
	11th to 20th	Night	38% 10% 16% 21% 14% 1	2682
	21st to end	Night	40% 9% 15% 19% 17% 1	2988
August	1st to 10th	Night	44% 10% 15% 22% 8% 1	2814
	11th to 20th	Night	38% 8% 13% 25% 15% 1	2873
	21st to end	Night	34% 7% 14% 28% 15% 2	3476
September	1st to 10th	Night	38% 6% 12% 21% 19% 4	1900
	11th to 20th	Night	34% 4 9% 27% 23% 2	1912
	21st to end	Night	43% 6% 11% 20% 18% 2	2190
October	1st to 10th	Night	40% 4 11% 22% 21% 3	3800
	11th to 20th	Night	24% 5% 11% 23% 31% 6%	3779
	21st to end	Night	24% 3 10% 32% 26% 5%	4120
November	1st to 10th	Night	31% 4 12% 26% 22% 5%	2401
	11th to 20th	Night	36% 5% 9% 24% 21% 5%	2680
	21st to end	Night	38% 5% 12% 25% 17% 3	2800
December	1st to 10th	Night	34% 5% 14% 30% 16% 1	4580
	11th to 20th	Night	46% 5% 13% 19% 14% 3	5208
	21st to end	Night	45% 9% 15% 18% 12%	6996

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	29% 24% 27% 16% 3 1	243
	11th to 20th	Day	28% 25% 30% 14% 4	300
	21st to end	Day	22% 26% 32% 14% 5% 1	300
February	1st to 10th	Day	24% 19% 37% 15% 4	270
	11th to 20th	Day	32% 22% 35% 9% 2	278
	21st to end	Day	30% 16% 32% 18% 4	382
March	1st to 10th	Day	24% 20% 27% 25% 4	460
	11th to 20th	Day	23% 19% 32% 17% 7% 2	460
	21st to end	Day	22% 18% 31% 22% 7% 1	506
April	1st to 10th	Day	22% 20% 28% 21% 8% 2	460
	11th to 20th	Day	16% 15% 32% 18% 14% 5%	452
	21st to end	Day	25% 12% 28% 22% 11% 2	460
May	1st to 10th	Day	22% 14% 26% 22% 12% 4	463
	11th to 20th	Day	21% 11% 25% 21% 16% 6%	487
	21st to end	Day	17% 4 23% 23% 22% 10%	529
June	1st to 10th	Day	14% 6% 25% 26% 19% 9%	490
	11th to 20th	Day	13% 6% 25% 28% 20% 8%	490
	21st to end	Day	9% 6% 25% 30% 21% 9%	490
July	1st to 10th	Day	10% 4 17% 27% 27% 15%	485
	11th to 20th	Day	6% 3 14% 32% 32% 13%	484
	21st to end	Day	12% 3 17% 30% 28% 10%	532
August	1st to 10th	Day	10% 3 18% 27% 28% 15%	486
	11th to 20th	Day	12% 4 18% 28% 25% 13%	479
	21st to end	Day	9% 3 18% 26% 27% 17%	539
September	1st to 10th	Day	17% 4 17% 22% 26% 13%	490
	11th to 20th	Day	14% 3 16% 29% 24% 14%	490
	21st to end	Day	26% 8% 19% 25% 17% 5%	490
October	1st to 10th	Day	19% 6% 18% 27% 23% 7%	490
	11th to 20th	Day	15% 10% 28% 27% 17% 4	490
	21st to end	Day	16% 9% 33% 26% 13% 3	539
November	1st to 10th	Day	16% 12% 27% 29% 13% 3	462
	11th to 20th	Day	21% 11% 31% 25% 10% 2	318
	21st to end	Day	21% 17% 32% 23% 7% 2	200
December	1st to 10th	Day	20% 20% 32% 23% 5% 2	200
	11th to 20th	Day	24% 17% 28% 24% 9%	200
	21st to end	Day	19% 13% 35% 24% 9%	220
January	1st to 10th	Night	17% 18% 40% 22% 3	800
	11th to 20th	Night	22% 12% 34% 28% 4	800
	21st to end	Night	18% 12% 42% 25% 2	841
February	1st to 10th	Night	17% 15% 35% 31% 2	760
	11th to 20th	Night	23% 17% 37% 22% 2	743
	21st to end	Night	23% 18% 36% 21% 3	628
March	1st to 10th	Night	20% 14% 37% 26% 3	760
	11th to 20th	Night	24% 15% 32% 24% 6%	760
	21st to end	Night	19% 12% 38% 26% 4 1	836
April	1st to 10th	Night	19% 16% 34% 26% 4	760
	11th to 20th	Night	16% 11% 34% 30% 8% 1	751
	21st to end	Night	19% 9% 33% 32% 7%	755
May	1st to 10th	Night	20% 5% 28% 35% 12%	570
	11th to 20th	Night	18% 5% 25% 39% 12% 1	482
	21st to end	Night	13% 3 21% 40% 21% 1	411
June	1st to 10th	Night	11% 3 23% 39% 23% 1	380
	11th to 20th	Night	12% 3 17% 44% 23% 1	380
	21st to end	Night	8% 1 21% 50% 19% 1	380
July	1st to 10th	Night	11% 3 14% 43% 25% 4	370
	11th to 20th	Night	6% 1 12% 40% 37% 3	487
	21st to end	Night	8% 2 13% 44% 31% 3	624
August	1st to 10th	Night	10% 1 12% 44% 32% 2	567
	11th to 20th	Night	12% 3 13% 37% 33% 4	599
	21st to end	Night	12% 3 13% 40% 28% 4	836
September	1st to 10th	Night	16% 1 14% 38% 27% 4	760
	11th to 20th	Night	12% 3 16% 35% 30% 4	760
	21st to end	Night	20% 4 21% 32% 21% 1	760
October	1st to 10th	Night	21% 5% 20% 30% 22% 3	760
	11th to 20th	Night	15% 6% 27% 36% 15% 1	760
	21st to end	Night	17% 8% 31% 32% 11%	836
November	1st to 10th	Night	13% 7% 31% 37% 11% 1	780
	11th to 20th	Night	18% 10% 32% 32% 8% 1	800
	21st to end	Night	17% 11% 33% 31% 8%	800
December	1st to 10th	Night	19% 12% 37% 26% 7%	800
	11th to 20th	Night	19% 8% 40% 30% 4	800
	21st to end	Night	18% 10% 33% 35% 3	880

Page generated at 2025-01-19 03:06:44. Version ac07

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