AACC Astronomy Lab
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the AACC Astronomy Lab clear sky chart from 2005-11-06 to 2025-05-11.

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 5% 4 4 4 4 6% 8% 44%	113822
	Night	15% 6% 2 6% 4 4 4 3 5% 7% 44%	115114
Transparency	Day	22% 42% 7% 15% 10% 3	117457
	Night	24% 40% 7% 15% 12% 3	119618
Seeing	Day	17% 8% 25% 27% 17% 6%	15623
	Night	17% 10% 28% 29% 14% 2	25195

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	14% 4 2 4 3 3 3 3 5% 6% 53%	3597
	11th to 20th	Day	14% 9% 3 6% 3 4 3 2 4 5% 47%	4037
	21st to end	Day	16% 7% 2 5% 4 4 4 3 4 6% 46%	4466
February	1st to 10th	Day	15% 7% 2 6% 4 5% 4 4 7% 7% 40%	1800
	11th to 20th	Day	19% 7% 2 5% 4 4 3 3 6% 6% 42%	1850
	21st to end	Day	18% 9% 2 6% 4 4 3 3 4 6% 41%	1650
March	1st to 10th	Day	19% 4 2 4 3 3 4 3 5% 6% 47%	4727
	11th to 20th	Day	12% 7% 2 5% 4 3 4 3 4 6% 51%	4980
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 7% 42%	5640
April	1st to 10th	Day	15% 6% 2 5% 4 4 4 5% 6% 7% 43%	2400
	11th to 20th	Day	12% 5% 3 6% 4 3 4 3 5% 6% 47%	2374
	21st to end	Day	16% 6% 2 4 4 4 4 4 6% 7% 43%	2400
May	1st to 10th	Day	25% 7% 2 6% 4 4 4 4 6% 8% 32%	4048
	11th to 20th	Day	22% 7% 2 6% 4 4 5% 4 6% 7% 34%	4385
	21st to end	Day	11% 4 1 5% 4 4 4 4 6% 10% 47%	4823
June	1st to 10th	Day	10% 4 2 5% 4 4 5% 4 8% 9% 44%	2660
	11th to 20th	Day	10% 4 1 6% 4 4 5% 5% 8% 9% 44%	2660
	21st to end	Day	9% 4 1 4 4 4 5% 5% 8% 11% 45%	2660
July	1st to 10th	Day	5% 4 2 5% 5% 5% 6% 5% 8% 11% 43%	4440
	11th to 20th	Day	8% 4 2 6% 5% 4 6% 5% 8% 11% 41%	4420
	21st to end	Day	9% 6% 2 7% 5% 5% 6% 6% 7% 10% 38%	4466
August	1st to 10th	Day	16% 5% 2 6% 4 5% 6% 5% 8% 9% 34%	4104
	11th to 20th	Day	10% 5% 2 5% 4 4 5% 4 7% 9% 44%	3804
	21st to end	Day	10% 4 1 5% 4 4 4 4 6% 8% 50%	4180
September	1st to 10th	Day	14% 5% 1 4 4 4 4 4 6% 8% 46%	2280
	11th to 20th	Day	12% 5% 2 5% 4 4 3 3 5% 7% 50%	2280
	21st to end	Day	20% 7% 2 6% 4 4 4 3 5% 6% 38%	2023
October	1st to 10th	Day	17% 5% 2 4 3 3 3 3 5% 6% 48%	1900
	11th to 20th	Day	18% 4 2 3 2 3 3 3 5% 6% 52%	1900
	21st to end	Day	14% 5% 2 5% 3 3 3 3 5% 6% 51%	2090
November	1st to 10th	Day	12% 7% 1 4 3 4 4 3 4 7% 53%	1803
	11th to 20th	Day	13% 6% 2 3 4 4 4 4 5% 6% 49%	1800
	21st to end	Day	14% 6% 2 4 4 4 4 4 5% 5% 48%	1625
December	1st to 10th	Day	12% 5% 2 6% 4 3 3 3 6% 7% 49%	2600
	11th to 20th	Day	19% 8% 2 4 3 3 3 3 4 6% 44%	2990
	21st to end	Day	15% 7% 2 6% 4 4 4 4 5% 7% 42%	3960
January	1st to 10th	Night	14% 6% 2 5% 4 4 4 3 4 5% 49%	6349
	11th to 20th	Night	14% 7% 2 4 3 2 3 3 4 5% 52%	5921
	21st to end	Night	15% 8% 2 6% 4 3 3 2 4 4 47%	6490
February	1st to 10th	Night	16% 8% 2 5% 4 4 4 4 6% 6% 41%	2600
	11th to 20th	Night	19% 7% 2 5% 3 4 3 3 4 5% 44%	2434
	21st to end	Night	17% 8% 2 5% 4 3 3 3 5% 5% 44%	1980
March	1st to 10th	Night	16% 6% 2 5% 3 3 4 2 4 5% 50%	5108
	11th to 20th	Night	11% 7% 1 4 3 3 4 3 5% 7% 51%	4980
	21st to end	Night	14% 7% 2 6% 4 4 4 3 5% 5% 46%	5290
April	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 6% 41%	2000
	11th to 20th	Night	14% 5% 2 6% 4 4 4 4 6% 6% 45%	1975
	21st to end	Night	18% 4 2 5% 3 4 4 3 5% 7% 45%	2000
May	1st to 10th	Night	21% 9% 2 7% 4 5% 4 3 5% 7% 32%	3083
	11th to 20th	Night	26% 8% 2 7% 4 3 5% 4 5% 6% 31%	2511
	21st to end	Night	12% 5% 2 6% 4 4 4 3 7% 10% 44%	2732
June	1st to 10th	Night	11% 5% 2 5% 5% 4 5% 3 6% 10% 45%	1520
	11th to 20th	Night	10% 5% 3 8% 4 5% 5% 4 6% 9% 42%	1520
	21st to end	Night	10% 4 2 6% 4 5% 4 4 7% 11% 42%	1520
July	1st to 10th	Night	7% 4 2 7% 6% 6% 5% 4 8% 10% 40%	2520
	11th to 20th	Night	9% 5% 3 7% 4 5% 5% 5% 7% 11% 38%	2536
	21st to end	Night	12% 7% 3 6% 5% 5% 5% 5% 7% 8% 37%	3087
August	1st to 10th	Night	17% 6% 3 6% 5% 5% 5% 3 6% 11% 33%	2825
	11th to 20th	Night	11% 7% 3 9% 6% 5% 6% 5% 7% 8% 34%	2993
	21st to end	Night	13% 4 2 5% 4 4 4 3 5% 8% 48%	3476
September	1st to 10th	Night	14% 6% 3 5% 4 4 4 4 5% 6% 45%	1900
	11th to 20th	Night	10% 6% 2 6% 4 3 4 3 5% 7% 50%	1900
	21st to end	Night	20% 7% 2 5% 5% 4 4 3 5% 6% 38%	2151
October	1st to 10th	Night	19% 6% 1 5% 4 4 4 3 4 6% 44%	2280
	11th to 20th	Night	16% 4 2 5% 3 3 3 3 4 6% 52%	2280
	21st to end	Night	16% 5% 1 4 3 4 3 3 4 7% 50%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 5% 5% 53%	2494
	11th to 20th	Night	16% 6% 2 4 4 3 4 3 4 6% 49%	2600
	21st to end	Night	13% 4 2 5% 4 3 4 3 6% 6% 48%	2770
December	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 5% 7% 52%	4580
	11th to 20th	Night	21% 6% 2 5% 4 4 4 3 5% 7% 40%	5205
	21st to end	Night	16% 7% 2 6% 4 4 5% 3 6% 7% 40%	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	24% 26% 4 21% 21% 5%	3598
	11th to 20th	Day	31% 26% 5% 16% 15% 7%	4037
	21st to end	Day	29% 29% 9% 21% 10% 2	4466
February	1st to 10th	Day	18% 62% 7% 9% 4 1	1800
	11th to 20th	Day	18% 60% 2 10% 8% 2	1859
	21st to end	Day	18% 61% 4 9% 6% 1	1650
March	1st to 10th	Day	29% 30% 5% 16% 12% 7%	4727
	11th to 20th	Day	24% 28% 6% 21% 16% 4	4980
	21st to end	Day	30% 30% 7% 13% 14% 6%	5640
April	1st to 10th	Day	18% 62% 4 8% 7% 2	2400
	11th to 20th	Day	15% 62% 5% 9% 7% 1	2374
	21st to end	Day	16% 62% 4 10% 8% 1	2400
May	1st to 10th	Day	36% 41% 6% 13% 4 1	4083
	11th to 20th	Day	31% 42% 9% 15% 2	4389
	21st to end	Day	19% 41% 10% 15% 12% 2	4837
June	1st to 10th	Day	14% 65% 5% 10% 6%	2660
	11th to 20th	Day	11% 66% 5% 9% 8% 1	2660
	21st to end	Day	10% 64% 6% 11% 8% 1	2660
July	1st to 10th	Day	20% 41% 16% 11% 7% 4	4370
	11th to 20th	Day	25% 38% 12% 19% 5% 1	4345
	21st to end	Day	28% 37% 9% 14% 11% 1	4324
August	1st to 10th	Day	29% 39% 11% 18% 3	4091
	11th to 20th	Day	21% 39% 9% 23% 8%	3842
	21st to end	Day	17% 38% 10% 21% 13% 1	4180
September	1st to 10th	Day	15% 60% 7% 10% 6% 2	2280
	11th to 20th	Day	14% 60% 4 12% 8% 2	2280
	21st to end	Day	23% 59% 2 10% 5%	2023
October	1st to 10th	Day	22% 37% 6% 11% 22% 4	3160
	11th to 20th	Day	11% 36% 4 21% 20% 8%	3140
	21st to end	Day	8% 36% 7% 22% 21% 6%	3428
November	1st to 10th	Day	17% 56% 7% 10% 8% 3	1803
	11th to 20th	Day	13% 57% 3 12% 13% 2	1800
	21st to end	Day	16% 56% 3 15% 9% 1	1625
December	1st to 10th	Day	23% 37% 11% 18% 8% 4	2600
	11th to 20th	Day	30% 32% 9% 21% 8% 1	2986
	21st to end	Day	31% 29% 12% 16% 12% 1	3960
January	1st to 10th	Night	28% 26% 5% 17% 20% 4	6349
	11th to 20th	Night	24% 26% 5% 19% 17% 7%	5921
	21st to end	Night	30% 28% 5% 20% 15% 2	6490
February	1st to 10th	Night	17% 62% 7% 10% 3 1	2600
	11th to 20th	Night	17% 60% 2 8% 10% 2	2436
	21st to end	Night	16% 61% 4 9% 6% 3	1980
March	1st to 10th	Night	30% 29% 4 18% 9% 9%	5108
	11th to 20th	Night	21% 29% 9% 21% 14% 6%	4980
	21st to end	Night	29% 29% 5% 14% 19% 3	5290
April	1st to 10th	Night	17% 61% 4 9% 8% 1	2000
	11th to 20th	Night	15% 63% 4 10% 7% 2	1975
	21st to end	Night	16% 61% 4 9% 9% 1	2000
May	1st to 10th	Night	36% 41% 6% 11% 5% 1	3113
	11th to 20th	Night	35% 42% 8% 12% 3	2512
	21st to end	Night	21% 41% 11% 15% 10% 2	2739
June	1st to 10th	Night	14% 65% 4 10% 7%	1520
	11th to 20th	Night	13% 66% 4 11% 6% 1	1520
	21st to end	Night	11% 64% 5% 12% 7% 1	1520
July	1st to 10th	Night	24% 40% 16% 10% 8% 2	2480
	11th to 20th	Night	26% 38% 15% 17% 4 1	2488
	21st to end	Night	32% 37% 8% 15% 8%	2988
August	1st to 10th	Night	33% 39% 12% 12% 4	2814
	11th to 20th	Night	29% 39% 8% 17% 6%	3021
	21st to end	Night	19% 38% 6% 22% 14% 1	3476
September	1st to 10th	Night	16% 60% 7% 10% 5% 2	1900
	11th to 20th	Night	14% 59% 4 12% 10% 2	1900
	21st to end	Night	22% 60% 4 9% 5%	2151
October	1st to 10th	Night	27% 37% 6% 12% 14% 3	3800
	11th to 20th	Night	10% 37% 7% 13% 26% 6%	3779
	21st to end	Night	10% 36% 7% 23% 18% 7%	4120
November	1st to 10th	Night	16% 56% 5% 11% 8% 3	2494
	11th to 20th	Night	15% 56% 4 9% 12% 3	2600
	21st to end	Night	14% 56% 6% 13% 9% 1	2770
December	1st to 10th	Night	20% 35% 9% 21% 12% 2	4580
	11th to 20th	Night	31% 31% 9% 12% 12% 4	5208
	21st to end	Night	32% 28% 11% 15% 13% 1	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	23% 19% 35% 17% 7% 1	200
	11th to 20th	Day	19% 24% 34% 20% 3	295
	21st to end	Day	20% 15% 42% 17% 6% 1	330
February	1st to 10th	Day	22% 11% 44% 16% 7%	300
	11th to 20th	Day	29% 17% 38% 13% 3	397
	21st to end	Day	26% 11% 38% 20% 5%	414
March	1st to 10th	Day	18% 19% 33% 22% 7% 1	500
	11th to 20th	Day	20% 15% 33% 22% 7% 2	500
	21st to end	Day	21% 12% 37% 22% 6% 2	550
April	1st to 10th	Day	21% 12% 36% 23% 7% 1	500
	11th to 20th	Day	15% 13% 34% 24% 10% 4	492
	21st to end	Day	20% 10% 30% 26% 12% 2	500
May	1st to 10th	Day	23% 8% 28% 27% 12% 2	503
	11th to 20th	Day	22% 3 29% 26% 15% 5%	487
	21st to end	Day	19% 3 16% 27% 26% 8%	529
June	1st to 10th	Day	13% 4 23% 27% 24% 9%	490
	11th to 20th	Day	12% 4 19% 32% 24% 8%	490
	21st to end	Day	10% 3 17% 37% 24% 8%	490
July	1st to 10th	Day	10% 1 10% 33% 32% 14%	485
	11th to 20th	Day	4 1 8% 38% 36% 12%	484
	21st to end	Day	10% 1 12% 37% 31% 8%	532
August	1st to 10th	Day	12% 1 10% 34% 31% 11%	486
	11th to 20th	Day	12% 1 12% 31% 32% 11%	479
	21st to end	Day	10% 1 16% 30% 28% 14%	539
September	1st to 10th	Day	19% 1 18% 23% 26% 13%	490
	11th to 20th	Day	13% 2 15% 33% 26% 10%	490
	21st to end	Day	26% 5% 18% 31% 16% 4	490
October	1st to 10th	Day	21% 6% 17% 29% 20% 8%	490
	11th to 20th	Day	18% 8% 26% 30% 15% 5%	490
	21st to end	Day	16% 8% 31% 28% 14% 4	539
November	1st to 10th	Day	16% 9% 34% 24% 12% 6%	417
	11th to 20th	Day	15% 14% 35% 26% 8% 2	400
	21st to end	Day	16% 17% 29% 21% 14% 1	225
December	1st to 10th	Day	20% 13% 37% 25% 5% 2	200
	11th to 20th	Day	21% 15% 31% 25% 9%	200
	21st to end	Day	22% 10% 30% 29% 9%	220
January	1st to 10th	Night	19% 19% 38% 20% 4	800
	11th to 20th	Night	23% 16% 32% 25% 6%	800
	21st to end	Night	18% 18% 35% 26% 2	880
February	1st to 10th	Night	20% 13% 36% 27% 4	800
	11th to 20th	Night	25% 18% 36% 19% 2	783
	21st to end	Night	22% 14% 38% 22% 3	660
March	1st to 10th	Night	17% 17% 37% 24% 5%	800
	11th to 20th	Night	19% 16% 34% 24% 6% 2	800
	21st to end	Night	19% 12% 38% 25% 5% 1	880
April	1st to 10th	Night	21% 15% 36% 22% 6%	800
	11th to 20th	Night	16% 10% 34% 30% 9% 1	791
	21st to end	Night	19% 8% 33% 29% 10% 1	800
May	1st to 10th	Night	21% 8% 28% 30% 12% 1	718
	11th to 20th	Night	18% 8% 25% 34% 14% 1	386
	21st to end	Night	14% 3 19% 36% 26% 3	411
June	1st to 10th	Night	11% 3 22% 38% 23% 2	380
	11th to 20th	Night	10% 4 22% 39% 22% 2	380
	21st to end	Night	11% 2 19% 44% 21% 3	380
July	1st to 10th	Night	10% 2 15% 38% 30% 5%	370
	11th to 20th	Night	5% 2 10% 42% 36% 4	371
	21st to end	Night	9% 1 16% 41% 29% 4	624
August	1st to 10th	Night	11% 2 11% 41% 31% 3	567
	11th to 20th	Night	12% 4 13% 36% 31% 5%	682
	21st to end	Night	14% 2 12% 36% 29% 8%	836
September	1st to 10th	Night	16% 3 15% 32% 27% 6%	760
	11th to 20th	Night	12% 3 14% 36% 30% 6%	760
	21st to end	Night	23% 5% 20% 31% 19% 3	760
October	1st to 10th	Night	22% 6% 21% 26% 21% 5%	760
	11th to 20th	Night	18% 9% 26% 29% 16% 2	760
	21st to end	Night	19% 11% 28% 27% 14% 1	836
November	1st to 10th	Night	14% 10% 30% 31% 14% 2	780
	11th to 20th	Night	19% 14% 30% 27% 10%	800
	21st to end	Night	16% 15% 33% 26% 9% 1	800
December	1st to 10th	Night	21% 15% 32% 22% 9% 1	800
	11th to 20th	Night	19% 13% 38% 24% 6%	800
	21st to end	Night	20% 11% 35% 30% 4	880

Page generated at 2025-05-08 09:33:55. Version ac07

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