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 2026-09-14.

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% 5% 2 5% 4 4 4 4 6% 8% 43%	149001
	Night	15% 6% 2 6% 4 4 4 3 5% 7% 43%	142076
Transparency	Day	26% 34% 9% 17% 12% 2	152593
	Night	27% 34% 8% 16% 12% 3	146555
Seeing	Day	17% 8% 25% 27% 17% 5%	17432
	Night	17% 10% 28% 29% 14% 2	26821

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 5% 4 4 4 3 4 6% 49%	4661
	11th to 20th	Day	15% 8% 3 5% 3 3 3 2 3 4 50%	5257
	21st to end	Day	14% 5% 2 5% 4 4 4 3 5% 6% 47%	5780
February	1st to 10th	Day	15% 7% 2 6% 4 5% 4 4 6% 7% 41%	1890
	11th to 20th	Day	19% 8% 2 5% 4 4 3 3 6% 6% 41%	1945
	21st to end	Day	18% 9% 2 6% 4 4 3 3 4 6% 41%	1730
March	1st to 10th	Day	19% 5% 2 4 3 3 4 3 5% 6% 46%	4832
	11th to 20th	Day	12% 7% 2 5% 4 3 4 3 4 6% 50%	5090
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 7% 43%	5765
April	1st to 10th	Day	15% 6% 2 5% 4 4 4 5% 6% 7% 43%	2520
	11th to 20th	Day	12% 5% 3 5% 4 3 4 3 5% 7% 47%	2494
	21st to end	Day	17% 6% 2 4 4 4 4 4 6% 7% 43%	2520
May	1st to 10th	Day	20% 6% 2 6% 4 3 4 4 6% 8% 37%	7317
	11th to 20th	Day	18% 5% 2 5% 3 4 4 4 6% 8% 40%	8195
	21st to end	Day	23% 5% 2 5% 4 4 4 3 5% 7% 39%	9045
June	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 8% 9% 45%	2940
	11th to 20th	Day	10% 4 1 5% 4 4 5% 5% 7% 9% 45%	2940
	21st to end	Day	10% 4 1 4 4 4 5% 4 8% 10% 46%	2940
July	1st to 10th	Day	9% 4 2 6% 5% 4 5% 4 7% 10% 44%	8280
	11th to 20th	Day	8% 5% 2 7% 5% 5% 6% 5% 9% 12% 35%	8246
	21st to end	Day	8% 5% 2 6% 5% 4 6% 5% 7% 10% 43%	8340
August	1st to 10th	Day	14% 5% 2 7% 5% 6% 6% 5% 8% 10% 31%	6014
	11th to 20th	Day	11% 5% 2 6% 5% 4 5% 4 7% 9% 41%	5606
	21st to end	Day	9% 4 1 5% 3 5% 5% 5% 7% 9% 47%	6080
September	1st to 10th	Day	14% 4 1 4 4 3 4 4 6% 8% 47%	2520
	11th to 20th	Day	12% 5% 2 6% 4 4 4 3 5% 7% 49%	2436
	21st to end	Day	20% 7% 2 6% 4 4 4 3 5% 6% 38%	2129
October	1st to 10th	Day	16% 5% 2 4 3 3 3 3 5% 6% 50%	2000
	11th to 20th	Day	18% 4 2 3 2 2 3 3 5% 6% 52%	2000
	21st to end	Day	14% 5% 2 5% 3 4 3 3 5% 6% 51%	2200
November	1st to 10th	Day	12% 6% 1 3 3 4 3 2 4 7% 53%	1895
	11th to 20th	Day	13% 6% 2 3 4 4 4 4 5% 6% 49%	1890
	21st to end	Day	14% 6% 2 4 4 4 4 4 5% 5% 48%	1706
December	1st to 10th	Day	12% 6% 2 6% 4 3 3 3 6% 7% 49%	2680
	11th to 20th	Day	18% 8% 2 4 3 3 3 3 4 6% 44%	3070
	21st to end	Day	15% 7% 2 6% 4 4 4 4 5% 7% 42%	4048
January	1st to 10th	Night	12% 7% 2 6% 4 4 5% 4 5% 6% 45%	8233
	11th to 20th	Night	15% 7% 2 5% 3 3 3 3 4 5% 51%	7701
	21st to end	Night	14% 8% 2 5% 4 3 3 3 4 5% 48%	8401
February	1st to 10th	Night	15% 8% 2 5% 4 4 4 4 6% 6% 41%	2730
	11th to 20th	Night	19% 7% 2 5% 3 4 3 3 4 5% 43%	2559
	21st to end	Night	17% 8% 2 5% 4 3 3 3 5% 6% 44%	2076
March	1st to 10th	Night	17% 6% 2 5% 3 3 4 2 4 5% 49%	5223
	11th to 20th	Night	11% 7% 1 4 3 3 4 3 5% 7% 51%	5090
	21st to end	Night	14% 7% 2 6% 4 4 4 3 5% 6% 45%	5407
April	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 6% 41%	2100
	11th to 20th	Night	13% 5% 2 6% 4 4 4 4 6% 6% 46%	2075
	21st to end	Night	18% 4 2 5% 3 4 4 3 5% 7% 45%	2100
May	1st to 10th	Night	18% 7% 2 8% 5% 5% 5% 4 6% 7% 34%	5579
	11th to 20th	Night	21% 6% 2 6% 3 3 4 4 6% 8% 36%	4662
	21st to end	Night	24% 7% 2 7% 3 4 4 3 5% 7% 34%	5108
June	1st to 10th	Night	10% 5% 2 6% 5% 4 5% 3 6% 9% 46%	1680
	11th to 20th	Night	11% 5% 3 8% 5% 5% 5% 4 6% 10% 40%	1680
	21st to end	Night	11% 5% 2 6% 4 5% 4 4 7% 11% 41%	1680
July	1st to 10th	Night	11% 5% 2 7% 5% 5% 5% 4 7% 8% 39%	4678
	11th to 20th	Night	10% 6% 3 8% 5% 5% 6% 5% 8% 11% 33%	4693
	21st to end	Night	10% 6% 3 6% 5% 5% 5% 5% 7% 9% 40%	5736
August	1st to 10th	Night	15% 6% 3 8% 6% 6% 6% 4 6% 11% 29%	4135
	11th to 20th	Night	12% 6% 3 9% 6% 5% 6% 5% 7% 9% 33%	4393
	21st to end	Night	12% 5% 2 6% 4 4 4 4 5% 8% 44%	5045
September	1st to 10th	Night	13% 5% 3 5% 4 4 4 4 5% 6% 46%	2100
	11th to 20th	Night	11% 6% 2 6% 4 4 4 3 5% 7% 48%	2031
	21st to end	Night	21% 7% 2 5% 5% 4 4 3 5% 6% 37%	2265
October	1st to 10th	Night	19% 6% 2 5% 4 3 4 3 4 6% 45%	2400
	11th to 20th	Night	17% 4 2 5% 3 3 3 3 4 6% 52%	2400
	21st to end	Night	16% 5% 1 4 3 4 3 3 4 7% 50%	2640
November	1st to 10th	Night	11% 5% 2 5% 3 3 3 4 5% 6% 53%	2622
	11th to 20th	Night	16% 6% 2 5% 4 3 4 3 4 6% 49%	2730
	21st to end	Night	13% 5% 2 5% 5% 3 4 3 6% 6% 48%	2909
December	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 5% 7% 51%	4720
	11th to 20th	Night	21% 6% 2 5% 4 4 4 3 5% 7% 41%	5345
	21st to end	Night	16% 7% 2 6% 4 4 4 4 6% 7% 40%	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	30% 20% 6% 22% 18% 4	4662
	11th to 20th	Day	31% 20% 4 18% 19% 8%	5257
	21st to end	Day	30% 23% 9% 26% 9% 2	5780
February	1st to 10th	Day	18% 60% 7% 9% 5% 1	1890
	11th to 20th	Day	20% 57% 3 10% 8% 2	1954
	21st to end	Day	20% 59% 5% 10% 6% 1	1730
March	1st to 10th	Day	30% 29% 6% 16% 13% 7%	4832
	11th to 20th	Day	25% 28% 6% 21% 16% 4	5090
	21st to end	Day	30% 29% 7% 14% 14% 6%	5765
April	1st to 10th	Day	18% 59% 5% 8% 8% 2	2520
	11th to 20th	Day	16% 59% 6% 11% 7% 1	2494
	21st to end	Day	17% 59% 5% 11% 7% 1	2520
May	1st to 10th	Day	36% 25% 11% 16% 11% 1	7349
	11th to 20th	Day	32% 25% 9% 24% 10%	8199
	21st to end	Day	38% 24% 8% 15% 14% 2	9061
June	1st to 10th	Day	15% 60% 5% 11% 9% 1	2940
	11th to 20th	Day	13% 60% 6% 10% 9% 1	2940
	21st to end	Day	13% 58% 7% 13% 9% 1	2940
July	1st to 10th	Day	29% 26% 15% 16% 11% 2	8210
	11th to 20th	Day	33% 25% 13% 21% 7%	8171
	21st to end	Day	29% 23% 10% 20% 18% 1	8156
August	1st to 10th	Day	35% 31% 14% 17% 3	6001
	11th to 20th	Day	28% 29% 13% 23% 7%	5644
	21st to end	Day	22% 29% 14% 21% 13% 1	6080
September	1st to 10th	Day	15% 55% 9% 13% 7% 2	2520
	11th to 20th	Day	17% 56% 4 13% 8% 2	2436
	21st to end	Day	25% 57% 3 10% 4	2129
October	1st to 10th	Day	21% 36% 6% 11% 22% 4	3260
	11th to 20th	Day	12% 35% 4 21% 20% 8%	3240
	21st to end	Day	8% 35% 7% 23% 22% 6%	3538
November	1st to 10th	Day	17% 53% 7% 11% 9% 3	1895
	11th to 20th	Day	14% 54% 3 13% 13% 2	1890
	21st to end	Day	17% 53% 4 15% 9% 1	1706
December	1st to 10th	Day	24% 36% 11% 18% 8% 3	2680
	11th to 20th	Day	29% 31% 9% 21% 8% 2	3066
	21st to end	Day	32% 28% 12% 16% 12% 1	4048
January	1st to 10th	Night	32% 21% 8% 19% 16% 3	8234
	11th to 20th	Night	28% 21% 5% 18% 19% 8%	7701
	21st to end	Night	31% 22% 5% 23% 16% 3	8401
February	1st to 10th	Night	19% 59% 6% 10% 5% 1	2730
	11th to 20th	Night	19% 57% 3 9% 10% 2	2561
	21st to end	Night	17% 58% 5% 10% 6% 3	2076
March	1st to 10th	Night	31% 29% 4 18% 9% 9%	5223
	11th to 20th	Night	21% 29% 9% 21% 14% 6%	5090
	21st to end	Night	29% 29% 6% 14% 18% 3	5407
April	1st to 10th	Night	18% 58% 5% 9% 8% 1	2100
	11th to 20th	Night	16% 60% 5% 11% 7% 1	2075
	21st to end	Night	17% 59% 5% 10% 8% 1	2100
May	1st to 10th	Night	40% 25% 9% 15% 9% 1	5609
	11th to 20th	Night	36% 25% 11% 20% 9%	4663
	21st to end	Night	43% 23% 8% 13% 11% 1	5115
June	1st to 10th	Night	16% 59% 4 10% 10%	1680
	11th to 20th	Night	17% 60% 5% 12% 6% 1	1680
	21st to end	Night	14% 58% 6% 13% 8% 1	1680
July	1st to 10th	Night	34% 26% 13% 15% 11% 1	4640
	11th to 20th	Night	38% 24% 14% 17% 7%	4648
	21st to end	Night	33% 23% 11% 19% 14%	5606
August	1st to 10th	Night	41% 30% 13% 11% 4	4124
	11th to 20th	Night	34% 30% 12% 17% 6%	4421
	21st to end	Night	27% 28% 8% 23% 13%	5045
September	1st to 10th	Night	17% 55% 7% 13% 6% 2	2100
	11th to 20th	Night	17% 55% 4 12% 10% 2	2031
	21st to end	Night	25% 58% 5% 8% 5%	2265
October	1st to 10th	Night	27% 36% 6% 12% 15% 3	3920
	11th to 20th	Night	11% 36% 7% 14% 26% 6%	3899
	21st to end	Night	10% 35% 7% 23% 18% 6%	4252
November	1st to 10th	Night	17% 53% 5% 12% 9% 3	2622
	11th to 20th	Night	17% 54% 4 10% 13% 4	2730
	21st to end	Night	15% 54% 6% 14% 10% 1	2909
December	1st to 10th	Night	21% 34% 10% 21% 12% 2	4720
	11th to 20th	Night	31% 31% 9% 13% 12% 4	5348
	21st to end	Night	32% 27% 11% 15% 13% 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	22% 18% 35% 17% 7%	220
	11th to 20th	Day	19% 25% 34% 19% 3	325
	21st to end	Day	19% 15% 44% 16% 5% 1	363
February	1st to 10th	Day	21% 12% 42% 17% 7%	330
	11th to 20th	Day	28% 17% 39% 13% 3	432
	21st to end	Day	25% 11% 38% 20% 5%	446
March	1st to 10th	Day	19% 19% 33% 21% 7% 1	540
	11th to 20th	Day	20% 17% 32% 22% 7% 2	540
	21st to end	Day	20% 12% 39% 22% 5% 2	594
April	1st to 10th	Day	20% 13% 35% 23% 9% 1	540
	11th to 20th	Day	15% 13% 34% 24% 11% 4	532
	21st to end	Day	21% 9% 29% 27% 12% 2	540
May	1st to 10th	Day	22% 8% 31% 27% 11% 1	543
	11th to 20th	Day	21% 4 29% 27% 14% 5%	567
	21st to end	Day	22% 4 18% 26% 24% 7%	617
June	1st to 10th	Day	12% 4 24% 28% 23% 9%	570
	11th to 20th	Day	13% 5% 21% 31% 24% 7%	570
	21st to end	Day	12% 3 16% 35% 25% 9%	570
July	1st to 10th	Day	11% 2 10% 32% 32% 14%	565
	11th to 20th	Day	5% 1 9% 36% 37% 11%	564
	21st to end	Day	10% 2 12% 36% 31% 9%	620
August	1st to 10th	Day	13% 1 11% 34% 31% 10%	566
	11th to 20th	Day	13% 2 12% 32% 31% 10%	559
	21st to end	Day	10% 1 17% 32% 27% 13%	627
September	1st to 10th	Day	17% 2 18% 27% 24% 12%	570
	11th to 20th	Day	14% 2 15% 33% 26% 10%	542
	21st to end	Day	26% 5% 19% 31% 15% 3	530
October	1st to 10th	Day	20% 6% 17% 28% 20% 9%	530
	11th to 20th	Day	19% 9% 26% 28% 14% 5%	530
	21st to end	Day	16% 10% 31% 27% 13% 4	583
November	1st to 10th	Day	16% 9% 35% 24% 11% 5%	449
	11th to 20th	Day	15% 15% 34% 26% 7% 2	430
	21st to end	Day	17% 18% 30% 20% 13% 1	246
December	1st to 10th	Day	20% 12% 37% 24% 5% 1	220
	11th to 20th	Day	19% 16% 31% 25% 8%	220
	21st to end	Day	21% 11% 32% 26% 9%	242
January	1st to 10th	Night	18% 19% 38% 21% 4	840
	11th to 20th	Night	22% 17% 32% 24% 5%	840
	21st to end	Night	18% 19% 36% 25% 2	924
February	1st to 10th	Night	19% 14% 36% 27% 4	840
	11th to 20th	Night	25% 18% 36% 20% 1	823
	21st to end	Night	22% 14% 38% 23% 3	692
March	1st to 10th	Night	18% 16% 37% 24% 5%	840
	11th to 20th	Night	19% 16% 34% 23% 6% 1	840
	21st to end	Night	18% 13% 38% 25% 5% 1	924
April	1st to 10th	Night	21% 16% 35% 23% 5%	840
	11th to 20th	Night	15% 10% 35% 30% 10%	831
	21st to end	Night	19% 8% 33% 30% 10% 1	840
May	1st to 10th	Night	20% 8% 30% 29% 12% 1	754
	11th to 20th	Night	18% 8% 26% 34% 14% 1	425
	21st to end	Night	16% 4 20% 34% 23% 3	455
June	1st to 10th	Night	10% 3 23% 39% 22% 2	420
	11th to 20th	Night	11% 5% 23% 38% 21% 1	420
	21st to end	Night	11% 3 19% 43% 22% 3	420
July	1st to 10th	Night	10% 2 15% 38% 30% 4	410
	11th to 20th	Night	6% 2 11% 42% 35% 4	411
	21st to end	Night	9% 2 17% 40% 28% 4	690
August	1st to 10th	Night	12% 3 11% 40% 31% 3	627
	11th to 20th	Night	12% 4 12% 36% 32% 5%	754
	21st to end	Night	13% 2 15% 35% 28% 7%	924
September	1st to 10th	Night	15% 3 16% 34% 27% 5%	840
	11th to 20th	Night	13% 3 14% 36% 29% 6%	813
	21st to end	Night	23% 5% 19% 32% 19% 2	800
October	1st to 10th	Night	21% 7% 20% 26% 21% 6%	800
	11th to 20th	Night	20% 10% 26% 28% 16% 2	800
	21st to end	Night	19% 11% 28% 26% 14% 1	880
November	1st to 10th	Night	14% 11% 30% 31% 13% 2	820
	11th to 20th	Night	19% 15% 30% 26% 9%	840
	21st to end	Night	16% 16% 33% 26% 8% 1	840
December	1st to 10th	Night	21% 15% 33% 22% 9% 1	840
	11th to 20th	Night	19% 14% 37% 24% 6%	840
	21st to end	Night	19% 12% 35% 30% 4	924

Page generated at 2026-09-11 04:11:57. Version ac07

Copyright © 2026 Attilla Danko.