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 2024-11-01.

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% 43%	101183
	Night	16% 6% 2 6% 4 4 4 3 5% 7% 43%	97711
Transparency	Day	21% 48% 7% 14% 8% 1	100980
	Night	22% 49% 6% 13% 8% 2	97584
Seeing	Day	17% 7% 25% 27% 17% 6%	15014
	Night	17% 10% 28% 29% 14% 2	24436

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% 3 3 2 3 5% 7% 49%	2517
	11th to 20th	Day	15% 11% 4 7% 4 4 3 3 4 6% 40%	2817
	21st to end	Day	18% 7% 3 5% 4 4 4 3 5% 7% 40%	3124
February	1st to 10th	Day	15% 7% 2 6% 4 5% 4 4 7% 7% 40%	1710
	11th to 20th	Day	19% 7% 2 5% 3 4 3 3 6% 6% 43%	1755
	21st to end	Day	18% 9% 2 6% 4 4 3 3 4 7% 39%	1570
March	1st to 10th	Day	23% 6% 2 5% 3 3 4 3 5% 7% 40%	3297
	11th to 20th	Day	10% 4 2 5% 4 4 4 4 5% 7% 51%	3480
	21st to end	Day	21% 7% 2 5% 4 3 3 2 4 6% 42%	3934
April	1st to 10th	Day	14% 6% 2 5% 4 4 4 5% 6% 7% 43%	2280
	11th to 20th	Day	12% 5% 3 6% 4 3 4 3 5% 7% 48%	2254
	21st to end	Day	17% 6% 2 4 4 4 4 4 6% 7% 42%	2280
May	1st to 10th	Day	25% 7% 2 6% 4 3 4 4 6% 8% 32%	3923
	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 4 6% 51%	2090
November	1st to 10th	Day	13% 7% 1 4 3 4 4 2 4 7% 52%	1711
	11th to 20th	Day	14% 6% 2 3 4 4 4 4 5% 6% 49%	1710
	21st to end	Day	14% 6% 2 4 4 4 4 4 5% 5% 48%	1544
December	1st to 10th	Day	16% 6% 2 5% 4 3 4 4 6% 7% 43%	1520
	11th to 20th	Day	16% 8% 2 5% 3 3 4 3 4 7% 45%	1910
	21st to end	Day	17% 7% 2 6% 5% 5% 4 4 4 6% 39%	2772
January	1st to 10th	Night	16% 8% 2 6% 4 4 4 3 5% 6% 43%	4429
	11th to 20th	Night	19% 8% 2 4 3 2 3 3 5% 5% 45%	4141
	21st to end	Night	19% 9% 3 6% 4 3 3 3 4 5% 42%	4532
February	1st to 10th	Night	16% 8% 2 5% 4 4 4 4 6% 6% 41%	2470
	11th to 20th	Night	19% 7% 2 5% 3 4 3 3 4 5% 44%	2309
	21st to end	Night	18% 8% 2 5% 4 3 3 3 5% 5% 43%	1884
March	1st to 10th	Night	22% 8% 2 5% 3 3 4 2 4 5% 42%	3552
	11th to 20th	Night	12% 5% 1 5% 4 3 4 3 6% 7% 50%	3480
	21st to end	Night	16% 7% 3 6% 4 3 3 3 3 5% 47%	3696
April	1st to 10th	Night	16% 7% 2 6% 4 4 4 3 5% 7% 42%	1900
	11th to 20th	Night	13% 6% 2 6% 4 4 4 4 6% 6% 46%	1875
	21st to end	Night	19% 5% 2 5% 3 3 4 3 5% 7% 45%	1900
May	1st to 10th	Night	21% 9% 2 7% 4 5% 4 3 5% 7% 32%	2987
	11th to 20th	Night	26% 8% 2 7% 4 3 5% 4 5% 6% 31%	2505
	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 6% 50%	2508
November	1st to 10th	Night	12% 6% 2 5% 3 3 3 4 4 5% 53%	2373
	11th to 20th	Night	16% 6% 2 4 4 3 4 3 4 6% 49%	2470
	21st to end	Night	13% 4 2 5% 4 3 4 3 6% 6% 48%	2631
December	1st to 10th	Night	18% 6% 3 6% 4 4 4 4 6% 6% 41%	2660
	11th to 20th	Night	17% 5% 2 5% 4 4 4 3 5% 7% 43%	3285
	21st to end	Night	16% 6% 2 6% 4 4 5% 4 6% 8% 37%	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	23% 36% 5% 22% 11% 4	2518
	11th to 20th	Day	31% 37% 6% 13% 10% 3	2817
	21st to end	Day	29% 41% 9% 14% 6% 1	3124
February	1st to 10th	Day	16% 66% 6% 7% 4 1	1710
	11th to 20th	Day	16% 63% 2 10% 7% 2	1764
	21st to end	Day	18% 65% 4 7% 6% 1	1570
March	1st to 10th	Day	32% 42% 5% 14% 5% 3	3297
	11th to 20th	Day	16% 40% 4 22% 16% 2	3480
	21st to end	Day	27% 41% 6% 12% 12% 3	3934
April	1st to 10th	Day	15% 65% 4 8% 7% 1	2280
	11th to 20th	Day	13% 65% 5% 9% 6% 1	2254
	21st to end	Day	15% 65% 4 9% 6% 1	2280
May	1st to 10th	Day	36% 42% 6% 13% 4 1	3958
	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	14% 59% 5% 11% 8% 1	1900
	11th to 20th	Day	11% 59% 4 12% 11% 3	1900
	21st to end	Day	11% 58% 4 13% 11% 2	2090
November	1st to 10th	Day	16% 58% 6% 9% 6% 3	1711
	11th to 20th	Day	13% 60% 2 11% 12% 2	1710
	21st to end	Day	15% 58% 3 14% 8% 1	1544
December	1st to 10th	Day	19% 60% 6% 8% 4 2	1520
	11th to 20th	Day	20% 47% 6% 18% 8% 2	1906
	21st to end	Day	30% 39% 10% 15% 7%	2772
January	1st to 10th	Night	28% 37% 6% 16% 11% 3	4429
	11th to 20th	Night	26% 36% 5% 17% 12% 3	4141
	21st to end	Night	31% 39% 4 16% 9% 1	4532
February	1st to 10th	Night	16% 65% 6% 10% 3 1	2470
	11th to 20th	Night	16% 63% 2 7% 10% 2	2311
	21st to end	Night	15% 64% 4 9% 5% 3	1884
March	1st to 10th	Night	34% 41% 3 15% 6% 1	3552
	11th to 20th	Night	16% 41% 6% 22% 13% 3	3480
	21st to end	Night	25% 40% 4 13% 15% 3	3696
April	1st to 10th	Night	15% 64% 3 9% 8% 1	1900
	11th to 20th	Night	13% 66% 4 10% 6% 2	1875
	21st to end	Night	16% 64% 4 8% 7% 1	1900
May	1st to 10th	Night	35% 42% 6% 11% 5%	3017
	11th to 20th	Night	35% 42% 8% 12% 3	2506
	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	18% 59% 5% 9% 8% 2	2280
	11th to 20th	Night	12% 60% 3 12% 11% 3	2280
	21st to end	Night	13% 59% 4 12% 10% 3	2508
November	1st to 10th	Night	16% 58% 5% 11% 7% 3	2373
	11th to 20th	Night	14% 59% 3 8% 12% 3	2470
	21st to end	Night	12% 59% 5% 13% 8% 2	2631
December	1st to 10th	Night	21% 59% 5% 10% 4 1	2660
	11th to 20th	Night	20% 48% 5% 11% 12% 3	3288
	21st to end	Night	29% 39% 11% 15% 6% 1	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	23% 15% 37% 17% 8% 1	180
	11th to 20th	Day	19% 23% 33% 21% 3	265
	21st to end	Day	21% 16% 39% 18% 6% 1	297
February	1st to 10th	Day	21% 10% 44% 17% 8%	270
	11th to 20th	Day	28% 15% 40% 14% 3	362
	21st to end	Day	27% 11% 38% 19% 5%	382
March	1st to 10th	Day	18% 18% 34% 22% 7% 1	460
	11th to 20th	Day	20% 15% 34% 22% 7% 2	460
	21st to end	Day	22% 11% 36% 24% 6% 2	506
April	1st to 10th	Day	19% 11% 37% 24% 8% 1	460
	11th to 20th	Day	15% 12% 33% 25% 10% 5%	452
	21st to end	Day	22% 9% 30% 27% 11% 2	460
May	1st to 10th	Day	24% 7% 28% 27% 12% 2	463
	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	17% 9% 32% 23% 12% 6%	385
	11th to 20th	Day	15% 15% 34% 25% 9% 2	370
	21st to end	Day	17% 15% 30% 22% 16% 1	204
December	1st to 10th	Day	21% 13% 34% 26% 6% 2	180
	11th to 20th	Day	21% 16% 31% 24% 9%	180
	21st to end	Day	24% 9% 29% 28% 9% 1	198
January	1st to 10th	Night	19% 17% 39% 21% 4	760
	11th to 20th	Night	23% 15% 31% 25% 6%	760
	21st to end	Night	19% 18% 35% 26% 2	836
February	1st to 10th	Night	19% 14% 35% 28% 4	760
	11th to 20th	Night	24% 17% 37% 20% 2	743
	21st to end	Night	23% 14% 38% 22% 3	628
March	1st to 10th	Night	18% 16% 37% 24% 5%	760
	11th to 20th	Night	20% 16% 34% 24% 6% 2	760
	21st to end	Night	19% 12% 38% 25% 6% 1	836
April	1st to 10th	Night	20% 15% 36% 23% 6%	760
	11th to 20th	Night	16% 10% 33% 30% 10% 1	751
	21st to end	Night	20% 8% 33% 29% 10% 1	760
May	1st to 10th	Night	21% 7% 28% 30% 13% 1	682
	11th to 20th	Night	18% 8% 25% 34% 14% 1	384
	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% 15% 1	836
November	1st to 10th	Night	15% 10% 30% 30% 14% 2	743
	11th to 20th	Night	19% 15% 28% 27% 10%	760
	21st to end	Night	16% 14% 33% 27% 9% 1	760
December	1st to 10th	Night	22% 15% 31% 22% 9% 1	760
	11th to 20th	Night	18% 12% 39% 25% 6%	760
	21st to end	Night	19% 11% 35% 30% 4	836

Page generated at 2024-10-29 11:20:49. Version ac07

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