Aurora Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Aurora Center clear sky chart from 2005-11-06 to 2026-04-02.

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	10% 7% 3 7% 4 4 4 4 5% 7% 45%	139498
	Night	8% 7% 3 7% 4 4 4 4 6% 7% 47%	139695
Transparency	Day	39% 4 14% 26% 16% 2	134184
	Night	37% 4 14% 26% 17% 2	136019
Seeing	Day	14% 9% 32% 29% 13% 2	17766
	Night	12% 9% 36% 34% 9% 1	27848

By Month

Percentage Cloud Cover SW (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	8% 14% 6% 14% 7% 5% 5% 3 4 5% 29%	4720
	11th to 20th	Day	4 10% 5% 12% 8% 7% 6% 4 8% 7% 30%	4740
	21st to end	Day	4 8% 4 10% 7% 6% 6% 4 6% 6% 40%	5501
February	1st to 10th	Day	8% 12% 4 11% 7% 4 5% 4 6% 6% 33%	1890
	11th to 20th	Day	8% 8% 4 9% 6% 5% 5% 4 5% 7% 39%	1976
	21st to end	Day	8% 10% 3 9% 4 3 4 3 6% 7% 41%	1730
March	1st to 10th	Day	8% 9% 3 7% 4 3 3 3 4 5% 51%	4679
	11th to 20th	Day	7% 5% 2 5% 4 4 3 4 6% 8% 52%	5090
	21st to end	Day	20% 12% 3 10% 5% 4 5% 3 5% 6% 26%	5585
April	1st to 10th	Day	16% 7% 2 7% 4 4 4 4 5% 7% 40%	2344
	11th to 20th	Day	18% 9% 3 6% 4 4 4 3 5% 5% 40%	2539
	21st to end	Day	18% 8% 2 6% 3 3 5% 4 5% 7% 39%	2600
May	1st to 10th	Day	10% 5% 2 5% 4 4 4 3 6% 7% 50%	5900
	11th to 20th	Day	16% 6% 2 5% 3 3 3 2 4 5% 49%	5862
	21st to end	Day	19% 7% 2 6% 4 4 4 4 6% 6% 38%	6638
June	1st to 10th	Day	10% 5% 2 5% 4 3 4 4 7% 9% 49%	2995
	11th to 20th	Day	11% 5% 2 5% 4 3 4 5% 6% 7% 47%	3000
	21st to end	Day	8% 4 2 5% 3 4 4 4 6% 8% 52%	2968
July	1st to 10th	Day	9% 4 2 5% 4 4 4 4 6% 9% 48%	6339
	11th to 20th	Day	8% 5% 2 5% 3 4 4 4 5% 7% 53%	6340
	21st to end	Day	6% 5% 2 5% 4 5% 6% 4 6% 9% 49%	6691
August	1st to 10th	Day	7% 5% 2 6% 4 5% 5% 5% 6% 8% 46%	5801
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 4 6% 8% 46%	5821
	21st to end	Day	8% 4 2 5% 3 3 3 3 5% 7% 56%	6445
September	1st to 10th	Day	11% 5% 2 4 3 3 3 3 4 6% 56%	2300
	11th to 20th	Day	8% 5% 2 5% 3 3 3 3 5% 8% 56%	2200
	21st to end	Day	11% 5% 2 3 3 2 3 3 5% 6% 58%	2200
October	1st to 10th	Day	8% 4 1 3 2 3 2 3 4 6% 64%	3495
	11th to 20th	Day	9% 5% 2 5% 3 3 4 3 4 6% 55%	3240
	21st to end	Day	10% 7% 3 7% 4 5% 4 3 6% 7% 44%	3221
November	1st to 10th	Day	8% 8% 2 6% 3 3 4 3 6% 7% 50%	1853
	11th to 20th	Day	6% 7% 3 8% 5% 4 5% 4 6% 6% 48%	1890
	21st to end	Day	8% 9% 4 8% 4 4 4 4 5% 7% 44%	1706
December	1st to 10th	Day	5% 9% 4 10% 7% 6% 6% 4 5% 6% 37%	2657
	11th to 20th	Day	7% 9% 5% 10% 5% 6% 6% 4 5% 7% 36%	2713
	21st to end	Day	12% 9% 4 12% 5% 4 5% 3 5% 6% 35%	3829
January	1st to 10th	Night	5% 11% 5% 13% 7% 6% 5% 5% 6% 7% 30%	8201
	11th to 20th	Night	3 8% 4 10% 7% 6% 8% 5% 8% 7% 36%	8250
	21st to end	Night	3 7% 4 11% 5% 4 4 4 6% 7% 45%	8710
February	1st to 10th	Night	9% 10% 5% 8% 5% 5% 5% 4 6% 6% 36%	2730
	11th to 20th	Night	7% 9% 3 8% 6% 4 5% 4 6% 6% 42%	2550
	21st to end	Night	8% 8% 2 7% 4 4 5% 5% 6% 7% 46%	2076
March	1st to 10th	Night	10% 8% 2 6% 4 3 4 3 5% 6% 49%	5411
	11th to 20th	Night	10% 5% 2 5% 3 3 4 4 5% 9% 50%	5090
	21st to end	Night	15% 11% 3 6% 4 4 4 4 5% 6% 36%	5588
April	1st to 10th	Night	13% 7% 3 6% 5% 4 5% 4 5% 7% 43%	2076
	11th to 20th	Night	15% 8% 2 5% 4 4 4 3 6% 8% 43%	1808
	21st to end	Night	15% 8% 2 6% 4 3 5% 4 5% 6% 42%	1800
May	1st to 10th	Night	12% 5% 1 5% 3 3 4 3 5% 7% 51%	4080
	11th to 20th	Night	13% 5% 2 5% 3 3 4 3 4 6% 52%	4053
	21st to end	Night	18% 7% 2 6% 4 3 4 4 5% 7% 41%	4243
June	1st to 10th	Night	7% 4 1 5% 3 4 3 4 6% 8% 55%	1400
	11th to 20th	Night	9% 5% 2 6% 4 4 3 4 6% 8% 50%	1400
	21st to end	Night	7% 3 1 4 4 3 4 3 6% 9% 56%	1426
July	1st to 10th	Night	6% 4 1 4 3 4 4 4 6% 10% 54%	3640
	11th to 20th	Night	7% 4 2 5% 3 4 3 3 5% 7% 57%	3640
	21st to end	Night	5% 4 1 5% 4 3 5% 4 7% 11% 53%	4180
August	1st to 10th	Night	6% 5% 3 6% 4 5% 5% 4 8% 11% 44%	4011
	11th to 20th	Night	8% 5% 2 6% 4 4 5% 4 7% 8% 49%	4028
	21st to end	Night	7% 4 2 5% 3 4 4 3 5% 8% 55%	4528
September	1st to 10th	Night	9% 3 2 3 3 3 4 3 4 6% 60%	2100
	11th to 20th	Night	7% 4 1 4 4 3 4 4 5% 7% 58%	2200
	21st to end	Night	8% 4 2 4 3 3 4 3 4 5% 61%	2200
October	1st to 10th	Night	8% 3 1 3 2 2 2 2 3 4 71%	3651
	11th to 20th	Night	9% 4 1 4 2 2 3 3 5% 8% 59%	3899
	21st to end	Night	10% 6% 2 5% 4 4 5% 3 6% 8% 48%	4574
November	1st to 10th	Night	6% 6% 2 6% 4 4 3 3 6% 8% 52%	2665
	11th to 20th	Night	8% 6% 2 7% 4 3 4 4 5% 7% 49%	2730
	21st to end	Night	7% 8% 2 5% 5% 4 4 4 5% 7% 48%	2910
December	1st to 10th	Night	4 6% 4 9% 5% 5% 4 3 4 5% 50%	4748
	11th to 20th	Night	6% 9% 4 8% 6% 5% 5% 4 5% 7% 41%	5714
	21st to end	Night	13% 11% 4 11% 5% 4 4 3 4 5% 36%	7385

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	59% 3 13% 19% 7%	4723
	11th to 20th	Day	51% 3 13% 19% 13% 1	4740
	21st to end	Day	44% 4 15% 23% 13% 1	5501
February	1st to 10th	Day	51% 5% 15% 18% 10%	1890
	11th to 20th	Day	44% 5% 13% 23% 13% 2	1985
	21st to end	Day	42% 7% 14% 22% 12% 3	1730
March	1st to 10th	Day	37% 4 9% 30% 18% 2	4679
	11th to 20th	Day	30% 3 11% 30% 23% 2	5090
	21st to end	Day	59% 4 14% 16% 5% 2	5585
April	1st to 10th	Day	42% 6% 13% 25% 12% 1	2344
	11th to 20th	Day	45% 5% 12% 22% 15% 3	2542
	21st to end	Day	44% 5% 13% 26% 11%	2600
May	1st to 10th	Day	31% 5% 11% 28% 24% 1	5900
	11th to 20th	Day	38% 5% 10% 21% 17% 8%	5866
	21st to end	Day	45% 5% 11% 25% 13% 1	6639
June	1st to 10th	Day	31% 6% 17% 32% 11% 2	2995
	11th to 20th	Day	34% 6% 16% 25% 16% 2	3000
	21st to end	Day	28% 7% 17% 29% 17% 3	2968
July	1st to 10th	Day	34% 3 17% 30% 14% 2	6271
	11th to 20th	Day	31% 2 12% 31% 20% 3	6265
	21st to end	Day	32% 4 18% 26% 17% 3	6498
August	1st to 10th	Day	33% 4 14% 30% 15% 4	4220
	11th to 20th	Day	43% 5% 14% 26% 12% 1	4210
	21st to end	Day	27% 4 18% 28% 20% 3	4642
September	1st to 10th	Day	30% 4 13% 29% 20% 4	2300
	11th to 20th	Day	27% 5% 16% 31% 19% 1	2200
	21st to end	Day	28% 5% 9% 33% 23% 3	2200
October	1st to 10th	Day	23% 3 8% 23% 38% 5%	3495
	11th to 20th	Day	31% 3 12% 31% 20% 3	3240
	21st to end	Day	40% 2 11% 25% 19% 2	3221
November	1st to 10th	Day	34% 5% 18% 26% 15% 3	1853
	11th to 20th	Day	35% 6% 15% 27% 14% 3	1890
	21st to end	Day	39% 4 16% 27% 12% 2	1706
December	1st to 10th	Day	47% 4 15% 23% 9% 1	2657
	11th to 20th	Day	48% 4 17% 21% 11%	2710
	21st to end	Day	52% 3 12% 27% 6%	3829
January	1st to 10th	Night	52% 4 13% 20% 9% 1	8202
	11th to 20th	Night	45% 4 15% 21% 14% 1	8250
	21st to end	Night	39% 4 15% 26% 16%	8710
February	1st to 10th	Night	47% 5% 15% 20% 11% 1	2730
	11th to 20th	Night	42% 5% 13% 23% 14% 3	2551
	21st to end	Night	36% 6% 14% 27% 13% 4	2076
March	1st to 10th	Night	36% 4 10% 29% 20% 2	5411
	11th to 20th	Night	31% 4 10% 29% 24% 2	5090
	21st to end	Night	48% 5% 13% 24% 10% 1	5588
April	1st to 10th	Night	40% 5% 14% 25% 15% 1	2076
	11th to 20th	Night	39% 5% 10% 25% 18% 2	1808
	21st to end	Night	41% 6% 12% 25% 14% 2	1800
May	1st to 10th	Night	31% 6% 12% 23% 24% 3	4080
	11th to 20th	Night	33% 5% 11% 21% 24% 6%	4054
	21st to end	Night	42% 6% 14% 26% 13%	4244
June	1st to 10th	Night	27% 5% 17% 30% 18% 3	1400
	11th to 20th	Night	32% 6% 15% 23% 20% 4	1400
	21st to end	Night	25% 5% 16% 33% 18% 3	1426
July	1st to 10th	Night	27% 3 20% 33% 14% 4	3600
	11th to 20th	Night	28% 3 11% 35% 19% 3	3592
	21st to end	Night	26% 4 22% 27% 17% 3	4060
August	1st to 10th	Night	32% 5% 18% 29% 12% 4	2916
	11th to 20th	Night	37% 4 15% 28% 14% 2	2915
	21st to end	Night	27% 4 17% 24% 23% 5%	3262
September	1st to 10th	Night	26% 5% 13% 28% 23% 5%	2100
	11th to 20th	Night	25% 5% 14% 33% 21% 2	2200
	21st to end	Night	26% 4 9% 28% 29% 4	2200
October	1st to 10th	Night	21% 2 7% 18% 46% 6%	3651
	11th to 20th	Night	25% 3 9% 34% 22% 7%	3899
	21st to end	Night	35% 3 14% 26% 20% 2	4574
November	1st to 10th	Night	30% 4 14% 26% 20% 4	2665
	11th to 20th	Night	33% 6% 15% 24% 19% 3	2730
	21st to end	Night	35% 4 15% 28% 16% 2	2910
December	1st to 10th	Night	37% 3 15% 32% 13% 1	4748
	11th to 20th	Night	43% 5% 15% 28% 8%	5716
	21st to end	Night	51% 3 13% 25% 8%	7385

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	17% 12% 27% 29% 14% 2	330
	11th to 20th	Day	13% 14% 38% 25% 8% 1	330
	21st to end	Day	15% 10% 45% 21% 8% 1	363
February	1st to 10th	Day	11% 15% 46% 22% 5% 1	330
	11th to 20th	Day	15% 9% 40% 25% 9% 1	330
	21st to end	Day	18% 9% 42% 27% 3 1	273
March	1st to 10th	Day	14% 12% 42% 24% 7% 2	330
	11th to 20th	Day	15% 11% 37% 25% 12% 1	330
	21st to end	Day	25% 12% 31% 21% 10% 2	363
April	1st to 10th	Day	22% 14% 38% 19% 7%	436
	11th to 20th	Day	24% 16% 31% 23% 6% 1	660
	21st to end	Day	21% 14% 30% 25% 8% 2	700
May	1st to 10th	Day	16% 11% 34% 25% 13% 1	703
	11th to 20th	Day	21% 9% 28% 26% 14% 2	725
	21st to end	Day	17% 8% 33% 27% 12% 3	789
June	1st to 10th	Day	10% 8% 25% 33% 18% 7%	730
	11th to 20th	Day	12% 8% 36% 30% 13% 2	730
	21st to end	Day	10% 6% 30% 36% 16% 2	730
July	1st to 10th	Day	9% 3 25% 38% 22% 4	721
	11th to 20th	Day	7% 4 29% 39% 19% 2	718
	21st to end	Day	7% 3 26% 44% 19% 2	794
August	1st to 10th	Day	8% 4 25% 40% 20% 3	726
	11th to 20th	Day	9% 3 29% 35% 20% 3	715
	21st to end	Day	7% 4 30% 34% 21% 4	784
September	1st to 10th	Day	11% 7% 27% 33% 17% 3	430
	11th to 20th	Day	12% 5% 33% 28% 16% 6%	330
	21st to end	Day	17% 12% 25% 29% 14% 4	330
October	1st to 10th	Day	17% 8% 24% 33% 13% 5%	330
	11th to 20th	Day	18% 12% 37% 22% 10% 1	330
	21st to end	Day	17% 17% 35% 18% 12% 1	363
November	1st to 10th	Day	18% 12% 35% 24% 8% 3	330
	11th to 20th	Day	13% 12% 43% 22% 9% 1	330
	21st to end	Day	15% 13% 36% 25% 10% 2	330
December	1st to 10th	Day	15% 8% 43% 25% 7% 1	330
	11th to 20th	Day	16% 13% 40% 24% 6% 1	330
	21st to end	Day	18% 11% 36% 27% 7%	363
January	1st to 10th	Night	10% 10% 38% 33% 8% 1	1050
	11th to 20th	Night	10% 14% 37% 32% 6% 1	1050
	21st to end	Night	11% 10% 41% 30% 7%	1155
February	1st to 10th	Night	14% 11% 41% 29% 6%	1050
	11th to 20th	Night	11% 10% 41% 33% 6%	916
	21st to end	Night	12% 9% 39% 30% 9% 1	692
March	1st to 10th	Night	14% 10% 39% 33% 5%	809
	11th to 20th	Night	17% 9% 37% 31% 6%	630
	21st to end	Night	18% 9% 41% 25% 8%	693
April	1st to 10th	Night	17% 12% 38% 30% 3	603
	11th to 20th	Night	20% 14% 34% 26% 5%	593
	21st to end	Night	19% 10% 34% 28% 8% 1	600
May	1st to 10th	Night	14% 8% 35% 33% 11% 1	600
	11th to 20th	Night	17% 8% 33% 30% 10% 1	591
	21st to end	Night	16% 7% 32% 36% 9% 1	650
June	1st to 10th	Night	9% 6% 28% 39% 17% 1	600
	11th to 20th	Night	11% 5% 37% 40% 8%	600
	21st to end	Night	8% 4 28% 49% 12% 1	600
July	1st to 10th	Night	5% 3 25% 46% 19% 1	585
	11th to 20th	Night	5% 1 33% 46% 15% 1	582
	21st to end	Night	4 2 30% 49% 15% 1	657
August	1st to 10th	Night	6% 2 29% 45% 17%	597
	11th to 20th	Night	7% 4 26% 45% 17% 1	594
	21st to end	Night	6% 3 31% 44% 16%	660
September	1st to 10th	Night	10% 4 31% 40% 15% 1	600
	11th to 20th	Night	8% 3 36% 38% 13% 2	600
	21st to end	Night	9% 8% 34% 34% 14% 1	600
October	1st to 10th	Night	16% 8% 33% 33% 9% 1	635
	11th to 20th	Night	16% 10% 35% 30% 10%	800
	21st to end	Night	16% 12% 35% 30% 8%	1076
November	1st to 10th	Night	11% 9% 40% 30% 9% 1	1025
	11th to 20th	Night	11% 11% 38% 31% 8% 1	1050
	21st to end	Night	10% 9% 39% 33% 8%	1050
December	1st to 10th	Night	11% 13% 38% 31% 7%	1050
	11th to 20th	Night	13% 10% 37% 31% 8%	1050
	21st to end	Night	18% 12% 34% 28% 7% 1	1155

Page generated at 2026-03-30 01:18:40. Version ac07

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