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 2024-12-09.

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 6% 4 4 4 4 5% 7% 46%	105227
	Night	9% 7% 3 7% 4 4 4 4 5% 7% 47%	103197
Transparency	Day	37% 5% 14% 26% 15% 2	104974
	Night	36% 5% 14% 26% 17% 3	103040
Seeing	Day	14% 9% 32% 30% 14% 2	16033
	Night	12% 8% 35% 34% 10% 1	25988

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	6% 14% 6% 10% 7% 5% 6% 4 5% 5% 31%	2538
	11th to 20th	Day	4 11% 5% 12% 6% 4 7% 4 7% 8% 30%	2540
	21st to end	Day	5% 9% 4 12% 7% 5% 5% 4 6% 6% 37%	2953
February	1st to 10th	Day	9% 12% 4 10% 6% 4 5% 4 6% 6% 33%	1710
	11th to 20th	Day	9% 9% 4 8% 6% 5% 5% 4 5% 8% 38%	1782
	21st to end	Day	9% 11% 4 9% 5% 4 4 3 6% 7% 39%	1570
March	1st to 10th	Day	8% 10% 3 8% 4 3 3 3 4 6% 48%	3182
	11th to 20th	Day	10% 6% 2 6% 4 4 3 3 5% 7% 49%	3480
	21st to end	Day	23% 12% 3 11% 5% 4 4 3 5% 5% 25%	3814
April	1st to 10th	Day	16% 7% 2 7% 4 4 4 4 5% 7% 41%	2216
	11th to 20th	Day	18% 9% 3 6% 4 4 4 3 5% 6% 39%	2410
	21st to end	Day	18% 8% 2 6% 3 3 4 4 5% 7% 39%	2470
May	1st to 10th	Day	14% 6% 2 7% 5% 5% 5% 4 6% 7% 41%	4120
	11th to 20th	Day	11% 5% 2 6% 4 4 3 3 4 6% 51%	4082
	21st to end	Day	16% 6% 2 6% 3 4 4 3 6% 7% 42%	4629
June	1st to 10th	Day	11% 5% 2 5% 3 3 4 4 7% 9% 49%	2845
	11th to 20th	Day	12% 5% 2 5% 4 4 3 5% 6% 7% 48%	2850
	21st to end	Day	8% 4 2 4 3 4 4 4 6% 9% 53%	2820
July	1st to 10th	Day	11% 5% 2 5% 4 4 4 5% 6% 8% 46%	4430
	11th to 20th	Day	4 3 2 5% 3 4 3 4 5% 7% 59%	4430
	21st to end	Day	4 3 1 4 3 4 5% 4 6% 10% 54%	4655
August	1st to 10th	Day	7% 5% 2 5% 4 4 5% 5% 6% 7% 49%	4103
	11th to 20th	Day	16% 7% 3 6% 4 4 5% 4 6% 7% 40%	4041
	21st to end	Day	6% 4 2 5% 3 4 4 3 5% 7% 57%	4501
September	1st to 10th	Day	11% 5% 2 4 3 3 3 3 3 6% 56%	2185
	11th to 20th	Day	8% 4 2 4 3 3 3 3 5% 7% 58%	2090
	21st to end	Day	12% 5% 2 3 3 2 3 3 4 6% 57%	2090
October	1st to 10th	Day	9% 4 1 3 2 2 2 3 4 6% 64%	3388
	11th to 20th	Day	8% 5% 2 5% 3 3 4 4 4 6% 56%	3140
	21st to end	Day	10% 7% 3 7% 4 5% 4 3 6% 7% 44%	3121
November	1st to 10th	Day	8% 7% 2 6% 3 3 4 3 6% 7% 49%	1763
	11th to 20th	Day	6% 7% 3 8% 5% 4 5% 4 6% 6% 48%	1800
	21st to end	Day	7% 9% 4 8% 4 4 4 4 6% 7% 45%	1625
December	1st to 10th	Day	8% 9% 3 8% 6% 5% 5% 5% 6% 7% 38%	1565
	11th to 20th	Day	6% 10% 5% 8% 6% 5% 5% 4 5% 7% 39%	1683
	21st to end	Day	15% 13% 5% 11% 4 4 5% 3 5% 5% 31%	2606
January	1st to 10th	Night	6% 11% 4 12% 8% 6% 5% 5% 7% 8% 29%	4417
	11th to 20th	Night	4 10% 5% 9% 5% 5% 9% 5% 9% 7% 33%	4430
	21st to end	Night	4 8% 5% 13% 5% 5% 4 4 6% 7% 38%	4711
February	1st to 10th	Night	9% 11% 5% 9% 5% 5% 5% 3 6% 6% 37%	2470
	11th to 20th	Night	7% 9% 3 8% 5% 4 5% 4 6% 6% 43%	2302
	21st to end	Night	8% 8% 2 7% 4 4 5% 5% 6% 6% 44%	1884
March	1st to 10th	Night	9% 10% 3 6% 4 4 4 3 5% 5% 47%	3680
	11th to 20th	Night	10% 6% 2 5% 4 4 4 4 5% 9% 49%	3480
	21st to end	Night	18% 12% 3 7% 5% 4 4 3 5% 7% 31%	3817
April	1st to 10th	Night	13% 7% 3 6% 5% 4 5% 4 5% 7% 43%	1962
	11th to 20th	Night	16% 8% 2 5% 4 4 4 3 5% 8% 42%	1717
	21st to end	Night	15% 7% 2 6% 5% 3 5% 5% 5% 6% 42%	1710
May	1st to 10th	Night	15% 5% 1 5% 3 4 5% 4 5% 8% 45%	2850
	11th to 20th	Night	10% 4 2 4 3 3 4 4 4 7% 54%	2823
	21st to end	Night	14% 6% 2 6% 4 3 4 3 6% 8% 45%	2955
June	1st to 10th	Night	7% 4 1 5% 3 4 3 4 6% 8% 55%	1330
	11th to 20th	Night	9% 5% 2 6% 4 4 3 4 6% 8% 50%	1330
	21st to end	Night	6% 3 1 3 3 3 4 3 6% 9% 58%	1354
July	1st to 10th	Night	8% 5% 1 5% 3 4 4 4 6% 10% 50%	2540
	11th to 20th	Night	4 2 1 4 3 4 4 3 5% 6% 63%	2540
	21st to end	Night	2 2 1 4 3 3 4 4 6% 10% 60%	2906
August	1st to 10th	Night	5% 5% 2 6% 4 4 5% 4 8% 10% 48%	2836
	11th to 20th	Night	11% 6% 2 6% 4 3 4 3 6% 6% 48%	2798
	21st to end	Night	6% 4 2 5% 3 3 4 3 5% 8% 58%	3162
September	1st to 10th	Night	9% 3 2 4 3 3 4 3 4 6% 60%	1995
	11th to 20th	Night	6% 4 1 4 4 3 3 3 5% 6% 60%	2090
	21st to end	Night	8% 4 2 4 3 3 4 3 4 5% 60%	2090
October	1st to 10th	Night	9% 3 1 3 2 2 2 2 3 4 70%	3539
	11th to 20th	Night	8% 4 1 4 2 2 3 3 5% 8% 60%	3779
	21st to end	Night	10% 5% 2 5% 4 4 5% 3 6% 8% 48%	4432
November	1st to 10th	Night	6% 6% 2 5% 4 4 3 3 6% 7% 52%	2535
	11th to 20th	Night	8% 6% 2 6% 4 3 4 4 5% 7% 49%	2600
	21st to end	Night	7% 8% 3 5% 5% 4 4 4 5% 8% 49%	2771
December	1st to 10th	Night	7% 8% 4 9% 5% 4 4 4 4 6% 45%	2793
	11th to 20th	Night	6% 9% 3 6% 5% 4 4 4 6% 7% 45%	3514
	21st to end	Night	17% 12% 4 11% 4 3 3 3 3 5% 33%	5055

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	54% 6% 11% 20% 9%	2539
	11th to 20th	Day	49% 5% 14% 24% 7% 1	2540
	21st to end	Day	47% 6% 16% 18% 12%	2953
February	1st to 10th	Day	50% 6% 15% 19% 10%	1710
	11th to 20th	Day	44% 5% 15% 23% 12% 2	1791
	21st to end	Day	44% 8% 14% 21% 11% 3	1570
March	1st to 10th	Day	38% 5% 10% 27% 18% 2	3182
	11th to 20th	Day	35% 4 12% 30% 17% 2	3480
	21st to end	Day	61% 6% 16% 14% 3 1	3814
April	1st to 10th	Day	41% 6% 14% 26% 12% 1	2216
	11th to 20th	Day	45% 5% 12% 21% 14% 3	2413
	21st to end	Day	44% 6% 13% 26% 11%	2470
May	1st to 10th	Day	40% 6% 12% 24% 16% 1	4120
	11th to 20th	Day	33% 7% 12% 27% 18% 3	4086
	21st to end	Day	40% 7% 13% 25% 14% 1	4629
June	1st to 10th	Day	31% 7% 17% 32% 11% 2	2845
	11th to 20th	Day	34% 7% 15% 25% 16% 2	2850
	21st to end	Day	27% 7% 17% 30% 16% 2	2820
July	1st to 10th	Day	35% 4 19% 26% 13% 2	4361
	11th to 20th	Day	25% 3 13% 34% 21% 4	4355
	21st to end	Day	25% 5% 17% 28% 21% 4	4507
August	1st to 10th	Day	33% 4 14% 30% 15% 4	4090
	11th to 20th	Day	43% 5% 14% 25% 11% 1	4080
	21st to end	Day	27% 4 19% 27% 20% 3	4500
September	1st to 10th	Day	31% 4 11% 29% 21% 4	2185
	11th to 20th	Day	26% 5% 15% 32% 20% 1	2090
	21st to end	Day	28% 5% 10% 33% 22% 3	2090
October	1st to 10th	Day	23% 3 8% 22% 38% 5%	3388
	11th to 20th	Day	31% 3 13% 31% 20% 3	3140
	21st to end	Day	40% 2 12% 24% 19% 2	3121
November	1st to 10th	Day	34% 5% 18% 25% 15% 3	1763
	11th to 20th	Day	35% 6% 14% 28% 14% 3	1800
	21st to end	Day	38% 4 16% 27% 13% 2	1625
December	1st to 10th	Day	42% 6% 15% 23% 12% 1	1565
	11th to 20th	Day	44% 5% 17% 25% 8%	1680
	21st to end	Day	56% 4 9% 24% 6% 1	2606
January	1st to 10th	Night	51% 6% 12% 21% 9% 1	4417
	11th to 20th	Night	45% 6% 16% 23% 9% 1	4430
	21st to end	Night	44% 7% 17% 20% 11% 1	4711
February	1st to 10th	Night	47% 6% 14% 21% 11% 1	2470
	11th to 20th	Night	41% 6% 13% 23% 14% 3	2303
	21st to end	Night	38% 7% 14% 25% 12% 4	1884
March	1st to 10th	Night	37% 5% 11% 23% 21% 2	3680
	11th to 20th	Night	32% 5% 11% 30% 19% 3	3480
	21st to end	Night	53% 6% 15% 18% 7% 1	3817
April	1st to 10th	Night	40% 5% 14% 25% 15% 1	1962
	11th to 20th	Night	41% 5% 10% 24% 18% 2	1717
	21st to end	Night	40% 7% 13% 25% 14% 2	1710
May	1st to 10th	Night	35% 8% 14% 23% 17% 4	2850
	11th to 20th	Night	28% 7% 12% 28% 23% 2	2824
	21st to end	Night	36% 8% 17% 25% 14% 1	2956
June	1st to 10th	Night	27% 6% 17% 29% 18% 3	1330
	11th to 20th	Night	32% 6% 15% 23% 20% 4	1330
	21st to end	Night	23% 5% 17% 33% 19% 4	1354
July	1st to 10th	Night	31% 4 23% 24% 14% 5%	2500
	11th to 20th	Night	21% 4 12% 36% 24% 5%	2492
	21st to end	Night	19% 4 19% 31% 22% 4	2815
August	1st to 10th	Night	31% 5% 18% 29% 12% 4	2826
	11th to 20th	Night	37% 4 15% 28% 14% 2	2825
	21st to end	Night	26% 4 17% 25% 23% 5%	3162
September	1st to 10th	Night	26% 5% 12% 28% 24% 6%	1995
	11th to 20th	Night	24% 5% 13% 35% 21% 2	2090
	21st to end	Night	27% 4 9% 28% 28% 4	2090
October	1st to 10th	Night	21% 2 7% 17% 47% 6%	3539
	11th to 20th	Night	24% 3 10% 34% 23% 7%	3779
	21st to end	Night	35% 3 14% 25% 20% 2	4432
November	1st to 10th	Night	30% 4 14% 26% 20% 5%	2535
	11th to 20th	Night	33% 6% 14% 24% 20% 3	2600
	21st to end	Night	35% 5% 15% 28% 16% 2	2771
December	1st to 10th	Night	41% 4 15% 28% 11% 1	2793
	11th to 20th	Night	37% 6% 15% 33% 9%	3516
	21st to end	Night	54% 4 11% 22% 8% 1	5055

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	16% 13% 28% 29% 14% 1	270
	11th to 20th	Day	13% 13% 37% 27% 9%	270
	21st to end	Day	15% 9% 43% 21% 10% 2	297
February	1st to 10th	Day	11% 14% 44% 26% 5%	270
	11th to 20th	Day	17% 11% 39% 25% 8% 1	270
	21st to end	Day	18% 8% 40% 30% 3 1	225
March	1st to 10th	Day	14% 13% 40% 23% 8% 2	270
	11th to 20th	Day	16% 9% 36% 26% 13% 1	270
	21st to end	Day	26% 10% 31% 20% 11% 2	297
April	1st to 10th	Day	22% 14% 39% 17% 7%	396
	11th to 20th	Day	24% 16% 31% 22% 6% 1	611
	21st to end	Day	21% 14% 30% 25% 8% 2	650
May	1st to 10th	Day	17% 12% 35% 25% 10% 1	653
	11th to 20th	Day	20% 9% 29% 26% 15% 2	675
	21st to end	Day	16% 9% 34% 27% 12% 3	734
June	1st to 10th	Day	10% 8% 25% 33% 18% 7%	680
	11th to 20th	Day	12% 8% 36% 28% 13% 2	680
	21st to end	Day	10% 6% 29% 37% 17% 2	680
July	1st to 10th	Day	8% 3 24% 38% 22% 4	671
	11th to 20th	Day	6% 4 29% 39% 19% 2	668
	21st to end	Day	6% 3 24% 45% 20% 2	739
August	1st to 10th	Day	8% 4 23% 41% 21% 3	676
	11th to 20th	Day	10% 4 29% 35% 20% 3	665
	21st to end	Day	7% 5% 30% 34% 21% 4	730
September	1st to 10th	Day	10% 8% 28% 34% 16% 4	395
	11th to 20th	Day	12% 5% 34% 27% 17% 6%	300
	21st to end	Day	19% 13% 25% 28% 13% 3	300
October	1st to 10th	Day	19% 8% 23% 32% 13% 5%	300
	11th to 20th	Day	16% 12% 37% 23% 11% 1	300
	21st to end	Day	17% 16% 35% 18% 12% 1	330
November	1st to 10th	Day	19% 11% 34% 24% 9% 3	300
	11th to 20th	Day	13% 13% 42% 23% 8% 1	300
	21st to end	Day	14% 14% 35% 26% 9% 2	300
December	1st to 10th	Day	15% 8% 41% 26% 8% 1	294
	11th to 20th	Day	14% 11% 41% 26% 7% 1	270
	21st to end	Day	21% 12% 34% 26% 7%	297
January	1st to 10th	Night	10% 11% 37% 33% 7% 1	950
	11th to 20th	Night	10% 13% 36% 33% 7% 1	950
	21st to end	Night	12% 10% 39% 31% 8%	1045
February	1st to 10th	Night	14% 11% 39% 29% 6%	950
	11th to 20th	Night	11% 10% 40% 32% 6%	828
	21st to end	Night	14% 8% 37% 31% 9% 1	628
March	1st to 10th	Night	14% 10% 38% 34% 5%	732
	11th to 20th	Night	16% 9% 37% 32% 6%	570
	21st to end	Night	19% 9% 39% 26% 9%	627
April	1st to 10th	Night	17% 11% 38% 30% 4	570
	11th to 20th	Night	21% 14% 33% 27% 6%	563
	21st to end	Night	18% 10% 34% 29% 8% 1	570
May	1st to 10th	Night	14% 8% 36% 32% 10% 1	570
	11th to 20th	Night	16% 8% 34% 30% 11% 1	561
	21st to end	Night	16% 7% 32% 36% 8% 1	617
June	1st to 10th	Night	9% 5% 28% 38% 17% 1	570
	11th to 20th	Night	11% 5% 37% 39% 8%	570
	21st to end	Night	6% 3 28% 50% 12% 1	570
July	1st to 10th	Night	5% 3 25% 46% 20% 1	555
	11th to 20th	Night	4 1 32% 46% 15% 1	552
	21st to end	Night	4 1 29% 50% 15% 1	624
August	1st to 10th	Night	5% 2 28% 47% 18%	567
	11th to 20th	Night	7% 4 26% 44% 17% 1	564
	21st to end	Night	6% 3 32% 43% 17%	627
September	1st to 10th	Night	10% 4 32% 39% 15% 1	570
	11th to 20th	Night	7% 3 36% 38% 13% 2	570
	21st to end	Night	9% 8% 35% 34% 13% 1	570
October	1st to 10th	Night	17% 7% 33% 33% 8% 1	603
	11th to 20th	Night	15% 10% 34% 30% 10%	760
	21st to end	Night	16% 11% 35% 30% 8%	1022
November	1st to 10th	Night	11% 9% 39% 32% 9% 1	975
	11th to 20th	Night	11% 12% 37% 31% 9% 1	1000
	21st to end	Night	10% 9% 39% 33% 8%	1000
December	1st to 10th	Night	11% 13% 39% 30% 7%	993
	11th to 20th	Night	12% 9% 37% 33% 9%	950
	21st to end	Night	19% 12% 32% 29% 7% 1	1045

Page generated at 2024-12-06 02:54:50. Version ac07

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