ASKC Dark Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the ASKC Dark Site clear sky chart from 2005-11-06 to 2024-11-03.

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	12% 7% 2 6% 4 4 4 3 5% 7% 45%	100905
	Night	12% 7% 2 6% 4 4 4 3 5% 6% 48%	98161
Transparency	Day	39% 4 12% 27% 16% 2	100717
	Night	37% 3 11% 27% 19% 3	98020
Seeing	Day	15% 9% 27% 28% 17% 4	15610
	Night	15% 8% 30% 33% 13% 1	25425

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	15% 9% 3 9% 5% 4 4 4 4 5% 39%	2847
	11th to 20th	Day	11% 11% 3 9% 6% 6% 4 3 3 5% 39%	2850
	21st to end	Day	17% 15% 4 7% 3 3 3 2 3 5% 36%	3135
February	1st to 10th	Day	15% 12% 3 7% 5% 4 3 2 3 6% 40%	1710
	11th to 20th	Day	13% 9% 3 7% 4 4 4 3 4 5% 43%	1664
	21st to end	Day	14% 9% 3 6% 4 4 4 3 5% 7% 41%	1644
March	1st to 10th	Day	15% 9% 2 6% 4 3 3 2 3 5% 48%	3436
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 5% 7% 51%	3480
	21st to end	Day	15% 11% 3 7% 5% 4 4 4 4 6% 36%	3814
April	1st to 10th	Day	13% 8% 2 6% 4 4 4 3 5% 6% 45%	2194
	11th to 20th	Day	15% 7% 3 6% 5% 4 4 3 6% 7% 41%	2254
	21st to end	Day	20% 9% 3 6% 5% 4 3 3 5% 6% 36%	2380
May	1st to 10th	Day	18% 9% 2 6% 5% 4 4 3 5% 6% 36%	4082
	11th to 20th	Day	14% 8% 3 7% 4 4 5% 4 5% 8% 39%	4082
	21st to end	Day	13% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	4477
June	1st to 10th	Day	6% 4 2 5% 4 5% 5% 5% 7% 9% 47%	2646
	11th to 20th	Day	7% 5% 1 5% 5% 4 5% 5% 6% 8% 49%	2660
	21st to end	Day	6% 4 1 4 4 3 4 4 7% 9% 55%	2567
July	1st to 10th	Day	10% 7% 3 7% 5% 5% 5% 4 6% 7% 42%	4120
	11th to 20th	Day	5% 4 2 5% 4 4 5% 4 6% 9% 52%	4120
	21st to end	Day	7% 4 2 5% 4 5% 6% 5% 9% 11% 42%	4465
August	1st to 10th	Day	5% 3 2 5% 4 4 4 4 6% 10% 52%	4104
	11th to 20th	Day	9% 6% 2 7% 4 5% 5% 3 6% 8% 46%	4041
	21st to end	Day	6% 4 1 4 4 4 4 4 6% 9% 56%	4226
September	1st to 10th	Day	12% 5% 1 5% 3 3 3 3 4 7% 54%	2280
	11th to 20th	Day	9% 5% 2 4 4 4 4 4 6% 6% 52%	2109
	21st to end	Day	9% 4 2 4 4 4 4 3 5% 6% 55%	2074
October	1st to 10th	Day	11% 5% 2 4 3 3 4 3 5% 7% 54%	1900
	11th to 20th	Day	12% 5% 2 4 3 2 3 3 3 5% 57%	1900
	21st to end	Day	17% 8% 2 5% 3 3 3 3 3 5% 49%	2090
November	1st to 10th	Day	9% 7% 2 5% 3 3 2 3 5% 6% 55%	1878
	11th to 20th	Day	15% 6% 3 5% 3 3 3 3 4 7% 50%	1711
	21st to end	Day	16% 8% 2 5% 3 2 3 2 4 5% 49%	1520
December	1st to 10th	Day	12% 8% 2 7% 4 3 4 3 4 7% 46%	1520
	11th to 20th	Day	13% 10% 3 8% 5% 3 4 2 3 5% 44%	1918
	21st to end	Day	18% 15% 6% 8% 4 4 4 3 5% 6% 28%	3007
January	1st to 10th	Night	11% 9% 3 10% 4 4 3 3 4 5% 43%	4108
	11th to 20th	Night	11% 8% 2 9% 6% 8% 6% 3 4 5% 38%	4120
	21st to end	Night	20% 11% 4 8% 3 3 3 3 3 5% 37%	4532
February	1st to 10th	Night	13% 11% 3 7% 4 3 4 3 5% 7% 39%	2470
	11th to 20th	Night	13% 9% 2 7% 4 3 4 4 4 7% 44%	2403
	21st to end	Night	14% 9% 3 6% 4 3 3 2 4 5% 47%	1824
March	1st to 10th	Night	15% 7% 2 6% 3 3 2 2 5% 6% 48%	3419
	11th to 20th	Night	10% 6% 2 5% 3 3 3 3 5% 6% 55%	3480
	21st to end	Night	14% 9% 3 7% 5% 4 4 4 5% 6% 38%	3817
April	1st to 10th	Night	10% 6% 3 6% 3 4 3 3 6% 6% 49%	1986
	11th to 20th	Night	14% 8% 2 5% 4 3 3 3 4 6% 47%	1875
	21st to end	Night	16% 7% 3 7% 4 3 3 3 4 7% 42%	1796
May	1st to 10th	Night	16% 7% 3 7% 4 4 4 3 5% 6% 41%	2823
	11th to 20th	Night	14% 8% 3 6% 4 3 4 3 6% 8% 42%	2823
	21st to end	Night	11% 5% 2 5% 3 3 3 3 5% 7% 52%	3090
June	1st to 10th	Night	7% 3 1 4 3 3 3 4 7% 10% 54%	1529
	11th to 20th	Night	3 4 2 5% 4 4 5% 6% 8% 8% 52%	1520
	21st to end	Night	5% 4 1 4 2 4 3 3 6% 8% 60%	1604
July	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 6% 7% 48%	2850
	11th to 20th	Night	3 3 2 4 3 4 4 4 6% 9% 59%	2850
	21st to end	Night	6% 4 2 4 4 4 5% 5% 8% 11% 47%	3098
August	1st to 10th	Night	3 2 1 3 2 4 4 3 6% 9% 63%	2836
	11th to 20th	Night	9% 5% 2 6% 4 4 4 3 5% 7% 51%	2798
	21st to end	Night	4 3 1 4 3 4 4 4 6% 8% 61%	3428
September	1st to 10th	Night	10% 4 1 5% 3 3 3 3 4 7% 57%	1900
	11th to 20th	Night	9% 4 1 4 3 4 5% 3 5% 7% 53%	2066
	21st to end	Night	8% 4 2 4 4 3 3 3 5% 7% 57%	2090
October	1st to 10th	Night	9% 3 2 4 3 4 3 3 4 5% 60%	2275
	11th to 20th	Night	13% 6% 2 5% 3 2 3 2 4 5% 57%	2280
	21st to end	Night	15% 6% 2 5% 4 2 3 2 4 4 52%	2508
November	1st to 10th	Night	10% 4 2 4 2 3 3 2 4 6% 61%	2245
	11th to 20th	Night	12% 7% 1 5% 3 3 3 3 4 6% 53%	2463
	21st to end	Night	17% 7% 2 4 3 2 3 3 4 5% 52%	2660
December	1st to 10th	Night	12% 8% 3 5% 3 4 3 3 5% 7% 46%	2660
	11th to 20th	Night	13% 8% 3 6% 3 4 3 3 6% 6% 45%	3279
	21st to end	Night	19% 16% 5% 8% 4 3 3 2 3 4 31%	4656

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	47% 3 9% 27% 14% 1	2848
	11th to 20th	Day	50% 3 11% 23% 12%	2850
	21st to end	Day	52% 2 10% 19% 16% 1	3135
February	1st to 10th	Day	49% 4 10% 25% 9% 3	1710
	11th to 20th	Day	45% 2 12% 22% 15% 3	1673
	21st to end	Day	44% 2 9% 25% 14% 6%	1644
March	1st to 10th	Day	41% 3 8% 23% 22% 3	3436
	11th to 20th	Day	35% 2 12% 29% 18% 4	3480
	21st to end	Day	50% 2 18% 22% 7% 1	3814
April	1st to 10th	Day	40% 4 12% 25% 17% 2	2194
	11th to 20th	Day	43% 3 11% 25% 16% 1	2254
	21st to end	Day	49% 3 11% 25% 11% 1	2380
May	1st to 10th	Day	49% 3 11% 20% 15% 1	4120
	11th to 20th	Day	43% 4 14% 24% 15%	4086
	21st to end	Day	41% 4 13% 28% 12% 1	4489
June	1st to 10th	Day	31% 5% 18% 27% 17% 1	2646
	11th to 20th	Day	32% 5% 14% 32% 16% 2	2660
	21st to end	Day	25% 4 16% 34% 18% 3	2567
July	1st to 10th	Day	39% 5% 12% 23% 17% 3	4056
	11th to 20th	Day	28% 5% 12% 37% 16% 2	4051
	21st to end	Day	31% 7% 21% 29% 12%	4325
August	1st to 10th	Day	27% 5% 15% 29% 22% 2	4090
	11th to 20th	Day	36% 5% 17% 27% 14% 1	4080
	21st to end	Day	25% 5% 12% 40% 14% 4	4225
September	1st to 10th	Day	32% 3 11% 28% 18% 7%	2280
	11th to 20th	Day	32% 3 9% 27% 25% 2	2109
	21st to end	Day	31% 3 9% 27% 26% 4	2074
October	1st to 10th	Day	32% 3 10% 26% 25% 4	1900
	11th to 20th	Day	31% 3 6% 29% 25% 6%	1900
	21st to end	Day	41% 2 9% 24% 19% 5%	2090
November	1st to 10th	Day	31% 3 11% 30% 21% 5%	1878
	11th to 20th	Day	36% 2 8% 26% 23% 4	1711
	21st to end	Day	40% 2 8% 28% 19% 1	1520
December	1st to 10th	Day	39% 3 10% 33% 13% 1	1520
	11th to 20th	Day	45% 3 10% 26% 14% 2	1915
	21st to end	Day	58% 2 11% 20% 8% 2	3007
January	1st to 10th	Night	44% 3 14% 25% 13% 1	4108
	11th to 20th	Night	50% 3 10% 23% 13% 1	4120
	21st to end	Night	51% 2 11% 22% 12% 2	4532
February	1st to 10th	Night	45% 4 11% 23% 15% 2	2470
	11th to 20th	Night	41% 2 11% 25% 17% 3	2404
	21st to end	Night	42% 3 7% 23% 18% 7%	1824
March	1st to 10th	Night	39% 2 10% 23% 21% 5%	3419
	11th to 20th	Night	31% 3 10% 26% 27% 3	3480
	21st to end	Night	46% 3 15% 25% 10% 1	3817
April	1st to 10th	Night	36% 3 10% 23% 23% 5%	1986
	11th to 20th	Night	39% 2 8% 26% 20% 4	1875
	21st to end	Night	44% 3 10% 26% 16% 1	1796
May	1st to 10th	Night	45% 4 15% 20% 15% 2	2850
	11th to 20th	Night	41% 3 13% 30% 12% 1	2824
	21st to end	Night	31% 4 13% 30% 21% 1	3099
June	1st to 10th	Night	25% 4 19% 30% 18% 3	1529
	11th to 20th	Night	25% 6% 16% 30% 20% 2	1520
	21st to end	Night	22% 4 13% 34% 23% 4	1604
July	1st to 10th	Night	36% 4 13% 25% 20% 3	2805
	11th to 20th	Night	21% 4 12% 37% 22% 2	2796
	21st to end	Night	27% 5% 20% 31% 15% 1	2999
August	1st to 10th	Night	18% 4 18% 33% 20% 7%	2826
	11th to 20th	Night	33% 5% 12% 29% 20% 1	2825
	21st to end	Night	21% 4 13% 40% 19% 4	3428
September	1st to 10th	Night	28% 4 10% 28% 21% 9%	1900
	11th to 20th	Night	30% 4 9% 26% 26% 5%	2066
	21st to end	Night	27% 3 8% 27% 30% 5%	2090
October	1st to 10th	Night	27% 3 9% 26% 28% 7%	2275
	11th to 20th	Night	32% 3 5% 27% 26% 8%	2280
	21st to end	Night	36% 3 7% 25% 24% 5%	2508
November	1st to 10th	Night	27% 3 11% 29% 24% 6%	2245
	11th to 20th	Night	34% 2 7% 27% 26% 4	2463
	21st to end	Night	37% 3 8% 27% 22% 3	2660
December	1st to 10th	Night	38% 3 12% 28% 18% 1	2660
	11th to 20th	Night	41% 3 9% 29% 17% 1	3281
	21st to end	Night	58% 3 7% 20% 10% 2	4656

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% 13% 36% 24% 4	270
	11th to 20th	Day	30% 13% 36% 20% 2	270
	21st to end	Day	18% 13% 43% 22% 4	297
February	1st to 10th	Day	23% 19% 37% 18% 4	270
	11th to 20th	Day	19% 18% 44% 14% 5% 1	270
	21st to end	Day	24% 24% 38% 12% 3	225
March	1st to 10th	Day	20% 17% 41% 19% 3	270
	11th to 20th	Day	25% 15% 33% 22% 5%	270
	21st to end	Day	30% 18% 36% 12% 2 1	297
April	1st to 10th	Day	19% 18% 36% 20% 6% 1	374
	11th to 20th	Day	19% 15% 35% 23% 7%	452
	21st to end	Day	24% 12% 34% 22% 8%	560
May	1st to 10th	Day	22% 12% 34% 23% 9% 1	653
	11th to 20th	Day	19% 8% 29% 28% 12% 3	675
	21st to end	Day	16% 7% 28% 28% 16% 5%	734
June	1st to 10th	Day	7% 3 18% 36% 27% 9%	680
	11th to 20th	Day	8% 4 24% 36% 23% 4	680
	21st to end	Day	6% 4 31% 35% 20% 3	680
July	1st to 10th	Day	11% 3 13% 30% 31% 12%	671
	11th to 20th	Day	4 1 20% 35% 32% 7%	668
	21st to end	Day	8% 2 12% 32% 35% 11%	739
August	1st to 10th	Day	8% 4 12% 38% 31% 7%	676
	11th to 20th	Day	8% 2 17% 36% 31% 6%	665
	21st to end	Day	9% 2 11% 30% 36% 12%	572
September	1st to 10th	Day	12% 2 13% 33% 31% 7%	490
	11th to 20th	Day	7% 1 20% 37% 28% 7%	319
	21st to end	Day	13% 5% 26% 34% 16% 7%	300
October	1st to 10th	Day	12% 8% 25% 29% 18% 8%	300
	11th to 20th	Day	12% 12% 39% 28% 8% 1	300
	21st to end	Day	25% 15% 32% 22% 6%	330
November	1st to 10th	Day	15% 9% 37% 24% 11% 4	276
	11th to 20th	Day	18% 17% 29% 25% 10% 1	270
	21st to end	Day	24% 14% 33% 22% 7% 1	270
December	1st to 10th	Day	14% 11% 42% 23% 6% 4	270
	11th to 20th	Day	20% 16% 32% 26% 7%	270
	21st to end	Day	23% 21% 39% 16% 1	297
January	1st to 10th	Night	15% 11% 37% 30% 6% 1	950
	11th to 20th	Night	17% 12% 39% 27% 4 1	950
	21st to end	Night	15% 13% 38% 30% 4	1045
February	1st to 10th	Night	19% 12% 41% 25% 3	950
	11th to 20th	Night	18% 13% 38% 28% 2	929
	21st to end	Night	21% 13% 39% 24% 3	568
March	1st to 10th	Night	18% 11% 40% 27% 4	570
	11th to 20th	Night	21% 11% 35% 28% 6% 1	570
	21st to end	Night	23% 12% 36% 26% 3	627
April	1st to 10th	Night	15% 12% 39% 29% 6%	570
	11th to 20th	Night	18% 13% 34% 28% 7%	563
	21st to end	Night	18% 8% 39% 26% 9%	570
May	1st to 10th	Night	18% 6% 32% 36% 8%	570
	11th to 20th	Night	17% 4 29% 34% 16% 1	561
	21st to end	Night	13% 6% 27% 33% 18% 2	617
June	1st to 10th	Night	6% 4 17% 44% 28% 1	570
	11th to 20th	Night	4 2 24% 50% 18% 2	570
	21st to end	Night	5% 4 25% 46% 19% 1	570
July	1st to 10th	Night	9% 3 11% 39% 35% 3	555
	11th to 20th	Night	4 1 14% 45% 33% 3	552
	21st to end	Night	8% 2 12% 39% 38% 2	624
August	1st to 10th	Night	6% 2 13% 45% 33% 2	567
	11th to 20th	Night	8% 1 12% 48% 28% 3	564
	21st to end	Night	7% 1 10% 41% 38% 3	627
September	1st to 10th	Night	13% 1 14% 43% 25% 4	570
	11th to 20th	Night	10% 2 18% 38% 27% 5%	570
	21st to end	Night	11% 3 21% 40% 21% 4	570
October	1st to 10th	Night	10% 4 25% 38% 22% 2	755
	11th to 20th	Night	15% 6% 38% 31% 9%	760
	21st to end	Night	20% 11% 32% 29% 8%	836
November	1st to 10th	Night	12% 9% 34% 32% 13% 1	749
	11th to 20th	Night	16% 10% 35% 29% 9%	911
	21st to end	Night	20% 10% 34% 29% 7%	950
December	1st to 10th	Night	16% 10% 38% 28% 6% 1	950
	11th to 20th	Night	21% 9% 34% 30% 5%	950
	21st to end	Night	19% 14% 36% 27% 4	1045

Page generated at 2024-10-31 02:39:21. Version ac07

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