Arthur
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arthur clear sky chart from 2005-11-06 to 2025-04-26.

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	7% 5% 2 5% 3 4 4 3 5% 7% 55%	117277
	Night	7% 6% 2 6% 4 4 4 4 5% 7% 52%	120155
Transparency	Day	29% 5% 12% 28% 21% 5%	117021
	Night	31% 5% 12% 27% 20% 4	120001
Seeing	Day	10% 10% 32% 31% 15% 3	16342
	Night	10% 9% 37% 35% 9%	26447

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	4 11% 3 8% 4 3 4 3 5% 5% 49%	3638
	11th to 20th	Day	4 8% 3 7% 5% 4 4 4 5% 6% 50%	3640
	21st to end	Day	3 4 2 7% 4 5% 4 3 5% 6% 56%	4004
February	1st to 10th	Day	6% 9% 4 8% 6% 5% 5% 3 6% 7% 42%	1926
	11th to 20th	Day	6% 7% 3 8% 5% 4 5% 4 5% 5% 48%	1949
	21st to end	Day	5% 8% 2 5% 3 3 4 3 5% 6% 56%	1650
March	1st to 10th	Day	9% 9% 2 4 3 3 3 2 4 6% 55%	4500
	11th to 20th	Day	7% 6% 2 4 3 4 3 3 4 6% 59%	4753
	21st to end	Day	9% 8% 2 6% 4 4 4 4 5% 7% 46%	5780
April	1st to 10th	Day	11% 7% 2 5% 3 3 3 3 4 7% 51%	2400
	11th to 20th	Day	13% 7% 3 6% 4 3 4 3 5% 6% 46%	2371
	21st to end	Day	13% 7% 2 5% 5% 3 4 3 5% 7% 46%	2516
May	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 6% 7% 46%	4120
	11th to 20th	Day	10% 5% 2 5% 3 4 4 4 5% 8% 51%	4099
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 7% 48%	4825
June	1st to 10th	Day	6% 4 1 4 3 3 3 3 7% 9% 57%	2660
	11th to 20th	Day	7% 5% 1 5% 4 4 4 3 5% 8% 55%	2660
	21st to end	Day	4 2 1 3 3 3 4 4 6% 10% 61%	2660
July	1st to 10th	Day	7% 4 1 4 3 3 4 4 6% 7% 56%	4430
	11th to 20th	Day	3 2 1 4 4 4 5% 5% 7% 10% 55%	4430
	21st to end	Day	3 2 1 4 3 4 4 4 6% 9% 60%	4802
August	1st to 10th	Day	11% 5% 1 4 3 3 4 4 7% 8% 49%	4164
	11th to 20th	Day	10% 5% 2 5% 4 3 3 4 6% 7% 49%	3892
	21st to end	Day	4 3 1 4 3 4 3 3 6% 8% 61%	4180
September	1st to 10th	Day	7% 4 1 3 3 3 3 3 5% 6% 62%	2280
	11th to 20th	Day	6% 4 1 3 3 3 4 3 5% 8% 61%	2114
	21st to end	Day	7% 4 1 3 3 3 3 2 4 5% 64%	2090
October	1st to 10th	Day	7% 3 1 2 1 2 2 2 2 4 75%	3480
	11th to 20th	Day	7% 4 2 4 3 3 3 3 4 6% 62%	3081
	21st to end	Day	7% 6% 2 4 3 4 3 3 4 8% 56%	3091
November	1st to 10th	Day	5% 4 2 5% 3 4 4 4 6% 7% 56%	1763
	11th to 20th	Day	3 6% 2 5% 3 4 3 3 4 7% 61%	1800
	21st to end	Day	6% 7% 2 4 3 3 3 3 5% 6% 58%	1800
December	1st to 10th	Day	3 4 1 4 3 3 3 3 4 7% 63%	2699
	11th to 20th	Day	3 4 1 4 4 3 3 3 5% 8% 62%	3026
	21st to end	Day	8% 10% 3 7% 3 4 4 3 5% 6% 47%	4004
January	1st to 10th	Night	5% 10% 4 10% 6% 4 4 3 5% 7% 42%	6327
	11th to 20th	Night	2 6% 3 7% 5% 5% 4 4 5% 7% 51%	6340
	21st to end	Night	2 5% 3 8% 5% 5% 4 3 5% 6% 53%	6974
February	1st to 10th	Night	7% 10% 4 8% 5% 4 4 4 6% 6% 43%	2489
	11th to 20th	Night	5% 9% 3 7% 5% 4 4 4 6% 7% 46%	2338
	21st to end	Night	4 7% 3 6% 3 3 4 3 6% 6% 53%	1980
March	1st to 10th	Night	10% 8% 2 5% 3 3 3 3 4 5% 56%	5374
	11th to 20th	Night	7% 7% 2 5% 4 3 3 3 5% 6% 54%	5216
	21st to end	Night	7% 9% 3 8% 4 5% 5% 4 6% 8% 40%	5162
April	1st to 10th	Night	11% 7% 2 6% 4 3 4 3 5% 8% 47%	2000
	11th to 20th	Night	12% 7% 2 5% 3 4 4 3 5% 6% 49%	1977
	21st to end	Night	14% 7% 2 5% 5% 4 4 3 5% 7% 44%	1780
May	1st to 10th	Night	15% 6% 2 5% 5% 4 5% 5% 5% 8% 41%	2850
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 7% 8% 49%	2803
	21st to end	Night	13% 6% 2 5% 5% 4 3 4 5% 8% 45%	2762
June	1st to 10th	Night	8% 4 2 5% 3 4 3 5% 6% 10% 51%	1520
	11th to 20th	Night	10% 5% 1 5% 4 4 4 4 7% 8% 48%	1520
	21st to end	Night	5% 3 1 5% 5% 5% 6% 5% 8% 9% 47%	1520
July	1st to 10th	Night	10% 3 2 5% 3 4 4 4 5% 7% 54%	2540
	11th to 20th	Night	3 4 2 5% 4 5% 5% 5% 6% 10% 52%	2540
	21st to end	Night	3 3 2 5% 4 4 4 4 7% 8% 55%	2761
August	1st to 10th	Night	9% 6% 3 7% 5% 5% 4 4 7% 9% 41%	2774
	11th to 20th	Night	11% 6% 2 5% 4 5% 5% 4 6% 7% 46%	2944
	21st to end	Night	4 4 1 5% 4 4 5% 4 7% 9% 54%	3487
September	1st to 10th	Night	7% 4 1 5% 3 3 3 2 4 7% 59%	1900
	11th to 20th	Night	7% 4 2 4 3 3 3 3 5% 6% 60%	2066
	21st to end	Night	9% 4 2 3 2 2 3 3 4 5% 63%	2090
October	1st to 10th	Night	7% 3 1 2 1 2 2 2 2 3 75%	3480
	11th to 20th	Night	9% 5% 1 3 2 2 2 2 4 6% 64%	3810
	21st to end	Night	6% 6% 2 5% 3 4 3 3 5% 6% 56%	4467
November	1st to 10th	Night	6% 5% 2 4 3 3 4 3 5% 8% 57%	2535
	11th to 20th	Night	4 5% 2 4 3 4 4 4 6% 7% 57%	2600
	21st to end	Night	6% 6% 2 5% 3 4 3 4 6% 7% 57%	2600
December	1st to 10th	Night	3 4 2 4 3 3 3 3 5% 6% 64%	4474
	11th to 20th	Night	3 5% 2 6% 4 3 4 4 6% 8% 56%	5181
	21st to end	Night	8% 10% 3 6% 3 4 4 3 5% 7% 45%	6974

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	37% 4 9% 32% 16% 1	3639
	11th to 20th	Day	35% 6% 10% 25% 21% 3	3640
	21st to end	Day	28% 6% 14% 26% 21% 5%	4004
February	1st to 10th	Day	41% 6% 14% 21% 13% 4	1926
	11th to 20th	Day	38% 4 13% 24% 18% 4	1959
	21st to end	Day	29% 4 12% 31% 18% 6%	1650
March	1st to 10th	Day	31% 3 7% 24% 30% 5%	4500
	11th to 20th	Day	27% 4 10% 27% 31% 2	4753
	21st to end	Day	36% 6% 18% 26% 13% 1	5780
April	1st to 10th	Day	33% 5% 14% 28% 16% 4	2400
	11th to 20th	Day	39% 5% 14% 22% 17% 3	2374
	21st to end	Day	37% 6% 14% 25% 17% 2	2516
May	1st to 10th	Day	32% 7% 15% 22% 17% 6%	4120
	11th to 20th	Day	30% 8% 15% 31% 16% 1	4103
	21st to end	Day	33% 8% 16% 29% 11% 3	4825
June	1st to 10th	Day	23% 7% 21% 32% 15% 2	2660
	11th to 20th	Day	27% 7% 14% 28% 18% 7%	2660
	21st to end	Day	18% 6% 13% 32% 25% 6%	2660
July	1st to 10th	Day	27% 5% 9% 25% 29% 5%	4361
	11th to 20th	Day	23% 6% 13% 29% 23% 6%	4355
	21st to end	Day	19% 6% 12% 24% 26% 13%	4651
August	1st to 10th	Day	31% 6% 13% 26% 19% 4	4149
	11th to 20th	Day	34% 6% 12% 22% 21% 4	3931
	21st to end	Day	23% 5% 15% 22% 23% 13%	4179
September	1st to 10th	Day	23% 4 8% 28% 24% 13%	2280
	11th to 20th	Day	22% 5% 9% 30% 27% 8%	2114
	21st to end	Day	24% 4 7% 30% 29% 7%	2090
October	1st to 10th	Day	17% 2 6% 22% 34% 19%	3480
	11th to 20th	Day	24% 4 10% 36% 21% 5%	3081
	21st to end	Day	29% 3 11% 32% 20% 6%	3091
November	1st to 10th	Day	26% 5% 14% 28% 20% 6%	1763
	11th to 20th	Day	24% 4 11% 29% 27% 6%	1800
	21st to end	Day	28% 4 12% 30% 22% 5%	1800
December	1st to 10th	Day	22% 4 11% 35% 24% 4	2699
	11th to 20th	Day	22% 4 17% 37% 17% 3	3024
	21st to end	Day	38% 4 9% 34% 14% 2	4004
January	1st to 10th	Night	41% 6% 12% 24% 14% 3	6327
	11th to 20th	Night	31% 5% 15% 24% 23% 2	6340
	21st to end	Night	31% 5% 11% 27% 21% 5%	6974
February	1st to 10th	Night	39% 7% 13% 24% 14% 3	2489
	11th to 20th	Night	37% 6% 13% 27% 14% 3	2338
	21st to end	Night	30% 4 12% 27% 21% 6%	1980
March	1st to 10th	Night	31% 4 7% 24% 29% 5%	5374
	11th to 20th	Night	30% 5% 7% 23% 30% 4	5216
	21st to end	Night	39% 9% 16% 24% 11% 1	5162
April	1st to 10th	Night	35% 6% 14% 24% 17% 4	2000
	11th to 20th	Night	36% 6% 12% 25% 18% 3	1977
	21st to end	Night	39% 7% 13% 27% 12% 2	1780
May	1st to 10th	Night	39% 8% 15% 21% 13% 4	2850
	11th to 20th	Night	29% 10% 18% 32% 9% 1	2804
	21st to end	Night	37% 9% 16% 28% 9% 1	2762
June	1st to 10th	Night	27% 9% 22% 27% 12% 2	1520
	11th to 20th	Night	32% 9% 17% 23% 14% 6%	1520
	21st to end	Night	28% 7% 16% 26% 20% 2	1520
July	1st to 10th	Night	31% 4 14% 28% 21% 2	2500
	11th to 20th	Night	27% 5% 16% 29% 19% 4	2492
	21st to end	Night	24% 6% 15% 24% 22% 9%	2674
August	1st to 10th	Night	38% 7% 17% 24% 12% 2	2766
	11th to 20th	Night	37% 6% 13% 25% 16% 3	2971
	21st to end	Night	26% 6% 13% 25% 19% 11%	3487
September	1st to 10th	Night	25% 5% 10% 25% 27% 8%	1900
	11th to 20th	Night	25% 5% 12% 26% 24% 8%	2066
	21st to end	Night	24% 5% 7% 28% 28% 8%	2090
October	1st to 10th	Night	17% 3 5% 19% 45% 11%	3480
	11th to 20th	Night	24% 3 9% 34% 24% 6%	3810
	21st to end	Night	29% 4 11% 27% 22% 6%	4467
November	1st to 10th	Night	25% 5% 11% 28% 23% 7%	2535
	11th to 20th	Night	25% 6% 12% 26% 25% 7%	2600
	21st to end	Night	26% 4 12% 30% 23% 5%	2600
December	1st to 10th	Night	21% 3 15% 35% 22% 4	4474
	11th to 20th	Night	26% 3 14% 37% 17% 2	5182
	21st to end	Night	39% 4 10% 31% 15% 1	6974

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	11% 10% 40% 25% 13% 2	300
	11th to 20th	Day	11% 14% 39% 30% 5% 1	300
	21st to end	Day	8% 11% 51% 20% 6% 3	330
February	1st to 10th	Day	9% 15% 53% 20% 3	300
	11th to 20th	Day	8% 14% 48% 22% 6% 1	300
	21st to end	Day	10% 20% 46% 20% 4	249
March	1st to 10th	Day	13% 12% 38% 27% 9% 1	300
	11th to 20th	Day	11% 13% 34% 31% 10% 1	300
	21st to end	Day	16% 14% 41% 20% 9% 1	465
April	1st to 10th	Day	14% 19% 37% 21% 8% 1	500
	11th to 20th	Day	17% 22% 31% 24% 5% 1	492
	21st to end	Day	18% 15% 30% 25% 10% 2	656
May	1st to 10th	Day	15% 13% 30% 27% 13% 2	653
	11th to 20th	Day	18% 7% 31% 28% 14% 2	675
	21st to end	Day	16% 10% 31% 27% 14% 2	734
June	1st to 10th	Day	8% 5% 26% 32% 24% 6%	680
	11th to 20th	Day	9% 8% 35% 31% 15% 2	680
	21st to end	Day	4 5% 31% 37% 20% 2	680
July	1st to 10th	Day	8% 4 24% 37% 23% 4	671
	11th to 20th	Day	3 4 22% 44% 24% 4	668
	21st to end	Day	6% 2 19% 43% 25% 4	739
August	1st to 10th	Day	9% 3 20% 39% 25% 4	676
	11th to 20th	Day	6% 4 27% 38% 22% 2	581
	21st to end	Day	4 2 27% 40% 21% 6%	539
September	1st to 10th	Day	6% 4 23% 42% 19% 6%	490
	11th to 20th	Day	3 4 22% 43% 21% 8%	324
	21st to end	Day	11% 12% 23% 30% 20% 4	300
October	1st to 10th	Day	14% 8% 21% 29% 23% 5%	300
	11th to 20th	Day	9% 14% 34% 28% 14% 1	300
	21st to end	Day	13% 16% 29% 28% 11% 3	330
November	1st to 10th	Day	6% 15% 38% 23% 14% 3	300
	11th to 20th	Day	9% 13% 37% 30% 10% 1	300
	21st to end	Day	11% 7% 39% 31% 8% 3	300
December	1st to 10th	Day	8% 11% 46% 27% 7% 1	300
	11th to 20th	Day	10% 14% 42% 26% 7% 1	300
	21st to end	Day	15% 16% 39% 24% 5% 1	330
January	1st to 10th	Night	8% 12% 40% 32% 9%	1000
	11th to 20th	Night	8% 15% 43% 28% 7% 1	1000
	21st to end	Night	7% 11% 45% 30% 6%	1100
February	1st to 10th	Night	11% 13% 45% 26% 5%	843
	11th to 20th	Night	11% 11% 48% 26% 4	784
	21st to end	Night	8% 12% 48% 29% 3	660
March	1st to 10th	Night	11% 11% 40% 33% 5%	800
	11th to 20th	Night	11% 8% 42% 32% 7%	706
	21st to end	Night	12% 10% 40% 31% 7%	660
April	1st to 10th	Night	15% 14% 40% 28% 4	600
	11th to 20th	Night	16% 14% 41% 26% 3	593
	21st to end	Night	18% 12% 31% 33% 5%	587
May	1st to 10th	Night	14% 8% 36% 34% 7%	570
	11th to 20th	Night	17% 6% 33% 33% 11%	561
	21st to end	Night	17% 8% 35% 30% 10%	617
June	1st to 10th	Night	11% 4 33% 38% 13% 2	570
	11th to 20th	Night	12% 7% 39% 35% 7% 1	570
	21st to end	Night	5% 4 30% 49% 12%	570
July	1st to 10th	Night	7% 3 30% 43% 16% 1	555
	11th to 20th	Night	4 3 28% 54% 11% 1	552
	21st to end	Night	7% 2 26% 50% 16%	624
August	1st to 10th	Night	9% 2 24% 48% 16% 1	567
	11th to 20th	Night	9% 1 28% 50% 12%	564
	21st to end	Night	5% 4 31% 45% 14%	627
September	1st to 10th	Night	8% 5% 33% 41% 14%	570
	11th to 20th	Night	8% 5% 32% 38% 15% 1	570
	21st to end	Night	11% 5% 32% 40% 11% 1	570
October	1st to 10th	Night	15% 6% 29% 38% 11%	570
	11th to 20th	Night	12% 9% 30% 37% 11% 1	767
	21st to end	Night	11% 11% 35% 34% 9% 1	1045
November	1st to 10th	Night	8% 7% 38% 38% 7% 1	975
	11th to 20th	Night	6% 11% 42% 33% 7%	1000
	21st to end	Night	9% 8% 36% 37% 9%	1000
December	1st to 10th	Night	8% 13% 43% 30% 6%	1000
	11th to 20th	Night	8% 11% 43% 31% 7%	1000
	21st to end	Night	13% 13% 38% 31% 5%	1100

Page generated at 2025-04-23 07:25:06. Version ac07

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