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 2026-08-30.

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% 54%	152375
	Night	7% 6% 2 6% 4 4 4 4 5% 7% 51%	147298
Transparency	Day	29% 5% 12% 28% 21% 5%	147227
	Night	32% 5% 12% 27% 20% 4	143471
Seeing	Day	10% 10% 32% 31% 15% 3	18341
	Night	10% 9% 38% 35% 8%	28179

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 10% 3 7% 4 3 3 3 4 5% 53%	4720
	11th to 20th	Day	3 7% 3 7% 4 4 4 4 5% 7% 52%	4740
	21st to end	Day	3 5% 3 9% 5% 5% 5% 4 5% 6% 51%	5188
February	1st to 10th	Day	5% 9% 4 7% 6% 6% 5% 3 6% 7% 43%	2023
	11th to 20th	Day	6% 7% 3 8% 5% 4 5% 4 5% 6% 48%	2049
	21st to end	Day	5% 7% 2 5% 3 3 4 3 5% 6% 57%	1730
March	1st to 10th	Day	9% 9% 2 4 3 3 3 2 4 6% 55%	4600
	11th to 20th	Day	7% 5% 2 4 3 4 3 3 4 6% 59%	4858
	21st to end	Day	9% 8% 2 6% 4 4 4 4 5% 7% 46%	5908
April	1st to 10th	Day	10% 7% 2 5% 3 3 3 3 4 7% 51%	2520
	11th to 20th	Day	13% 7% 3 6% 4 3 4 3 5% 6% 47%	2491
	21st to end	Day	14% 7% 2 5% 4 3 4 3 5% 7% 46%	2709
May	1st to 10th	Day	9% 5% 2 5% 3 4 4 4 6% 7% 51%	7655
	11th to 20th	Day	11% 5% 2 4 3 3 4 4 5% 7% 53%	7644
	21st to end	Day	14% 6% 2 5% 4 4 4 3 5% 7% 45%	9026
June	1st to 10th	Day	7% 4 1 4 3 3 3 3 6% 9% 56%	2940
	11th to 20th	Day	7% 5% 1 5% 4 4 3 3 5% 8% 55%	2940
	21st to end	Day	5% 3 1 3 3 3 4 4 6% 9% 59%	2940
July	1st to 10th	Day	6% 4 1 4 3 3 4 4 6% 8% 57%	8249
	11th to 20th	Day	4 3 1 3 3 3 4 4 5% 8% 62%	8250
	21st to end	Day	5% 4 2 5% 4 5% 5% 4 7% 9% 50%	8957
August	1st to 10th	Day	11% 5% 1 4 3 3 4 4 6% 8% 51%	6020
	11th to 20th	Day	7% 4 2 5% 3 3 4 4 6% 7% 55%	5728
	21st to end	Day	8% 6% 2 6% 4 4 4 4 6% 8% 48%	6093
September	1st to 10th	Day	7% 4 1 3 3 3 3 3 5% 6% 62%	2400
	11th to 20th	Day	7% 4 1 3 3 3 4 3 5% 8% 59%	2225
	21st to end	Day	7% 4 1 3 3 3 3 2 4 5% 64%	2200
October	1st to 10th	Day	7% 3 1 2 2 2 2 2 2 4 74%	3590
	11th to 20th	Day	7% 4 2 4 3 3 3 3 4 6% 62%	3179
	21st to end	Day	7% 6% 2 4 3 4 3 2 4 8% 56%	3190
November	1st to 10th	Day	5% 4 2 5% 3 4 4 4 6% 8% 56%	1853
	11th to 20th	Day	3 6% 2 4 3 4 3 3 4 7% 61%	1890
	21st to end	Day	6% 7% 2 4 4 3 3 3 5% 6% 57%	1890
December	1st to 10th	Day	3 5% 2 4 3 3 3 3 5% 7% 63%	2782
	11th to 20th	Day	3 4 1 5% 4 3 3 3 5% 8% 62%	3106
	21st to end	Day	8% 9% 3 7% 3 4 4 3 5% 6% 47%	4092
January	1st to 10th	Night	5% 10% 3 9% 6% 4 4 3 5% 7% 45%	8201
	11th to 20th	Night	2 5% 2 7% 5% 5% 5% 4 5% 8% 52%	8250
	21st to end	Night	2 6% 3 8% 5% 5% 5% 3 6% 5% 50%	9026
February	1st to 10th	Night	7% 10% 4 7% 4 4 4 4 6% 6% 44%	2613
	11th to 20th	Night	5% 9% 3 7% 5% 5% 4 4 6% 7% 46%	2458
	21st to end	Night	4 7% 3 6% 3 3 4 4 6% 6% 54%	2076
March	1st to 10th	Night	10% 8% 2 5% 3 3 3 3 4 5% 55%	5494
	11th to 20th	Night	7% 7% 2 6% 4 3 3 3 5% 6% 54%	5331
	21st to end	Night	7% 8% 3 8% 5% 5% 5% 4 6% 8% 40%	5276
April	1st to 10th	Night	11% 7% 2 6% 4 3 4 3 5% 8% 47%	2100
	11th to 20th	Night	12% 7% 2 5% 3 4 4 3 5% 6% 49%	2077
	21st to end	Night	14% 6% 2 5% 5% 4 4 3 5% 7% 44%	1911
May	1st to 10th	Night	12% 6% 2 6% 4 4 4 4 4 6% 47%	5292
	11th to 20th	Night	9% 6% 2 5% 4 4 4 4 6% 7% 51%	5232
	21st to end	Night	17% 7% 2 5% 4 4 4 4 5% 7% 41%	5181
June	1st to 10th	Night	8% 4 2 5% 4 4 4 4 6% 10% 50%	1680
	11th to 20th	Night	10% 5% 1 5% 4 4 4 4 7% 9% 47%	1680
	21st to end	Night	7% 4 1 5% 5% 5% 6% 5% 8% 9% 46%	1680
July	1st to 10th	Night	7% 4 2 6% 4 4 5% 4 7% 9% 47%	4740
	11th to 20th	Night	5% 4 1 4 3 3 3 4 5% 8% 58%	4740
	21st to end	Night	6% 6% 2 6% 5% 5% 5% 5% 7% 8% 43%	5152
August	1st to 10th	Night	10% 6% 3 8% 6% 5% 6% 4 7% 9% 36%	4011
	11th to 20th	Night	8% 4 1 5% 4 5% 5% 5% 7% 9% 48%	4324
	21st to end	Night	7% 8% 2 7% 4 4 4 4 6% 8% 44%	5083
September	1st to 10th	Night	7% 5% 1 5% 3 3 3 3 4 7% 59%	2000
	11th to 20th	Night	7% 4 2 4 3 4 3 3 5% 7% 59%	2175
	21st to end	Night	9% 4 2 4 2 3 3 3 5% 5% 62%	2200
October	1st to 10th	Night	7% 3 1 2 1 2 2 2 2 3 75%	3590
	11th to 20th	Night	9% 5% 1 3 3 2 2 3 4 6% 63%	3930
	21st to end	Night	6% 6% 2 5% 3 4 3 3 5% 6% 56%	4610
November	1st to 10th	Night	6% 5% 2 4 3 3 4 3 5% 8% 58%	2665
	11th to 20th	Night	4 5% 1 4 3 4 4 4 6% 7% 57%	2730
	21st to end	Night	6% 6% 2 5% 3 3 3 4 6% 7% 56%	2730
December	1st to 10th	Night	3 4 2 5% 3 3 3 3 5% 7% 64%	4611
	11th to 20th	Night	3 5% 2 6% 4 3 4 4 6% 8% 56%	5321
	21st to end	Night	8% 10% 3 6% 3 4 4 3 5% 7% 45%	7128

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	34% 3 11% 30% 20% 1	4723
	11th to 20th	Day	32% 5% 11% 28% 20% 4	4740
	21st to end	Day	34% 5% 13% 26% 19% 4	5188
February	1st to 10th	Day	41% 6% 13% 22% 14% 4	2023
	11th to 20th	Day	37% 4 12% 24% 18% 4	2059
	21st to end	Day	28% 4 12% 32% 19% 6%	1730
March	1st to 10th	Day	31% 3 7% 24% 29% 5%	4600
	11th to 20th	Day	27% 4 10% 26% 31% 2	4858
	21st to end	Day	36% 6% 18% 26% 13% 2	5908
April	1st to 10th	Day	34% 5% 15% 28% 15% 4	2520
	11th to 20th	Day	38% 5% 14% 22% 18% 4	2494
	21st to end	Day	37% 5% 13% 25% 17% 2	2709
May	1st to 10th	Day	30% 5% 12% 30% 20% 4	7655
	11th to 20th	Day	30% 5% 11% 32% 19% 3	7648
	21st to end	Day	38% 6% 13% 27% 14% 3	9027
June	1st to 10th	Day	25% 7% 20% 31% 15% 2	2940
	11th to 20th	Day	27% 6% 14% 27% 19% 7%	2940
	21st to end	Day	21% 6% 12% 31% 24% 6%	2940
July	1st to 10th	Day	25% 4 10% 29% 27% 4	8181
	11th to 20th	Day	21% 4 11% 28% 23% 13%	8175
	21st to end	Day	29% 6% 16% 21% 21% 8%	8759
August	1st to 10th	Day	31% 6% 13% 26% 19% 4	4413
	11th to 20th	Day	34% 6% 13% 23% 21% 4	4182
	21st to end	Day	24% 5% 16% 21% 22% 12%	4420
September	1st to 10th	Day	23% 4 9% 29% 24% 12%	2400
	11th to 20th	Day	23% 5% 10% 29% 26% 7%	2225
	21st to end	Day	23% 4 7% 30% 29% 6%	2200
October	1st to 10th	Day	17% 2 6% 22% 34% 19%	3590
	11th to 20th	Day	25% 4 10% 36% 20% 5%	3179
	21st to end	Day	29% 3 11% 31% 20% 6%	3190
November	1st to 10th	Day	25% 5% 15% 28% 21% 6%	1853
	11th to 20th	Day	24% 4 11% 29% 27% 6%	1890
	21st to end	Day	28% 4 13% 30% 22% 4	1890
December	1st to 10th	Day	23% 4 11% 35% 24% 4	2782
	11th to 20th	Day	23% 4 17% 37% 17% 3	3104
	21st to end	Day	38% 4 9% 34% 14% 2	4092
January	1st to 10th	Night	40% 5% 11% 26% 16% 3	8202
	11th to 20th	Night	30% 5% 16% 28% 20% 2	8250
	21st to end	Night	35% 4 10% 27% 20% 4	9026
February	1st to 10th	Night	38% 6% 12% 26% 15% 3	2613
	11th to 20th	Night	36% 6% 13% 27% 15% 4	2458
	21st to end	Night	29% 4 12% 27% 22% 6%	2076
March	1st to 10th	Night	31% 4 7% 24% 29% 5%	5494
	11th to 20th	Night	30% 5% 7% 23% 30% 4	5331
	21st to end	Night	39% 9% 16% 24% 12% 1	5276
April	1st to 10th	Night	36% 6% 14% 24% 16% 4	2100
	11th to 20th	Night	36% 6% 12% 25% 19% 3	2077
	21st to end	Night	39% 7% 13% 26% 12% 2	1911
May	1st to 10th	Night	37% 5% 12% 27% 17% 2	5293
	11th to 20th	Night	31% 7% 13% 28% 20% 3	5233
	21st to end	Night	43% 7% 13% 26% 11% 1	5182
June	1st to 10th	Night	29% 8% 22% 27% 12% 2	1680
	11th to 20th	Night	32% 8% 17% 23% 15% 5%	1680
	21st to end	Night	31% 7% 16% 25% 19% 3	1680
July	1st to 10th	Night	33% 4 15% 28% 18% 1	4700
	11th to 20th	Night	25% 4 14% 26% 21% 10%	4692
	21st to end	Night	37% 6% 16% 22% 16% 5%	5038
August	1st to 10th	Night	37% 7% 17% 24% 13% 2	2942
	11th to 20th	Night	37% 6% 14% 25% 15% 3	3159
	21st to end	Night	27% 6% 14% 24% 18% 10%	3687
September	1st to 10th	Night	26% 5% 10% 26% 26% 8%	2000
	11th to 20th	Night	26% 5% 12% 26% 23% 8%	2175
	21st to end	Night	24% 5% 7% 28% 28% 7%	2200
October	1st to 10th	Night	17% 3 5% 20% 44% 11%	3590
	11th to 20th	Night	24% 3 9% 34% 24% 6%	3930
	21st to end	Night	29% 4 11% 27% 22% 6%	4610
November	1st to 10th	Night	25% 5% 11% 29% 23% 7%	2665
	11th to 20th	Night	25% 5% 12% 26% 25% 6%	2730
	21st to end	Night	27% 4 12% 30% 22% 5%	2730
December	1st to 10th	Night	22% 3 15% 35% 21% 4	4611
	11th to 20th	Night	26% 3 14% 37% 17% 3	5322
	21st to end	Night	38% 4 10% 31% 16% 1	7128

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% 11% 41% 23% 12% 2	330
	11th to 20th	Day	12% 15% 39% 28% 4 1	330
	21st to end	Day	8% 11% 52% 21% 6% 3	363
February	1st to 10th	Day	9% 14% 53% 21% 3	330
	11th to 20th	Day	8% 13% 48% 23% 6% 1	330
	21st to end	Day	10% 19% 47% 21% 3	273
March	1st to 10th	Day	13% 12% 40% 25% 9% 1	330
	11th to 20th	Day	11% 13% 35% 30% 10% 1	330
	21st to end	Day	15% 14% 41% 21% 8% 1	505
April	1st to 10th	Day	13% 19% 38% 21% 7% 1	540
	11th to 20th	Day	16% 23% 33% 23% 5% 1	532
	21st to end	Day	19% 15% 32% 23% 9% 2	729
May	1st to 10th	Day	14% 13% 30% 27% 13% 3	753
	11th to 20th	Day	19% 8% 31% 28% 13% 2	775
	21st to end	Day	17% 9% 31% 27% 13% 2	844
June	1st to 10th	Day	9% 6% 27% 31% 22% 6%	780
	11th to 20th	Day	9% 9% 34% 32% 14% 2	780
	21st to end	Day	7% 5% 33% 35% 19% 2	780
July	1st to 10th	Day	8% 4 25% 38% 22% 4	771
	11th to 20th	Day	4 3 21% 44% 24% 4	768
	21st to end	Day	6% 2 19% 43% 25% 4	849
August	1st to 10th	Day	9% 4 21% 39% 23% 4	776
	11th to 20th	Day	6% 4 27% 38% 22% 3	672
	21st to end	Day	5% 2 27% 39% 21% 5%	620
September	1st to 10th	Day	6% 4 25% 41% 19% 5%	530
	11th to 20th	Day	5% 4 22% 43% 19% 7%	355
	21st to end	Day	11% 11% 22% 31% 20% 5%	330
October	1st to 10th	Day	13% 8% 23% 30% 22% 5%	330
	11th to 20th	Day	10% 16% 34% 26% 12% 1	330
	21st to end	Day	13% 15% 31% 28% 11% 3	363
November	1st to 10th	Day	6% 15% 39% 22% 14% 3	330
	11th to 20th	Day	8% 12% 38% 31% 10% 1	330
	21st to end	Day	12% 7% 38% 31% 8% 3	330
December	1st to 10th	Day	8% 11% 47% 26% 6% 1	330
	11th to 20th	Day	11% 15% 42% 25% 6% 1	330
	21st to end	Day	13% 15% 41% 25% 4 1	363
January	1st to 10th	Night	8% 12% 40% 31% 9%	1050
	11th to 20th	Night	8% 16% 43% 27% 6%	1050
	21st to end	Night	7% 11% 45% 30% 6%	1155
February	1st to 10th	Night	10% 13% 46% 27% 5%	885
	11th to 20th	Night	10% 11% 48% 26% 4	824
	21st to end	Night	8% 11% 50% 28% 3	692
March	1st to 10th	Night	11% 11% 40% 33% 5%	840
	11th to 20th	Night	12% 9% 42% 31% 6%	741
	21st to end	Night	12% 11% 41% 30% 6%	693
April	1st to 10th	Night	15% 14% 40% 27% 4	630
	11th to 20th	Night	16% 14% 41% 26% 3	623
	21st to end	Night	18% 13% 31% 33% 5%	630
May	1st to 10th	Night	14% 8% 35% 35% 7%	630
	11th to 20th	Night	17% 7% 35% 31% 10%	621
	21st to end	Night	18% 8% 35% 30% 10%	683
June	1st to 10th	Night	10% 4 35% 37% 13% 2	630
	11th to 20th	Night	12% 7% 40% 35% 7%	630
	21st to end	Night	7% 5% 31% 47% 11%	630
July	1st to 10th	Night	7% 3 31% 43% 15%	615
	11th to 20th	Night	4 3 28% 54% 11% 1	612
	21st to end	Night	7% 2 28% 48% 14%	690
August	1st to 10th	Night	9% 2 25% 48% 15% 1	627
	11th to 20th	Night	9% 2 28% 50% 12%	624
	21st to end	Night	6% 4 30% 46% 13%	687
September	1st to 10th	Night	8% 5% 33% 41% 13% 1	600
	11th to 20th	Night	8% 5% 32% 38% 16% 1	600
	21st to end	Night	10% 5% 33% 39% 12% 1	600
October	1st to 10th	Night	14% 6% 30% 39% 11%	600
	11th to 20th	Night	12% 10% 31% 36% 10%	807
	21st to end	Night	11% 11% 35% 34% 8% 1	1100
November	1st to 10th	Night	8% 8% 38% 37% 7% 1	1025
	11th to 20th	Night	6% 10% 42% 34% 7%	1050
	21st to end	Night	9% 8% 36% 37% 9%	1050
December	1st to 10th	Night	8% 12% 44% 29% 6%	1050
	11th to 20th	Night	8% 12% 43% 30% 6%	1050
	21st to end	Night	12% 13% 39% 31% 5%	1155

Page generated at 2026-08-27 02:07:16. Version ac07

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