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 2024-10-25.

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%	101157
	Night	7% 6% 2 6% 4 4 4 4 5% 7% 51%	97861
Transparency	Day	29% 6% 13% 27% 20% 5%	100901
	Night	31% 6% 13% 27% 18% 4	97707
Seeing	Day	10% 9% 31% 31% 15% 3	15758
	Night	10% 9% 37% 35% 9%	25658

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 8% 3 8% 4 3 4 2 5% 5% 55%	2538
	11th to 20th	Day	5% 8% 3 6% 4 4 5% 4 6% 7% 48%	2540
	21st to end	Day	4 4 2 6% 3 4 4 3 6% 7% 56%	2794
February	1st to 10th	Day	6% 9% 3 8% 6% 6% 5% 3 6% 6% 42%	1829
	11th to 20th	Day	6% 6% 3 8% 5% 4 5% 4 5% 5% 49%	1849
	21st to end	Day	6% 8% 2 6% 3 3 4 3 5% 6% 55%	1570
March	1st to 10th	Day	7% 7% 2 4 3 3 3 2 5% 7% 56%	3130
	11th to 20th	Day	8% 6% 2 4 2 3 3 2 3 5% 60%	3318
	21st to end	Day	8% 9% 3 6% 5% 4 4 4 5% 7% 44%	4032
April	1st to 10th	Day	11% 7% 2 5% 3 3 3 3 4 8% 51%	2280
	11th to 20th	Day	14% 7% 3 6% 4 4 4 3 5% 6% 45%	2251
	21st to end	Day	13% 7% 2 5% 5% 3 4 3 5% 7% 46%	2451
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	11% 5% 1 3 2 3 2 2 4 5% 60%	2090
	11th to 20th	Day	8% 5% 1 4 2 3 2 3 4 5% 61%	1870
	21st to end	Day	9% 6% 2 4 3 3 2 2 4 8% 56%	1823
November	1st to 10th	Day	4 4 2 5% 3 4 4 4 6% 7% 57%	1673
	11th to 20th	Day	4 6% 2 5% 3 4 3 3 5% 7% 60%	1710
	21st to end	Day	6% 7% 2 4 3 3 2 3 5% 6% 58%	1710
December	1st to 10th	Day	5% 7% 2 5% 5% 3 4 4 5% 8% 52%	1573
	11th to 20th	Day	4 5% 2 5% 4 3 3 3 5% 7% 58%	1926
	21st to end	Day	10% 12% 3 7% 3 4 3 3 5% 6% 46%	2794
January	1st to 10th	Night	5% 9% 3 9% 6% 3 4 3 6% 8% 45%	4417
	11th to 20th	Night	3 7% 3 8% 4 4 3 3 5% 8% 51%	4430
	21st to end	Night	3 6% 2 7% 5% 5% 5% 3 6% 7% 51%	4873
February	1st to 10th	Night	7% 10% 3 7% 5% 4 4 4 5% 6% 44%	2365
	11th to 20th	Night	5% 8% 3 7% 5% 4 4 4 6% 7% 47%	2218
	21st to end	Night	4 8% 3 7% 3 3 5% 4 6% 6% 52%	1884
March	1st to 10th	Night	9% 10% 2 5% 3 3 3 3 4 5% 53%	3734
	11th to 20th	Night	7% 7% 2 5% 4 3 3 3 5% 6% 54%	3646
	21st to end	Night	8% 9% 3 7% 4 4 5% 4 6% 8% 42%	3603
April	1st to 10th	Night	11% 7% 2 6% 4 3 4 3 5% 8% 46%	1900
	11th to 20th	Night	12% 7% 2 5% 3 4 4 3 5% 6% 49%	1877
	21st to end	Night	14% 6% 2 5% 5% 4 4 3 5% 7% 45%	1729
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	12% 5% 1 4 2 2 3 3 4 5% 59%	2090
	11th to 20th	Night	8% 5% 1 3 3 2 2 3 4 6% 61%	2287
	21st to end	Night	8% 6% 2 5% 3 3 2 3 4 6% 59%	2639
November	1st to 10th	Night	5% 4 2 4 3 3 4 3 5% 8% 59%	2405
	11th to 20th	Night	4 5% 2 5% 3 4 4 4 6% 7% 56%	2470
	21st to end	Night	6% 6% 2 4 3 4 3 4 6% 7% 56%	2470
December	1st to 10th	Night	5% 6% 2 6% 4 4 4 3 6% 7% 55%	2603
	11th to 20th	Night	4 5% 2 5% 4 2 4 4 6% 7% 57%	3271
	21st to end	Night	10% 11% 3 6% 3 3 4 3 5% 6% 47%	4873

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	31% 4 11% 33% 20% 1	2539
	11th to 20th	Day	34% 8% 12% 27% 17% 3	2540
	21st to end	Day	26% 8% 19% 30% 14% 4	2794
February	1st to 10th	Day	41% 6% 14% 22% 13% 4	1829
	11th to 20th	Day	36% 4 13% 25% 18% 5%	1859
	21st to end	Day	30% 4 13% 30% 17% 6%	1570
March	1st to 10th	Day	28% 4 10% 26% 28% 5%	3130
	11th to 20th	Day	28% 4 13% 32% 20% 3	3318
	21st to end	Day	38% 8% 17% 29% 7% 1	4032
April	1st to 10th	Day	33% 5% 15% 27% 15% 4	2280
	11th to 20th	Day	40% 5% 14% 22% 16% 3	2254
	21st to end	Day	37% 6% 14% 25% 17% 2	2451
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	28% 3 10% 25% 27% 7%	2090
	11th to 20th	Day	26% 5% 9% 30% 22% 8%	1870
	21st to end	Day	29% 3 12% 29% 20% 7%	1823
November	1st to 10th	Day	25% 5% 15% 28% 21% 6%	1673
	11th to 20th	Day	24% 4 12% 29% 26% 5%	1710
	21st to end	Day	28% 4 12% 30% 22% 5%	1710
December	1st to 10th	Day	31% 6% 11% 30% 18% 3	1573
	11th to 20th	Day	26% 5% 15% 37% 16% 2	1924
	21st to end	Day	40% 4 10% 29% 14% 2	2794
January	1st to 10th	Night	37% 6% 13% 27% 16% 1	4417
	11th to 20th	Night	31% 6% 18% 25% 18% 2	4430
	21st to end	Night	31% 7% 14% 30% 17% 1	4873
February	1st to 10th	Night	39% 7% 13% 25% 14% 3	2365
	11th to 20th	Night	35% 6% 14% 27% 14% 3	2218
	21st to end	Night	31% 5% 13% 25% 20% 6%	1884
March	1st to 10th	Night	32% 5% 10% 21% 29% 4	3734
	11th to 20th	Night	30% 7% 10% 29% 22% 2	3646
	21st to end	Night	37% 11% 17% 26% 8% 1	3603
April	1st to 10th	Night	36% 6% 14% 22% 17% 4	1900
	11th to 20th	Night	36% 6% 12% 25% 17% 3	1877
	21st to end	Night	38% 8% 13% 27% 12% 2	1729
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	28% 5% 8% 26% 28% 6%	2090
	11th to 20th	Night	25% 4 10% 29% 23% 9%	2287
	21st to end	Night	28% 4 11% 25% 23% 8%	2639
November	1st to 10th	Night	23% 5% 12% 29% 24% 7%	2405
	11th to 20th	Night	26% 6% 12% 26% 24% 6%	2470
	21st to end	Night	26% 4 11% 29% 23% 5%	2470
December	1st to 10th	Night	28% 5% 14% 29% 19% 4	2603
	11th to 20th	Night	26% 4 13% 37% 19% 2	3272
	21st to end	Night	38% 4 11% 31% 14% 2	4873

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	10% 10% 41% 25% 13% 2	270
	11th to 20th	Day	12% 13% 36% 33% 4 1	270
	21st to end	Day	9% 12% 49% 20% 7% 3	297
February	1st to 10th	Day	10% 14% 52% 21% 3	270
	11th to 20th	Day	6% 15% 48% 23% 6% 1	270
	21st to end	Day	12% 20% 44% 20% 4	225
March	1st to 10th	Day	11% 12% 38% 28% 9% 1	270
	11th to 20th	Day	11% 11% 32% 33% 10% 1	270
	21st to end	Day	16% 12% 41% 20% 9% 1	425
April	1st to 10th	Day	14% 19% 38% 21% 8% 1	460
	11th to 20th	Day	17% 21% 32% 24% 5% 1	452
	21st to end	Day	17% 15% 31% 24% 10% 2	631
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% 12% 3	311
November	1st to 10th	Day	4 14% 40% 23% 16% 3	270
	11th to 20th	Day	9% 12% 36% 32% 10% 1	270
	21st to end	Day	12% 7% 40% 29% 9% 3	270
December	1st to 10th	Day	9% 10% 47% 27% 6% 1	270
	11th to 20th	Day	10% 13% 42% 26% 7% 1	270
	21st to end	Day	15% 16% 40% 24% 5% 1	297
January	1st to 10th	Night	8% 12% 40% 31% 9%	950
	11th to 20th	Night	8% 15% 41% 28% 7% 1	950
	21st to end	Night	7% 11% 45% 30% 6%	1045
February	1st to 10th	Night	11% 12% 45% 27% 5%	801
	11th to 20th	Night	10% 10% 49% 26% 5%	744
	21st to end	Night	8% 11% 48% 29% 3	628
March	1st to 10th	Night	11% 11% 40% 32% 6%	760
	11th to 20th	Night	11% 8% 42% 32% 7%	671
	21st to end	Night	12% 9% 41% 31% 7%	627
April	1st to 10th	Night	15% 14% 39% 28% 4	570
	11th to 20th	Night	16% 14% 41% 26% 3	563
	21st to end	Night	17% 13% 31% 34% 6%	570
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	1015
November	1st to 10th	Night	7% 7% 38% 38% 8% 1	925
	11th to 20th	Night	6% 11% 42% 33% 7%	950
	21st to end	Night	9% 8% 36% 36% 10%	950
December	1st to 10th	Night	8% 13% 43% 29% 6%	950
	11th to 20th	Night	8% 11% 42% 32% 7%	950
	21st to end	Night	13% 13% 38% 31% 5%	1045

Page generated at 2024-10-22 18:21:00. Version ac07

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