Arrakis Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Arrakis Observatory clear sky chart from 2005-11-06 to 2024-04-03.

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% 8% 4 7% 6% 3 5% 5% 5% 7% 39%	78950
	Night	10% 8% 4 8% 6% 3 5% 5% 5% 7% 41%	81193
Transparency	Day	44% 6% 13% 24% 12% 1	78652
	Night	43% 5% 13% 24% 13% 2	81012
Seeing	Day	16% 10% 32% 27% 12% 3	15029
	Night	14% 11% 36% 29% 9% 1	25151

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	9% 14% 4 13% 14% 6% 5% 4 4 4 23%	2232
	11th to 20th	Day	6% 12% 6% 9% 7% 5% 9% 7% 9% 7% 22%	2815
	21st to end	Day	6% 11% 12% 13% 8% 4 5% 5% 5% 6% 25%	3135
February	1st to 10th	Day	9% 9% 5% 10% 10% 5% 6% 6% 5% 8% 26%	1710
	11th to 20th	Day	9% 10% 4 9% 8% 4 7% 6% 5% 6% 31%	1664
	21st to end	Day	11% 12% 4 8% 8% 3 6% 5% 5% 6% 32%	1517
March	1st to 10th	Day	14% 10% 5% 9% 6% 3 4 4 5% 7% 32%	2048
	11th to 20th	Day	13% 10% 5% 7% 7% 3 4 5% 5% 7% 34%	2090
	21st to end	Day	18% 13% 5% 9% 6% 3 4 4 4 6% 28%	2299
April	1st to 10th	Day	18% 8% 3 6% 6% 3 5% 4 4 8% 35%	2100
	11th to 20th	Day	18% 10% 4 6% 5% 3 4 4 4 6% 35%	2266
	21st to end	Day	22% 9% 3 5% 4 3 3 4 5% 6% 35%	2340
May	1st to 10th	Day	20% 8% 3 5% 6% 4 5% 4 5% 6% 33%	2340
	11th to 20th	Day	18% 7% 3 6% 5% 3 5% 4 5% 8% 36%	2397
	21st to end	Day	15% 6% 3 5% 5% 4 5% 5% 6% 8% 38%	2725
June	1st to 10th	Day	12% 5% 2 5% 5% 3 4 5% 6% 9% 45%	2556
	11th to 20th	Day	14% 6% 2 5% 5% 3 5% 5% 5% 8% 41%	2700
	21st to end	Day	8% 4 2 5% 5% 3 5% 6% 6% 9% 47%	2700
July	1st to 10th	Day	7% 2 1 4 4 3 4 5% 6% 10% 53%	2700
	11th to 20th	Day	5% 3 2 4 4 2 4 5% 6% 8% 56%	2561
	21st to end	Day	6% 3 2 4 3 3 6% 5% 6% 9% 53%	2546
August	1st to 10th	Day	10% 5% 3 6% 6% 3 5% 5% 7% 8% 41%	2328
	11th to 20th	Day	11% 5% 2 4 5% 4 4 5% 6% 9% 46%	2301
	21st to end	Day	8% 4 2 4 4 2 3 4 5% 8% 56%	2552
September	1st to 10th	Day	12% 5% 3 4 4 3 4 3 4 7% 51%	2156
	11th to 20th	Day	10% 5% 2 5% 4 3 4 4 6% 7% 50%	1980
	21st to end	Day	13% 6% 3 4 4 3 3 4 5% 6% 49%	1980
October	1st to 10th	Day	15% 7% 3 5% 5% 2 4 4 5% 7% 44%	1810
	11th to 20th	Day	14% 8% 3 5% 4 3 3 4 4 6% 48%	1800
	21st to end	Day	16% 9% 3 6% 3 3 3 3 4 6% 43%	1946
November	1st to 10th	Day	12% 9% 3 6% 5% 2 3 4 4 7% 43%	1626
	11th to 20th	Day	9% 10% 4 11% 7% 4 3 4 5% 5% 36%	1520
	21st to end	Day	11% 10% 4 7% 6% 4 5% 4 5% 8% 36%	1520
December	1st to 10th	Day	10% 10% 4 10% 9% 5% 6% 4 5% 8% 31%	1520
	11th to 20th	Day	10% 11% 6% 9% 6% 3 5% 5% 6% 8% 31%	1918
	21st to end	Day	18% 9% 6% 13% 8% 3 4 4 5% 7% 24%	2552
January	1st to 10th	Night	9% 11% 5% 11% 12% 7% 6% 5% 5% 5% 25%	4705
	11th to 20th	Night	4 9% 5% 11% 11% 5% 8% 9% 9% 7% 22%	4149
	21st to end	Night	6% 12% 8% 15% 7% 3 4 4 4 6% 29%	4532
February	1st to 10th	Night	7% 11% 5% 9% 8% 5% 6% 6% 5% 7% 33%	2470
	11th to 20th	Night	8% 7% 5% 7% 7% 4 6% 6% 5% 7% 38%	2403
	21st to end	Night	9% 8% 4 9% 8% 3 5% 5% 5% 7% 36%	1955
March	1st to 10th	Night	10% 9% 5% 8% 7% 3 4 4 5% 7% 39%	2130
	11th to 20th	Night	10% 11% 4 8% 6% 3 5% 4 5% 6% 38%	2090
	21st to end	Night	19% 10% 5% 7% 6% 3 4 4 5% 6% 32%	2299
April	1st to 10th	Night	15% 9% 3 5% 5% 3 5% 4 4 7% 40%	1904
	11th to 20th	Night	17% 10% 3 5% 5% 2 4 3 5% 6% 39%	1640
	21st to end	Night	18% 7% 3 4 5% 2 4 4 6% 7% 40%	1620
May	1st to 10th	Night	15% 7% 3 5% 5% 3 4 3 5% 6% 43%	1620
	11th to 20th	Night	15% 7% 2 5% 4 2 4 4 5% 7% 44%	1496
	21st to end	Night	13% 5% 2 6% 6% 4 4 5% 5% 7% 43%	1551
June	1st to 10th	Night	8% 4 1 3 3 3 4 4 5% 9% 55%	1400
	11th to 20th	Night	9% 4 3 3 4 2 5% 4 6% 8% 51%	1260
	21st to end	Night	6% 4 1 3 5% 3 3 4 5% 10% 58%	1260
July	1st to 10th	Night	4 3 1 4 3 2 3 4 7% 8% 61%	1260
	11th to 20th	Night	3 2 1 3 3 2 4 3 4 8% 66%	1391
	21st to end	Night	4 3 2 3 3 2 4 4 6% 9% 60%	1772
August	1st to 10th	Night	6% 4 2 4 5% 3 4 5% 7% 9% 51%	1611
	11th to 20th	Night	9% 4 2 4 2 2 4 4 5% 7% 58%	1600
	21st to end	Night	5% 3 1 4 4 2 3 4 4 6% 62%	1803
September	1st to 10th	Night	8% 4 2 3 3 2 4 4 4 7% 58%	1800
	11th to 20th	Night	9% 5% 2 3 4 2 4 4 4 7% 55%	1980
	21st to end	Night	10% 5% 2 4 3 2 4 4 4 7% 55%	1980
October	1st to 10th	Night	16% 6% 3 4 3 2 3 3 4 6% 50%	2131
	11th to 20th	Night	14% 7% 3 5% 4 2 3 4 4 5% 48%	2160
	21st to end	Night	15% 9% 3 6% 4 2 3 3 4 6% 44%	2407
November	1st to 10th	Night	10% 7% 3 6% 5% 3 4 5% 4 6% 48%	2450
	11th to 20th	Night	10% 9% 4 9% 7% 3 4 4 4 6% 40%	2660
	21st to end	Night	10% 9% 3 7% 5% 3 4 4 5% 7% 42%	2660
December	1st to 10th	Night	8% 10% 4 9% 8% 4 6% 5% 5% 7% 35%	2660
	11th to 20th	Night	11% 9% 4 9% 7% 4 5% 6% 5% 8% 32%	3279
	21st to end	Night	18% 10% 6% 14% 7% 3 5% 4 4 5% 23%	5105

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	64% 5% 11% 14% 6%	2233
	11th to 20th	Day	55% 5% 9% 23% 7% 1	2815
	21st to end	Day	59% 4 12% 17% 7%	3135
February	1st to 10th	Day	55% 5% 14% 18% 8% 1	1710
	11th to 20th	Day	52% 6% 12% 19% 10% 2	1673
	21st to end	Day	50% 7% 12% 18% 11% 3	1517
March	1st to 10th	Day	52% 5% 12% 21% 8% 1	2048
	11th to 20th	Day	50% 5% 13% 24% 8% 1	2090
	21st to end	Day	59% 6% 12% 16% 6% 1	2299
April	1st to 10th	Day	48% 6% 14% 22% 10% 1	2100
	11th to 20th	Day	51% 4 11% 22% 11% 2	2266
	21st to end	Day	50% 6% 12% 24% 8%	2340
May	1st to 10th	Day	50% 7% 11% 21% 11%	2340
	11th to 20th	Day	46% 7% 14% 22% 11% 1	2401
	21st to end	Day	45% 7% 12% 23% 11% 1	2725
June	1st to 10th	Day	35% 6% 17% 28% 13% 1	2556
	11th to 20th	Day	41% 7% 13% 23% 14% 2	2700
	21st to end	Day	32% 8% 17% 28% 14% 2	2700
July	1st to 10th	Day	25% 7% 17% 36% 14% 1	2626
	11th to 20th	Day	25% 6% 15% 29% 21% 5%	2481
	21st to end	Day	26% 8% 16% 29% 18% 2	2405
August	1st to 10th	Day	39% 8% 16% 24% 12% 3	2314
	11th to 20th	Day	34% 7% 16% 30% 11% 2	2301
	21st to end	Day	27% 5% 13% 33% 18% 3	2552
September	1st to 10th	Day	35% 5% 11% 30% 19% 1	2156
	11th to 20th	Day	33% 6% 17% 29% 13% 1	1980
	21st to end	Day	35% 6% 10% 30% 17% 1	1980
October	1st to 10th	Day	40% 6% 12% 25% 16% 1	1810
	11th to 20th	Day	39% 4 12% 28% 16% 2	1800
	21st to end	Day	44% 3 12% 24% 14% 2	1946
November	1st to 10th	Day	40% 5% 13% 23% 16% 4	1626
	11th to 20th	Day	49% 6% 13% 20% 12% 1	1520
	21st to end	Day	45% 5% 16% 20% 13% 2	1520
December	1st to 10th	Day	52% 5% 14% 16% 10% 2	1520
	11th to 20th	Day	50% 6% 18% 19% 6%	1915
	21st to end	Day	60% 4 12% 15% 8% 2	2552
January	1st to 10th	Night	61% 5% 9% 18% 8%	4705
	11th to 20th	Night	55% 5% 12% 19% 8% 1	4149
	21st to end	Night	56% 4 11% 19% 9% 1	4532
February	1st to 10th	Night	50% 4 12% 22% 10% 2	2470
	11th to 20th	Night	44% 6% 12% 22% 13% 3	2404
	21st to end	Night	46% 6% 12% 19% 13% 5%	1955
March	1st to 10th	Night	45% 5% 15% 24% 10% 2	2130
	11th to 20th	Night	48% 5% 10% 24% 12% 1	2090
	21st to end	Night	54% 5% 14% 19% 7% 2	2299
April	1st to 10th	Night	45% 5% 11% 26% 12% 1	1904
	11th to 20th	Night	47% 4 9% 22% 16% 2	1640
	21st to end	Night	43% 6% 12% 26% 11% 2	1620
May	1st to 10th	Night	41% 5% 14% 25% 14% 1	1620
	11th to 20th	Night	40% 6% 12% 25% 16% 2	1497
	21st to end	Night	39% 7% 16% 22% 15% 2	1552
June	1st to 10th	Night	27% 8% 17% 29% 18% 2	1400
	11th to 20th	Night	30% 7% 17% 24% 19% 3	1260
	21st to end	Night	23% 5% 17% 32% 20% 2	1260
July	1st to 10th	Night	20% 5% 16% 37% 19% 3	1225
	11th to 20th	Night	18% 5% 16% 34% 21% 6%	1349
	21st to end	Night	20% 6% 16% 33% 21% 4	1673
August	1st to 10th	Night	28% 8% 18% 27% 16% 4	1601
	11th to 20th	Night	25% 7% 14% 38% 16% 2	1600
	21st to end	Night	23% 5% 14% 30% 23% 4	1803
September	1st to 10th	Night	27% 4 14% 31% 22% 3	1800
	11th to 20th	Night	30% 5% 13% 33% 18% 1	1980
	21st to end	Night	30% 5% 11% 32% 21% 1	1980
October	1st to 10th	Night	36% 4 11% 28% 19% 2	2131
	11th to 20th	Night	38% 4 11% 26% 19% 3	2160
	21st to end	Night	43% 4 14% 23% 16% 1	2407
November	1st to 10th	Night	37% 5% 13% 23% 18% 3	2450
	11th to 20th	Night	44% 5% 11% 24% 13% 2	2660
	21st to end	Night	42% 5% 16% 23% 14% 1	2660
December	1st to 10th	Night	47% 6% 16% 21% 9% 1	2660
	11th to 20th	Night	49% 6% 18% 20% 6%	3281
	21st to end	Night	62% 4 8% 18% 7% 1	5105

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% 12% 39% 23% 8% 3	270
	11th to 20th	Day	16% 14% 39% 22% 7% 1	270
	21st to end	Day	12% 15% 36% 25% 10% 1	297
February	1st to 10th	Day	16% 18% 43% 20% 3	270
	11th to 20th	Day	13% 19% 41% 22% 5%	270
	21st to end	Day	19% 18% 44% 17% 2	225
March	1st to 10th	Day	19% 15% 39% 23% 4	270
	11th to 20th	Day	22% 14% 34% 23% 5% 1	270
	21st to end	Day	27% 14% 27% 22% 9% 1	297
April	1st to 10th	Day	26% 14% 34% 19% 6% 1	344
	11th to 20th	Day	28% 13% 28% 23% 7% 1	544
	21st to end	Day	23% 15% 30% 20% 10% 2	600
May	1st to 10th	Day	24% 11% 31% 24% 9% 1	603
	11th to 20th	Day	23% 10% 30% 22% 12% 3	625
	21st to end	Day	17% 9% 31% 28% 13% 1	679
June	1st to 10th	Day	9% 8% 26% 32% 19% 6%	630
	11th to 20th	Day	16% 7% 36% 28% 9% 4	630
	21st to end	Day	9% 6% 34% 31% 17% 2	630
July	1st to 10th	Day	8% 5% 23% 36% 22% 6%	621
	11th to 20th	Day	6% 6% 34% 30% 17% 7%	618
	21st to end	Day	6% 4 26% 40% 20% 5%	684
August	1st to 10th	Day	11% 3 21% 38% 22% 5%	626
	11th to 20th	Day	13% 4 22% 33% 23% 5%	615
	21st to end	Day	8% 5% 32% 34% 18% 4	671
September	1st to 10th	Day	16% 7% 27% 31% 15% 3	446
	11th to 20th	Day	10% 5% 35% 33% 13% 4	270
	21st to end	Day	17% 14% 36% 20% 10% 3	270
October	1st to 10th	Day	19% 10% 30% 24% 13% 4	270
	11th to 20th	Day	17% 20% 35% 21% 6% 1	270
	21st to end	Day	24% 15% 36% 17% 6% 1	297
November	1st to 10th	Day	21% 16% 35% 20% 7% 1	270
	11th to 20th	Day	16% 21% 44% 16% 3	270
	21st to end	Day	19% 17% 37% 23% 4	270
December	1st to 10th	Day	12% 16% 41% 24% 7%	270
	11th to 20th	Day	16% 17% 44% 19% 5%	270
	21st to end	Day	26% 16% 37% 18% 3	297
January	1st to 10th	Night	12% 15% 43% 23% 7% 1	950
	11th to 20th	Night	11% 17% 40% 26% 5% 1	950
	21st to end	Night	16% 14% 39% 25% 6%	1045
February	1st to 10th	Night	13% 14% 44% 24% 5%	950
	11th to 20th	Night	14% 16% 40% 24% 6% 1	929
	21st to end	Night	15% 13% 41% 23% 7% 1	699
March	1st to 10th	Night	13% 13% 43% 25% 6% 1	610
	11th to 20th	Night	16% 15% 39% 25% 6%	570
	21st to end	Night	24% 10% 29% 27% 9% 1	627
April	1st to 10th	Night	21% 11% 37% 26% 5%	546
	11th to 20th	Night	23% 13% 32% 23% 8% 1	533
	21st to end	Night	22% 11% 32% 24% 9% 1	540
May	1st to 10th	Night	19% 9% 35% 27% 9% 1	540
	11th to 20th	Night	18% 8% 33% 30% 10% 1	531
	21st to end	Night	17% 6% 33% 31% 13% 1	584
June	1st to 10th	Night	9% 6% 28% 38% 19%	540
	11th to 20th	Night	11% 4 36% 38% 10%	540
	21st to end	Night	6% 4 27% 48% 14%	540
July	1st to 10th	Night	5% 3 23% 44% 23% 2	525
	11th to 20th	Night	4 3 28% 45% 19% 1	522
	21st to end	Night	4 2 27% 46% 20% 2	591
August	1st to 10th	Night	8% 2 23% 46% 20% 1	537
	11th to 20th	Night	10% 5% 20% 39% 23% 2	534
	21st to end	Night	6% 2 30% 39% 20% 2	594
September	1st to 10th	Night	10% 4 30% 38% 16% 2	540
	11th to 20th	Night	11% 6% 31% 36% 14% 2	540
	21st to end	Night	11% 13% 33% 32% 11% 1	540
October	1st to 10th	Night	18% 10% 33% 29% 9%	691
	11th to 20th	Night	19% 13% 38% 24% 7%	720
	21st to end	Night	20% 14% 32% 27% 7%	823
November	1st to 10th	Night	15% 13% 38% 27% 7%	925
	11th to 20th	Night	14% 15% 41% 24% 6% 1	950
	21st to end	Night	15% 14% 41% 25% 5%	950
December	1st to 10th	Night	13% 14% 42% 24% 6%	950
	11th to 20th	Night	19% 12% 41% 23% 6% 1	950
	21st to end	Night	23% 14% 38% 20% 5%	1045

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