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 2024-11-04.

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% 3 8% 5% 4 4 4 6% 7% 40%	101711
	Night	11% 8% 3 8% 5% 4 4 4 5% 7% 42%	97368
Transparency	Day	42% 6% 13% 24% 13% 1	105319
	Night	41% 5% 13% 25% 15% 2	101890
Seeing	Day	16% 11% 32% 27% 12% 3	15971
	Night	14% 11% 36% 29% 9% 1	25837

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% 15% 4 16% 12% 7% 5% 3 4 5% 22%	2232
	11th to 20th	Day	5% 13% 5% 11% 5% 6% 10% 5% 10% 7% 22%	2815
	21st to end	Day	5% 12% 12% 14% 7% 5% 5% 4 5% 6% 25%	3135
February	1st to 10th	Day	9% 10% 4 11% 9% 7% 6% 5% 5% 7% 26%	1710
	11th to 20th	Day	9% 11% 4 10% 7% 6% 7% 5% 5% 6% 31%	1664
	21st to end	Day	11% 12% 3 10% 6% 5% 4 4 5% 6% 32%	1517
March	1st to 10th	Day	12% 9% 3 9% 5% 4 4 4 5% 6% 38%	3373
	11th to 20th	Day	10% 10% 3 8% 6% 4 4 3 5% 7% 40%	3480
	21st to end	Day	24% 14% 4 13% 4 3 3 3 3 5% 23%	3814
April	1st to 10th	Day	18% 8% 3 7% 5% 4 4 3 5% 7% 36%	2194
	11th to 20th	Day	18% 11% 3 7% 4 4 4 3 5% 6% 34%	2394
	21st to end	Day	22% 10% 3 6% 3 4 3 3 5% 6% 35%	2470
May	1st to 10th	Day	16% 7% 2 6% 5% 5% 5% 4 6% 8% 36%	4082
	11th to 20th	Day	14% 6% 3 6% 4 4 4 3 5% 6% 46%	4261
	21st to end	Day	17% 7% 2 6% 4 4 4 4 6% 8% 39%	4824
June	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 6% 8% 46%	2698
	11th to 20th	Day	15% 6% 2 5% 4 5% 4 4 5% 8% 41%	2850
	21st to end	Day	9% 4 2 5% 4 5% 5% 4 6% 9% 46%	2850
July	1st to 10th	Day	9% 5% 2 6% 5% 5% 5% 5% 8% 10% 41%	4750
	11th to 20th	Day	4 3 2 5% 3 3 4 4 6% 8% 59%	4504
	21st to end	Day	5% 3 2 5% 4 5% 5% 4 7% 9% 52%	4465
August	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 6% 8% 42%	4104
	11th to 20th	Day	13% 6% 2 6% 5% 6% 5% 5% 7% 8% 37%	4041
	21st to end	Day	8% 4 2 5% 4 4 4 4 6% 8% 51%	4482
September	1st to 10th	Day	12% 5% 2 4 4 3 3 3 5% 7% 53%	2275
	11th to 20th	Day	9% 5% 2 6% 3 4 4 3 6% 7% 51%	2090
	21st to end	Day	12% 6% 2 4 3 4 3 3 5% 6% 51%	2090
October	1st to 10th	Day	14% 6% 2 6% 4 3 3 4 5% 7% 45%	1910
	11th to 20th	Day	13% 8% 2 6% 3 4 4 3 4 6% 47%	1900
	21st to end	Day	16% 9% 3 6% 3 4 3 3 4 6% 43%	2054
November	1st to 10th	Day	12% 9% 3 8% 4 3 3 4 5% 7% 42%	1653
	11th to 20th	Day	9% 11% 4 13% 5% 5% 3 3 5% 5% 36%	1520
	21st to end	Day	11% 11% 4 8% 5% 4 4 3 6% 8% 36%	1520
December	1st to 10th	Day	10% 10% 4 11% 8% 7% 4 3 5% 8% 31%	1520
	11th to 20th	Day	9% 12% 6% 11% 5% 5% 5% 4 7% 7% 31%	1918
	21st to end	Day	18% 10% 5% 15% 6% 4 3 3 6% 7% 24%	2552
January	1st to 10th	Night	8% 12% 4 13% 10% 8% 5% 4 5% 5% 25%	4705
	11th to 20th	Night	4 10% 5% 13% 8% 7% 9% 6% 9% 7% 22%	4149
	21st to end	Night	6% 12% 8% 17% 5% 5% 4 3 5% 5% 29%	4532
February	1st to 10th	Night	7% 12% 4 11% 6% 7% 5% 5% 5% 6% 33%	2470
	11th to 20th	Night	8% 8% 4 8% 6% 6% 6% 4 5% 7% 38%	2403
	21st to end	Night	9% 9% 4 10% 7% 5% 5% 4 5% 7% 36%	1955
March	1st to 10th	Night	7% 8% 3 7% 5% 4 4 3 5% 7% 47%	3481
	11th to 20th	Night	8% 8% 3 7% 5% 5% 4 4 6% 7% 43%	3480
	21st to end	Night	27% 12% 4 9% 5% 3 3 2 4 5% 26%	3817
April	1st to 10th	Night	15% 10% 3 6% 4 4 5% 3 4 7% 39%	1986
	11th to 20th	Night	16% 10% 3 6% 4 4 4 3 5% 6% 39%	1732
	21st to end	Night	17% 8% 2 5% 4 3 4 3 5% 6% 40%	1710
May	1st to 10th	Night	18% 7% 2 5% 4 3 3 2 5% 6% 46%	2823
	11th to 20th	Night	10% 5% 2 6% 4 3 4 4 6% 7% 49%	2642
	21st to end	Night	15% 6% 2 6% 4 4 3 4 4 6% 46%	2743
June	1st to 10th	Night	8% 5% 1 4 3 3 5% 3 6% 8% 55%	1477
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 6% 8% 49%	1330
	21st to end	Night	6% 4 1 4 4 3 3 3 5% 9% 57%	1330
July	1st to 10th	Night	9% 5% 1 6% 4 4 4 4 7% 8% 47%	2220
	11th to 20th	Night	3 2 1 3 3 3 3 3 5% 8% 66%	2458
	21st to end	Night	4 3 2 4 3 3 4 4 7% 9% 57%	3098
August	1st to 10th	Night	11% 7% 2 5% 4 3 3 3 5% 7% 50%	2836
	11th to 20th	Night	11% 6% 2 5% 3 3 3 4 6% 7% 51%	2798
	21st to end	Night	7% 5% 2 4 4 3 4 4 6% 7% 54%	3182
September	1st to 10th	Night	8% 5% 1 4 3 3 4 3 4 7% 60%	1900
	11th to 20th	Night	9% 5% 1 4 4 4 4 3 5% 7% 55%	2090
	21st to end	Night	10% 5% 2 4 3 3 3 3 4 7% 56%	2090
October	1st to 10th	Night	15% 6% 3 4 2 3 3 3 4 7% 52%	2250
	11th to 20th	Night	14% 8% 2 5% 4 3 3 3 5% 5% 49%	2280
	21st to end	Night	15% 9% 2 7% 4 3 3 2 5% 6% 44%	2541
November	1st to 10th	Night	11% 7% 2 7% 5% 4 4 4 4 5% 48%	2496
	11th to 20th	Night	10% 10% 3 10% 6% 4 3 4 5% 6% 40%	2660
	21st to end	Night	10% 10% 3 8% 4 4 4 4 5% 7% 41%	2660
December	1st to 10th	Night	8% 10% 4 10% 7% 5% 5% 4 6% 6% 35%	2660
	11th to 20th	Night	11% 10% 4 10% 6% 5% 6% 5% 5% 8% 32%	3279
	21st to end	Night	18% 11% 5% 15% 6% 4 5% 3 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	46% 5% 11% 27% 11% 1	3373
	11th to 20th	Day	43% 4 10% 26% 14% 2	3480
	21st to end	Day	64% 6% 10% 13% 6% 1	3814
April	1st to 10th	Day	47% 5% 13% 22% 11% 1	2194
	11th to 20th	Day	51% 5% 10% 22% 11% 1	2394
	21st to end	Day	50% 6% 12% 23% 9%	2470
May	1st to 10th	Day	43% 8% 10% 24% 14%	4120
	11th to 20th	Day	38% 6% 10% 28% 15% 3	4265
	21st to end	Day	43% 6% 11% 25% 12% 1	4837
June	1st to 10th	Day	35% 6% 16% 28% 14% 1	2698
	11th to 20th	Day	42% 7% 13% 23% 14% 2	2850
	21st to end	Day	32% 8% 16% 27% 14% 2	2850
July	1st to 10th	Day	35% 8% 20% 25% 11% 1	4676
	11th to 20th	Day	23% 6% 15% 34% 20% 3	4424
	21st to end	Day	26% 8% 17% 28% 20% 2	4325
August	1st to 10th	Day	40% 7% 12% 27% 13% 2	4090
	11th to 20th	Day	42% 7% 15% 28% 7% 1	4080
	21st to end	Day	31% 6% 18% 28% 15% 2	4481
September	1st to 10th	Day	33% 5% 11% 32% 18% 1	2275
	11th to 20th	Day	32% 6% 17% 30% 14% 1	2090
	21st to end	Day	33% 6% 10% 30% 19% 2	2090
October	1st to 10th	Day	28% 4 8% 29% 31% 1	3170
	11th to 20th	Day	36% 4 13% 24% 21% 2	3140
	21st to end	Day	42% 3 13% 26% 15% 1	3370
November	1st to 10th	Day	40% 5% 13% 23% 15% 3	1653
	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	36% 4 11% 29% 18% 2	3481
	11th to 20th	Night	38% 5% 10% 30% 15% 1	3480
	21st to end	Night	62% 5% 13% 14% 6% 1	3817
April	1st to 10th	Night	45% 5% 11% 26% 12% 1	1986
	11th to 20th	Night	45% 5% 9% 22% 17% 2	1732
	21st to end	Night	43% 6% 11% 25% 13% 2	1710
May	1st to 10th	Night	40% 5% 13% 22% 18% 2	2850
	11th to 20th	Night	32% 7% 11% 27% 20% 4	2643
	21st to end	Night	39% 5% 12% 27% 15% 2	2751
June	1st to 10th	Night	27% 8% 17% 28% 17% 3	1477
	11th to 20th	Night	32% 6% 17% 23% 19% 3	1330
	21st to end	Night	24% 5% 16% 33% 20% 2	1330
July	1st to 10th	Night	32% 6% 21% 26% 13% 2	2185
	11th to 20th	Night	19% 5% 13% 38% 22% 3	2416
	21st to end	Night	21% 8% 18% 32% 19% 2	2999
August	1st to 10th	Night	34% 6% 15% 26% 17% 3	2826
	11th to 20th	Night	32% 7% 16% 32% 12% 1	2825
	21st to end	Night	29% 6% 17% 27% 18% 3	3182
September	1st to 10th	Night	26% 4 13% 33% 22% 3	1900
	11th to 20th	Night	29% 5% 13% 33% 18% 2	2090
	21st to end	Night	29% 5% 10% 32% 22% 2	2090
October	1st to 10th	Night	22% 3 6% 27% 38% 4	3757
	11th to 20th	Night	27% 3 13% 30% 23% 3	3779
	21st to end	Night	40% 3 12% 29% 14% 1	4177
November	1st to 10th	Night	38% 5% 14% 23% 18% 3	2496
	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	26% 14% 27% 23% 9% 1	297
April	1st to 10th	Day	27% 14% 35% 19% 5% 1	374
	11th to 20th	Day	27% 15% 29% 22% 7% 1	592
	21st to end	Day	24% 16% 29% 20% 10% 2	650
May	1st to 10th	Day	23% 12% 32% 23% 9% 1	653
	11th to 20th	Day	21% 11% 30% 23% 12% 3	675
	21st to end	Day	17% 10% 31% 28% 13% 1	734
June	1st to 10th	Day	10% 9% 26% 31% 19% 5%	680
	11th to 20th	Day	17% 8% 36% 26% 9% 4	680
	21st to end	Day	10% 7% 34% 30% 16% 2	680
July	1st to 10th	Day	8% 5% 23% 35% 21% 6%	671
	11th to 20th	Day	6% 5% 33% 31% 18% 7%	668
	21st to end	Day	6% 4 26% 39% 20% 5%	739
August	1st to 10th	Day	12% 4 22% 37% 21% 4	676
	11th to 20th	Day	14% 4 23% 32% 22% 5%	665
	21st to end	Day	8% 4 33% 34% 17% 4	724
September	1st to 10th	Day	15% 7% 28% 31% 15% 3	485
	11th to 20th	Day	9% 5% 35% 34% 12% 5%	300
	21st to end	Day	16% 13% 34% 21% 12% 4	300
October	1st to 10th	Day	18% 11% 29% 24% 14% 4	300
	11th to 20th	Day	16% 18% 36% 23% 6% 1	300
	21st to end	Day	24% 15% 37% 17% 5% 1	330
November	1st to 10th	Day	21% 16% 36% 19% 7% 1	279
	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% 12% 38% 25% 4	570
	11th to 20th	Night	22% 14% 32% 24% 8% 1	563
	21st to end	Night	22% 12% 31% 25% 9% 1	570
May	1st to 10th	Night	19% 10% 35% 27% 9% 1	570
	11th to 20th	Night	17% 8% 33% 31% 10% 1	561
	21st to end	Night	17% 6% 33% 31% 12% 1	617
June	1st to 10th	Night	9% 6% 29% 38% 18%	570
	11th to 20th	Night	12% 4 36% 38% 10%	570
	21st to end	Night	7% 4 27% 48% 14%	570
July	1st to 10th	Night	6% 4 24% 43% 23% 2	555
	11th to 20th	Night	4 3 26% 45% 20% 1	552
	21st to end	Night	4 2 26% 46% 20% 2	624
August	1st to 10th	Night	9% 2 23% 46% 19% 1	567
	11th to 20th	Night	11% 5% 21% 39% 22% 2	564
	21st to end	Night	7% 2 31% 39% 19% 2	627
September	1st to 10th	Night	9% 4 30% 39% 16% 2	570
	11th to 20th	Night	10% 6% 31% 36% 14% 2	570
	21st to end	Night	10% 13% 33% 32% 12% 1	570
October	1st to 10th	Night	17% 10% 33% 30% 10%	730
	11th to 20th	Night	18% 13% 38% 24% 6%	760
	21st to end	Night	20% 14% 33% 26% 6%	869
November	1st to 10th	Night	16% 13% 38% 27% 7%	943
	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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