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 2025-08-05.

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% 5% 4 4 6% 7% 40%	126204
	Night	10% 8% 3 8% 5% 4 4 4 5% 6% 41%	122089
Transparency	Day	42% 5% 13% 25% 13% 2	129773
	Night	41% 5% 12% 25% 14% 2	126586
Seeing	Day	16% 11% 32% 26% 12% 3	17030
	Night	14% 11% 36% 29% 9% 1	26887

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	7% 14% 4 18% 12% 7% 6% 3 4 5% 20%	3206
	11th to 20th	Day	5% 11% 5% 10% 6% 7% 10% 6% 11% 8% 21%	4032
	21st to end	Day	4 9% 9% 13% 6% 5% 5% 4 5% 5% 36%	4488
February	1st to 10th	Day	9% 11% 5% 11% 9% 7% 6% 5% 5% 7% 26%	1800
	11th to 20th	Day	9% 11% 4 10% 7% 6% 7% 5% 5% 6% 31%	1754
	21st to end	Day	11% 12% 3 10% 6% 5% 4 4 5% 6% 34%	1595
March	1st to 10th	Day	11% 7% 3 7% 5% 4 4 4 5% 7% 43%	4847
	11th to 20th	Day	9% 9% 3 7% 5% 4 4 3 5% 9% 44%	4980
	21st to end	Day	22% 12% 4 12% 5% 4 4 3 4 6% 24%	5462
April	1st to 10th	Day	18% 8% 3 6% 5% 4 4 3 5% 7% 36%	2310
	11th to 20th	Day	17% 11% 3 7% 4 4 4 3 5% 6% 34%	2522
	21st to end	Day	21% 10% 3 6% 4 4 3 3 5% 6% 35%	2600
May	1st to 10th	Day	12% 5% 2 5% 4 4 4 3 5% 7% 48%	5862
	11th to 20th	Day	18% 7% 2 5% 3 3 3 3 4 6% 45%	6125
	21st to end	Day	17% 8% 3 7% 4 5% 5% 4 6% 7% 35%	6935
June	1st to 10th	Day	11% 5% 2 6% 4 4 4 4 6% 8% 46%	2840
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 4 5% 8% 40%	3000
	21st to end	Day	10% 4 2 5% 4 5% 5% 4 6% 9% 45%	3000
July	1st to 10th	Day	8% 4 2 6% 4 5% 5% 5% 7% 10% 44%	6800
	11th to 20th	Day	8% 5% 2 5% 3 3 3 3 5% 7% 55%	6447
	21st to end	Day	7% 5% 2 5% 4 4 5% 4 7% 9% 48%	6422
August	1st to 10th	Day	13% 6% 2 6% 4 4 4 4 6% 8% 42%	4157
	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	13% 9% 3 8% 4 3 3 4 5% 7% 42%	1713
	11th to 20th	Day	9% 11% 4 13% 5% 5% 3 3 5% 5% 36%	1600
	21st to end	Day	11% 11% 4 9% 5% 4 4 3 6% 8% 35%	1600
December	1st to 10th	Day	6% 8% 5% 11% 8% 9% 5% 3 5% 8% 33%	2610
	11th to 20th	Day	9% 13% 6% 13% 7% 8% 7% 3 6% 5% 24%	3008
	21st to end	Day	15% 11% 5% 18% 6% 4 4 3 5% 6% 22%	3647
January	1st to 10th	Night	6% 10% 4 15% 11% 9% 6% 5% 6% 5% 23%	6741
	11th to 20th	Night	3 8% 4 12% 8% 7% 10% 7% 9% 7% 23%	5929
	21st to end	Night	4 10% 7% 14% 4 4 5% 3 5% 5% 39%	6490
February	1st to 10th	Night	7% 11% 4 11% 7% 7% 5% 5% 5% 6% 32%	2600
	11th to 20th	Night	8% 8% 5% 9% 6% 6% 6% 4 5% 7% 37%	2533
	21st to end	Night	9% 9% 3 10% 7% 5% 5% 4 5% 7% 37%	2054
March	1st to 10th	Night	8% 9% 3 7% 4 3 3 2 4 7% 50%	5009
	11th to 20th	Night	9% 7% 2 6% 5% 4 4 4 6% 9% 44%	4980
	21st to end	Night	22% 11% 4 9% 5% 3 4 3 4 5% 31%	5467
April	1st to 10th	Night	15% 10% 3 6% 4 4 5% 3 4 7% 40%	2090
	11th to 20th	Night	15% 10% 3 6% 3 4 4 3 6% 6% 40%	1824
	21st to end	Night	18% 8% 3 6% 4 3 4 3 5% 7% 39%	1800
May	1st to 10th	Night	13% 5% 1 4 3 3 3 2 4 6% 57%	4053
	11th to 20th	Night	13% 7% 2 6% 3 3 3 3 5% 6% 49%	3788
	21st to end	Night	17% 8% 2 6% 4 4 3 3 4 6% 43%	3942
June	1st to 10th	Night	9% 5% 1 4 3 3 5% 3 6% 8% 54%	1554
	11th to 20th	Night	9% 5% 2 5% 4 4 4 4 6% 8% 49%	1400
	21st to end	Night	8% 4 1 4 4 3 3 3 5% 10% 56%	1400
July	1st to 10th	Night	8% 4 1 6% 4 4 4 4 7% 9% 50%	3178
	11th to 20th	Night	6% 4 2 5% 3 4 4 3 5% 7% 56%	3522
	21st to end	Night	9% 5% 2 4 3 3 4 4 6% 8% 52%	4449
August	1st to 10th	Night	11% 7% 2 5% 4 3 3 3 5% 7% 50%	2872
	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% 8% 2 7% 5% 4 4 4 4 5% 47%	2583
	11th to 20th	Night	10% 9% 3 10% 6% 4 3 4 4 6% 41%	2800
	21st to end	Night	9% 11% 3 9% 4 4 4 4 5% 7% 40%	2800
December	1st to 10th	Night	5% 7% 4 12% 7% 8% 5% 4 5% 6% 35%	4580
	11th to 20th	Night	10% 10% 4 12% 9% 7% 5% 4 5% 6% 29%	5199
	21st to end	Night	15% 12% 5% 15% 7% 4 5% 4 5% 5% 22%	7321

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	66% 4 10% 12% 6%	3207
	11th to 20th	Day	54% 4 9% 19% 13% 1	4032
	21st to end	Day	50% 4 14% 19% 12% 1	4488
February	1st to 10th	Day	56% 5% 13% 17% 9% 1	1800
	11th to 20th	Day	52% 6% 12% 18% 11% 2	1763
	21st to end	Day	49% 6% 13% 19% 11% 3	1595
March	1st to 10th	Day	41% 4 13% 28% 13% 2	4847
	11th to 20th	Day	38% 3 11% 31% 15% 2	4980
	21st to end	Day	62% 5% 10% 16% 6%	5464
April	1st to 10th	Day	47% 5% 13% 22% 11% 1	2310
	11th to 20th	Day	50% 5% 10% 22% 11% 1	2522
	21st to end	Day	50% 6% 12% 23% 9%	2600
May	1st to 10th	Day	34% 6% 10% 29% 20% 1	5900
	11th to 20th	Day	40% 5% 9% 22% 16% 8%	6129
	21st to end	Day	47% 6% 10% 25% 11% 1	6949
June	1st to 10th	Day	35% 6% 16% 28% 13% 1	2840
	11th to 20th	Day	42% 7% 13% 23% 13% 2	3000
	21st to end	Day	33% 7% 16% 27% 15% 2	3000
July	1st to 10th	Day	32% 7% 21% 29% 10% 1	6726
	11th to 20th	Day	28% 5% 14% 34% 17% 2	6367
	21st to end	Day	31% 7% 18% 27% 16% 1	6240
August	1st to 10th	Day	40% 6% 12% 27% 13% 2	4143
	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	41% 5% 13% 23% 15% 3	1713
	11th to 20th	Day	49% 5% 13% 21% 12% 1	1600
	21st to end	Day	46% 5% 15% 20% 12% 2	1600
December	1st to 10th	Day	49% 4 15% 22% 9% 1	2610
	11th to 20th	Day	61% 5% 14% 15% 5%	3005
	21st to end	Day	62% 3 12% 14% 9% 1	3647
January	1st to 10th	Night	60% 4 9% 19% 8%	6741
	11th to 20th	Night	54% 4 14% 15% 12% 2	5929
	21st to end	Night	47% 4 12% 23% 14% 1	6490
February	1st to 10th	Night	50% 4 12% 21% 10% 2	2600
	11th to 20th	Night	45% 6% 12% 22% 13% 3	2534
	21st to end	Night	45% 6% 12% 20% 13% 4	2054
March	1st to 10th	Night	35% 3 9% 34% 16% 2	5009
	11th to 20th	Night	36% 4 11% 32% 15% 1	4980
	21st to end	Night	56% 4 13% 19% 7% 1	5467
April	1st to 10th	Night	45% 5% 11% 26% 12% 1	2090
	11th to 20th	Night	44% 4 9% 23% 17% 2	1824
	21st to end	Night	44% 6% 11% 25% 12% 2	1800
May	1st to 10th	Night	30% 4 11% 31% 21% 2	4080
	11th to 20th	Night	36% 5% 8% 21% 20% 9%	3789
	21st to end	Night	42% 5% 10% 30% 12% 1	3950
June	1st to 10th	Night	27% 7% 17% 28% 17% 3	1554
	11th to 20th	Night	32% 6% 17% 24% 18% 3	1400
	21st to end	Night	25% 5% 16% 32% 19% 2	1400
July	1st to 10th	Night	30% 5% 18% 31% 14% 2	3145
	11th to 20th	Night	28% 5% 14% 35% 17% 2	3483
	21st to end	Night	29% 7% 18% 31% 14% 2	4320
August	1st to 10th	Night	34% 6% 15% 26% 17% 3	2862
	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% 13% 23% 18% 3	2583
	11th to 20th	Night	43% 5% 12% 24% 14% 3	2800
	21st to end	Night	43% 4 16% 23% 13% 1	2800
December	1st to 10th	Night	47% 5% 16% 24% 8%	4580
	11th to 20th	Night	55% 5% 15% 20% 5%	5201
	21st to end	Night	63% 4 9% 18% 6%	7321

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	15% 12% 40% 22% 8% 2	300
	11th to 20th	Day	16% 14% 40% 21% 7% 1	300
	21st to end	Day	12% 17% 35% 25% 9% 1	330
February	1st to 10th	Day	18% 18% 43% 18% 3	300
	11th to 20th	Day	13% 18% 41% 22% 6%	300
	21st to end	Day	18% 17% 46% 16% 2	249
March	1st to 10th	Day	18% 15% 40% 23% 4	300
	11th to 20th	Day	21% 15% 34% 24% 4 1	300
	21st to end	Day	27% 14% 28% 22% 8% 1	330
April	1st to 10th	Day	27% 14% 35% 19% 5%	410
	11th to 20th	Day	27% 15% 29% 22% 6% 1	640
	21st to end	Day	24% 16% 30% 20% 10% 2	700
May	1st to 10th	Day	22% 12% 31% 23% 11% 2	703
	11th to 20th	Day	22% 11% 29% 23% 13% 3	725
	21st to end	Day	18% 9% 30% 29% 13% 1	789
June	1st to 10th	Day	10% 9% 27% 31% 18% 5%	730
	11th to 20th	Day	17% 8% 37% 26% 9% 4	730
	21st to end	Day	11% 7% 35% 29% 16% 2	730
July	1st to 10th	Day	8% 5% 24% 36% 21% 6%	721
	11th to 20th	Day	7% 6% 33% 31% 17% 7%	718
	21st to end	Day	6% 4 27% 39% 19% 5%	794
August	1st to 10th	Day	12% 4 21% 38% 21% 4	697
	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	23% 15% 35% 19% 7% 1	300
	11th to 20th	Day	18% 20% 43% 15% 3	300
	21st to end	Day	19% 17% 37% 23% 5%	300
December	1st to 10th	Day	13% 15% 41% 23% 7%	300
	11th to 20th	Day	16% 18% 42% 19% 4	300
	21st to end	Day	26% 15% 37% 19% 3	330
January	1st to 10th	Night	12% 14% 43% 23% 7% 1	1000
	11th to 20th	Night	11% 17% 41% 25% 5% 1	1000
	21st to end	Night	15% 15% 40% 24% 5%	1100
February	1st to 10th	Night	13% 15% 44% 24% 5%	1000
	11th to 20th	Night	13% 16% 41% 24% 5% 1	979
	21st to end	Night	14% 14% 42% 23% 7% 1	734
March	1st to 10th	Night	13% 13% 43% 24% 6% 1	642
	11th to 20th	Night	16% 16% 39% 24% 5%	600
	21st to end	Night	24% 11% 30% 26% 8% 1	660
April	1st to 10th	Night	21% 13% 37% 25% 5%	600
	11th to 20th	Night	21% 15% 33% 24% 7% 1	593
	21st to end	Night	23% 12% 32% 24% 9% 1	600
May	1st to 10th	Night	18% 10% 34% 29% 9% 1	600
	11th to 20th	Night	18% 8% 32% 31% 10% 1	591
	21st to end	Night	18% 6% 32% 32% 12% 1	650
June	1st to 10th	Night	9% 6% 29% 38% 17%	600
	11th to 20th	Night	12% 5% 36% 38% 9%	600
	21st to end	Night	9% 4 28% 46% 13%	600
July	1st to 10th	Night	6% 3 25% 43% 22% 2	585
	11th to 20th	Night	5% 3 26% 45% 20% 1	582
	21st to end	Night	5% 2 26% 46% 19% 2	657
August	1st to 10th	Night	9% 2 23% 46% 19% 1	579
	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% 12% 38% 26% 7%	975
	11th to 20th	Night	14% 16% 41% 23% 6% 1	1000
	21st to end	Night	15% 15% 41% 24% 5%	1000
December	1st to 10th	Night	13% 15% 43% 24% 5%	1000
	11th to 20th	Night	19% 13% 40% 22% 5% 1	1000
	21st to end	Night	22% 13% 39% 20% 5% 1	1100

Page generated at 2025-08-02 00:32:22. Version ac07

Copyright © 2025 Attilla Danko.