Uranias' Slave Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Uranias' Slave Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	13% 7% 2 6% 5% 5% 5% 4 6% 8% 40%	130902
	Night	14% 7% 2 6% 4 4 4 4 6% 7% 42%	129942
Transparency	Day	40% 6% 12% 22% 16% 4	134651
	Night	39% 6% 11% 22% 17% 4	134262
Seeing	Day	17% 9% 23% 23% 19% 9%	16359
	Night	17% 9% 23% 29% 19% 4	26446

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	20% 10% 2 7% 4 3 3 3 4 5% 40%	5258
	11th to 20th	Day	13% 6% 4 6% 4 4 3 3 3 5% 50%	5280
	21st to end	Day	21% 9% 3 7% 3 4 4 3 5% 6% 36%	6059
February	1st to 10th	Day	18% 8% 3 6% 4 4 4 4 5% 6% 37%	2100
	11th to 20th	Day	22% 8% 3 6% 4 4 5% 3 5% 6% 35%	1998
	21st to end	Day	19% 8% 2 7% 5% 3 3 3 4 6% 39%	1650
March	1st to 10th	Day	19% 9% 3 6% 5% 4 5% 3 6% 6% 34%	4480
	11th to 20th	Day	13% 6% 2 5% 4 4 4 4 4 5% 50%	4980
	21st to end	Day	14% 7% 3 6% 5% 4 4 3 5% 6% 43%	5465
April	1st to 10th	Day	12% 6% 2 5% 4 5% 5% 5% 6% 7% 43%	2360
	11th to 20th	Day	13% 6% 2 6% 4 4 3 3 5% 6% 46%	2374
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 4 6% 8% 42%	2534
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 5% 7% 9% 31%	5862
	11th to 20th	Day	16% 9% 3 9% 5% 5% 6% 4 6% 7% 30%	5861
	21st to end	Day	15% 7% 2 7% 4 4 5% 4 6% 7% 39%	6433
June	1st to 10th	Day	9% 5% 2 6% 5% 6% 5% 5% 8% 10% 39%	2600
	11th to 20th	Day	5% 3 2 5% 5% 5% 6% 5% 9% 11% 43%	2600
	21st to end	Day	6% 4 2 6% 6% 5% 7% 6% 8% 11% 40%	2600
July	1st to 10th	Day	5% 4 2 7% 5% 6% 6% 5% 8% 12% 38%	5900
	11th to 20th	Day	6% 5% 2 6% 5% 6% 6% 6% 9% 12% 35%	5900
	21st to end	Day	11% 9% 3 9% 7% 7% 7% 5% 8% 10% 24%	6424
August	1st to 10th	Day	6% 4 2 6% 4 5% 7% 5% 9% 10% 43%	4234
	11th to 20th	Day	7% 4 2 6% 5% 6% 6% 5% 7% 11% 42%	3858
	21st to end	Day	8% 4 2 6% 4 5% 5% 4 6% 7% 50%	4312
September	1st to 10th	Day	10% 6% 2 6% 5% 5% 6% 5% 5% 8% 42%	2270
	11th to 20th	Day	14% 5% 2 6% 4 4 5% 3 6% 8% 42%	2200
	21st to end	Day	15% 5% 2 5% 4 4 4 3 5% 6% 48%	2200
October	1st to 10th	Day	11% 5% 2 5% 3 4 4 3 4 5% 55%	2200
	11th to 20th	Day	11% 5% 1 4 3 4 3 3 4 6% 56%	2122
	21st to end	Day	15% 5% 2 4 3 3 4 3 4 6% 51%	2108
November	1st to 10th	Day	12% 5% 2 5% 3 4 4 3 4 5% 53%	1852
	11th to 20th	Day	13% 5% 2 6% 4 3 4 3 5% 6% 49%	1890
	21st to end	Day	15% 7% 2 5% 4 4 4 4 5% 7% 43%	1890
December	1st to 10th	Day	17% 7% 2 6% 4 4 3 3 4 5% 44%	3020
	11th to 20th	Day	19% 10% 4 8% 5% 5% 4 3 5% 4 33%	3463
	21st to end	Day	19% 9% 3 6% 4 4 4 4 5% 7% 35%	4565
January	1st to 10th	Night	17% 8% 3 6% 4 4 3 3 4 5% 43%	7635
	11th to 20th	Night	15% 8% 3 6% 3 4 3 3 5% 5% 46%	7680
	21st to end	Night	18% 10% 3 6% 4 4 4 4 4 5% 37%	8106
February	1st to 10th	Night	15% 8% 3 7% 5% 4 4 3 5% 7% 40%	2520
	11th to 20th	Night	23% 8% 3 6% 4 4 4 3 3 4 37%	2399
	21st to end	Night	23% 8% 2 6% 4 3 4 4 5% 6% 36%	1980
March	1st to 10th	Night	22% 10% 3 6% 4 4 4 3 5% 6% 33%	5358
	11th to 20th	Night	11% 6% 2 5% 3 4 4 4 5% 7% 49%	4980
	21st to end	Night	15% 8% 3 7% 4 5% 4 3 4 5% 42%	5467
April	1st to 10th	Night	13% 5% 2 5% 4 3 4 3 5% 8% 49%	2055
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 4 7% 50%	1975
	21st to end	Night	13% 5% 2 4 4 4 4 4 5% 7% 49%	1865
May	1st to 10th	Night	13% 7% 3 9% 5% 6% 6% 4 6% 8% 33%	4033
	11th to 20th	Night	19% 8% 3 8% 5% 5% 5% 4 6% 7% 30%	4032
	21st to end	Night	14% 6% 3 7% 4 4 5% 4 7% 8% 39%	4421
June	1st to 10th	Night	7% 5% 3 7% 5% 6% 6% 5% 7% 10% 41%	1800
	11th to 20th	Night	4 3 2 5% 4 4 4 4 9% 12% 49%	1800
	21st to end	Night	6% 3 2 5% 4 5% 6% 6% 7% 11% 45%	1800
July	1st to 10th	Night	7% 5% 2 7% 5% 5% 6% 5% 9% 11% 37%	4058
	11th to 20th	Night	7% 5% 2 6% 5% 5% 6% 5% 8% 13% 38%	4057
	21st to end	Night	10% 8% 3 9% 6% 6% 6% 5% 9% 11% 27%	4427
August	1st to 10th	Night	5% 3 2 6% 5% 5% 6% 4 8% 10% 46%	2915
	11th to 20th	Night	6% 4 2 5% 4 5% 5% 4 7% 9% 50%	3186
	21st to end	Night	6% 4 1 5% 4 3 5% 4 7% 9% 53%	3586
September	1st to 10th	Night	10% 6% 3 8% 5% 4 5% 4 6% 9% 42%	2110
	11th to 20th	Night	12% 5% 2 7% 5% 4 4 3 6% 8% 43%	2200
	21st to end	Night	13% 6% 2 4 3 4 4 3 5% 7% 49%	2200
October	1st to 10th	Night	12% 4 1 5% 3 3 3 2 4 7% 56%	2200
	11th to 20th	Night	11% 5% 1 6% 4 3 4 3 4 5% 55%	2276
	21st to end	Night	14% 6% 2 4 3 2 3 3 4 5% 54%	2730
November	1st to 10th	Night	14% 5% 2 5% 3 3 4 3 5% 6% 52%	2666
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 51%	2730
	21st to end	Night	16% 7% 2 6% 4 4 4 3 5% 6% 44%	2730
December	1st to 10th	Night	17% 9% 3 6% 3 4 5% 3 4 5% 40%	4380
	11th to 20th	Night	21% 8% 3 5% 4 3 4 4 5% 5% 38%	4952
	21st to end	Night	21% 8% 2 6% 4 4 5% 4 6% 7% 33%	6633

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	46% 5% 8% 26% 15% 1	5259
	11th to 20th	Day	36% 4 6% 20% 29% 5%	5280
	21st to end	Day	49% 4 7% 23% 14% 4	6059
February	1st to 10th	Day	47% 4 8% 22% 16% 3	2100
	11th to 20th	Day	50% 5% 7% 19% 16% 3	2008
	21st to end	Day	48% 3 7% 17% 19% 6%	1650
March	1st to 10th	Day	48% 5% 9% 13% 17% 8%	4480
	11th to 20th	Day	36% 4 7% 19% 23% 11%	4980
	21st to end	Day	42% 3 8% 30% 15% 2	5465
April	1st to 10th	Day	38% 6% 12% 24% 17% 3	2360
	11th to 20th	Day	38% 4 8% 21% 23% 6%	2374
	21st to end	Day	39% 5% 11% 28% 15% 2	2534
May	1st to 10th	Day	47% 8% 13% 20% 10% 1	5897
	11th to 20th	Day	51% 8% 14% 18% 8% 1	5865
	21st to end	Day	43% 6% 9% 22% 17% 3	6447
June	1st to 10th	Day	37% 6% 17% 24% 13% 2	2600
	11th to 20th	Day	31% 8% 19% 26% 14% 2	2600
	21st to end	Day	34% 10% 20% 19% 14% 4	2600
July	1st to 10th	Day	34% 11% 19% 22% 12% 1	5835
	11th to 20th	Day	35% 13% 27% 18% 5% 1	5831
	21st to end	Day	50% 12% 17% 16% 4	6240
August	1st to 10th	Day	31% 9% 17% 33% 10% 1	4221
	11th to 20th	Day	32% 8% 18% 26% 14% 1	3894
	21st to end	Day	32% 7% 12% 31% 17% 2	4311
September	1st to 10th	Day	38% 7% 14% 19% 17% 6%	2270
	11th to 20th	Day	39% 6% 11% 22% 20% 2	2200
	21st to end	Day	37% 5% 12% 21% 20% 6%	2200
October	1st to 10th	Day	24% 5% 7% 19% 34% 11%	3590
	11th to 20th	Day	21% 3 5% 25% 33% 13%	3440
	21st to end	Day	28% 4 8% 26% 26% 8%	3384
November	1st to 10th	Day	34% 4 8% 21% 27% 6%	1852
	11th to 20th	Day	36% 3 6% 24% 25% 5%	1890
	21st to end	Day	41% 4 8% 24% 20% 4	1890
December	1st to 10th	Day	42% 3 6% 24% 22% 2	3020
	11th to 20th	Day	52% 4 8% 20% 12% 3	3460
	21st to end	Day	45% 5% 12% 27% 9% 2	4565
January	1st to 10th	Night	42% 5% 13% 24% 15% 1	7636
	11th to 20th	Night	39% 4 6% 20% 27% 4	7680
	21st to end	Night	48% 4 8% 20% 15% 6%	8106
February	1st to 10th	Night	44% 4 10% 22% 18% 3	2520
	11th to 20th	Night	51% 3 7% 19% 15% 5%	2400
	21st to end	Night	49% 4 6% 18% 18% 5%	1980
March	1st to 10th	Night	50% 5% 7% 15% 18% 4	5358
	11th to 20th	Night	33% 5% 7% 20% 22% 13%	4980
	21st to end	Night	44% 4 7% 25% 17% 3	5467
April	1st to 10th	Night	34% 4 12% 24% 22% 4	2055
	11th to 20th	Night	34% 3 8% 25% 22% 8%	1975
	21st to end	Night	35% 5% 13% 27% 19% 2	1865
May	1st to 10th	Night	47% 7% 15% 19% 10% 1	4060
	11th to 20th	Night	52% 8% 14% 18% 8%	4033
	21st to end	Night	40% 8% 14% 23% 14% 1	4429
June	1st to 10th	Night	37% 8% 16% 23% 14% 2	1800
	11th to 20th	Night	25% 8% 22% 28% 16% 2	1800
	21st to end	Night	30% 10% 20% 22% 14% 5%	1800
July	1st to 10th	Night	37% 11% 20% 22% 9% 1	4015
	11th to 20th	Night	34% 11% 28% 20% 5% 2	4006
	21st to end	Night	47% 11% 21% 15% 5% 1	4297
August	1st to 10th	Night	29% 8% 16% 32% 13% 1	2904
	11th to 20th	Night	28% 9% 22% 26% 14% 1	3216
	21st to end	Night	26% 7% 16% 31% 18% 1	3586
September	1st to 10th	Night	38% 7% 16% 20% 14% 5%	2110
	11th to 20th	Night	37% 8% 12% 21% 20% 1	2200
	21st to end	Night	34% 6% 11% 22% 21% 6%	2200
October	1st to 10th	Night	21% 5% 8% 22% 29% 14%	3590
	11th to 20th	Night	20% 3 5% 21% 35% 16%	3697
	21st to end	Night	25% 3 6% 28% 28% 11%	4404
November	1st to 10th	Night	33% 4 8% 24% 23% 8%	2666
	11th to 20th	Night	35% 3 6% 22% 26% 8%	2730
	21st to end	Night	42% 3 7% 23% 22% 4	2730
December	1st to 10th	Night	43% 4 8% 21% 22% 2	4380
	11th to 20th	Night	46% 4 9% 21% 18% 2	4954
	21st to end	Night	47% 6% 10% 25% 11% 1	6633

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	28% 23% 27% 19% 3 1	330
	11th to 20th	Day	26% 22% 33% 12% 5% 1	330
	21st to end	Day	25% 14% 45% 13% 2	363
February	1st to 10th	Day	25% 18% 36% 16% 5% 1	330
	11th to 20th	Day	38% 20% 28% 12% 2 1	315
	21st to end	Day	32% 18% 37% 10% 2	249
March	1st to 10th	Day	26% 19% 33% 15% 5% 1	300
	11th to 20th	Day	24% 18% 30% 21% 7% 1	500
	21st to end	Day	21% 15% 37% 21% 6% 1	550
April	1st to 10th	Day	19% 15% 32% 21% 10% 3	500
	11th to 20th	Day	16% 15% 33% 22% 12% 2	492
	21st to end	Day	16% 11% 32% 25% 13% 3	500
May	1st to 10th	Day	14% 12% 32% 23% 14% 5%	503
	11th to 20th	Day	15% 7% 24% 25% 20% 10%	527
	21st to end	Day	15% 6% 19% 24% 21% 14%	573
June	1st to 10th	Day	11% 5% 18% 29% 24% 13%	530
	11th to 20th	Day	7% 3 14% 30% 28% 18%	530
	21st to end	Day	8% 3 14% 30% 26% 20%	530
July	1st to 10th	Day	9% 3 13% 24% 28% 23%	525
	11th to 20th	Day	10% 3 6% 25% 32% 23%	524
	21st to end	Day	10% 4 7% 24% 30% 25%	576
August	1st to 10th	Day	12% 4 9% 24% 33% 19%	526
	11th to 20th	Day	11% 5% 9% 29% 32% 14%	519
	21st to end	Day	13% 3 9% 22% 32% 21%	583
September	1st to 10th	Day	11% 3 9% 25% 34% 18%	530
	11th to 20th	Day	16% 3 10% 21% 31% 18%	530
	21st to end	Day	17% 5% 14% 26% 30% 8%	530
October	1st to 10th	Day	13% 5% 18% 26% 28% 11%	530
	11th to 20th	Day	11% 8% 28% 30% 20% 3	530
	21st to end	Day	20% 8% 22% 29% 18% 3	491
November	1st to 10th	Day	21% 8% 26% 28% 12% 5%	330
	11th to 20th	Day	12% 14% 38% 27% 9% 1	330
	21st to end	Day	16% 12% 38% 22% 10% 1	330
December	1st to 10th	Day	31% 14% 32% 18% 5% 1	330
	11th to 20th	Day	23% 15% 35% 19% 7% 1	330
	21st to end	Day	27% 16% 31% 19% 6% 1	363
January	1st to 10th	Night	24% 17% 33% 20% 5%	840
	11th to 20th	Night	21% 17% 32% 24% 5%	840
	21st to end	Night	21% 15% 35% 27% 2	924
February	1st to 10th	Night	20% 14% 34% 28% 5%	840
	11th to 20th	Night	28% 18% 31% 21% 2	804
	21st to end	Night	28% 15% 33% 21% 2	660
March	1st to 10th	Night	20% 13% 32% 30% 5%	800
	11th to 20th	Night	20% 12% 32% 27% 8% 1	800
	21st to end	Night	20% 11% 33% 29% 8%	880
April	1st to 10th	Night	16% 11% 31% 30% 11%	655
	11th to 20th	Night	16% 9% 33% 31% 11%	593
	21st to end	Night	15% 7% 27% 37% 13% 1	600
May	1st to 10th	Night	11% 6% 27% 35% 19% 2	600
	11th to 20th	Night	16% 5% 20% 31% 24% 5%	591
	21st to end	Night	12% 4 13% 33% 33% 5%	650
June	1st to 10th	Night	9% 3 13% 37% 32% 6%	600
	11th to 20th	Night	5% 2 11% 35% 35% 12%	600
	21st to end	Night	7% 2 8% 31% 41% 11%	600
July	1st to 10th	Night	12% 3 7% 26% 41% 12%	585
	11th to 20th	Night	10% 2 5% 27% 41% 16%	582
	21st to end	Night	9% 2 8% 26% 42% 14%	657
August	1st to 10th	Night	8% 3 5% 30% 43% 11%	597
	11th to 20th	Night	10% 2 9% 31% 41% 7%	594
	21st to end	Night	12% 2 8% 26% 42% 11%	660
September	1st to 10th	Night	11% 3 10% 32% 36% 9%	710
	11th to 20th	Night	16% 3 10% 28% 37% 7%	800
	21st to end	Night	15% 3 15% 31% 33% 3	800
October	1st to 10th	Night	14% 4 17% 30% 29% 8%	800
	11th to 20th	Night	14% 7% 23% 35% 20% 1	800
	21st to end	Night	16% 8% 21% 37% 16% 2	880
November	1st to 10th	Night	15% 9% 20% 36% 18% 2	820
	11th to 20th	Night	16% 10% 31% 31% 11% 1	840
	21st to end	Night	19% 12% 28% 29% 11% 1	840
December	1st to 10th	Night	26% 14% 26% 27% 8%	840
	11th to 20th	Night	22% 9% 32% 29% 8%	840
	21st to end	Night	24% 13% 27% 29% 6%	924

Page generated at 2026-02-13 01:42:17. Version ac07

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