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 2024-11-30.

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%	101145
	Night	14% 7% 2 6% 4 4 4 4 6% 7% 43%	98398
Transparency	Day	40% 6% 11% 23% 17% 4	104938
	Night	38% 5% 11% 23% 18% 4	102743
Seeing	Day	17% 9% 22% 24% 19% 10%	14802
	Night	17% 8% 22% 29% 20% 4	24831

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	17% 12% 2 7% 4 3 3 4 5% 6% 36%	2836
	11th to 20th	Day	21% 8% 4 7% 5% 3 4 4 3 6% 37%	2840
	21st to end	Day	24% 10% 2 5% 3 3 3 3 4 6% 37%	3264
February	1st to 10th	Day	19% 9% 3 6% 4 4 4 4 5% 6% 36%	1900
	11th to 20th	Day	21% 8% 3 6% 4 4 5% 3 4 6% 35%	1848
	21st to end	Day	20% 9% 2 7% 5% 3 3 3 4 6% 37%	1570
March	1st to 10th	Day	25% 10% 3 6% 4 4 4 3 5% 5% 31%	3120
	11th to 20th	Day	14% 7% 2 5% 4 3 3 3 4 5% 49%	3480
	21st to end	Day	15% 8% 3 7% 6% 4 5% 3 6% 6% 38%	3815
April	1st to 10th	Day	12% 6% 2 5% 5% 5% 5% 5% 6% 7% 43%	2242
	11th to 20th	Day	14% 6% 2 6% 4 4 3 3 5% 6% 45%	2254
	21st to end	Day	12% 5% 2 6% 4 5% 5% 4 6% 8% 44%	2407
May	1st to 10th	Day	12% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	4082
	11th to 20th	Day	16% 7% 3 7% 4 5% 4 4 6% 8% 36%	4081
	21st to end	Day	10% 5% 2 7% 4 5% 5% 4 7% 9% 42%	4476
June	1st to 10th	Day	9% 5% 2 6% 5% 6% 5% 5% 8% 10% 38%	2470
	11th to 20th	Day	5% 3 2 5% 5% 5% 6% 5% 9% 10% 44%	2470
	21st to end	Day	7% 4 2 6% 6% 5% 7% 6% 8% 10% 40%	2470
July	1st to 10th	Day	6% 4 2 7% 6% 7% 7% 6% 9% 13% 33%	4120
	11th to 20th	Day	8% 6% 2 6% 5% 6% 6% 5% 8% 11% 37%	4120
	21st to end	Day	14% 11% 4 10% 7% 7% 7% 5% 8% 7% 19%	4466
August	1st to 10th	Day	5% 4 2 5% 4 5% 7% 5% 8% 10% 44%	4104
	11th to 20th	Day	7% 4 2 6% 6% 6% 6% 5% 7% 11% 42%	3738
	21st to end	Day	8% 4 2 6% 4 5% 5% 4 5% 7% 50%	4180
September	1st to 10th	Day	10% 6% 2 6% 5% 5% 6% 5% 5% 8% 41%	2157
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 3 6% 7% 41%	2090
	21st to end	Day	15% 5% 2 5% 4 4 4 3 5% 6% 48%	2090
October	1st to 10th	Day	11% 5% 2 5% 3 3 4 3 3 5% 56%	2090
	11th to 20th	Day	11% 5% 1 4 4 4 3 3 4 6% 55%	2016
	21st to end	Day	14% 5% 1 4 3 3 4 3 4 7% 51%	2003
November	1st to 10th	Day	12% 5% 2 5% 3 3 4 3 4 5% 54%	1762
	11th to 20th	Day	13% 5% 2 6% 4 3 4 3 5% 6% 48%	1800
	21st to end	Day	15% 7% 2 5% 3 4 4 4 5% 7% 44%	1797
December	1st to 10th	Day	22% 7% 3 7% 5% 5% 3 3 4 5% 35%	1710
	11th to 20th	Day	14% 8% 4 7% 4 5% 4 4 6% 5% 39%	2153
	21st to end	Day	19% 10% 3 7% 4 4 4 4 6% 7% 33%	3124
January	1st to 10th	Night	18% 9% 3 5% 4 4 4 3 5% 6% 39%	4109
	11th to 20th	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	4120
	21st to end	Night	23% 12% 2 6% 4 3 3 3 4 5% 35%	4376
February	1st to 10th	Night	16% 8% 2 7% 4 4 4 3 5% 6% 41%	2280
	11th to 20th	Night	21% 8% 3 6% 5% 4 4 3 3 4 38%	2218
	21st to end	Night	24% 8% 2 6% 5% 4 4 4 5% 5% 35%	1884
March	1st to 10th	Night	25% 9% 3 6% 4 4 4 3 5% 6% 32%	3718
	11th to 20th	Night	11% 6% 2 5% 4 3 4 4 5% 8% 48%	3480
	21st to end	Night	16% 8% 3 7% 5% 5% 4 3 4 5% 40%	3817
April	1st to 10th	Night	12% 5% 2 4 4 3 4 3 5% 8% 50%	1952
	11th to 20th	Night	12% 6% 2 5% 3 3 3 3 5% 8% 50%	1875
	21st to end	Night	12% 5% 2 4 4 4 4 4 5% 7% 49%	1772
May	1st to 10th	Night	12% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	2813
	11th to 20th	Night	16% 6% 2 6% 5% 4 4 4 6% 7% 39%	2812
	21st to end	Night	8% 5% 2 6% 4 4 5% 5% 8% 10% 43%	3079
June	1st to 10th	Night	8% 5% 3 7% 5% 5% 6% 5% 7% 10% 40%	1710
	11th to 20th	Night	4 3 1 4 4 3 4 5% 9% 12% 51%	1710
	21st to end	Night	6% 3 2 5% 4 5% 6% 6% 7% 12% 44%	1710
July	1st to 10th	Night	8% 5% 2 8% 6% 6% 6% 5% 9% 10% 35%	2840
	11th to 20th	Night	9% 5% 2 5% 5% 4 5% 4 8% 11% 42%	2840
	21st to end	Night	13% 9% 4 9% 7% 6% 5% 5% 8% 8% 25%	3087
August	1st to 10th	Night	4 3 2 6% 5% 5% 5% 4 8% 10% 47%	2825
	11th to 20th	Night	6% 4 2 5% 4 5% 5% 4 6% 9% 50%	3087
	21st to end	Night	6% 4 1 5% 4 3 5% 4 7% 9% 53%	3476
September	1st to 10th	Night	10% 6% 3 8% 5% 4 4 3 6% 9% 42%	2004
	11th to 20th	Night	13% 5% 2 7% 5% 4 4 3 6% 8% 42%	2090
	21st to end	Night	12% 6% 2 4 3 4 4 3 5% 7% 50%	2090
October	1st to 10th	Night	12% 4 1 4 3 3 3 2 4 7% 58%	2090
	11th to 20th	Night	12% 5% 1 6% 4 3 4 3 4 5% 54%	2162
	21st to end	Night	14% 6% 2 4 3 2 3 3 4 5% 54%	2593
November	1st to 10th	Night	14% 4 1 5% 3 3 4 2 5% 6% 52%	2536
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 51%	2600
	21st to end	Night	16% 7% 2 6% 4 4 3 3 5% 6% 44%	2599
December	1st to 10th	Night	20% 8% 2 7% 4 5% 5% 3 5% 5% 36%	2470
	11th to 20th	Night	16% 7% 3 5% 4 3 4 3 5% 6% 45%	3042
	21st to end	Night	20% 8% 2 7% 5% 5% 5% 5% 6% 8% 30%	4532

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	47% 5% 6% 22% 18% 2	2837
	11th to 20th	Day	50% 5% 7% 19% 19% 1	2840
	21st to end	Day	49% 4 7% 24% 15% 1	3264
February	1st to 10th	Day	48% 4 8% 21% 16% 3	1900
	11th to 20th	Day	50% 4 7% 19% 17% 4	1858
	21st to end	Day	50% 3 8% 16% 18% 6%	1570
March	1st to 10th	Day	54% 5% 8% 13% 13% 8%	3120
	11th to 20th	Day	38% 4 8% 23% 22% 6%	3480
	21st to end	Day	46% 4 9% 27% 12% 2	3815
April	1st to 10th	Day	39% 5% 11% 23% 18% 4	2242
	11th to 20th	Day	39% 4 7% 20% 23% 6%	2254
	21st to end	Day	38% 5% 10% 29% 16% 2	2407
May	1st to 10th	Day	43% 7% 12% 23% 14% 2	4120
	11th to 20th	Day	45% 7% 13% 23% 11% 1	4085
	21st to end	Day	37% 7% 11% 23% 20% 2	4489
June	1st to 10th	Day	38% 6% 17% 24% 13% 2	2470
	11th to 20th	Day	30% 8% 19% 27% 15% 2	2470
	21st to end	Day	35% 9% 20% 19% 14% 4	2470
July	1st to 10th	Day	38% 12% 21% 19% 9% 1	4055
	11th to 20th	Day	37% 10% 23% 21% 7% 2	4051
	21st to end	Day	59% 9% 12% 14% 5%	4324
August	1st to 10th	Day	30% 9% 17% 33% 10% 1	4091
	11th to 20th	Day	32% 8% 18% 26% 14% 1	3774
	21st to end	Day	32% 7% 12% 31% 16% 2	4179
September	1st to 10th	Day	39% 7% 14% 18% 17% 6%	2157
	11th to 20th	Day	40% 6% 10% 21% 20% 2	2090
	21st to end	Day	36% 4 12% 21% 20% 7%	2090
October	1st to 10th	Day	23% 5% 7% 19% 35% 11%	3480
	11th to 20th	Day	21% 3 5% 25% 33% 13%	3334
	21st to end	Day	27% 5% 8% 26% 26% 8%	3279
November	1st to 10th	Day	33% 4 7% 21% 28% 6%	1762
	11th to 20th	Day	37% 3 5% 23% 25% 6%	1800
	21st to end	Day	41% 4 7% 24% 20% 4	1797
December	1st to 10th	Day	52% 3 6% 21% 14% 3	1710
	11th to 20th	Day	46% 4 9% 22% 17% 3	2150
	21st to end	Day	48% 5% 8% 25% 11% 2	3124
January	1st to 10th	Night	46% 3 7% 24% 17% 2	4109
	11th to 20th	Night	45% 4 7% 20% 18% 5%	4120
	21st to end	Night	52% 4 6% 20% 17% 2	4376
February	1st to 10th	Night	45% 4 9% 21% 18% 3	2280
	11th to 20th	Night	51% 3 7% 18% 16% 5%	2219
	21st to end	Night	50% 4 7% 17% 17% 5%	1884
March	1st to 10th	Night	52% 5% 9% 14% 15% 4	3718
	11th to 20th	Night	34% 5% 8% 24% 19% 10%	3480
	21st to end	Night	46% 4 7% 23% 18% 2	3817
April	1st to 10th	Night	34% 4 11% 25% 22% 4	1952
	11th to 20th	Night	34% 3 8% 25% 22% 9%	1875
	21st to end	Night	34% 5% 13% 27% 19% 2	1772
May	1st to 10th	Night	43% 6% 14% 23% 13% 1	2840
	11th to 20th	Night	42% 7% 15% 24% 12%	2813
	21st to end	Night	32% 8% 17% 25% 16% 1	3087
June	1st to 10th	Night	38% 7% 16% 23% 14% 2	1710
	11th to 20th	Night	23% 7% 22% 29% 16% 2	1710
	21st to end	Night	30% 9% 20% 22% 14% 5%	1710
July	1st to 10th	Night	39% 11% 21% 20% 7% 2	2795
	11th to 20th	Night	33% 8% 25% 24% 6% 3	2786
	21st to end	Night	55% 9% 15% 15% 6% 1	2988
August	1st to 10th	Night	28% 8% 17% 33% 13% 1	2814
	11th to 20th	Night	28% 8% 22% 26% 14% 1	3117
	21st to end	Night	26% 7% 16% 31% 18% 1	3476
September	1st to 10th	Night	38% 6% 16% 20% 14% 5%	2004
	11th to 20th	Night	39% 7% 12% 20% 20% 2	2090
	21st to end	Night	33% 6% 11% 22% 22% 6%	2090
October	1st to 10th	Night	21% 5% 8% 22% 30% 15%	3480
	11th to 20th	Night	20% 3 5% 21% 34% 16%	3583
	21st to end	Night	24% 3 6% 28% 28% 11%	4267
November	1st to 10th	Night	33% 4 8% 24% 23% 9%	2536
	11th to 20th	Night	35% 3 6% 22% 26% 8%	2600
	21st to end	Night	41% 3 7% 23% 22% 4	2599
December	1st to 10th	Night	50% 4 8% 20% 16% 3	2470
	11th to 20th	Night	41% 4 9% 21% 24% 2	3044
	21st to end	Night	49% 7% 9% 22% 12% 1	4532

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	26% 23% 27% 20% 3 1	270
	11th to 20th	Day	28% 21% 30% 13% 6% 1	270
	21st to end	Day	24% 13% 44% 14% 3	297
February	1st to 10th	Day	27% 17% 33% 17% 5% 1	270
	11th to 20th	Day	36% 18% 31% 13% 2 1	270
	21st to end	Day	34% 17% 38% 9% 1	225
March	1st to 10th	Day	26% 17% 34% 16% 6% 1	270
	11th to 20th	Day	23% 18% 30% 22% 7% 1	460
	21st to end	Day	21% 13% 37% 21% 6% 1	506
April	1st to 10th	Day	19% 14% 31% 22% 11% 3	460
	11th to 20th	Day	16% 15% 33% 22% 12% 2	452
	21st to end	Day	17% 10% 32% 26% 12% 4	460
May	1st to 10th	Day	13% 11% 32% 24% 15% 6%	463
	11th to 20th	Day	16% 5% 22% 26% 21% 10%	487
	21st to end	Day	14% 5% 18% 25% 22% 15%	529
June	1st to 10th	Day	12% 4 17% 29% 24% 13%	490
	11th to 20th	Day	7% 2 12% 31% 28% 19%	490
	21st to end	Day	9% 3 14% 31% 25% 18%	490
July	1st to 10th	Day	9% 3 13% 23% 28% 24%	485
	11th to 20th	Day	11% 3 6% 26% 31% 23%	484
	21st to end	Day	10% 4 8% 24% 30% 24%	532
August	1st to 10th	Day	12% 3 9% 24% 33% 18%	486
	11th to 20th	Day	10% 5% 9% 31% 33% 12%	479
	21st to end	Day	13% 3 8% 20% 33% 23%	539
September	1st to 10th	Day	10% 3 8% 25% 34% 19%	490
	11th to 20th	Day	17% 3 10% 21% 30% 18%	490
	21st to end	Day	18% 4 13% 26% 31% 9%	490
October	1st to 10th	Day	12% 4 19% 27% 27% 11%	490
	11th to 20th	Day	11% 9% 28% 29% 20% 4	490
	21st to end	Day	19% 8% 21% 30% 19% 4	452
November	1st to 10th	Day	21% 7% 26% 29% 13% 5%	300
	11th to 20th	Day	13% 13% 37% 27% 9% 1	300
	21st to end	Day	16% 11% 37% 23% 11% 1	299
December	1st to 10th	Day	31% 12% 30% 20% 6% 1	270
	11th to 20th	Day	21% 15% 35% 20% 8% 1	270
	21st to end	Day	28% 15% 33% 18% 5% 1	297
January	1st to 10th	Night	24% 17% 33% 20% 5%	760
	11th to 20th	Night	22% 17% 31% 25% 6%	760
	21st to end	Night	22% 14% 34% 28% 2	836
February	1st to 10th	Night	20% 13% 34% 27% 5%	760
	11th to 20th	Night	27% 17% 32% 22% 2	743
	21st to end	Night	29% 15% 33% 19% 3	628
March	1st to 10th	Night	20% 12% 33% 30% 5%	760
	11th to 20th	Night	20% 12% 33% 26% 8% 1	760
	21st to end	Night	20% 11% 33% 28% 8%	836
April	1st to 10th	Night	16% 10% 32% 30% 12%	622
	11th to 20th	Night	16% 9% 32% 31% 12%	563
	21st to end	Night	15% 7% 27% 37% 13% 1	570
May	1st to 10th	Night	11% 6% 26% 35% 20% 2	570
	11th to 20th	Night	16% 4 19% 32% 25% 5%	561
	21st to end	Night	10% 3 12% 34% 35% 6%	617
June	1st to 10th	Night	9% 3 13% 36% 32% 6%	570
	11th to 20th	Night	5% 2 10% 36% 35% 12%	570
	21st to end	Night	7% 2 9% 33% 39% 10%	570
July	1st to 10th	Night	12% 3 7% 27% 40% 11%	555
	11th to 20th	Night	11% 2 5% 27% 41% 15%	552
	21st to end	Night	9% 2 8% 26% 42% 13%	624
August	1st to 10th	Night	7% 2 4 30% 44% 12%	567
	11th to 20th	Night	11% 2 9% 32% 41% 6%	564
	21st to end	Night	12% 2 7% 25% 44% 11%	627
September	1st to 10th	Night	11% 3 10% 31% 37% 9%	674
	11th to 20th	Night	17% 3 10% 28% 36% 7%	760
	21st to end	Night	15% 3 15% 31% 33% 3	760
October	1st to 10th	Night	14% 4 17% 29% 28% 8%	760
	11th to 20th	Night	15% 7% 24% 33% 20% 1	760
	21st to end	Night	15% 8% 20% 37% 16% 2	836
November	1st to 10th	Night	15% 9% 20% 35% 19% 2	780
	11th to 20th	Night	17% 10% 31% 30% 11% 1	800
	21st to end	Night	19% 11% 28% 31% 11% 1	800
December	1st to 10th	Night	26% 13% 25% 28% 8%	760
	11th to 20th	Night	21% 9% 33% 28% 9%	760
	21st to end	Night	24% 13% 27% 30% 6%	836

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