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 2024-07-02.

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	14% 7% 2 6% 5% 5% 5% 4 6% 7% 40%	89573
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 42%	89547
Transparency	Day	42% 6% 11% 22% 16% 3	89347
	Night	40% 5% 11% 22% 18% 4	89377
Seeing	Day	17% 9% 22% 24% 19% 10%	14230
	Night	17% 8% 22% 29% 20% 4	24290

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	8% 5% 2 8% 6% 7% 7% 6% 7% 10% 33%	2353
	11th to 20th	Day	10% 6% 2 6% 6% 6% 6% 6% 8% 12% 32%	2340
	21st to end	Day	7% 6% 3 7% 7% 6% 7% 6% 9% 10% 31%	2547
August	1st to 10th	Day	9% 5% 2 7% 5% 6% 9% 6% 9% 10% 31%	2327
	11th to 20th	Day	9% 5% 2 7% 7% 6% 7% 5% 7% 10% 34%	2134
	21st to end	Day	13% 6% 2 8% 6% 6% 6% 5% 7% 8% 33%	2376
September	1st to 10th	Day	11% 6% 2 6% 5% 5% 6% 4 5% 8% 41%	2044
	11th to 20th	Day	15% 5% 2 6% 4 4 5% 3 6% 8% 42%	1980
	21st to end	Day	13% 5% 2 5% 4 4 4 3 5% 6% 50%	1980
October	1st to 10th	Day	11% 5% 2 5% 3 4 4 3 3 5% 55%	1980
	11th to 20th	Day	11% 5% 1 4 4 4 3 3 4 6% 54%	1910
	21st to end	Day	14% 4 2 4 3 3 4 3 4 7% 51%	1898
November	1st to 10th	Day	12% 5% 2 5% 3 3 4 3 4 5% 55%	1672
	11th to 20th	Day	13% 5% 2 5% 5% 3 4 3 4 6% 49%	1710
	21st to end	Day	16% 7% 2 5% 3 3 4 3 5% 7% 44%	1710
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% 11% 45%	1710
July	1st to 10th	Night	9% 6% 2 7% 5% 6% 5% 5% 8% 10% 37%	1635
	11th to 20th	Night	8% 5% 2 6% 5% 4 5% 5% 8% 12% 40%	1620
	21st to end	Night	7% 5% 2 6% 6% 5% 5% 5% 9% 11% 38%	1771
August	1st to 10th	Night	6% 4 2 6% 5% 5% 7% 5% 9% 11% 40%	1611
	11th to 20th	Night	10% 5% 2 6% 5% 6% 5% 4 6% 9% 42%	1763
	21st to end	Night	9% 6% 2 6% 5% 3 6% 4 8% 10% 39%	1980
September	1st to 10th	Night	10% 6% 3 8% 5% 4 5% 4 6% 9% 42%	1898
	11th to 20th	Night	12% 5% 2 7% 5% 3 4 3 6% 8% 43%	1980
	21st to end	Night	11% 6% 2 4 3 4 4 3 5% 7% 52%	1980
October	1st to 10th	Night	12% 4 1 4 3 3 3 2 4 7% 57%	1980
	11th to 20th	Night	13% 5% 1 6% 4 3 4 3 4 5% 52%	2048
	21st to end	Night	14% 6% 2 4 3 2 3 3 4 5% 54%	2456
November	1st to 10th	Night	13% 4 1 4 3 3 4 2 5% 6% 54%	2406
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 5% 6% 51%	2470
	21st to end	Night	17% 7% 2 6% 4 4 4 3 5% 6% 44%	2470
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	43% 10% 16% 19% 11% 2	2288
	11th to 20th	Day	40% 9% 20% 17% 10% 4	2271
	21st to end	Day	41% 10% 17% 22% 9% 1	2405
August	1st to 10th	Day	41% 11% 20% 20% 8% 1	2314
	11th to 20th	Day	43% 9% 15% 19% 13% 1	2134
	21st to end	Day	46% 7% 15% 17% 12% 3	2376
September	1st to 10th	Day	39% 7% 14% 18% 16% 5%	2044
	11th to 20th	Day	40% 6% 10% 22% 20% 2	1980
	21st to end	Day	35% 4 11% 22% 21% 7%	1980
October	1st to 10th	Day	32% 5% 9% 22% 25% 8%	1980
	11th to 20th	Day	31% 4 8% 25% 24% 8%	1910
	21st to end	Day	34% 4 6% 24% 27% 5%	1898
November	1st to 10th	Day	32% 4 7% 21% 29% 6%	1672
	11th to 20th	Day	37% 3 5% 23% 25% 6%	1710
	21st to end	Day	41% 3 7% 24% 21% 4	1710
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% 8% 21% 18% 12% 3	1590
	11th to 20th	Night	35% 8% 21% 19% 11% 6%	1566
	21st to end	Night	35% 8% 22% 22% 11% 2	1672
August	1st to 10th	Night	35% 9% 20% 27% 9% 1	1600
	11th to 20th	Night	37% 9% 20% 19% 14% 2	1763
	21st to end	Night	38% 9% 19% 20% 13% 2	1980
September	1st to 10th	Night	38% 7% 17% 20% 14% 4	1898
	11th to 20th	Night	38% 7% 13% 20% 21% 2	1980
	21st to end	Night	31% 6% 10% 23% 23% 7%	1980
October	1st to 10th	Night	29% 4 9% 23% 27% 9%	1980
	11th to 20th	Night	34% 4 8% 22% 23% 9%	2048
	21st to end	Night	35% 3 4 24% 26% 9%	2456
November	1st to 10th	Night	32% 4 7% 24% 24% 9%	2406
	11th to 20th	Night	35% 3 6% 23% 26% 8%	2470
	21st to end	Night	41% 3 6% 23% 22% 4	2470
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	10% 3 11% 23% 27% 26%	449
	11th to 20th	Day	12% 2 6% 25% 32% 24%	444
	21st to end	Day	9% 3 7% 23% 32% 26%	488
August	1st to 10th	Day	13% 3 8% 24% 33% 20%	446
	11th to 20th	Day	11% 4 6% 30% 35% 13%	439
	21st to end	Day	14% 3 9% 21% 33% 21%	495
September	1st to 10th	Day	11% 3 7% 24% 35% 20%	450
	11th to 20th	Day	17% 3 10% 21% 30% 19%	450
	21st to end	Day	15% 4 14% 26% 32% 9%	450
October	1st to 10th	Day	14% 5% 17% 26% 28% 11%	450
	11th to 20th	Day	12% 9% 28% 28% 19% 4	450
	21st to end	Day	20% 8% 21% 29% 17% 4	413
November	1st to 10th	Day	20% 7% 25% 29% 13% 6%	270
	11th to 20th	Day	12% 13% 37% 28% 9% 1	270
	21st to end	Day	16% 10% 38% 23% 12% 1	270
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% 2 7% 26% 41% 12%	530
	11th to 20th	Night	11% 2 5% 27% 40% 16%	522
	21st to end	Night	8% 2 7% 25% 44% 13%	591
August	1st to 10th	Night	8% 2 4 29% 45% 12%	537
	11th to 20th	Night	11% 2 6% 32% 42% 6%	534
	21st to end	Night	12% 2 7% 25% 44% 10%	594
September	1st to 10th	Night	11% 2 9% 31% 38% 9%	638
	11th to 20th	Night	17% 2 10% 27% 36% 7%	720
	21st to end	Night	14% 3 15% 31% 34% 4	720
October	1st to 10th	Night	15% 4 17% 29% 28% 8%	720
	11th to 20th	Night	16% 7% 24% 32% 20% 1	720
	21st to end	Night	16% 8% 21% 37% 16% 2	792
November	1st to 10th	Night	15% 9% 19% 36% 20% 2	740
	11th to 20th	Night	16% 10% 32% 30% 11% 1	760
	21st to end	Night	19% 11% 27% 31% 11% 1	760
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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