Stillwaters Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stillwaters Observatory clear sky chart from 2005-11-06 to 2024-01-12.

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% 9% 4 7% 6% 3 5% 5% 5% 7% 35%	74856
	Night	16% 9% 4 7% 6% 3 4 4 5% 7% 36%	74877
Transparency	Day	48% 6% 12% 20% 12% 2	74562
	Night	49% 6% 11% 20% 13% 2	74692
Seeing	Day	17% 11% 27% 23% 15% 6%	14498
	Night	19% 9% 29% 29% 13% 1	24453

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% 14% 7% 12% 8% 3 4 4 5% 6% 19%	1517
	11th to 20th	Day	16% 18% 8% 10% 7% 4 5% 4 4 5% 18%	1453
	21st to end	Day	15% 18% 9% 11% 7% 4 5% 4 4 6% 17%	1638
February	1st to 10th	Day	17% 16% 6% 10% 8% 4 5% 4 4 5% 20%	1724
	11th to 20th	Day	17% 11% 6% 11% 7% 4 5% 5% 4 6% 23%	1748
	21st to end	Day	17% 15% 7% 9% 6% 3 4 4 5% 5% 25%	1480
March	1st to 10th	Day	18% 12% 5% 8% 7% 4 4 4 4 6% 28%	1800
	11th to 20th	Day	17% 10% 4 6% 7% 4 5% 4 6% 7% 30%	1931
	21st to end	Day	24% 11% 4 8% 7% 4 5% 4 4 7% 24%	2239
April	1st to 10th	Day	17% 11% 4 7% 6% 3 5% 4 5% 7% 31%	2160
	11th to 20th	Day	18% 9% 3 6% 6% 3 5% 4 5% 6% 35%	2160
	21st to end	Day	19% 9% 4 6% 5% 3 5% 4 5% 8% 32%	2340
May	1st to 10th	Day	18% 8% 4 6% 6% 4 5% 5% 5% 7% 33%	2340
	11th to 20th	Day	20% 8% 3 6% 6% 3 5% 4 5% 6% 34%	2301
	21st to end	Day	11% 5% 3 5% 5% 4 6% 6% 7% 8% 39%	2617
June	1st to 10th	Day	11% 5% 2 5% 6% 3 6% 5% 6% 7% 43%	2520
	11th to 20th	Day	10% 5% 3 6% 5% 3 5% 6% 6% 9% 44%	2520
	21st to end	Day	11% 4 3 6% 5% 4 5% 6% 7% 9% 40%	2520
July	1st to 10th	Day	8% 5% 2 5% 6% 3 5% 5% 6% 9% 47%	2520
	11th to 20th	Day	8% 4 2 5% 4 3 6% 6% 6% 11% 45%	2520
	21st to end	Day	7% 4 3 5% 5% 3 6% 6% 7% 10% 43%	2685
August	1st to 10th	Day	7% 4 2 5% 6% 4 5% 6% 7% 10% 46%	2327
	11th to 20th	Day	9% 4 2 5% 6% 3 5% 5% 7% 10% 43%	2292
	21st to end	Day	9% 4 2 5% 5% 3 5% 6% 6% 10% 46%	2376
September	1st to 10th	Day	11% 5% 3 5% 6% 4 5% 5% 5% 7% 44%	2116
	11th to 20th	Day	10% 4 2 4 4 3 5% 5% 6% 9% 47%	1980
	21st to end	Day	12% 5% 2 6% 5% 3 4 5% 5% 6% 46%	1980
October	1st to 10th	Day	14% 5% 3 5% 5% 3 5% 5% 6% 7% 43%	1980
	11th to 20th	Day	17% 6% 3 6% 6% 3 5% 5% 5% 7% 38%	1806
	21st to end	Day	21% 10% 4 7% 6% 3 4 5% 6% 6% 29%	1782
November	1st to 10th	Day	12% 8% 3 5% 3 3 4 4 5% 7% 46%	1672
	11th to 20th	Day	18% 13% 4 7% 6% 3 3 3 5% 7% 32%	1710
	21st to end	Day	20% 12% 5% 9% 5% 3 4 5% 5% 5% 28%	1710
December	1st to 10th	Day	17% 15% 6% 9% 7% 5% 4 4 4 5% 23%	1691
	11th to 20th	Day	14% 16% 7% 11% 7% 3 4 4 5% 5% 23%	1918
	21st to end	Day	25% 22% 8% 11% 7% 3 4 3 3 3 11%	2783
January	1st to 10th	Night	18% 13% 6% 10% 7% 3 4 5% 5% 5% 24%	2648
	11th to 20th	Night	19% 14% 6% 10% 8% 3 4 3 3 5% 24%	2547
	21st to end	Night	16% 15% 7% 10% 7% 4 4 5% 5% 5% 22%	2715
February	1st to 10th	Night	19% 14% 5% 9% 7% 3 4 4 4 6% 24%	2232
	11th to 20th	Night	14% 13% 6% 7% 7% 3 5% 4 5% 7% 28%	2098
	21st to end	Night	22% 11% 4 9% 6% 2 4 4 4 5% 30%	1776
March	1st to 10th	Night	16% 10% 4 6% 7% 4 5% 4 4 6% 34%	2160
	11th to 20th	Night	15% 8% 3 6% 5% 4 4 4 5% 7% 38%	2028
	21st to end	Night	21% 10% 3 6% 5% 2 4 3 6% 6% 34%	2133
April	1st to 10th	Night	15% 8% 3 5% 5% 3 4 5% 6% 7% 38%	1800
	11th to 20th	Night	19% 8% 2 5% 5% 3 5% 4 5% 7% 38%	1748
	21st to end	Night	20% 7% 4 6% 4 3 4 4 6% 7% 36%	1620
May	1st to 10th	Night	18% 9% 4 6% 6% 3 5% 5% 6% 7% 30%	1620
	11th to 20th	Night	21% 6% 3 5% 5% 3 4 4 5% 7% 37%	1592
	21st to end	Night	9% 6% 2 4 5% 3 7% 6% 6% 9% 43%	1654
June	1st to 10th	Night	9% 5% 2 5% 5% 3 4 4 5% 9% 49%	1440
	11th to 20th	Night	8% 5% 3 6% 5% 3 5% 4 5% 8% 48%	1440
	21st to end	Night	10% 4 3 5% 5% 4 5% 5% 5% 10% 44%	1440
July	1st to 10th	Night	7% 5% 2 5% 5% 3 5% 4 5% 8% 51%	1440
	11th to 20th	Night	9% 3 2 4 4 3 4 4 6% 9% 52%	1440
	21st to end	Night	5% 4 2 4 6% 3 5% 4 7% 9% 50%	1629
August	1st to 10th	Night	7% 3 2 5% 4 3 4 4 7% 11% 50%	1611
	11th to 20th	Night	8% 5% 2 4 5% 3 5% 5% 5% 8% 50%	1606
	21st to end	Night	8% 4 2 4 5% 3 4 4 5% 8% 54%	1980
September	1st to 10th	Night	10% 5% 2 5% 4 3 4 4 5% 8% 50%	1826
	11th to 20th	Night	11% 3 2 4 4 2 4 3 4 8% 56%	1980
	21st to end	Night	13% 6% 3 5% 4 3 4 4 4 6% 49%	1980
October	1st to 10th	Night	12% 6% 2 4 5% 3 4 4 5% 6% 49%	1980
	11th to 20th	Night	18% 9% 2 6% 5% 3 4 4 4 6% 40%	2147
	21st to end	Night	21% 8% 3 5% 5% 3 4 3 5% 6% 36%	2574
November	1st to 10th	Night	13% 7% 3 6% 5% 3 4 3 5% 6% 45%	2406
	11th to 20th	Night	16% 12% 3 6% 6% 3 4 5% 6% 7% 34%	2470
	21st to end	Night	19% 12% 5% 7% 6% 2 5% 4 4 6% 28%	2470
December	1st to 10th	Night	17% 14% 6% 8% 6% 3 5% 5% 5% 6% 26%	2484
	11th to 20th	Night	18% 14% 6% 10% 7% 4 4 4 5% 5% 23%	3279
	21st to end	Night	27% 17% 7% 10% 6% 3 4 4 4 5% 13%	4884

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	65% 4 11% 12% 7%	1518
	11th to 20th	Day	68% 6% 12% 9% 5%	1453
	21st to end	Day	69% 5% 11% 10% 5%	1638
February	1st to 10th	Day	67% 3 10% 13% 6% 1	1724
	11th to 20th	Day	62% 6% 9% 15% 8% 1	1758
	21st to end	Day	61% 5% 10% 16% 6% 1	1480
March	1st to 10th	Day	58% 5% 10% 19% 8%	1800
	11th to 20th	Day	53% 6% 12% 18% 10% 1	1931
	21st to end	Day	62% 4 10% 18% 5% 1	2239
April	1st to 10th	Day	53% 6% 10% 19% 11%	2160
	11th to 20th	Day	49% 7% 9% 20% 12% 3	2160
	21st to end	Day	50% 5% 12% 18% 11% 3	2340
May	1st to 10th	Day	50% 8% 12% 20% 9% 1	2340
	11th to 20th	Day	51% 7% 9% 19% 14% 1	2305
	21st to end	Day	40% 9% 12% 23% 14% 2	2617
June	1st to 10th	Day	38% 7% 12% 25% 17% 2	2520
	11th to 20th	Day	35% 8% 11% 25% 18% 3	2520
	21st to end	Day	38% 11% 15% 19% 16% 2	2520
July	1st to 10th	Day	35% 7% 12% 23% 18% 4	2451
	11th to 20th	Day	31% 8% 15% 27% 17% 2	2445
	21st to end	Day	34% 8% 14% 25% 17% 3	2536
August	1st to 10th	Day	32% 9% 15% 29% 14% 2	2314
	11th to 20th	Day	35% 8% 16% 25% 14% 2	2292
	21st to end	Day	32% 7% 13% 23% 20% 5%	2376
September	1st to 10th	Day	38% 7% 16% 19% 17% 3	2116
	11th to 20th	Day	33% 6% 13% 27% 20% 1	1980
	21st to end	Day	38% 6% 11% 24% 19% 1	1980
October	1st to 10th	Day	40% 7% 11% 24% 17% 2	1980
	11th to 20th	Day	46% 5% 11% 21% 17% 1	1806
	21st to end	Day	55% 6% 12% 17% 10% 1	1782
November	1st to 10th	Day	38% 6% 13% 26% 16% 1	1672
	11th to 20th	Day	53% 5% 10% 19% 11% 1	1710
	21st to end	Day	59% 5% 11% 18% 7% 1	1710
December	1st to 10th	Day	64% 4 12% 14% 5%	1691
	11th to 20th	Day	62% 5% 12% 17% 3	1915
	21st to end	Day	79% 4 6% 6% 4	2783
January	1st to 10th	Night	62% 5% 10% 15% 8%	2648
	11th to 20th	Night	64% 5% 11% 14% 6%	2547
	21st to end	Night	64% 5% 11% 14% 6% 1	2715
February	1st to 10th	Night	64% 5% 10% 13% 7% 1	2232
	11th to 20th	Night	56% 5% 11% 18% 10% 1	2099
	21st to end	Night	58% 4 10% 16% 9% 2	1776
March	1st to 10th	Night	52% 5% 11% 20% 11% 1	2160
	11th to 20th	Night	45% 6% 12% 21% 13% 3	2028
	21st to end	Night	52% 6% 9% 22% 10% 1	2133
April	1st to 10th	Night	45% 6% 13% 21% 14% 1	1800
	11th to 20th	Night	46% 6% 9% 20% 17% 2	1748
	21st to end	Night	47% 7% 10% 20% 13% 3	1620
May	1st to 10th	Night	53% 7% 9% 19% 10% 2	1620
	11th to 20th	Night	47% 6% 10% 21% 15% 2	1593
	21st to end	Night	35% 9% 15% 22% 15% 3	1655
June	1st to 10th	Night	32% 7% 12% 27% 20% 3	1440
	11th to 20th	Night	35% 7% 11% 24% 20% 3	1440
	21st to end	Night	36% 7% 17% 22% 15% 3	1440
July	1st to 10th	Night	33% 7% 13% 24% 19% 5%	1400
	11th to 20th	Night	29% 8% 15% 25% 20% 3	1392
	21st to end	Night	29% 7% 15% 24% 19% 6%	1538
August	1st to 10th	Night	28% 7% 14% 31% 19% 2	1600
	11th to 20th	Night	32% 8% 13% 24% 20% 2	1606
	21st to end	Night	29% 8% 13% 24% 22% 5%	1980
September	1st to 10th	Night	32% 7% 14% 23% 18% 6%	1826
	11th to 20th	Night	28% 6% 10% 30% 24% 2	1980
	21st to end	Night	36% 7% 9% 26% 20% 2	1980
October	1st to 10th	Night	36% 5% 9% 28% 20% 1	1980
	11th to 20th	Night	46% 4 9% 23% 17% 1	2147
	21st to end	Night	49% 5% 10% 22% 13% 1	2574
November	1st to 10th	Night	40% 4 11% 25% 18% 1	2406
	11th to 20th	Night	50% 6% 11% 21% 12% 1	2470
	21st to end	Night	57% 5% 11% 19% 7%	2470
December	1st to 10th	Night	59% 6% 10% 17% 7%	2484
	11th to 20th	Night	62% 5% 12% 14% 6%	3281
	21st to end	Night	75% 4 10% 7% 4	4884

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	22% 22% 33% 16% 7% 1	270
	11th to 20th	Day	24% 14% 41% 16% 4 1	245
	21st to end	Day	28% 18% 32% 17% 5%	264
February	1st to 10th	Day	29% 20% 34% 16% 1	240
	11th to 20th	Day	28% 22% 33% 14% 3	240
	21st to end	Day	25% 26% 37% 9% 2	198
March	1st to 10th	Day	22% 23% 36% 15% 4	240
	11th to 20th	Day	24% 13% 33% 23% 6% 1	371
	21st to end	Day	32% 15% 27% 18% 5% 2	462
April	1st to 10th	Day	20% 15% 34% 22% 9% 1	420
	11th to 20th	Day	21% 17% 31% 18% 11% 3	412
	21st to end	Day	25% 14% 31% 18% 10% 3	420
May	1st to 10th	Day	25% 13% 32% 18% 9% 3	423
	11th to 20th	Day	23% 11% 26% 22% 14% 5%	447
	21st to end	Day	13% 6% 27% 26% 18% 9%	571
June	1st to 10th	Day	11% 9% 27% 31% 17% 5%	630
	11th to 20th	Day	12% 5% 30% 27% 20% 6%	630
	21st to end	Day	12% 6% 28% 30% 17% 7%	630
July	1st to 10th	Day	9% 4 19% 30% 23% 15%	621
	11th to 20th	Day	8% 4 23% 31% 24% 11%	618
	21st to end	Day	9% 4 21% 31% 26% 10%	626
August	1st to 10th	Day	8% 1 15% 28% 29% 18%	446
	11th to 20th	Day	11% 4 18% 29% 29% 10%	439
	21st to end	Day	11% 2 16% 27% 29% 15%	495
September	1st to 10th	Day	14% 5% 19% 26% 27% 9%	450
	11th to 20th	Day	10% 4 14% 24% 32% 15%	450
	21st to end	Day	15% 10% 20% 24% 25% 7%	450
October	1st to 10th	Day	15% 7% 24% 28% 16% 9%	450
	11th to 20th	Day	19% 18% 29% 25% 9%	396
	21st to end	Day	27% 17% 34% 19% 1 1	297
November	1st to 10th	Day	13% 14% 36% 24% 10% 3	270
	11th to 20th	Day	27% 15% 31% 18% 9% 1	270
	21st to end	Day	27% 17% 37% 16% 3	270
December	1st to 10th	Day	24% 21% 33% 14% 8%	270
	11th to 20th	Day	21% 21% 37% 16% 4	270
	21st to end	Day	29% 21% 33% 14% 2	297
January	1st to 10th	Night	25% 14% 35% 21% 4	950
	11th to 20th	Night	25% 12% 37% 21% 4 1	910
	21st to end	Night	22% 13% 37% 24% 4	933
February	1st to 10th	Night	26% 12% 38% 21% 3	720
	11th to 20th	Night	22% 14% 39% 22% 3	703
	21st to end	Night	28% 14% 37% 18% 3	592
March	1st to 10th	Night	20% 14% 34% 25% 7%	720
	11th to 20th	Night	18% 12% 37% 27% 5% 1	720
	21st to end	Night	25% 10% 31% 28% 5%	747
April	1st to 10th	Night	18% 12% 36% 26% 7% 1	540
	11th to 20th	Night	21% 10% 35% 26% 8% 1	533
	21st to end	Night	21% 7% 36% 25% 9% 1	540
May	1st to 10th	Night	22% 7% 32% 31% 6%	540
	11th to 20th	Night	22% 5% 26% 30% 16% 1	531
	21st to end	Night	12% 3 25% 38% 20% 1	584
June	1st to 10th	Night	11% 2 25% 45% 17%	540
	11th to 20th	Night	10% 4 25% 41% 19% 1	540
	21st to end	Night	9% 2 25% 37% 24% 2	540
July	1st to 10th	Night	9% 2 15% 42% 30% 3	525
	11th to 20th	Night	9% 2 16% 40% 30% 2	522
	21st to end	Night	7% 3 15% 41% 33% 2	591
August	1st to 10th	Night	7% 2 14% 43% 29% 5%	537
	11th to 20th	Night	9% 2 14% 40% 31% 4	534
	21st to end	Night	10% 2 15% 34% 35% 4	594
September	1st to 10th	Night	10% 5% 17% 34% 30% 4	566
	11th to 20th	Night	11% 4 14% 35% 33% 3	720
	21st to end	Night	14% 6% 24% 33% 20% 3	720
October	1st to 10th	Night	14% 6% 25% 33% 19% 3	720
	11th to 20th	Night	21% 13% 29% 28% 9% 1	720
	21st to end	Night	26% 12% 32% 24% 6%	792
November	1st to 10th	Night	16% 11% 34% 26% 11% 1	740
	11th to 20th	Night	22% 15% 33% 24% 6% 1	760
	21st to end	Night	25% 12% 31% 25% 6%	760
December	1st to 10th	Night	25% 15% 34% 21% 5%	774
	11th to 20th	Night	25% 13% 34% 22% 5%	950
	21st to end	Night	31% 14% 32% 20% 3	1045

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