Stacy's Stellar Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Stacy's Stellar Observatory clear sky chart from 2005-11-06 to 2025-01-07.

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% 9% 3 9% 6% 5% 5% 4 6% 7% 35%	105171
	Night	11% 10% 4 10% 6% 5% 5% 4 5% 6% 34%	104051
Transparency	Day	46% 7% 13% 24% 9% 1	108708
	Night	48% 6% 12% 22% 10% 1	108605
Seeing	Day	15% 11% 31% 26% 13% 3	16314
	Night	15% 10% 33% 30% 11% 1	26315

By Month

Percentage Cloud Cover NE (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	6% 15% 6% 16% 7% 6% 6% 4 5% 7% 21%	2260
	11th to 20th	Day	6% 9% 5% 16% 12% 9% 9% 7% 7% 5% 16%	2257
	21st to end	Day	6% 13% 11% 17% 8% 6% 5% 4 5% 6% 18%	3112
February	1st to 10th	Day	7% 14% 6% 10% 6% 6% 6% 5% 5% 7% 28%	1710
	11th to 20th	Day	8% 11% 4 8% 7% 6% 6% 4 7% 6% 32%	1664
	21st to end	Day	9% 10% 4 11% 8% 8% 4 3 4 8% 30%	1413
March	1st to 10th	Day	10% 11% 4 9% 6% 5% 5% 4 5% 6% 35%	3406
	11th to 20th	Day	9% 9% 4 10% 6% 4 4 4 6% 8% 36%	3480
	21st to end	Day	18% 14% 3 9% 6% 4 4 3 4 6% 28%	3814
April	1st to 10th	Day	15% 11% 4 9% 6% 5% 4 3 6% 5% 33%	2269
	11th to 20th	Day	17% 12% 3 8% 5% 4 4 4 6% 7% 31%	2376
	21st to end	Day	21% 10% 3 6% 4 4 4 3 4 6% 33%	2470
May	1st to 10th	Day	19% 8% 3 6% 4 5% 4 4 6% 7% 35%	4294
	11th to 20th	Day	15% 7% 2 5% 4 4 5% 4 6% 8% 39%	4399
	21st to end	Day	17% 8% 2 7% 5% 5% 4 4 6% 7% 36%	4837
June	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 7% 9% 41%	2838
	11th to 20th	Day	14% 7% 3 7% 5% 4 5% 4 6% 7% 38%	2850
	21st to end	Day	11% 5% 2 6% 4 5% 6% 5% 8% 9% 38%	2850
July	1st to 10th	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 10% 42%	4750
	11th to 20th	Day	6% 4 1 4 4 4 4 4 7% 10% 51%	4750
	21st to end	Day	6% 5% 2 6% 4 4 5% 4 7% 10% 47%	4804
August	1st to 10th	Day	11% 5% 2 5% 4 4 5% 5% 7% 9% 44%	4104
	11th to 20th	Day	13% 5% 2 6% 4 4 5% 4 6% 9% 43%	4041
	21st to end	Day	14% 7% 3 6% 3 4 4 4 5% 7% 43%	4471
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 6% 9% 47%	2280
	11th to 20th	Day	12% 7% 2 7% 4 5% 4 4 6% 7% 41%	2190
	21st to end	Day	16% 9% 2 6% 4 3 4 3 5% 6% 42%	2040
October	1st to 10th	Day	17% 10% 2 7% 5% 3 3 4 5% 5% 38%	1900
	11th to 20th	Day	17% 14% 3 7% 4 5% 3 3 4 4 34%	1900
	21st to end	Day	18% 12% 5% 10% 6% 4 4 3 4 6% 28%	2027
November	1st to 10th	Day	12% 15% 5% 10% 6% 4 4 4 4 6% 30%	1567
	11th to 20th	Day	12% 16% 5% 16% 7% 5% 3 3 4 5% 24%	1600
	21st to end	Day	12% 12% 6% 13% 8% 7% 5% 4 6% 6% 22%	1600
December	1st to 10th	Day	9% 13% 5% 14% 14% 10% 6% 4 4 6% 15%	2610
	11th to 20th	Day	7% 17% 6% 13% 6% 5% 4 3 5% 5% 27%	2740
	21st to end	Day	16% 18% 7% 18% 10% 5% 3 2 4 4 14%	3498
January	1st to 10th	Night	7% 11% 5% 13% 10% 7% 6% 5% 6% 7% 25%	4828
	11th to 20th	Night	5% 12% 5% 13% 10% 8% 8% 6% 7% 7% 18%	4710
	21st to end	Night	7% 14% 11% 17% 7% 5% 5% 4 5% 6% 20%	4550
February	1st to 10th	Night	7% 10% 4 9% 5% 7% 6% 5% 7% 6% 33%	2470
	11th to 20th	Night	6% 10% 4 9% 6% 5% 6% 5% 7% 7% 35%	2403
	21st to end	Night	9% 11% 4 9% 7% 6% 5% 4 6% 6% 31%	2041
March	1st to 10th	Night	8% 9% 3 8% 5% 4 4 3 5% 7% 42%	3463
	11th to 20th	Night	8% 9% 3 11% 7% 5% 6% 4 5% 6% 36%	3479
	21st to end	Night	18% 16% 6% 11% 4 4 3 3 4 4 28%	3817
April	1st to 10th	Night	15% 10% 3 6% 6% 5% 5% 3 5% 5% 37%	1910
	11th to 20th	Night	17% 11% 4 8% 3 4 4 3 5% 6% 37%	1750
	21st to end	Night	19% 10% 3 7% 4 4 4 4 5% 6% 35%	1710
May	1st to 10th	Night	16% 9% 3 7% 4 4 4 3 4 7% 39%	2654
	11th to 20th	Night	14% 8% 2 6% 3 3 4 4 5% 7% 44%	2506
	21st to end	Night	17% 8% 3 8% 5% 4 4 3 5% 7% 36%	2750
June	1st to 10th	Night	11% 6% 1 5% 5% 4 4 5% 8% 7% 44%	1339
	11th to 20th	Night	9% 7% 2 5% 5% 4 5% 4 5% 8% 45%	1330
	21st to end	Night	10% 5% 2 5% 5% 5% 4 3 7% 9% 47%	1330
July	1st to 10th	Night	8% 4 2 5% 4 5% 5% 6% 8% 11% 44%	2220
	11th to 20th	Night	5% 3 1 3 3 3 3 4 5% 8% 62%	2220
	21st to end	Night	5% 4 2 4 4 4 4 3 6% 10% 54%	2756
August	1st to 10th	Night	8% 6% 2 6% 4 4 4 4 5% 7% 51%	2836
	11th to 20th	Night	14% 5% 2 4 3 3 4 3 6% 9% 47%	2798
	21st to end	Night	12% 7% 2 6% 3 3 4 3 6% 7% 46%	3193
September	1st to 10th	Night	9% 4 2 6% 4 3 4 4 6% 7% 51%	1900
	11th to 20th	Night	8% 6% 1 7% 5% 3 4 3 5% 8% 51%	1987
	21st to end	Night	14% 6% 2 7% 4 3 3 2 4 6% 48%	2120
October	1st to 10th	Night	15% 9% 3 6% 4 4 4 4 5% 6% 41%	2280
	11th to 20th	Night	16% 9% 3 8% 5% 4 4 4 5% 5% 36%	2280
	21st to end	Night	18% 11% 4 9% 5% 5% 4 3 5% 6% 30%	2568
November	1st to 10th	Night	12% 12% 4 9% 5% 5% 4 4 5% 6% 33%	2731
	11th to 20th	Night	14% 13% 5% 13% 6% 4 4 3 5% 7% 25%	2800
	21st to end	Night	12% 10% 4 11% 8% 6% 5% 5% 6% 6% 27%	2800
December	1st to 10th	Night	6% 11% 6% 15% 12% 11% 8% 4 5% 6% 16%	4580
	11th to 20th	Night	9% 14% 6% 16% 8% 6% 5% 4 6% 6% 21%	5462
	21st to end	Night	18% 18% 6% 16% 6% 4 4 3 4 4 16%	7480

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	60% 7% 10% 17% 6%	2261
	11th to 20th	Day	64% 7% 8% 16% 4	2257
	21st to end	Day	65% 7% 9% 13% 6%	3112
February	1st to 10th	Day	54% 6% 10% 20% 10%	1710
	11th to 20th	Day	49% 8% 12% 20% 10% 2	1673
	21st to end	Day	54% 6% 12% 16% 12% 1	1413
March	1st to 10th	Day	47% 7% 11% 25% 9%	3408
	11th to 20th	Day	44% 6% 14% 26% 9%	3480
	21st to end	Day	56% 8% 11% 21% 4	3814
April	1st to 10th	Day	52% 6% 10% 18% 12% 2	2269
	11th to 20th	Day	52% 7% 12% 18% 11% 1	2376
	21st to end	Day	51% 6% 11% 21% 10%	2470
May	1st to 10th	Day	44% 8% 15% 21% 12% 1	4305
	11th to 20th	Day	40% 8% 15% 26% 9% 3	4403
	21st to end	Day	43% 10% 12% 26% 7% 1	4837
June	1st to 10th	Day	37% 9% 18% 26% 9% 1	2838
	11th to 20th	Day	44% 8% 14% 22% 12% 1	2850
	21st to end	Day	39% 9% 15% 27% 9%	2850
July	1st to 10th	Day	34% 10% 19% 25% 12% 1	4676
	11th to 20th	Day	26% 8% 17% 37% 11% 1	4669
	21st to end	Day	31% 8% 18% 33% 10% 1	4651
August	1st to 10th	Day	34% 7% 16% 29% 13% 1	4090
	11th to 20th	Day	37% 7% 17% 30% 8% 1	4080
	21st to end	Day	40% 6% 16% 28% 9% 1	4470
September	1st to 10th	Day	34% 7% 17% 32% 10%	2280
	11th to 20th	Day	40% 8% 19% 25% 7%	2190
	21st to end	Day	42% 6% 13% 30% 7% 1	2040
October	1st to 10th	Day	36% 4 14% 31% 14% 1	3160
	11th to 20th	Day	46% 4 9% 21% 19% 1	3140
	21st to end	Day	54% 4 13% 23% 5%	3320
November	1st to 10th	Day	54% 6% 12% 17% 10% 1	1567
	11th to 20th	Day	64% 5% 11% 13% 7% 1	1600
	21st to end	Day	59% 8% 13% 14% 5% 1	1600
December	1st to 10th	Day	71% 5% 10% 11% 3 1	2610
	11th to 20th	Day	58% 4 8% 19% 11%	2741
	21st to end	Day	75% 4 6% 11% 4	3498
January	1st to 10th	Night	57% 7% 11% 19% 6%	4828
	11th to 20th	Night	62% 8% 11% 13% 5% 1	4710
	21st to end	Night	64% 6% 8% 17% 5%	4550
February	1st to 10th	Night	48% 6% 12% 24% 10% 1	2470
	11th to 20th	Night	45% 8% 11% 23% 12% 1	2404
	21st to end	Night	52% 7% 9% 21% 10% 1	2041
March	1st to 10th	Night	39% 8% 16% 22% 13% 2	3474
	11th to 20th	Night	46% 8% 12% 22% 12% 1	3480
	21st to end	Night	60% 5% 9% 17% 8% 1	3817
April	1st to 10th	Night	49% 5% 11% 20% 15% 1	1910
	11th to 20th	Night	49% 5% 9% 21% 15% 1	1750
	21st to end	Night	49% 5% 11% 22% 13% 1	1710
May	1st to 10th	Night	43% 9% 12% 25% 11% 1	2663
	11th to 20th	Night	38% 8% 11% 24% 15% 3	2507
	21st to end	Night	45% 9% 12% 22% 11% 1	2751
June	1st to 10th	Night	33% 9% 20% 27% 9% 2	1339
	11th to 20th	Night	37% 7% 15% 24% 16% 2	1330
	21st to end	Night	32% 8% 19% 29% 10% 2	1330
July	1st to 10th	Night	31% 11% 21% 24% 13% 1	2185
	11th to 20th	Night	20% 6% 16% 41% 15% 2	2178
	21st to end	Night	24% 8% 18% 35% 13% 1	2669
August	1st to 10th	Night	32% 6% 18% 27% 15% 2	2826
	11th to 20th	Night	34% 6% 16% 34% 9% 1	2825
	21st to end	Night	36% 7% 18% 27% 11% 1	3193
September	1st to 10th	Night	32% 8% 15% 33% 12% 1	1900
	11th to 20th	Night	33% 6% 20% 30% 11%	1987
	21st to end	Night	37% 5% 12% 35% 11%	2120
October	1st to 10th	Night	30% 4 9% 36% 17% 4	3800
	11th to 20th	Night	43% 4 10% 24% 18% 1	3779
	21st to end	Night	50% 5% 12% 22% 10%	4228
November	1st to 10th	Night	51% 6% 12% 22% 9% 1	2731
	11th to 20th	Night	58% 6% 13% 15% 7% 1	2800
	21st to end	Night	55% 8% 15% 17% 5% 1	2800
December	1st to 10th	Night	68% 6% 12% 11% 3	4580
	11th to 20th	Night	63% 5% 10% 14% 9% 1	5460
	21st to end	Night	72% 5% 7% 12% 5%	7480

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	13% 14% 33% 28% 10% 2	288
	11th to 20th	Day	13% 10% 38% 30% 9%	270
	21st to end	Day	12% 8% 38% 25% 14% 2	297
February	1st to 10th	Day	14% 16% 44% 19% 6% 1	270
	11th to 20th	Day	17% 16% 41% 16% 9% 2	270
	21st to end	Day	12% 13% 40% 28% 6%	225
March	1st to 10th	Day	16% 12% 44% 21% 6% 1	270
	11th to 20th	Day	15% 10% 39% 20% 13% 1	270
	21st to end	Day	21% 12% 35% 17% 12% 3	297
April	1st to 10th	Day	21% 14% 35% 23% 6%	449
	11th to 20th	Day	24% 14% 29% 22% 10% 1	574
	21st to end	Day	24% 10% 31% 23% 13%	650
May	1st to 10th	Day	20% 14% 31% 23% 9% 2	653
	11th to 20th	Day	18% 12% 29% 28% 11% 3	675
	21st to end	Day	18% 11% 28% 29% 12% 3	734
June	1st to 10th	Day	12% 9% 30% 30% 17% 3	680
	11th to 20th	Day	15% 12% 33% 27% 11% 2	680
	21st to end	Day	14% 8% 30% 29% 14% 6%	680
July	1st to 10th	Day	9% 6% 23% 28% 23% 11%	671
	11th to 20th	Day	10% 7% 27% 26% 21% 9%	668
	21st to end	Day	7% 6% 24% 32% 24% 7%	739
August	1st to 10th	Day	10% 6% 23% 32% 22% 6%	676
	11th to 20th	Day	13% 6% 22% 31% 20% 9%	665
	21st to end	Day	13% 10% 23% 30% 19% 5%	713
September	1st to 10th	Day	10% 7% 33% 29% 18% 3	490
	11th to 20th	Day	12% 7% 30% 32% 16% 3	400
	21st to end	Day	20% 13% 32% 20% 13% 2	300
October	1st to 10th	Day	18% 14% 30% 22% 11% 5%	300
	11th to 20th	Day	17% 19% 35% 25% 5%	300
	21st to end	Day	22% 19% 38% 13% 8% 1	330
November	1st to 10th	Day	19% 13% 34% 25% 9% 1	300
	11th to 20th	Day	18% 16% 42% 19% 4	300
	21st to end	Day	19% 14% 39% 24% 3	300
December	1st to 10th	Day	13% 14% 41% 24% 7% 1	300
	11th to 20th	Day	15% 21% 32% 28% 4	300
	21st to end	Day	23% 9% 35% 25% 6% 2	330
January	1st to 10th	Night	12% 11% 37% 30% 9% 2	981
	11th to 20th	Night	14% 11% 37% 30% 8% 1	950
	21st to end	Night	16% 11% 37% 27% 8% 1	1045
February	1st to 10th	Night	11% 12% 37% 30% 9% 1	950
	11th to 20th	Night	11% 13% 42% 24% 8% 2	929
	21st to end	Night	13% 11% 37% 27% 9% 2	785
March	1st to 10th	Night	14% 13% 36% 25% 11% 1	614
	11th to 20th	Night	15% 11% 39% 29% 6% 1	570
	21st to end	Night	21% 10% 28% 27% 12% 3	627
April	1st to 10th	Night	19% 11% 35% 29% 6% 1	570
	11th to 20th	Night	22% 10% 32% 27% 8% 1	563
	21st to end	Night	22% 9% 28% 31% 10% 1	570
May	1st to 10th	Night	19% 11% 32% 29% 9% 1	570
	11th to 20th	Night	16% 9% 30% 34% 10% 1	561
	21st to end	Night	14% 9% 29% 35% 13%	617
June	1st to 10th	Night	13% 7% 31% 34% 15% 1	570
	11th to 20th	Night	12% 6% 33% 41% 8%	570
	21st to end	Night	11% 6% 25% 40% 17% 1	570
July	1st to 10th	Night	6% 2 24% 45% 21% 2	555
	11th to 20th	Night	9% 3 27% 38% 22% 1	552
	21st to end	Night	5% 2 27% 40% 23% 3	624
August	1st to 10th	Night	7% 3 22% 47% 19% 2	567
	11th to 20th	Night	13% 4 20% 39% 22% 3	564
	21st to end	Night	11% 5% 26% 35% 21% 2	627
September	1st to 10th	Night	11% 5% 27% 39% 17% 1	570
	11th to 20th	Night	10% 7% 32% 33% 16% 2	570
	21st to end	Night	15% 7% 37% 28% 11% 2	600
October	1st to 10th	Night	20% 9% 32% 28% 11% 1	760
	11th to 20th	Night	20% 12% 35% 26% 6% 1	760
	21st to end	Night	25% 11% 33% 26% 4	879
November	1st to 10th	Night	18% 11% 34% 28% 9%	975
	11th to 20th	Night	19% 13% 35% 25% 6% 1	1000
	21st to end	Night	17% 16% 38% 23% 6%	1000
December	1st to 10th	Night	14% 14% 35% 28% 8% 1	1000
	11th to 20th	Night	19% 12% 37% 24% 6% 1	1000
	21st to end	Night	23% 10% 35% 24% 7% 1	1100

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