YCAS Stewartstown Observing Site
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the YCAS Stewartstown Observing Site clear sky chart from 2005-11-06 to 2026-07-16.

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	15% 6% 2 6% 4 4 5% 4 6% 8% 40%	138203
	Night	14% 7% 2 6% 4 4 4 4 5% 7% 43%	134482
Transparency	Day	28% 35% 7% 17% 11% 2	141795
	Night	27% 35% 7% 17% 12% 2	138962
Seeing	Day	20% 12% 28% 23% 13% 5%	17097
	Night	18% 10% 30% 29% 11% 1	26486

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	13% 6% 2 6% 4 5% 5% 4 6% 6% 44%	4661
	11th to 20th	Day	14% 8% 3 8% 4 4 4 3 4 5% 44%	5142
	21st to end	Day	14% 6% 2 6% 5% 5% 6% 4 6% 6% 40%	5780
February	1st to 10th	Day	13% 8% 3 6% 5% 4 4 4 7% 7% 39%	1890
	11th to 20th	Day	16% 9% 2 7% 5% 4 4 4 6% 6% 38%	1922
	21st to end	Day	17% 10% 2 7% 4 4 4 3 5% 5% 39%	1730
March	1st to 10th	Day	19% 5% 2 4 4 4 4 3 4 6% 45%	4832
	11th to 20th	Day	11% 6% 2 7% 4 4 4 4 6% 7% 45%	5090
	21st to end	Day	16% 6% 2 6% 4 4 4 3 5% 7% 41%	5764
April	1st to 10th	Day	14% 8% 2 6% 4 4 5% 4 6% 7% 38%	2520
	11th to 20th	Day	13% 5% 2 6% 4 4 4 4 6% 7% 43%	2494
	21st to end	Day	17% 6% 2 6% 4 4 4 3 5% 7% 41%	2520
May	1st to 10th	Day	22% 6% 2 6% 4 4 4 4 6% 8% 33%	7486
	11th to 20th	Day	20% 6% 2 6% 4 4 5% 4 6% 8% 36%	8221
	21st to end	Day	24% 6% 2 6% 4 4 4 3 6% 7% 36%	9045
June	1st to 10th	Day	11% 6% 2 6% 4 5% 5% 5% 7% 9% 39%	2940
	11th to 20th	Day	13% 6% 3 6% 5% 5% 5% 4 7% 9% 39%	2940
	21st to end	Day	11% 4 2 6% 5% 5% 5% 4 6% 10% 42%	2940
July	1st to 10th	Day	8% 5% 2 6% 5% 5% 6% 4 7% 10% 41%	6500
	11th to 20th	Day	10% 7% 3 8% 6% 6% 7% 5% 8% 10% 31%	6430
	21st to end	Day	9% 6% 2 7% 5% 5% 5% 4 7% 10% 39%	6481
August	1st to 10th	Day	18% 6% 2 7% 5% 5% 5% 5% 6% 8% 34%	4234
	11th to 20th	Day	11% 5% 3 6% 4 5% 6% 5% 7% 8% 40%	3977
	21st to end	Day	12% 5% 2 5% 4 4 4 4 6% 7% 48%	4312
September	1st to 10th	Day	14% 6% 2 5% 4 4 4 3 4 6% 46%	2400
	11th to 20th	Day	12% 5% 2 5% 4 4 4 3 5% 8% 48%	2400
	21st to end	Day	21% 7% 3 6% 4 4 4 4 5% 6% 37%	2123
October	1st to 10th	Day	17% 6% 2 5% 4 3 4 3 5% 7% 46%	2000
	11th to 20th	Day	15% 6% 2 5% 3 3 4 3 5% 6% 47%	2000
	21st to end	Day	15% 6% 2 6% 4 3 4 3 6% 6% 46%	2200
November	1st to 10th	Day	11% 6% 2 5% 3 3 3 2 5% 8% 51%	1877
	11th to 20th	Day	15% 5% 2 4 3 3 5% 4 5% 6% 48%	1874
	21st to end	Day	14% 7% 2 4 5% 4 4 3 5% 6% 44%	1680
December	1st to 10th	Day	12% 6% 2 6% 5% 4 4 4 6% 7% 43%	2680
	11th to 20th	Day	18% 10% 2 5% 3 3 4 4 5% 7% 40%	3070
	21st to end	Day	16% 8% 3 7% 4 4 5% 4 6% 7% 36%	4048
January	1st to 10th	Night	10% 8% 3 6% 4 4 4 4 6% 8% 42%	8233
	11th to 20th	Night	13% 8% 3 6% 3 3 3 3 4 6% 49%	7800
	21st to end	Night	13% 7% 2 5% 3 3 4 3 5% 7% 47%	8401
February	1st to 10th	Night	15% 9% 2 7% 4 4 3 4 6% 6% 40%	2730
	11th to 20th	Night	17% 8% 3 6% 4 3 4 3 5% 6% 40%	2582
	21st to end	Night	16% 8% 3 7% 5% 4 4 3 5% 5% 41%	2076
March	1st to 10th	Night	16% 7% 2 6% 4 3 3 3 4 5% 46%	5225
	11th to 20th	Night	10% 5% 3 6% 4 3 4 3 5% 6% 51%	5090
	21st to end	Night	13% 8% 3 6% 4 3 4 3 5% 6% 45%	5409
April	1st to 10th	Night	16% 8% 3 7% 5% 4 4 3 6% 6% 38%	2100
	11th to 20th	Night	13% 6% 2 6% 4 4 5% 3 6% 8% 42%	2075
	21st to end	Night	18% 7% 2 5% 3 3 4 3 5% 7% 43%	2100
May	1st to 10th	Night	21% 8% 3 7% 5% 4 4 4 5% 7% 33%	5420
	11th to 20th	Night	20% 8% 3 6% 4 3 4 3 6% 9% 34%	4639
	21st to end	Night	24% 7% 2 7% 4 3 4 3 5% 6% 34%	5108
June	1st to 10th	Night	12% 4 2 6% 3 4 4 4 6% 8% 46%	1680
	11th to 20th	Night	12% 5% 2 6% 4 5% 5% 5% 8% 9% 40%	1680
	21st to end	Night	10% 5% 3 4 4 5% 5% 5% 6% 10% 43%	1680
July	1st to 10th	Night	8% 6% 3 7% 5% 5% 5% 5% 7% 9% 39%	3678
	11th to 20th	Night	9% 8% 3 9% 6% 6% 6% 5% 8% 9% 30%	3637
	21st to end	Night	10% 6% 2 5% 4 4 4 4 6% 8% 45%	4354
August	1st to 10th	Night	19% 5% 2 6% 4 4 5% 4 7% 9% 35%	2915
	11th to 20th	Night	11% 7% 3 7% 5% 5% 6% 4 5% 8% 40%	3039
	21st to end	Night	12% 5% 2 5% 3 3 4 4 5% 8% 48%	3586
September	1st to 10th	Night	13% 6% 2 6% 4 3 3 3 6% 7% 48%	2000
	11th to 20th	Night	12% 6% 2 5% 3 3 4 3 4 7% 52%	2000
	21st to end	Night	19% 8% 2 6% 4 4 3 4 4 6% 40%	2271
October	1st to 10th	Night	18% 7% 2 6% 3 3 3 3 5% 5% 46%	2400
	11th to 20th	Night	15% 5% 2 4 3 3 3 3 4 6% 51%	2400
	21st to end	Night	16% 5% 2 4 3 3 4 3 5% 6% 50%	2640
November	1st to 10th	Night	11% 6% 2 4 4 3 4 4 5% 6% 53%	2638
	11th to 20th	Night	15% 8% 2 5% 4 4 3 3 4 6% 47%	2741
	21st to end	Night	13% 5% 2 5% 5% 4 4 3 6% 7% 46%	2940
December	1st to 10th	Night	10% 7% 2 6% 4 4 4 3 6% 8% 46%	4720
	11th to 20th	Night	20% 7% 2 5% 4 4 4 3 5% 6% 40%	5345
	21st to end	Night	16% 8% 3 7% 5% 5% 5% 4 6% 7% 36%	7150

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	32% 22% 7% 22% 15% 2	4662
	11th to 20th	Day	35% 21% 4 17% 16% 7%	5142
	21st to end	Day	34% 23% 10% 23% 9% 2	5780
February	1st to 10th	Day	18% 59% 7% 10% 4 1	1890
	11th to 20th	Day	20% 58% 4 10% 7% 1	1931
	21st to end	Day	19% 59% 4 11% 5% 1	1730
March	1st to 10th	Day	31% 29% 4 16% 12% 6%	4832
	11th to 20th	Day	27% 29% 7% 22% 11% 4	5090
	21st to end	Day	31% 28% 7% 14% 15% 4	5764
April	1st to 10th	Day	20% 59% 4 8% 7% 1	2520
	11th to 20th	Day	16% 60% 7% 10% 6% 1	2494
	21st to end	Day	18% 59% 4 11% 8% 1	2520
May	1st to 10th	Day	39% 23% 10% 17% 10% 1	7519
	11th to 20th	Day	35% 24% 9% 23% 9%	8225
	21st to end	Day	39% 23% 7% 16% 14% 1	9061
June	1st to 10th	Day	18% 60% 4 10% 9% 1	2940
	11th to 20th	Day	17% 60% 4 11% 7% 1	2940
	21st to end	Day	16% 58% 6% 12% 8%	2940
July	1st to 10th	Day	29% 29% 12% 16% 12% 2	6430
	11th to 20th	Day	37% 29% 13% 16% 5%	6355
	21st to end	Day	29% 27% 8% 19% 17% 1	6295
August	1st to 10th	Day	33% 37% 8% 17% 5%	4221
	11th to 20th	Day	25% 37% 7% 22% 9%	4016
	21st to end	Day	20% 36% 7% 21% 15% 1	4312
September	1st to 10th	Day	17% 57% 6% 11% 8% 2	2400
	11th to 20th	Day	17% 57% 3 13% 8% 2	2400
	21st to end	Day	24% 56% 5% 8% 7%	2123
October	1st to 10th	Day	24% 35% 3 12% 22% 4	3260
	11th to 20th	Day	13% 36% 6% 20% 16% 8%	3240
	21st to end	Day	11% 35% 7% 21% 23% 3	3538
November	1st to 10th	Day	17% 53% 7% 10% 10% 3	1877
	11th to 20th	Day	14% 54% 3 14% 13% 1	1874
	21st to end	Day	18% 54% 5% 16% 6% 1	1680
December	1st to 10th	Day	27% 35% 11% 18% 7% 2	2680
	11th to 20th	Day	32% 31% 9% 20% 7% 1	3066
	21st to end	Day	36% 28% 10% 14% 11% 1	4048
January	1st to 10th	Night	33% 21% 9% 22% 13% 3	8234
	11th to 20th	Night	30% 20% 7% 18% 18% 7%	7800
	21st to end	Night	30% 22% 6% 24% 16% 2	8401
February	1st to 10th	Night	18% 58% 7% 11% 5% 1	2730
	11th to 20th	Night	20% 57% 3 8% 10% 1	2584
	21st to end	Night	17% 59% 5% 10% 7% 2	2076
March	1st to 10th	Night	33% 29% 4 18% 8% 8%	5225
	11th to 20th	Night	23% 28% 9% 25% 10% 5%	5090
	21st to end	Night	30% 28% 6% 14% 20% 2	5409
April	1st to 10th	Night	20% 59% 4 10% 7% 1	2100
	11th to 20th	Night	17% 59% 6% 9% 7% 1	2075
	21st to end	Night	18% 59% 4 10% 8% 1	2100
May	1st to 10th	Night	42% 24% 8% 15% 10% 2	5450
	11th to 20th	Night	37% 24% 10% 23% 6%	4640
	21st to end	Night	43% 23% 6% 17% 9% 1	5115
June	1st to 10th	Night	16% 60% 3 11% 10%	1680
	11th to 20th	Night	17% 60% 5% 10% 8%	1680
	21st to end	Night	16% 59% 5% 12% 7% 1	1680
July	1st to 10th	Night	30% 29% 13% 14% 12% 2	3640
	11th to 20th	Night	40% 28% 12% 14% 5%	3592
	21st to end	Night	29% 27% 8% 20% 16%	4226
August	1st to 10th	Night	34% 37% 12% 13% 4	2904
	11th to 20th	Night	26% 37% 6% 22% 8%	3066
	21st to end	Night	20% 36% 6% 25% 12% 1	3586
September	1st to 10th	Night	15% 57% 7% 12% 8% 2	2000
	11th to 20th	Night	17% 56% 5% 11% 10% 2	2000
	21st to end	Night	23% 57% 5% 8% 7%	2271
October	1st to 10th	Night	28% 36% 7% 12% 16% 2	3920
	11th to 20th	Night	13% 35% 6% 19% 22% 5%	3899
	21st to end	Night	13% 35% 6% 24% 18% 4	4252
November	1st to 10th	Night	16% 54% 6% 13% 10% 2	2638
	11th to 20th	Night	16% 54% 3 11% 13% 3	2741
	21st to end	Night	16% 54% 6% 14% 8% 1	2940
December	1st to 10th	Night	23% 35% 10% 22% 9% 1	4720
	11th to 20th	Night	32% 30% 8% 15% 12% 3	5348
	21st to end	Night	38% 28% 7% 13% 14% 1	7150

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% 25% 30% 13% 3	220
	11th to 20th	Day	35% 22% 29% 12% 2	305
	21st to end	Day	23% 26% 37% 9% 5% 1	363
February	1st to 10th	Day	25% 21% 39% 12% 3	330
	11th to 20th	Day	31% 22% 36% 9% 1	409
	21st to end	Day	28% 18% 36% 15% 2	446
March	1st to 10th	Day	26% 20% 31% 20% 4	540
	11th to 20th	Day	24% 22% 32% 17% 4 1	540
	21st to end	Day	24% 21% 33% 16% 5% 1	594
April	1st to 10th	Day	25% 17% 34% 17% 6% 1	540
	11th to 20th	Day	20% 19% 30% 19% 8% 4	532
	21st to end	Day	25% 14% 31% 21% 7% 2	540
May	1st to 10th	Day	25% 17% 28% 18% 8% 3	543
	11th to 20th	Day	22% 12% 29% 23% 11% 3	567
	21st to end	Day	23% 5% 23% 24% 18% 7%	617
June	1st to 10th	Day	15% 11% 29% 25% 15% 6%	570
	11th to 20th	Day	18% 7% 27% 27% 15% 5%	570
	21st to end	Day	16% 5% 24% 32% 16% 7%	570
July	1st to 10th	Day	14% 3 16% 31% 25% 11%	565
	11th to 20th	Day	6% 3 20% 34% 27% 10%	544
	21st to end	Day	12% 4 22% 34% 22% 6%	576
August	1st to 10th	Day	13% 3 22% 28% 24% 9%	526
	11th to 20th	Day	14% 3 16% 33% 24% 10%	519
	21st to end	Day	11% 4 20% 30% 21% 13%	583
September	1st to 10th	Day	18% 6% 21% 21% 23% 12%	530
	11th to 20th	Day	15% 5% 17% 29% 24% 11%	530
	21st to end	Day	26% 8% 22% 25% 13% 5%	530
October	1st to 10th	Day	20% 8% 24% 22% 18% 8%	530
	11th to 20th	Day	19% 13% 28% 25% 12% 4	530
	21st to end	Day	17% 12% 37% 23% 9% 2	583
November	1st to 10th	Day	17% 14% 30% 26% 10% 2	439
	11th to 20th	Day	20% 17% 32% 22% 9%	414
	21st to end	Day	28% 21% 31% 15% 5%	220
December	1st to 10th	Day	25% 18% 40% 13% 3 2	220
	11th to 20th	Day	28% 19% 34% 13% 6%	220
	21st to end	Day	23% 20% 36% 14% 7%	242
January	1st to 10th	Night	18% 16% 43% 20% 3	840
	11th to 20th	Night	23% 18% 33% 23% 3	840
	21st to end	Night	18% 15% 45% 20% 2	924
February	1st to 10th	Night	21% 15% 37% 25% 2	840
	11th to 20th	Night	24% 20% 38% 17% 1	823
	21st to end	Night	22% 18% 38% 19% 2	692
March	1st to 10th	Night	21% 16% 38% 22% 3	840
	11th to 20th	Night	22% 16% 36% 21% 5%	840
	21st to end	Night	21% 13% 39% 21% 5%	924
April	1st to 10th	Night	23% 15% 39% 19% 4	840
	11th to 20th	Night	18% 12% 36% 27% 8%	831
	21st to end	Night	23% 10% 32% 27% 8%	840
May	1st to 10th	Night	21% 9% 31% 30% 9% 1	698
	11th to 20th	Night	19% 7% 32% 32% 9%	414
	21st to end	Night	18% 4 21% 39% 18% 1	455
June	1st to 10th	Night	13% 2 27% 38% 20%	420
	11th to 20th	Night	13% 5% 24% 43% 15% 1	420
	21st to end	Night	11% 3 21% 48% 17%	420
July	1st to 10th	Night	11% 2 18% 41% 26% 3	410
	11th to 20th	Night	7% 2 14% 44% 29% 3	398
	21st to end	Night	12% 2 15% 44% 24% 3	623
August	1st to 10th	Night	12% 2 10% 47% 26% 2	597
	11th to 20th	Night	13% 2 14% 42% 27% 3	693
	21st to end	Night	14% 1 15% 40% 26% 3	880
September	1st to 10th	Night	14% 3 21% 34% 24% 4	800
	11th to 20th	Night	15% 3 14% 38% 27% 5%	800
	21st to end	Night	24% 4 20% 32% 19% 2	800
October	1st to 10th	Night	21% 6% 21% 28% 21% 5%	800
	11th to 20th	Night	18% 10% 28% 32% 13% 1	800
	21st to end	Night	19% 8% 34% 27% 12%	880
November	1st to 10th	Night	15% 7% 35% 34% 10% 1	820
	11th to 20th	Night	19% 14% 33% 26% 8% 1	840
	21st to end	Night	17% 16% 35% 26% 7%	840
December	1st to 10th	Night	22% 15% 36% 21% 6% 1	840
	11th to 20th	Night	19% 15% 39% 23% 4	840
	21st to end	Night	20% 15% 35% 27% 2	924

Page generated at 2026-07-13 03:14:46. Version ac07

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