Gwynedd Mercy College
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Gwynedd Mercy College clear sky chart from 2005-11-06 to 2025-10-04.

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	16% 7% 2 6% 4 4 4 4 6% 7% 40%	126549
	Night	16% 7% 2 6% 4 4 4 3 5% 6% 43%	122808
Transparency	Day	41% 6% 12% 23% 16% 3	130078
	Night	40% 6% 11% 23% 17% 3	127366
Seeing	Day	21% 15% 29% 22% 10% 3	16162
	Night	19% 12% 31% 28% 9% 1	25382

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	13% 6% 2 5% 4 4 4 3 5% 6% 50%	3597
	11th to 20th	Day	13% 9% 3 8% 5% 4 4 4 4 5% 40%	3661
	21st to end	Day	18% 8% 3 7% 4 4 4 3 4 6% 40%	4466
February	1st to 10th	Day	16% 8% 3 6% 5% 3 4 4 6% 7% 38%	1800
	11th to 20th	Day	17% 9% 2 7% 4 4 5% 3 4 6% 39%	1898
	21st to end	Day	18% 9% 3 7% 5% 3 4 4 4 6% 39%	1650
March	1st to 10th	Day	20% 5% 2 5% 4 3 4 3 4 5% 44%	4476
	11th to 20th	Day	12% 7% 2 6% 4 5% 4 4 5% 7% 45%	4978
	21st to end	Day	18% 6% 2 6% 4 4 4 3 5% 6% 43%	5786
April	1st to 10th	Day	16% 7% 3 7% 5% 4 4 4 5% 7% 39%	2400
	11th to 20th	Day	15% 7% 2 5% 5% 4 4 4 6% 7% 41%	2374
	21st to end	Day	15% 5% 3 7% 5% 4 5% 5% 5% 7% 40%	2400
May	1st to 10th	Day	26% 7% 3 6% 4 4 5% 4 5% 7% 29%	5847
	11th to 20th	Day	24% 8% 3 6% 4 4 4 3 5% 6% 32%	6318
	21st to end	Day	19% 6% 2 6% 4 4 4 4 6% 9% 36%	6949
June	1st to 10th	Day	13% 5% 3 7% 4 5% 5% 5% 7% 8% 39%	2800
	11th to 20th	Day	13% 6% 2 6% 5% 5% 6% 4 7% 9% 37%	2800
	21st to end	Day	11% 5% 2 5% 4 4 5% 5% 7% 10% 41%	2800
July	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 4 8% 11% 38%	6360
	11th to 20th	Day	11% 8% 3 7% 5% 6% 6% 5% 8% 10% 31%	6360
	21st to end	Day	11% 7% 2 7% 4 4 5% 4 6% 9% 40%	6836
August	1st to 10th	Day	19% 6% 2 6% 5% 5% 5% 4 6% 8% 35%	4234
	11th to 20th	Day	13% 5% 2 6% 4 3 4 4 7% 9% 43%	3857
	21st to end	Day	11% 5% 2 5% 4 3 4 3 5% 9% 49%	4312
September	1st to 10th	Day	14% 6% 2 6% 4 4 4 4 5% 6% 46%	2400
	11th to 20th	Day	13% 5% 2 4 3 3 4 3 4 9% 51%	2368
	21st to end	Day	18% 7% 3 7% 5% 5% 5% 3 5% 6% 37%	2200
October	1st to 10th	Day	17% 6% 2 5% 2 4 4 4 5% 7% 43%	1931
	11th to 20th	Day	15% 5% 2 4 4 3 4 3 6% 6% 48%	1900
	21st to end	Day	19% 6% 2 4 3 3 3 3 5% 5% 46%	2019
November	1st to 10th	Day	12% 7% 2 5% 3 3 3 3 4 6% 53%	1761
	11th to 20th	Day	16% 5% 2 5% 4 3 3 3 5% 6% 48%	1800
	21st to end	Day	15% 7% 2 4 4 3 4 4 6% 8% 44%	1665
December	1st to 10th	Day	14% 9% 2 6% 4 3 4 4 5% 7% 43%	2600
	11th to 20th	Day	20% 10% 2 4 3 2 3 3 4 6% 42%	2986
	21st to end	Day	16% 6% 2 6% 4 5% 5% 4 6% 7% 38%	3960
January	1st to 10th	Night	13% 6% 2 6% 4 4 4 3 4 6% 49%	6349
	11th to 20th	Night	15% 7% 2 6% 3 3 4 3 4 6% 48%	6209
	21st to end	Night	17% 9% 2 5% 4 3 4 3 4 5% 43%	6490
February	1st to 10th	Night	17% 9% 3 5% 4 4 4 4 4 6% 40%	2600
	11th to 20th	Night	18% 8% 2 7% 5% 4 3 3 5% 5% 40%	2386
	21st to end	Night	17% 8% 3 7% 3 4 4 3 4 6% 42%	1980
March	1st to 10th	Night	18% 8% 2 5% 3 3 4 3 5% 5% 44%	5364
	11th to 20th	Night	11% 7% 3 6% 3 4 3 3 4 6% 50%	4979
	21st to end	Night	16% 8% 3 6% 4 4 3 3 4 6% 43%	5132
April	1st to 10th	Night	17% 7% 2 6% 5% 4 4 4 6% 6% 39%	2000
	11th to 20th	Night	16% 6% 3 7% 4 4 4 3 5% 6% 41%	1975
	21st to end	Night	18% 5% 2 5% 3 4 4 3 6% 8% 43%	2000
May	1st to 10th	Night	23% 9% 2 7% 4 4 5% 4 5% 8% 29%	4122
	11th to 20th	Night	22% 9% 3 8% 4 3 4 3 5% 7% 32%	3575
	21st to end	Night	21% 6% 2 6% 3 4 3 4 6% 8% 37%	3926
June	1st to 10th	Night	15% 7% 2 6% 4 4 4 3 6% 7% 42%	1600
	11th to 20th	Night	11% 8% 3 6% 6% 5% 4 5% 6% 8% 40%	1600
	21st to end	Night	11% 5% 2 5% 4 4 5% 6% 7% 9% 42%	1600
July	1st to 10th	Night	10% 6% 3 8% 6% 5% 5% 4 7% 9% 38%	3600
	11th to 20th	Night	14% 9% 3 9% 5% 5% 5% 5% 7% 10% 30%	3600
	21st to end	Night	12% 6% 2 6% 4 4 4 4 7% 8% 44%	4011
August	1st to 10th	Night	22% 7% 3 5% 4 4 4 4 6% 8% 33%	2915
	11th to 20th	Night	13% 7% 3 6% 5% 4 4 4 5% 7% 42%	3157
	21st to end	Night	11% 5% 2 5% 3 4 4 3 5% 8% 49%	3586
September	1st to 10th	Night	13% 7% 3 6% 5% 3 4 3 4 6% 47%	2000
	11th to 20th	Night	11% 6% 2 5% 3 3 3 3 5% 6% 52%	2031
	21st to end	Night	18% 7% 3 6% 4 4 4 4 5% 7% 39%	2200
October	1st to 10th	Night	18% 7% 2 6% 4 4 3 4 4 5% 43%	2312
	11th to 20th	Night	14% 5% 2 5% 3 3 4 4 4 6% 51%	2280
	21st to end	Night	17% 7% 2 5% 2 3 3 3 4 6% 48%	2577
November	1st to 10th	Night	11% 6% 2 5% 3 3 3 3 4 6% 54%	2537
	11th to 20th	Night	17% 7% 2 5% 4 3 3 3 4 5% 48%	2600
	21st to end	Night	14% 5% 2 5% 4 4 5% 4 5% 7% 45%	2731
December	1st to 10th	Night	13% 8% 3 6% 4 4 4 3 5% 6% 47%	4580
	11th to 20th	Night	23% 7% 2 5% 3 3 4 3 4 6% 41%	5208
	21st to end	Night	19% 8% 3 6% 4 3 4 3 5% 5% 40%	6996

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	34% 4 10% 28% 22% 3	3598
	11th to 20th	Day	43% 6% 8% 22% 15% 5%	3691
	21st to end	Day	45% 6% 11% 23% 15% 1	4466
February	1st to 10th	Day	43% 7% 14% 24% 11% 2	1800
	11th to 20th	Day	45% 4 10% 24% 15% 2	1907
	21st to end	Day	45% 5% 11% 24% 12% 3	1650
March	1st to 10th	Day	40% 5% 9% 24% 15% 7%	4479
	11th to 20th	Day	37% 6% 10% 29% 13% 5%	4978
	21st to end	Day	43% 4 10% 16% 23% 5%	5786
April	1st to 10th	Day	43% 6% 11% 21% 15% 5%	2400
	11th to 20th	Day	41% 6% 12% 22% 16% 4	2374
	21st to end	Day	40% 7% 11% 20% 18% 4	2400
May	1st to 10th	Day	52% 7% 14% 17% 9% 1	5857
	11th to 20th	Day	51% 6% 9% 22% 11% 1	6322
	21st to end	Day	44% 7% 11% 23% 14% 1	6949
June	1st to 10th	Day	40% 8% 14% 23% 15% 1	2800
	11th to 20th	Day	41% 9% 13% 24% 11% 2	2800
	21st to end	Day	36% 9% 14% 24% 16% 1	2800
July	1st to 10th	Day	40% 9% 14% 20% 15% 2	6290
	11th to 20th	Day	45% 8% 17% 19% 10% 1	6285
	21st to end	Day	39% 7% 10% 21% 22% 1	6640
August	1st to 10th	Day	46% 8% 13% 23% 10%	4221
	11th to 20th	Day	36% 7% 11% 30% 14% 1	3893
	21st to end	Day	33% 6% 10% 27% 22% 2	4312
September	1st to 10th	Day	38% 7% 11% 20% 21% 3	2400
	11th to 20th	Day	32% 5% 10% 26% 23% 4	2368
	21st to end	Day	48% 5% 8% 17% 19% 2	2200
October	1st to 10th	Day	36% 5% 9% 18% 27% 5%	3191
	11th to 20th	Day	24% 5% 11% 25% 28% 7%	3140
	21st to end	Day	25% 4 14% 25% 29% 5%	3309
November	1st to 10th	Day	34% 4 13% 25% 21% 3	1761
	11th to 20th	Day	37% 5% 10% 26% 19% 3	1800
	21st to end	Day	37% 5% 14% 30% 11% 2	1665
December	1st to 10th	Day	40% 4 16% 27% 11% 2	2600
	11th to 20th	Day	43% 4 11% 29% 10% 2	2986
	21st to end	Day	41% 7% 12% 22% 16% 1	3960
January	1st to 10th	Night	35% 5% 9% 25% 21% 5%	6349
	11th to 20th	Night	38% 4 11% 24% 16% 6%	6254
	21st to end	Night	43% 4 10% 22% 18% 3	6490
February	1st to 10th	Night	44% 6% 12% 25% 11% 2	2600
	11th to 20th	Night	45% 5% 9% 21% 17% 4	2388
	21st to end	Night	43% 4 8% 22% 18% 4	1980
March	1st to 10th	Night	42% 5% 9% 24% 12% 9%	5374
	11th to 20th	Night	36% 6% 13% 27% 14% 5%	4980
	21st to end	Night	43% 4 7% 17% 24% 5%	5132
April	1st to 10th	Night	44% 6% 10% 20% 16% 4	2000
	11th to 20th	Night	43% 6% 10% 20% 19% 3	1975
	21st to end	Night	39% 5% 10% 21% 19% 5%	2000
May	1st to 10th	Night	52% 7% 11% 16% 10% 2	4128
	11th to 20th	Night	51% 7% 12% 17% 13% 1	3576
	21st to end	Night	44% 8% 12% 22% 13% 2	3927
June	1st to 10th	Night	40% 7% 10% 21% 20% 2	1600
	11th to 20th	Night	39% 9% 13% 25% 13% 1	1600
	21st to end	Night	34% 11% 15% 24% 14% 2	1600
July	1st to 10th	Night	42% 9% 16% 18% 14% 2	3560
	11th to 20th	Night	48% 8% 17% 18% 9% 1	3552
	21st to end	Night	36% 7% 14% 23% 20% 1	3893
August	1st to 10th	Night	47% 8% 17% 17% 9% 1	2904
	11th to 20th	Night	40% 7% 12% 26% 14% 1	3187
	21st to end	Night	33% 6% 10% 28% 21% 2	3586
September	1st to 10th	Night	39% 6% 11% 20% 21% 3	2000
	11th to 20th	Night	33% 5% 10% 22% 26% 4	2031
	21st to end	Night	43% 7% 8% 20% 20% 2	2200
October	1st to 10th	Night	42% 4 11% 21% 21% 2	3832
	11th to 20th	Night	24% 4 10% 29% 29% 5%	3779
	21st to end	Night	27% 4 11% 31% 23% 4	4237
November	1st to 10th	Night	32% 5% 13% 24% 21% 5%	2537
	11th to 20th	Night	40% 4 9% 23% 20% 5%	2600
	21st to end	Night	38% 5% 13% 27% 15% 2	2731
December	1st to 10th	Night	39% 4 12% 30% 13% 2	4580
	11th to 20th	Night	44% 4 10% 25% 13% 3	5208
	21st to end	Night	45% 5% 10% 21% 17% 2	6996

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% 29% 29% 11% 3 2	200
	11th to 20th	Day	38% 24% 28% 9% 1	220
	21st to end	Day	27% 29% 28% 12% 5%	330
February	1st to 10th	Day	30% 26% 33% 8% 4	300
	11th to 20th	Day	33% 23% 33% 10% 1	446
	21st to end	Day	29% 24% 32% 12% 3	414
March	1st to 10th	Day	29% 20% 31% 16% 3	500
	11th to 20th	Day	25% 24% 33% 14% 3 1	500
	21st to end	Day	27% 25% 30% 13% 5% 1	550
April	1st to 10th	Day	30% 22% 29% 13% 5% 1	500
	11th to 20th	Day	23% 24% 30% 15% 6% 2	492
	21st to end	Day	24% 20% 32% 18% 4 2	500
May	1st to 10th	Day	29% 19% 29% 16% 6% 1	503
	11th to 20th	Day	24% 13% 30% 23% 8% 2	527
	21st to end	Day	21% 9% 26% 25% 15% 3	573
June	1st to 10th	Day	17% 12% 29% 26% 12% 3	530
	11th to 20th	Day	17% 12% 28% 27% 12% 3	530
	21st to end	Day	14% 10% 30% 30% 12% 4	530
July	1st to 10th	Day	11% 6% 23% 32% 22% 6%	525
	11th to 20th	Day	9% 4 25% 32% 26% 5%	524
	21st to end	Day	13% 6% 28% 33% 16% 4	576
August	1st to 10th	Day	11% 6% 26% 33% 18% 6%	526
	11th to 20th	Day	15% 5% 19% 34% 20% 7%	519
	21st to end	Day	12% 8% 26% 30% 17% 7%	583
September	1st to 10th	Day	17% 10% 23% 27% 16% 7%	530
	11th to 20th	Day	15% 6% 25% 30% 15% 9%	530
	21st to end	Day	25% 13% 22% 23% 12% 5%	530
October	1st to 10th	Day	24% 8% 26% 21% 14% 7%	502
	11th to 20th	Day	17% 16% 29% 24% 10% 3	490
	21st to end	Day	22% 15% 31% 18% 12% 2	502
November	1st to 10th	Day	19% 12% 33% 23% 11% 2	395
	11th to 20th	Day	21% 19% 35% 21% 5% 1	400
	21st to end	Day	25% 19% 34% 17% 4	265
December	1st to 10th	Day	29% 25% 33% 8% 6%	200
	11th to 20th	Day	34% 20% 31% 11% 5%	200
	21st to end	Day	27% 19% 33% 14% 7%	220
January	1st to 10th	Night	21% 20% 38% 18% 3	800
	11th to 20th	Night	25% 18% 31% 23% 3	800
	21st to end	Night	18% 20% 38% 22% 1	880
February	1st to 10th	Night	22% 16% 38% 23% 1	800
	11th to 20th	Night	27% 21% 35% 16% 1	783
	21st to end	Night	23% 19% 39% 17% 2	660
March	1st to 10th	Night	23% 19% 34% 21% 3	800
	11th to 20th	Night	22% 19% 36% 19% 5%	800
	21st to end	Night	21% 14% 41% 19% 5%	880
April	1st to 10th	Night	22% 20% 38% 16% 5%	800
	11th to 20th	Night	20% 14% 37% 23% 5%	791
	21st to end	Night	21% 12% 34% 28% 5%	800
May	1st to 10th	Night	23% 12% 34% 25% 6%	642
	11th to 20th	Night	21% 9% 37% 27% 6%	394
	21st to end	Night	17% 7% 25% 38% 12% 1	433
June	1st to 10th	Night	17% 5% 30% 37% 10% 1	400
	11th to 20th	Night	12% 6% 30% 40% 12% 1	400
	21st to end	Night	10% 4 28% 47% 11%	400
July	1st to 10th	Night	10% 5% 23% 43% 17% 2	390
	11th to 20th	Night	8% 5% 21% 44% 21% 1	388
	21st to end	Night	9% 3 26% 40% 21% 1	473
August	1st to 10th	Night	14% 2 16% 44% 22% 2	597
	11th to 20th	Night	13% 4 19% 38% 25% 2	762
	21st to end	Night	14% 4 20% 38% 21% 2	880
September	1st to 10th	Night	16% 5% 26% 35% 17% 2	800
	11th to 20th	Night	14% 4 21% 36% 21% 4	800
	21st to end	Night	23% 5% 22% 32% 18% 1	800
October	1st to 10th	Night	22% 8% 24% 26% 16% 4	773
	11th to 20th	Night	17% 11% 27% 32% 13%	760
	21st to end	Night	20% 13% 31% 26% 9%	836
November	1st to 10th	Night	15% 9% 36% 28% 11% 1	780
	11th to 20th	Night	21% 16% 30% 26% 7% 1	800
	21st to end	Night	17% 16% 35% 25% 7% 1	800
December	1st to 10th	Night	24% 16% 34% 20% 6% 1	800
	11th to 20th	Night	22% 14% 37% 23% 5%	800
	21st to end	Night	22% 16% 33% 27% 3	880

Page generated at 2025-10-01 01:39:02. Version ac07

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