Young's Astronomy Club and Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Young's Astronomy Club and Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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	13% 11% 4 10% 6% 5% 5% 4 5% 6% 32%	127266
	Night	13% 11% 4 10% 6% 5% 4 4 5% 6% 32%	123373
Transparency	Day	50% 6% 11% 21% 11% 1	130697
	Night	51% 6% 12% 20% 10% 1	128028
Seeing	Day	22% 16% 32% 19% 8% 3	17501
	Night	20% 13% 32% 25% 9% 1	27352

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	7% 12% 7% 19% 14% 8% 5% 3 5% 6% 16%	3617
	11th to 20th	Day	7% 16% 6% 11% 12% 7% 9% 5% 6% 6% 14%	3589
	21st to end	Day	9% 15% 9% 18% 5% 5% 5% 3 5% 5% 20%	3982
February	1st to 10th	Day	9% 13% 5% 12% 7% 7% 6% 4 6% 6% 26%	1729
	11th to 20th	Day	8% 13% 5% 12% 8% 7% 6% 3 6% 6% 25%	1858
	21st to end	Day	12% 15% 6% 12% 6% 6% 5% 3 6% 6% 23%	1650
March	1st to 10th	Day	13% 11% 3 8% 5% 5% 4 3 5% 6% 37%	4478
	11th to 20th	Day	11% 13% 3 13% 8% 5% 5% 4 5% 6% 27%	4691
	21st to end	Day	21% 14% 6% 13% 5% 4 3 2 4 4 23%	5969
April	1st to 10th	Day	17% 15% 4 9% 5% 5% 5% 4 5% 7% 26%	2400
	11th to 20th	Day	20% 13% 5% 10% 4 4 3 3 4 5% 29%	2374
	21st to end	Day	17% 12% 3 7% 6% 5% 5% 4 5% 6% 29%	2584
May	1st to 10th	Day	14% 8% 3 7% 4 4 3 3 4 5% 45%	6244
	11th to 20th	Day	12% 10% 3 7% 4 4 4 4 5% 6% 40%	6319
	21st to end	Day	12% 10% 4 9% 5% 5% 5% 3 6% 7% 34%	6949
June	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 4 5% 8% 37%	2800
	11th to 20th	Day	15% 7% 3 8% 6% 5% 5% 4 6% 8% 34%	2800
	21st to end	Day	14% 7% 2 7% 6% 5% 5% 5% 7% 7% 34%	2800
July	1st to 10th	Day	12% 5% 2 6% 4 5% 5% 4 7% 8% 41%	6359
	11th to 20th	Day	10% 4 2 6% 4 4 4 4 7% 9% 47%	6360
	21st to end	Day	8% 6% 2 6% 5% 4 5% 5% 6% 9% 44%	6926
August	1st to 10th	Day	11% 6% 2 8% 6% 5% 6% 5% 7% 7% 38%	4563
	11th to 20th	Day	13% 6% 2 6% 4 4 3 4 6% 7% 46%	4198
	21st to end	Day	16% 8% 2 6% 5% 4 4 3 4 7% 41%	4312
September	1st to 10th	Day	15% 7% 2 6% 5% 5% 4 4 5% 8% 40%	2400
	11th to 20th	Day	13% 9% 3 8% 5% 4 5% 3 5% 7% 40%	2309
	21st to end	Day	16% 10% 3 8% 5% 4 5% 4 5% 8% 35%	2200
October	1st to 10th	Day	17% 10% 3 7% 4 3 3 4 6% 8% 35%	2070
	11th to 20th	Day	20% 15% 5% 10% 5% 5% 4 4 6% 5% 22%	1937
	21st to end	Day	20% 18% 4 9% 5% 4 4 3 4 6% 23%	1980
November	1st to 10th	Day	16% 15% 4 8% 5% 5% 4 4 5% 6% 28%	1852
	11th to 20th	Day	16% 17% 7% 13% 7% 5% 4 3 5% 5% 19%	1771
	21st to end	Day	15% 15% 7% 15% 7% 6% 4 3 5% 6% 17%	1600
December	1st to 10th	Day	8% 15% 7% 27% 10% 6% 4 3 4 3 14%	2610
	11th to 20th	Day	10% 22% 7% 18% 7% 4 4 3 3 5% 17%	3004
	21st to end	Day	19% 25% 7% 11% 4 4 3 2 3 5% 16%	3982
January	1st to 10th	Night	10% 13% 5% 17% 12% 6% 5% 3 4 6% 19%	6348
	11th to 20th	Night	8% 15% 6% 14% 9% 8% 7% 5% 6% 6% 16%	6316
	21st to end	Night	8% 13% 7% 16% 8% 6% 5% 4 5% 6% 22%	6996
February	1st to 10th	Night	9% 13% 5% 11% 6% 6% 6% 5% 7% 7% 26%	2670
	11th to 20th	Night	8% 11% 4 12% 8% 7% 6% 5% 6% 6% 26%	2429
	21st to end	Night	12% 16% 5% 9% 7% 5% 4 4 5% 7% 26%	1980
March	1st to 10th	Night	11% 8% 3 9% 6% 5% 5% 3 6% 8% 36%	5362
	11th to 20th	Night	10% 13% 5% 12% 5% 5% 5% 4 5% 7% 29%	5298
	21st to end	Night	20% 11% 5% 10% 7% 5% 4 3 4 6% 25%	4962
April	1st to 10th	Night	17% 14% 4 9% 4 4 5% 4 5% 6% 29%	2000
	11th to 20th	Night	20% 10% 4 9% 4 4 2 3 5% 5% 34%	1975
	21st to end	Night	16% 10% 3 9% 5% 5% 4 3 4 6% 35%	1815
May	1st to 10th	Night	13% 9% 3 6% 3 4 3 3 4 6% 46%	3711
	11th to 20th	Night	12% 10% 4 7% 5% 3 4 3 4 6% 42%	3596
	21st to end	Night	14% 8% 2 7% 4 5% 4 4 6% 6% 39%	3949
June	1st to 10th	Night	12% 8% 2 6% 5% 4 4 3 5% 8% 43%	1600
	11th to 20th	Night	13% 8% 3 7% 5% 4 4 5% 5% 7% 39%	1600
	21st to end	Night	14% 7% 2 6% 4 4 4 4 7% 9% 40%	1600
July	1st to 10th	Night	12% 5% 2 5% 4 3 4 3 6% 9% 46%	3620
	11th to 20th	Night	10% 5% 2 5% 3 3 3 3 5% 8% 52%	3620
	21st to end	Night	8% 6% 2 6% 3 4 4 3 6% 8% 51%	3948
August	1st to 10th	Night	11% 7% 2 7% 4 4 3 3 6% 8% 44%	2597
	11th to 20th	Night	13% 6% 2 5% 3 4 4 2 5% 7% 49%	2851
	21st to end	Night	15% 8% 3 6% 4 3 4 3 4 5% 46%	3586
September	1st to 10th	Night	12% 7% 2 5% 3 5% 3 3 5% 8% 47%	2000
	11th to 20th	Night	13% 6% 2 5% 4 4 4 3 5% 8% 46%	2070
	21st to end	Night	14% 9% 3 7% 5% 4 4 4 5% 6% 40%	2200
October	1st to 10th	Night	15% 10% 2 6% 4 5% 3 3 5% 6% 41%	2325
	11th to 20th	Night	17% 12% 4 8% 4 4 5% 3 5% 7% 30%	2461
	21st to end	Night	19% 15% 3 8% 5% 4 5% 3 5% 5% 26%	2860
November	1st to 10th	Night	16% 12% 4 8% 5% 5% 5% 3 6% 7% 29%	2666
	11th to 20th	Night	15% 14% 6% 12% 6% 4 4 3 4 6% 25%	2783
	21st to end	Night	15% 13% 6% 13% 7% 5% 4 3 6% 7% 20%	2800
December	1st to 10th	Night	12% 12% 6% 20% 8% 6% 4 4 6% 5% 17%	4580
	11th to 20th	Night	11% 16% 7% 16% 9% 6% 4 4 4 4 20%	5203
	21st to end	Night	20% 20% 6% 11% 6% 4 4 3 4 4 17%	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	68% 6% 8% 12% 5%	3618
	11th to 20th	Day	69% 7% 9% 11% 3 1	3620
	21st to end	Day	65% 6% 10% 12% 7% 1	3982
February	1st to 10th	Day	57% 6% 11% 16% 8% 1	1729
	11th to 20th	Day	58% 6% 10% 18% 8%	1867
	21st to end	Day	59% 7% 9% 15% 9% 1	1650
March	1st to 10th	Day	47% 5% 15% 17% 15% 2	4480
	11th to 20th	Day	57% 6% 12% 15% 9% 1	4691
	21st to end	Day	65% 5% 8% 12% 9%	5969
April	1st to 10th	Day	58% 6% 10% 17% 9%	2400
	11th to 20th	Day	58% 6% 7% 20% 8% 1	2374
	21st to end	Day	54% 6% 8% 18% 12% 2	2584
May	1st to 10th	Day	42% 5% 8% 25% 19% 2	6253
	11th to 20th	Day	43% 6% 9% 27% 11% 3	6323
	21st to end	Day	48% 6% 8% 19% 17% 1	6949
June	1st to 10th	Day	42% 7% 13% 24% 13% 1	2800
	11th to 20th	Day	45% 8% 14% 19% 14% 1	2800
	21st to end	Day	45% 9% 13% 22% 10% 1	2800
July	1st to 10th	Day	38% 7% 14% 30% 10% 1	6291
	11th to 20th	Day	32% 6% 16% 31% 13% 1	6285
	21st to end	Day	35% 6% 12% 31% 15%	6728
August	1st to 10th	Day	42% 7% 15% 24% 11% 1	4548
	11th to 20th	Day	36% 7% 16% 30% 10%	4240
	21st to end	Day	43% 5% 14% 24% 13% 1	4311
September	1st to 10th	Day	41% 6% 14% 28% 10%	2400
	11th to 20th	Day	42% 7% 19% 25% 6%	2309
	21st to end	Day	47% 7% 13% 25% 9%	2200
October	1st to 10th	Day	36% 5% 12% 33% 14%	3369
	11th to 20th	Day	57% 5% 10% 17% 11%	3127
	21st to end	Day	59% 5% 14% 18% 4	3180
November	1st to 10th	Day	56% 6% 10% 18% 8% 1	1852
	11th to 20th	Day	66% 6% 10% 12% 6%	1771
	21st to end	Day	68% 7% 12% 10% 3 1	1600
December	1st to 10th	Day	75% 5% 7% 10% 3	2610
	11th to 20th	Day	70% 6% 6% 10% 8%	3005
	21st to end	Day	71% 4 5% 14% 6%	3982
January	1st to 10th	Night	66% 6% 8% 16% 4	6348
	11th to 20th	Night	65% 8% 9% 12% 6%	6360
	21st to end	Night	62% 6% 11% 16% 5%	6996
February	1st to 10th	Night	56% 5% 11% 16% 10% 1	2670
	11th to 20th	Night	56% 7% 11% 19% 7% 1	2430
	21st to end	Night	55% 6% 13% 18% 7%	1980
March	1st to 10th	Night	44% 6% 18% 17% 13% 1	5373
	11th to 20th	Night	53% 6% 14% 19% 8%	5299
	21st to end	Night	60% 6% 9% 15% 9% 1	4962
April	1st to 10th	Night	54% 6% 10% 16% 12% 1	2000
	11th to 20th	Night	52% 5% 9% 21% 11% 2	1975
	21st to end	Night	50% 5% 11% 18% 15% 1	1815
May	1st to 10th	Night	39% 5% 13% 25% 16% 2	3718
	11th to 20th	Night	42% 7% 10% 21% 18% 2	3597
	21st to end	Night	43% 8% 11% 21% 15% 2	3950
June	1st to 10th	Night	38% 8% 13% 27% 13% 1	1600
	11th to 20th	Night	43% 8% 13% 22% 13% 1	1600
	21st to end	Night	40% 7% 16% 26% 11% 2	1600
July	1st to 10th	Night	35% 7% 19% 28% 10% 2	3580
	11th to 20th	Night	30% 6% 17% 35% 11% 1	3572
	21st to end	Night	30% 6% 16% 29% 18% 1	3832
August	1st to 10th	Night	38% 6% 15% 23% 17% 1	2588
	11th to 20th	Night	36% 6% 17% 26% 12% 2	2875
	21st to end	Night	40% 6% 12% 26% 14% 1	3586
September	1st to 10th	Night	35% 7% 14% 30% 13% 1	2000
	11th to 20th	Night	37% 7% 14% 32% 8%	2070
	21st to end	Night	44% 8% 14% 25% 9% 1	2200
October	1st to 10th	Night	35% 4 9% 36% 15% 1	3806
	11th to 20th	Night	45% 5% 12% 26% 12%	4010
	21st to end	Night	57% 6% 11% 21% 6%	4610
November	1st to 10th	Night	53% 6% 12% 19% 9%	2666
	11th to 20th	Night	60% 7% 13% 12% 7% 1	2783
	21st to end	Night	62% 8% 14% 13% 3 1	2800
December	1st to 10th	Night	66% 8% 13% 11% 2	4580
	11th to 20th	Night	67% 5% 8% 13% 7%	5201
	21st to end	Night	70% 5% 5% 13% 7%	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	21% 15% 37% 21% 4 2	300
	11th to 20th	Day	24% 18% 36% 18% 4	300
	21st to end	Day	22% 15% 45% 15% 3	330
February	1st to 10th	Day	22% 25% 39% 10% 4 1	300
	11th to 20th	Day	20% 33% 33% 12% 3	300
	21st to end	Day	27% 29% 36% 8% 1	249
March	1st to 10th	Day	23% 24% 40% 12% 1	300
	11th to 20th	Day	29% 23% 31% 14% 3	307
	21st to end	Day	30% 20% 29% 13% 7% 1	550
April	1st to 10th	Day	32% 16% 31% 17% 4 1	500
	11th to 20th	Day	30% 20% 28% 15% 5% 1	492
	21st to end	Day	30% 19% 25% 20% 5% 3	500
May	1st to 10th	Day	24% 19% 31% 17% 7% 2	651
	11th to 20th	Day	22% 18% 34% 16% 6% 3	725
	21st to end	Day	20% 16% 35% 19% 8% 3	789
June	1st to 10th	Day	18% 18% 33% 21% 7% 4	730
	11th to 20th	Day	22% 12% 36% 20% 8% 2	730
	21st to end	Day	20% 12% 33% 22% 10% 3	730
July	1st to 10th	Day	10% 11% 37% 25% 13% 5%	721
	11th to 20th	Day	14% 11% 35% 25% 11% 5%	718
	21st to end	Day	9% 11% 36% 24% 14% 5%	794
August	1st to 10th	Day	14% 10% 31% 24% 16% 5%	726
	11th to 20th	Day	17% 8% 24% 27% 18% 6%	612
	21st to end	Day	21% 12% 24% 22% 16% 5%	583
September	1st to 10th	Day	20% 13% 26% 22% 14% 5%	530
	11th to 20th	Day	15% 11% 30% 29% 12% 3	530
	21st to end	Day	20% 14% 26% 25% 12% 3	530
October	1st to 10th	Day	25% 13% 33% 19% 9% 2	400
	11th to 20th	Day	27% 23% 36% 11% 3	330
	21st to end	Day	31% 24% 33% 10% 2 1	363
November	1st to 10th	Day	30% 20% 32% 14% 4 1	330
	11th to 20th	Day	29% 25% 33% 9% 3 1	321
	21st to end	Day	27% 21% 38% 9% 4	300
December	1st to 10th	Day	20% 20% 40% 15% 5% 1	300
	11th to 20th	Day	29% 26% 31% 11% 3	300
	21st to end	Day	32% 22% 31% 14% 2 1	330
January	1st to 10th	Night	18% 18% 41% 20% 3 1	1000
	11th to 20th	Night	20% 17% 40% 15% 7% 1	1000
	21st to end	Night	19% 14% 40% 21% 6%	1100
February	1st to 10th	Night	18% 17% 40% 18% 6% 1	1000
	11th to 20th	Night	18% 21% 33% 21% 6%	875
	21st to end	Night	21% 20% 39% 17% 3	660
March	1st to 10th	Night	19% 15% 35% 24% 6% 1	800
	11th to 20th	Night	19% 20% 34% 22% 4	758
	21st to end	Night	21% 12% 31% 27% 8% 1	660
April	1st to 10th	Night	23% 15% 33% 23% 5% 1	600
	11th to 20th	Night	26% 14% 27% 25% 6% 1	593
	21st to end	Night	21% 14% 33% 25% 8%	600
May	1st to 10th	Night	21% 13% 30% 29% 7% 1	600
	11th to 20th	Night	20% 10% 34% 28% 7% 1	591
	21st to end	Night	15% 10% 30% 34% 11%	650
June	1st to 10th	Night	16% 10% 28% 35% 11%	600
	11th to 20th	Night	17% 9% 29% 35% 10% 1	600
	21st to end	Night	18% 8% 28% 34% 12% 1	600
July	1st to 10th	Night	10% 4 28% 41% 16%	585
	11th to 20th	Night	11% 6% 30% 35% 17% 1	582
	21st to end	Night	8% 4 30% 38% 18% 2	657
August	1st to 10th	Night	12% 4 26% 37% 19% 2	597
	11th to 20th	Night	14% 3 20% 35% 26% 2	594
	21st to end	Night	16% 6% 26% 31% 20% 1	660
September	1st to 10th	Night	17% 7% 23% 32% 18% 3	600
	11th to 20th	Night	17% 6% 26% 34% 16% 1	670
	21st to end	Night	20% 9% 28% 30% 13% 1	800
October	1st to 10th	Night	21% 12% 31% 24% 11% 1	800
	11th to 20th	Night	25% 16% 29% 24% 6%	800
	21st to end	Night	29% 13% 26% 25% 7%	880
November	1st to 10th	Night	23% 13% 36% 21% 5% 1	820
	11th to 20th	Night	25% 20% 34% 18% 4	920
	21st to end	Night	24% 20% 31% 21% 4	1000
December	1st to 10th	Night	21% 19% 38% 17% 5% 1	1000
	11th to 20th	Night	20% 17% 36% 20% 5% 2	1000
	21st to end	Night	29% 16% 34% 17% 4	1100

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