James Wylie Shepherd Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the James Wylie Shepherd Observatory clear sky chart from 2005-11-06 to 2026-01-02.

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% 6% 2 6% 4 5% 5% 4 6% 8% 42%	126927
	Night	13% 7% 2 6% 4 4 4 4 5% 7% 45%	124455
Transparency	Day	38% 8% 11% 20% 18% 6%	130723
	Night	36% 7% 10% 20% 20% 7%	128733
Seeing	Day	14% 10% 23% 25% 19% 9%	16017
	Night	14% 8% 23% 30% 21% 4	26011

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	16% 9% 4 5% 3 3 2 3 5% 4 46%	4065
	11th to 20th	Day	12% 7% 5% 5% 3 3 3 2 4 5% 51%	4317
	21st to end	Day	22% 10% 2 5% 3 3 3 2 4 5% 39%	4972
February	1st to 10th	Day	18% 10% 3 6% 4 4 4 3 5% 5% 39%	2000
	11th to 20th	Day	22% 8% 2 5% 4 3 4 3 4 5% 40%	1948
	21st to end	Day	19% 8% 3 7% 4 3 4 3 4 4 40%	1650
March	1st to 10th	Day	20% 9% 2 6% 4 5% 4 3 5% 5% 36%	4480
	11th to 20th	Day	12% 6% 2 6% 4 4 4 4 5% 7% 47%	4764
	21st to end	Day	15% 7% 3 7% 5% 5% 4 3 4 7% 40%	5773
April	1st to 10th	Day	12% 6% 2 5% 4 4 4 3 5% 7% 47%	2400
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 3 5% 7% 46%	2374
	21st to end	Day	8% 5% 2 5% 4 5% 5% 4 6% 8% 49%	2400
May	1st to 10th	Day	13% 6% 3 7% 5% 4 5% 4 6% 9% 37%	5720
	11th to 20th	Day	16% 7% 2 7% 4 5% 5% 4 7% 9% 33%	5861
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 6% 9% 41%	6433
June	1st to 10th	Day	10% 7% 3 7% 5% 5% 6% 6% 7% 11% 34%	2600
	11th to 20th	Day	7% 4 2 6% 6% 4 6% 6% 8% 10% 42%	2600
	21st to end	Day	6% 5% 2 7% 6% 5% 7% 6% 8% 10% 39%	2600
July	1st to 10th	Day	6% 5% 2 7% 6% 7% 7% 6% 9% 13% 32%	5900
	11th to 20th	Day	6% 5% 2 6% 6% 6% 7% 6% 9% 14% 33%	5900
	21st to end	Day	7% 6% 3 8% 6% 7% 7% 7% 10% 13% 25%	6181
August	1st to 10th	Day	5% 3 1 4 4 5% 6% 5% 8% 11% 49%	3905
	11th to 20th	Day	7% 4 2 6% 5% 5% 5% 4 6% 9% 48%	3849
	21st to end	Day	7% 3 2 6% 5% 6% 6% 5% 7% 8% 46%	4312
September	1st to 10th	Day	10% 5% 2 7% 6% 6% 4 4 7% 8% 41%	2355
	11th to 20th	Day	13% 7% 2 6% 5% 4 4 4 5% 6% 43%	2200
	21st to end	Day	10% 5% 1 5% 5% 4 4 3 4 6% 52%	2200
October	1st to 10th	Day	9% 5% 2 5% 4 4 4 4 5% 7% 52%	2200
	11th to 20th	Day	9% 4 2 4 4 2 3 2 5% 7% 59%	2200
	21st to end	Day	12% 6% 1 5% 3 3 3 3 4 6% 54%	2088
November	1st to 10th	Day	12% 6% 2 4 3 4 4 2 4 5% 54%	1852
	11th to 20th	Day	13% 5% 2 4 4 3 3 3 4 6% 54%	1890
	21st to end	Day	14% 7% 2 5% 4 2 4 4 5% 6% 47%	1890
December	1st to 10th	Day	20% 7% 2 5% 4 3 3 3 4 6% 43%	3020
	11th to 20th	Day	14% 8% 3 7% 4 4 4 3 4 5% 45%	3463
	21st to end	Day	20% 8% 3 6% 3 3 3 2 4 6% 42%	4565
January	1st to 10th	Night	16% 10% 4 6% 2 3 3 2 4 4 46%	5903
	11th to 20th	Night	15% 8% 4 6% 3 3 3 3 4 5% 45%	5627
	21st to end	Night	20% 11% 2 5% 3 3 3 3 3 4 42%	5984
February	1st to 10th	Night	16% 8% 3 6% 4 3 3 4 5% 6% 42%	2400
	11th to 20th	Night	21% 8% 2 6% 3 3 3 3 4 5% 42%	2338
	21st to end	Night	18% 7% 2 6% 4 3 3 3 5% 6% 42%	1980
March	1st to 10th	Night	23% 9% 2 6% 4 3 4 3 5% 6% 35%	5373
	11th to 20th	Night	13% 7% 2 4 3 3 4 4 5% 7% 50%	5194
	21st to end	Night	14% 6% 2 7% 5% 4 4 3 4 6% 45%	5197
April	1st to 10th	Night	11% 6% 2 5% 4 5% 4 4 6% 7% 47%	2000
	11th to 20th	Night	13% 5% 2 5% 4 4 3 3 4 7% 51%	1975
	21st to end	Night	9% 5% 2 5% 4 4 4 3 6% 6% 53%	2000
May	1st to 10th	Night	12% 7% 3 7% 5% 5% 5% 4 6% 8% 38%	4174
	11th to 20th	Night	16% 8% 3 7% 5% 5% 4 4 6% 7% 34%	4032
	21st to end	Night	9% 5% 2 5% 4 5% 5% 5% 8% 10% 43%	4421
June	1st to 10th	Night	6% 5% 2 8% 6% 5% 7% 5% 8% 10% 38%	1800
	11th to 20th	Night	4 4 1 4 4 4 4 4 8% 11% 49%	1800
	21st to end	Night	5% 5% 2 5% 4 5% 6% 6% 8% 12% 43%	1800
July	1st to 10th	Night	7% 6% 3 7% 5% 7% 6% 5% 8% 11% 35%	4058
	11th to 20th	Night	6% 5% 3 7% 5% 5% 6% 5% 9% 15% 35%	4057
	21st to end	Night	7% 6% 3 8% 7% 7% 7% 6% 9% 11% 29%	4651
August	1st to 10th	Night	3 3 2 4 4 4 4 5% 8% 11% 52%	3244
	11th to 20th	Night	5% 4 1 5% 5% 3 4 4 7% 9% 52%	3200
	21st to end	Night	6% 4 1 5% 4 4 5% 4 7% 9% 51%	3586
September	1st to 10th	Night	9% 5% 3 6% 5% 5% 5% 4 6% 8% 43%	2025
	11th to 20th	Night	12% 6% 2 5% 4 4 5% 3 6% 9% 46%	2200
	21st to end	Night	7% 4 2 6% 5% 4 5% 4 6% 7% 51%	2200
October	1st to 10th	Night	8% 5% 2 5% 3 3 4 3 4 5% 59%	2200
	11th to 20th	Night	10% 6% 2 3 2 3 3 3 4 6% 59%	2200
	21st to end	Night	14% 6% 2 4 3 3 2 3 3 4 56%	2745
November	1st to 10th	Night	10% 4 2 5% 3 3 3 3 5% 6% 57%	2666
	11th to 20th	Night	11% 6% 2 4 3 2 3 3 5% 6% 56%	2730
	21st to end	Night	13% 6% 2 4 4 3 4 4 6% 6% 48%	2730
December	1st to 10th	Night	19% 7% 2 4 3 3 3 2 3 6% 49%	4380
	11th to 20th	Night	16% 8% 3 6% 4 3 3 3 4 5% 48%	4952
	21st to end	Night	20% 7% 3 6% 3 3 3 3 5% 6% 41%	6633

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	42% 4 5% 25% 21% 3	4066
	11th to 20th	Day	35% 4 5% 22% 29% 4	4317
	21st to end	Day	48% 3 6% 14% 17% 11%	4972
February	1st to 10th	Day	46% 4 8% 18% 18% 6%	2000
	11th to 20th	Day	47% 3 5% 16% 21% 7%	1958
	21st to end	Day	48% 5% 5% 13% 20% 8%	1650
March	1st to 10th	Day	48% 5% 7% 13% 20% 6%	4480
	11th to 20th	Day	36% 5% 7% 16% 19% 16%	4764
	21st to end	Day	43% 5% 8% 18% 20% 6%	5775
April	1st to 10th	Day	38% 5% 10% 20% 20% 8%	2400
	11th to 20th	Day	37% 6% 7% 18% 24% 8%	2374
	21st to end	Day	32% 7% 11% 22% 25% 3	2400
May	1st to 10th	Day	41% 9% 12% 23% 13% 2	5756
	11th to 20th	Day	44% 9% 16% 21% 8% 1	5865
	21st to end	Day	37% 9% 13% 24% 16% 1	6447
June	1st to 10th	Day	42% 10% 13% 19% 14% 1	2600
	11th to 20th	Day	32% 11% 18% 23% 15% 1	2600
	21st to end	Day	35% 10% 19% 22% 10% 4	2600
July	1st to 10th	Day	38% 14% 20% 20% 7% 1	5835
	11th to 20th	Day	35% 17% 28% 16% 4	5831
	21st to end	Day	44% 16% 20% 17% 4	6003
August	1st to 10th	Day	25% 9% 14% 32% 20% 1	3893
	11th to 20th	Day	31% 9% 16% 27% 14% 3	3885
	21st to end	Day	31% 11% 16% 20% 19% 3	4311
September	1st to 10th	Day	39% 9% 14% 16% 18% 5%	2355
	11th to 20th	Day	40% 8% 10% 19% 19% 4	2200
	21st to end	Day	34% 5% 11% 21% 22% 7%	2200
October	1st to 10th	Day	24% 4 6% 13% 30% 22%	3590
	11th to 20th	Day	17% 2 4 15% 30% 30%	3568
	21st to end	Day	23% 4 5% 18% 37% 12%	3351
November	1st to 10th	Day	34% 4 7% 17% 26% 12%	1852
	11th to 20th	Day	32% 4 7% 18% 28% 12%	1890
	21st to end	Day	37% 6% 8% 17% 24% 8%	1890
December	1st to 10th	Day	43% 4 5% 18% 26% 4	3020
	11th to 20th	Day	43% 4 8% 19% 20% 6%	3460
	21st to end	Day	44% 4 5% 23% 22% 2	4565
January	1st to 10th	Night	42% 4 4 23% 24% 3	5903
	11th to 20th	Night	41% 3 4 19% 24% 9%	5627
	21st to end	Night	47% 3 3 15% 26% 6%	5984
February	1st to 10th	Night	42% 4 7% 18% 20% 9%	2400
	11th to 20th	Night	46% 4 4 16% 19% 10%	2339
	21st to end	Night	42% 6% 8% 16% 18% 10%	1980
March	1st to 10th	Night	49% 5% 6% 15% 17% 9%	5373
	11th to 20th	Night	33% 6% 8% 15% 21% 17%	5194
	21st to end	Night	41% 4 6% 22% 19% 8%	5197
April	1st to 10th	Night	35% 7% 11% 19% 19% 9%	2000
	11th to 20th	Night	34% 4 7% 20% 24% 11%	1975
	21st to end	Night	31% 6% 11% 24% 22% 6%	2000
May	1st to 10th	Night	39% 11% 13% 20% 15% 2	4203
	11th to 20th	Night	46% 8% 15% 21% 8% 1	4033
	21st to end	Night	32% 10% 14% 25% 17% 2	4429
June	1st to 10th	Night	36% 12% 15% 18% 17% 1	1800
	11th to 20th	Night	26% 9% 20% 26% 16% 3	1800
	21st to end	Night	29% 12% 19% 20% 15% 4	1800
July	1st to 10th	Night	38% 12% 21% 21% 7% 1	4015
	11th to 20th	Night	33% 16% 30% 16% 4	4006
	21st to end	Night	44% 15% 21% 16% 4	4516
August	1st to 10th	Night	21% 10% 15% 36% 16% 2	3232
	11th to 20th	Night	26% 11% 18% 28% 14% 3	3230
	21st to end	Night	27% 8% 16% 21% 24% 4	3586
September	1st to 10th	Night	37% 9% 15% 17% 15% 7%	2025
	11th to 20th	Night	34% 9% 12% 20% 19% 5%	2200
	21st to end	Night	30% 7% 10% 23% 23% 7%	2200
October	1st to 10th	Night	23% 4 7% 14% 27% 25%	3590
	11th to 20th	Night	17% 3 4 16% 29% 31%	3571
	21st to end	Night	22% 3 4 21% 32% 17%	4432
November	1st to 10th	Night	29% 4 9% 17% 26% 14%	2666
	11th to 20th	Night	30% 4 7% 18% 28% 13%	2730
	21st to end	Night	36% 5% 7% 19% 24% 10%	2730
December	1st to 10th	Night	39% 3 6% 21% 26% 5%	4380
	11th to 20th	Night	40% 4 10% 16% 25% 5%	4954
	21st to end	Night	43% 3 7% 24% 21% 2	6633

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	23% 25% 32% 17% 3	303
	11th to 20th	Day	21% 24% 32% 16% 6%	300
	21st to end	Day	25% 25% 35% 13% 1 1	330
February	1st to 10th	Day	22% 23% 36% 16% 4	300
	11th to 20th	Day	35% 21% 31% 12% 1	300
	21st to end	Day	20% 24% 42% 11% 2	249
March	1st to 10th	Day	23% 24% 36% 12% 5% 1	300
	11th to 20th	Day	21% 20% 35% 18% 6%	401
	21st to end	Day	21% 17% 34% 19% 8%	550
April	1st to 10th	Day	20% 16% 31% 20% 12% 1	500
	11th to 20th	Day	16% 14% 35% 22% 11% 2	492
	21st to end	Day	11% 12% 29% 32% 15% 2	500
May	1st to 10th	Day	13% 8% 28% 32% 15% 4	503
	11th to 20th	Day	9% 5% 28% 30% 18% 9%	527
	21st to end	Day	10% 6% 24% 29% 17% 14%	573
June	1st to 10th	Day	11% 5% 16% 32% 24% 12%	530
	11th to 20th	Day	8% 4 14% 25% 31% 19%	530
	21st to end	Day	7% 2 13% 25% 33% 19%	530
July	1st to 10th	Day	7% 2 8% 24% 36% 23%	525
	11th to 20th	Day	5% 2 6% 24% 33% 30%	524
	21st to end	Day	5% 2 8% 22% 40% 23%	576
August	1st to 10th	Day	9% 3 8% 25% 32% 23%	526
	11th to 20th	Day	9% 1 9% 28% 34% 20%	519
	21st to end	Day	11% 3 11% 26% 29% 19%	583
September	1st to 10th	Day	6% 4 13% 36% 28% 13%	530
	11th to 20th	Day	13% 4 12% 27% 32% 11%	530
	21st to end	Day	10% 4 16% 30% 31% 9%	530
October	1st to 10th	Day	11% 8% 22% 32% 23% 5%	530
	11th to 20th	Day	9% 6% 26% 35% 20% 3	530
	21st to end	Day	18% 13% 24% 28% 14% 3	383
November	1st to 10th	Day	20% 9% 29% 29% 10% 4	330
	11th to 20th	Day	16% 10% 40% 26% 8%	330
	21st to end	Day	14% 20% 36% 21% 8% 1	330
December	1st to 10th	Day	28% 19% 32% 16% 4	330
	11th to 20th	Day	22% 18% 37% 18% 5% 1	330
	21st to end	Day	25% 18% 33% 20% 5%	363
January	1st to 10th	Night	21% 17% 34% 24% 5%	805
	11th to 20th	Night	21% 17% 27% 27% 8%	800
	21st to end	Night	20% 15% 35% 26% 3	880
February	1st to 10th	Night	20% 14% 33% 28% 5%	800
	11th to 20th	Night	26% 15% 31% 24% 4	783
	21st to end	Night	20% 15% 37% 25% 3	660
March	1st to 10th	Night	20% 15% 31% 30% 6%	800
	11th to 20th	Night	17% 14% 35% 26% 8%	800
	21st to end	Night	16% 12% 33% 29% 10%	765
April	1st to 10th	Night	13% 11% 36% 26% 13%	600
	11th to 20th	Night	14% 10% 33% 28% 13% 2	593
	21st to end	Night	10% 8% 30% 34% 17% 2	600
May	1st to 10th	Night	8% 7% 28% 32% 23% 3	600
	11th to 20th	Night	12% 4 23% 34% 23% 4	591
	21st to end	Night	8% 3 11% 36% 35% 7%	650
June	1st to 10th	Night	8% 2 11% 41% 34% 5%	600
	11th to 20th	Night	5% 2 11% 28% 43% 12%	600
	21st to end	Night	6% 2 7% 27% 46% 12%	600
July	1st to 10th	Night	7% 1 6% 27% 44% 15%	585
	11th to 20th	Night	5% 2 4 23% 43% 22%	582
	21st to end	Night	5% 2 7% 24% 48% 15%	657
August	1st to 10th	Night	5% 5% 26% 48% 16%	597
	11th to 20th	Night	8% 1 7% 30% 44% 10%	594
	21st to end	Night	11% 2 7% 26% 40% 14%	660
September	1st to 10th	Night	9% 3 9% 33% 36% 10%	625
	11th to 20th	Night	13% 3 11% 29% 40% 5%	800
	21st to end	Night	10% 2 15% 33% 37% 4	800
October	1st to 10th	Night	9% 4 23% 34% 27% 4	800
	11th to 20th	Night	12% 5% 23% 36% 23% 2	800
	21st to end	Night	15% 8% 20% 36% 18% 2	880
November	1st to 10th	Night	12% 8% 21% 37% 19% 3	820
	11th to 20th	Night	14% 10% 28% 33% 14% 1	840
	21st to end	Night	16% 12% 28% 33% 11% 1	840
December	1st to 10th	Night	22% 12% 28% 29% 9% 1	840
	11th to 20th	Night	23% 10% 29% 28% 9%	840
	21st to end	Night	25% 11% 29% 28% 7%	924

Page generated at 2025-12-30 00:14:07. Version ac07

Copyright © 2025 Attilla Danko.