Shank Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Shank Observatory clear sky chart from 2005-11-06 to 2025-09-04.

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	13% 6% 2 6% 5% 5% 5% 4 6% 8% 41%	125621
	Night	13% 6% 2 6% 4 4 4 4 5% 7% 45%	123043
Transparency	Day	40% 5% 11% 23% 17% 5%	129227
	Night	38% 4 12% 24% 18% 5%	127506
Seeing	Day	18% 9% 24% 26% 17% 7%	16036
	Night	17% 6% 24% 35% 16% 2	25800

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	11% 7% 2 6% 5% 5% 4 4 4 5% 48%	4056
	11th to 20th	Day	10% 8% 3 8% 4 4 4 3 4 5% 47%	4060
	21st to end	Day	13% 7% 3 6% 3 4 3 3 5% 5% 46%	4466
February	1st to 10th	Day	11% 9% 3 8% 5% 4 4 4 5% 7% 41%	1800
	11th to 20th	Day	14% 8% 3 6% 5% 5% 4 3 5% 7% 41%	1753
	21st to end	Day	15% 7% 3 5% 4 4 5% 3 5% 7% 42%	1712
March	1st to 10th	Day	15% 7% 2 5% 4 4 5% 4 5% 6% 43%	4935
	11th to 20th	Day	10% 6% 2 5% 4 4 4 3 5% 6% 52%	4980
	21st to end	Day	12% 8% 3 7% 5% 5% 5% 4 5% 6% 41%	5465
April	1st to 10th	Day	11% 7% 2 6% 5% 5% 5% 3 5% 6% 44%	2337
	11th to 20th	Day	13% 5% 2 6% 4 4 4 3 6% 7% 46%	2374
	21st to end	Day	16% 6% 3 6% 4 4 4 4 5% 6% 42%	2490
May	1st to 10th	Day	14% 6% 2 7% 6% 6% 7% 5% 7% 10% 30%	5862
	11th to 20th	Day	22% 7% 2 6% 4 5% 5% 4 6% 7% 33%	5861
	21st to end	Day	16% 5% 2 6% 5% 5% 5% 5% 7% 8% 37%	6680
June	1st to 10th	Day	11% 5% 2 5% 5% 5% 5% 5% 6% 10% 42%	2800
	11th to 20th	Day	10% 6% 2 6% 6% 6% 6% 5% 7% 10% 36%	2800
	21st to end	Day	8% 4 2 6% 5% 5% 6% 5% 8% 11% 42%	2800
July	1st to 10th	Day	7% 4 2 7% 6% 6% 6% 5% 7% 11% 40%	5959
	11th to 20th	Day	8% 4 2 7% 7% 7% 7% 7% 9% 13% 28%	5900
	21st to end	Day	9% 6% 3 8% 6% 6% 6% 5% 8% 10% 33%	6424
August	1st to 10th	Day	13% 6% 2 8% 6% 5% 7% 5% 8% 10% 31%	4234
	11th to 20th	Day	10% 4 2 6% 6% 6% 7% 5% 8% 10% 36%	4155
	21st to end	Day	8% 4 2 5% 5% 5% 6% 5% 7% 9% 46%	4312
September	1st to 10th	Day	13% 5% 2 6% 4 4 5% 4 5% 7% 45%	2316
	11th to 20th	Day	14% 7% 2 5% 3 4 4 4 5% 6% 46%	2157
	21st to end	Day	23% 5% 2 5% 5% 3 4 3 6% 7% 36%	2049
October	1st to 10th	Day	16% 5% 2 6% 3 4 4 3 5% 6% 46%	1900
	11th to 20th	Day	15% 4 1 5% 4 4 4 3 4 5% 50%	1900
	21st to end	Day	14% 5% 1 4 4 3 4 3 5% 6% 51%	2090
November	1st to 10th	Day	10% 5% 2 5% 3 4 4 3 4 6% 55%	1957
	11th to 20th	Day	14% 5% 2 4 4 3 4 4 6% 6% 50%	1753
	21st to end	Day	13% 5% 1 4 4 4 4 4 5% 7% 49%	1600
December	1st to 10th	Day	10% 6% 1 5% 4 4 5% 3 5% 6% 51%	2600
	11th to 20th	Day	18% 5% 2 4 4 4 4 3 5% 6% 45%	2990
	21st to end	Day	18% 6% 2 6% 4 4 5% 4 6% 7% 36%	4094
January	1st to 10th	Night	10% 7% 2 5% 5% 4 4 3 4 5% 52%	5889
	11th to 20th	Night	12% 7% 2 5% 3 3 3 2 4 5% 53%	5900
	21st to end	Night	15% 7% 2 4 3 3 3 3 4 5% 52%	6490
February	1st to 10th	Night	11% 7% 3 6% 4 4 5% 4 6% 6% 44%	2600
	11th to 20th	Night	16% 8% 2 6% 4 3 4 4 5% 6% 41%	2533
	21st to end	Night	13% 8% 3 6% 4 4 4 3 6% 8% 43%	1914
March	1st to 10th	Night	17% 6% 2 6% 3 3 4 2 4 5% 48%	4918
	11th to 20th	Night	9% 4 1 4 3 4 4 3 6% 9% 53%	4980
	21st to end	Night	11% 6% 2 7% 4 5% 4 3 4 7% 47%	5467
April	1st to 10th	Night	15% 7% 3 5% 4 4 4 4 5% 5% 44%	2062
	11th to 20th	Night	12% 4 2 5% 4 4 5% 4 6% 8% 46%	1975
	21st to end	Night	16% 4 2 5% 4 4 4 3 5% 7% 46%	1925
May	1st to 10th	Night	17% 8% 2 6% 5% 5% 5% 4 6% 8% 35%	4033
	11th to 20th	Night	23% 6% 2 6% 4 4 4 4 6% 7% 34%	4032
	21st to end	Night	15% 4 2 6% 4 5% 5% 4 7% 8% 39%	4158
June	1st to 10th	Night	10% 5% 2 5% 4 4 4 5% 7% 11% 45%	1600
	11th to 20th	Night	11% 5% 2 5% 4 5% 5% 4 7% 8% 44%	1600
	21st to end	Night	9% 4 1 5% 4 4 5% 5% 8% 10% 46%	1600
July	1st to 10th	Night	6% 5% 2 7% 5% 5% 5% 5% 6% 10% 44%	3965
	11th to 20th	Night	9% 6% 3 9% 7% 6% 7% 5% 7% 10% 30%	4057
	21st to end	Night	7% 6% 3 8% 6% 6% 6% 5% 8% 10% 34%	4427
August	1st to 10th	Night	14% 5% 2 7% 5% 5% 5% 5% 6% 9% 37%	2915
	11th to 20th	Night	11% 5% 2 6% 5% 6% 6% 5% 8% 7% 40%	2875
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 8% 48%	3571
September	1st to 10th	Night	13% 4 1 5% 4 4 4 4 5% 6% 49%	1931
	11th to 20th	Night	14% 5% 2 6% 3 4 4 3 5% 7% 46%	2019
	21st to end	Night	23% 7% 2 5% 4 3 3 3 4 5% 41%	2130
October	1st to 10th	Night	17% 5% 2 5% 3 4 4 3 5% 7% 45%	2280
	11th to 20th	Night	14% 5% 1 4 3 3 4 3 5% 5% 53%	2280
	21st to end	Night	14% 3 2 4 4 3 3 3 5% 7% 51%	2508
November	1st to 10th	Night	11% 6% 2 5% 3 4 4 3 4 6% 53%	2341
	11th to 20th	Night	13% 5% 2 4 4 3 4 3 5% 6% 50%	2639
	21st to end	Night	14% 5% 2 3 4 4 4 4 5% 7% 49%	2800
December	1st to 10th	Night	9% 5% 2 5% 4 4 4 3 5% 6% 53%	4580
	11th to 20th	Night	15% 8% 3 6% 4 4 5% 3 5% 5% 43%	5205
	21st to end	Night	15% 5% 2 6% 4 4 5% 5% 6% 8% 40%	6844

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	39% 3 4 21% 24% 10%	4057
	11th to 20th	Day	41% 2 5% 17% 21% 15%	4060
	21st to end	Day	40% 2 6% 25% 22% 5%	4466
February	1st to 10th	Day	43% 3 10% 22% 18% 4	1800
	11th to 20th	Day	44% 3 8% 21% 19% 5%	1762
	21st to end	Day	42% 3 11% 20% 19% 6%	1712
March	1st to 10th	Day	41% 4 8% 20% 17% 11%	4935
	11th to 20th	Day	33% 3 7% 26% 23% 9%	4980
	21st to end	Day	45% 3 9% 18% 19% 7%	5465
April	1st to 10th	Day	41% 3 10% 20% 19% 8%	2337
	11th to 20th	Day	38% 2 11% 22% 21% 6%	2374
	21st to end	Day	43% 3 11% 23% 16% 4	2490
May	1st to 10th	Day	47% 6% 13% 21% 10% 3	5897
	11th to 20th	Day	51% 5% 9% 21% 12% 1	5865
	21st to end	Day	43% 5% 14% 19% 15% 3	6695
June	1st to 10th	Day	39% 5% 14% 25% 17% 1	2800
	11th to 20th	Day	42% 6% 16% 21% 14% 2	2800
	21st to end	Day	35% 6% 15% 28% 14% 2	2800
July	1st to 10th	Day	36% 10% 16% 21% 12% 4	5894
	11th to 20th	Day	41% 13% 18% 22% 6% 1	5831
	21st to end	Day	42% 7% 12% 25% 13% 1	6240
August	1st to 10th	Day	45% 8% 16% 24% 8%	4221
	11th to 20th	Day	38% 8% 14% 28% 11% 1	4194
	21st to end	Day	33% 6% 17% 29% 13% 1	4312
September	1st to 10th	Day	39% 4 14% 19% 19% 5%	2316
	11th to 20th	Day	40% 3 7% 27% 20% 4	2157
	21st to end	Day	48% 3 8% 21% 18% 3	2049
October	1st to 10th	Day	32% 5% 6% 21% 28% 7%	3160
	11th to 20th	Day	25% 2 8% 29% 23% 13%	3140
	21st to end	Day	23% 2 9% 31% 24% 12%	3428
November	1st to 10th	Day	32% 2 9% 26% 26% 5%	1957
	11th to 20th	Day	35% 3 9% 21% 23% 8%	1753
	21st to end	Day	36% 3 9% 27% 23% 3	1600
December	1st to 10th	Day	33% 4 9% 26% 21% 6%	2600
	11th to 20th	Day	39% 5% 11% 22% 17% 5%	2986
	21st to end	Day	46% 4 12% 20% 16% 2	4094
January	1st to 10th	Night	35% 3 5% 22% 27% 9%	5889
	11th to 20th	Night	35% 2 6% 23% 23% 12%	5900
	21st to end	Night	37% 2 7% 27% 22% 5%	6490
February	1st to 10th	Night	39% 4 12% 27% 16% 3	2600
	11th to 20th	Night	44% 3 8% 21% 17% 6%	2535
	21st to end	Night	41% 3 11% 23% 16% 7%	1914
March	1st to 10th	Night	39% 2 7% 24% 19% 9%	4918
	11th to 20th	Night	29% 3 9% 26% 26% 7%	4980
	21st to end	Night	38% 3 12% 20% 20% 5%	5467
April	1st to 10th	Night	42% 3 10% 21% 20% 4	2062
	11th to 20th	Night	35% 3 14% 22% 21% 5%	1975
	21st to end	Night	39% 2 12% 27% 16% 3	1925
May	1st to 10th	Night	47% 6% 16% 20% 10% 3	4060
	11th to 20th	Night	49% 4 11% 25% 10%	4033
	21st to end	Night	41% 6% 18% 25% 7% 3	4165
June	1st to 10th	Night	33% 7% 16% 28% 15% 1	1600
	11th to 20th	Night	37% 6% 16% 25% 13% 2	1600
	21st to end	Night	31% 7% 18% 23% 19% 2	1600
July	1st to 10th	Night	35% 10% 20% 21% 12% 2	3922
	11th to 20th	Night	46% 11% 17% 18% 7% 1	4006
	21st to end	Night	39% 10% 18% 22% 10%	4297
August	1st to 10th	Night	43% 7% 18% 25% 7% 1	2904
	11th to 20th	Night	38% 7% 16% 24% 14%	2902
	21st to end	Night	34% 5% 17% 32% 11% 2	3571
September	1st to 10th	Night	35% 6% 15% 21% 19% 5%	1931
	11th to 20th	Night	38% 3 8% 29% 19% 3	2019
	21st to end	Night	46% 4 8% 22% 18% 2	2130
October	1st to 10th	Night	40% 4 8% 20% 23% 4	3800
	11th to 20th	Night	24% 3 10% 23% 30% 10%	3779
	21st to end	Night	21% 2 10% 30% 28% 8%	4120
November	1st to 10th	Night	34% 3 9% 24% 24% 7%	2341
	11th to 20th	Night	35% 2 7% 25% 24% 6%	2639
	21st to end	Night	35% 3 10% 25% 22% 5%	2800
December	1st to 10th	Night	31% 4 10% 29% 23% 3	4580
	11th to 20th	Night	43% 3 12% 19% 16% 7%	5208
	21st to end	Night	42% 4 12% 22% 19% 1	6844

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	20% 24% 32% 19% 5% 1	300
	11th to 20th	Day	24% 21% 35% 17% 4	300
	21st to end	Day	24% 19% 35% 18% 3 1	330
February	1st to 10th	Day	22% 22% 34% 17% 5%	300
	11th to 20th	Day	33% 21% 37% 8% 1	300
	21st to end	Day	25% 15% 36% 20% 4	366
March	1st to 10th	Day	19% 19% 31% 25% 6%	500
	11th to 20th	Day	18% 19% 34% 21% 7% 1	500
	21st to end	Day	19% 17% 32% 24% 8% 1	550
April	1st to 10th	Day	23% 18% 28% 21% 9% 1	500
	11th to 20th	Day	18% 13% 27% 23% 14% 4	492
	21st to end	Day	24% 10% 29% 22% 12% 3	500
May	1st to 10th	Day	21% 12% 27% 25% 11% 5%	503
	11th to 20th	Day	23% 6% 27% 22% 17% 4	527
	21st to end	Day	19% 5% 19% 27% 20% 10%	573
June	1st to 10th	Day	12% 4 24% 28% 23% 9%	530
	11th to 20th	Day	11% 6% 20% 30% 21% 11%	530
	21st to end	Day	8% 3 20% 37% 24% 8%	530
July	1st to 10th	Day	12% 2 14% 28% 30% 14%	525
	11th to 20th	Day	6% 2 12% 33% 30% 17%	524
	21st to end	Day	11% 2 10% 32% 29% 16%	576
August	1st to 10th	Day	15% 3 13% 27% 25% 17%	526
	11th to 20th	Day	15% 2 13% 29% 26% 16%	519
	21st to end	Day	10% 2 11% 29% 30% 19%	583
September	1st to 10th	Day	17% 2 14% 24% 25% 17%	502
	11th to 20th	Day	16% 3 11% 27% 28% 14%	490
	21st to end	Day	29% 6% 16% 24% 20% 6%	490
October	1st to 10th	Day	21% 5% 15% 29% 23% 7%	490
	11th to 20th	Day	14% 11% 25% 28% 18% 4	490
	21st to end	Day	16% 8% 30% 31% 12% 3	539
November	1st to 10th	Day	15% 7% 29% 31% 15% 3	495
	11th to 20th	Day	19% 12% 31% 26% 10% 2	308
	21st to end	Day	20% 14% 33% 23% 9% 1	200
December	1st to 10th	Day	20% 14% 38% 22% 6% 2	200
	11th to 20th	Day	25% 10% 32% 25% 9% 1	200
	21st to end	Day	21% 15% 32% 22% 8% 2	248
January	1st to 10th	Night	18% 12% 38% 28% 4	800
	11th to 20th	Night	19% 10% 33% 34% 5%	800
	21st to end	Night	16% 9% 42% 30% 3	880
February	1st to 10th	Night	17% 10% 36% 34% 3	800
	11th to 20th	Night	24% 12% 37% 25% 1	783
	21st to end	Night	21% 11% 37% 28% 3	660
March	1st to 10th	Night	16% 12% 36% 32% 5%	800
	11th to 20th	Night	20% 12% 32% 28% 7% 1	800
	21st to end	Night	20% 9% 32% 32% 6% 1	880
April	1st to 10th	Night	20% 13% 31% 28% 8%	800
	11th to 20th	Night	14% 9% 31% 33% 12% 1	791
	21st to end	Night	18% 5% 26% 40% 10% 1	725
May	1st to 10th	Night	18% 6% 20% 38% 17% 1	600
	11th to 20th	Night	21% 2 20% 36% 18% 2	591
	21st to end	Night	15% 2 14% 40% 26% 2	542
June	1st to 10th	Night	10% 1 15% 48% 24% 2	400
	11th to 20th	Night	11% 3 11% 46% 26% 3	400
	21st to end	Night	9% 2 11% 47% 28% 3	400
July	1st to 10th	Night	13% 2 6% 35% 38% 6%	544
	11th to 20th	Night	6% 1 6% 38% 42% 6%	582
	21st to end	Night	9% 1 6% 40% 39% 5%	657
August	1st to 10th	Night	13% 2 6% 40% 34% 5%	597
	11th to 20th	Night	15% 1 5% 38% 37% 4	594
	21st to end	Night	13% 2 8% 38% 32% 6%	865
September	1st to 10th	Night	16% 1 9% 34% 34% 7%	773
	11th to 20th	Night	16% 2 7% 38% 31% 6%	760
	21st to end	Night	26% 3 14% 32% 23% 2	760
October	1st to 10th	Night	19% 4 16% 33% 25% 4	760
	11th to 20th	Night	17% 5% 26% 35% 16% 1	760
	21st to end	Night	16% 6% 24% 39% 13% 2	836
November	1st to 10th	Night	16% 5% 22% 42% 13% 2	780
	11th to 20th	Night	17% 9% 26% 37% 11% 1	800
	21st to end	Night	17% 8% 30% 35% 9% 2	800
December	1st to 10th	Night	19% 8% 31% 35% 6% 1	800
	11th to 20th	Night	15% 8% 32% 39% 6%	800
	21st to end	Night	18% 8% 28% 41% 5%	880

Page generated at 2025-09-01 01:51:02. Version ac07

Copyright © 2025 Attilla Danko.