EKU Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the EKU Observatory clear sky chart from 2005-11-06 to 2025-07-21.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 36%	120163
	Night	14% 9% 3 8% 5% 4 4 4 5% 7% 38%	118980
Transparency	Day	45% 5% 10% 23% 15% 2	123905
	Night	45% 4 9% 23% 16% 3	123374
Seeing	Day	17% 9% 22% 24% 18% 10%	15653
	Night	18% 7% 23% 31% 18% 2	25701

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	15% 16% 7% 10% 6% 4 4 3 5% 6% 24%	4056
	11th to 20th	Day	13% 14% 7% 15% 4 4 3 2 3 4 31%	4060
	21st to end	Day	20% 11% 6% 7% 5% 4 4 3 5% 5% 31%	4466
February	1st to 10th	Day	17% 12% 5% 9% 5% 4 4 3 4 5% 30%	1945
	11th to 20th	Day	20% 11% 5% 8% 5% 5% 3 3 4 5% 31%	1948
	21st to end	Day	19% 10% 3 8% 5% 4 4 3 4 5% 33%	1650
March	1st to 10th	Day	18% 12% 3 7% 5% 4 4 3 4 5% 35%	4562
	11th to 20th	Day	12% 8% 3 6% 4 4 4 4 5% 7% 43%	4980
	21st to end	Day	14% 9% 3 7% 4 4 4 4 5% 7% 37%	5465
April	1st to 10th	Day	14% 8% 2 8% 5% 4 4 4 6% 8% 38%	2325
	11th to 20th	Day	14% 9% 3 7% 4 4 4 3 4 7% 41%	2408
	21st to end	Day	15% 7% 4 7% 5% 5% 5% 4 6% 7% 36%	2600
May	1st to 10th	Day	16% 10% 3 9% 6% 6% 7% 4 7% 8% 25%	5862
	11th to 20th	Day	15% 9% 4 9% 5% 5% 6% 4 6% 7% 31%	5861
	21st to end	Day	12% 7% 3 7% 5% 5% 6% 5% 8% 10% 32%	6433
June	1st to 10th	Day	10% 5% 2 7% 6% 5% 6% 5% 6% 8% 40%	2600
	11th to 20th	Day	7% 5% 2 6% 4 6% 6% 6% 8% 10% 41%	2600
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 11% 41%	2600
July	1st to 10th	Day	9% 6% 3 7% 6% 5% 6% 4 7% 10% 38%	4250
	11th to 20th	Day	7% 5% 2 7% 5% 5% 6% 5% 7% 11% 41%	4250
	21st to end	Day	17% 11% 4 10% 6% 6% 6% 5% 7% 8% 22%	4466
August	1st to 10th	Day	8% 5% 2 6% 5% 5% 5% 4 7% 10% 43%	4104
	11th to 20th	Day	9% 6% 2 7% 5% 6% 5% 5% 8% 11% 37%	4017
	21st to end	Day	5% 3 2 4 4 4 4 4 6% 9% 55%	4180
September	1st to 10th	Day	13% 7% 2 6% 4 5% 5% 4 5% 7% 42%	2140
	11th to 20th	Day	11% 6% 2 5% 4 4 4 4 6% 8% 45%	2090
	21st to end	Day	16% 6% 2 5% 4 4 4 3 4 5% 46%	2090
October	1st to 10th	Day	10% 4 2 5% 4 5% 4 2 5% 7% 52%	2090
	11th to 20th	Day	12% 6% 2 5% 4 4 4 4 4 6% 50%	1939
	21st to end	Day	19% 7% 2 5% 3 3 3 3 5% 6% 44%	1995
November	1st to 10th	Day	12% 6% 2 4 4 3 3 3 4 5% 53%	1762
	11th to 20th	Day	15% 10% 2 6% 4 3 4 3 4 6% 44%	1800
	21st to end	Day	16% 11% 3 8% 4 3 3 3 5% 6% 39%	1800
December	1st to 10th	Day	16% 11% 3 8% 5% 4 4 3 4 4 38%	2930
	11th to 20th	Day	18% 14% 5% 9% 5% 4 4 3 4 6% 28%	3373
	21st to end	Day	16% 15% 5% 11% 6% 5% 5% 4 5% 6% 22%	4466
January	1st to 10th	Night	16% 14% 6% 15% 6% 4 4 3 4 5% 24%	5889
	11th to 20th	Night	18% 14% 5% 11% 6% 4 4 2 3 4 30%	5900
	21st to end	Night	19% 10% 3 7% 4 4 4 3 4 5% 37%	6490
February	1st to 10th	Night	19% 11% 4 10% 4 5% 4 3 4 6% 29%	2452
	11th to 20th	Night	21% 11% 3 9% 4 4 3 3 4 6% 31%	2338
	21st to end	Night	22% 12% 3 8% 4 4 3 3 4 4 31%	1980
March	1st to 10th	Night	18% 14% 3 6% 4 4 4 3 4 5% 36%	5290
	11th to 20th	Night	12% 6% 2 6% 4 4 4 3 5% 6% 48%	4980
	21st to end	Night	13% 9% 3 9% 6% 5% 5% 4 6% 6% 34%	5467
April	1st to 10th	Night	13% 6% 2 6% 4 4 5% 4 6% 6% 42%	2090
	11th to 20th	Night	13% 7% 2 6% 4 4 4 4 5% 7% 44%	1938
	21st to end	Night	15% 8% 3 7% 5% 3 4 4 6% 8% 38%	1800
May	1st to 10th	Night	16% 10% 3 9% 5% 5% 6% 5% 6% 9% 26%	4033
	11th to 20th	Night	15% 9% 3 8% 5% 5% 5% 4 7% 8% 30%	4032
	21st to end	Night	10% 7% 3 8% 5% 5% 5% 5% 6% 9% 36%	4421
June	1st to 10th	Night	9% 5% 2 5% 4 4 3 4 7% 10% 46%	1800
	11th to 20th	Night	5% 4 2 6% 4 5% 6% 5% 7% 11% 46%	1800
	21st to end	Night	6% 4 1 6% 3 5% 5% 5% 8% 12% 43%	1800
July	1st to 10th	Night	5% 5% 2 7% 5% 5% 6% 5% 7% 10% 44%	2930
	11th to 20th	Night	6% 5% 2 6% 5% 4 5% 3 6% 10% 47%	2930
	21st to end	Night	13% 10% 3 9% 5% 5% 6% 5% 7% 9% 29%	3087
August	1st to 10th	Night	7% 5% 2 7% 5% 4 5% 4 6% 9% 46%	2825
	11th to 20th	Night	7% 6% 3 7% 5% 6% 5% 4 7% 9% 43%	2808
	21st to end	Night	4 3 1 4 3 3 4 3 6% 9% 60%	3476
September	1st to 10th	Night	11% 5% 2 6% 3 5% 4 3 5% 6% 48%	2019
	11th to 20th	Night	10% 5% 2 6% 4 3 3 4 6% 8% 48%	2090
	21st to end	Night	11% 6% 2 5% 4 3 4 3 5% 6% 50%	2090
October	1st to 10th	Night	10% 5% 2 4 3 3 4 4 5% 6% 54%	2090
	11th to 20th	Night	14% 6% 2 5% 4 4 3 3 4 7% 49%	2239
	21st to end	Night	17% 6% 2 6% 4 3 3 4 4 5% 46%	2598
November	1st to 10th	Night	12% 7% 3 6% 3 4 3 3 5% 5% 50%	2536
	11th to 20th	Night	15% 9% 3 5% 4 3 4 2 5% 6% 45%	2600
	21st to end	Night	17% 10% 3 5% 3 4 3 3 4 5% 41%	2600
December	1st to 10th	Night	16% 13% 5% 9% 4 3 3 2 3 4 37%	4250
	11th to 20th	Night	19% 11% 5% 9% 4 4 4 2 4 5% 33%	4822
	21st to end	Night	15% 15% 5% 10% 6% 6% 6% 4 5% 6% 21%	6490

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	61% 3 8% 20% 7% 1	4057
	11th to 20th	Day	59% 2 5% 18% 14% 2	4060
	21st to end	Day	56% 2 8% 21% 12% 2	4466
February	1st to 10th	Day	56% 3 7% 20% 12% 2	1945
	11th to 20th	Day	57% 3 6% 21% 11% 3	1958
	21st to end	Day	54% 3 8% 19% 12% 4	1650
March	1st to 10th	Day	52% 2 5% 17% 17% 6%	4562
	11th to 20th	Day	40% 4 9% 24% 21% 2	4980
	21st to end	Day	46% 4 9% 29% 11% 1	5465
April	1st to 10th	Day	44% 4 11% 24% 14% 3	2325
	11th to 20th	Day	44% 4 8% 24% 16% 4	2408
	21st to end	Day	46% 4 11% 25% 11% 2	2600
May	1st to 10th	Day	54% 5% 13% 19% 9% 1	5897
	11th to 20th	Day	52% 5% 13% 21% 10%	5865
	21st to end	Day	44% 6% 15% 19% 14% 2	6447
June	1st to 10th	Day	40% 6% 11% 26% 16% 2	2600
	11th to 20th	Day	37% 7% 15% 23% 17% 2	2600
	21st to end	Day	34% 8% 16% 21% 17% 3	2600
July	1st to 10th	Day	39% 7% 16% 22% 14% 1	4185
	11th to 20th	Day	36% 6% 14% 33% 10% 2	4181
	21st to end	Day	58% 6% 11% 15% 9% 1	4324
August	1st to 10th	Day	34% 8% 13% 31% 13% 1	4091
	11th to 20th	Day	38% 7% 13% 28% 12% 2	4056
	21st to end	Day	26% 6% 12% 37% 17% 2	4179
September	1st to 10th	Day	41% 5% 11% 19% 17% 5%	2140
	11th to 20th	Day	36% 4 11% 23% 22% 3	2090
	21st to end	Day	40% 4 8% 25% 20% 3	2090
October	1st to 10th	Day	23% 4 7% 24% 40% 3	3480
	11th to 20th	Day	25% 3 10% 28% 32% 3	3205
	21st to end	Day	30% 3 8% 29% 20% 9%	3271
November	1st to 10th	Day	34% 3 9% 25% 24% 4	1762
	11th to 20th	Day	44% 2 7% 24% 20% 4	1800
	21st to end	Day	48% 3 8% 22% 18% 2	1800
December	1st to 10th	Day	49% 3 7% 24% 16% 1	2930
	11th to 20th	Day	59% 3 8% 18% 12% 1	3370
	21st to end	Day	63% 4 9% 18% 5% 1	4466
January	1st to 10th	Night	65% 2 6% 18% 8% 1	5889
	11th to 20th	Night	60% 3 4 17% 13% 4	5900
	21st to end	Night	51% 2 8% 22% 15% 2	6490
February	1st to 10th	Night	56% 4 10% 19% 10% 1	2452
	11th to 20th	Night	57% 3 6% 19% 13% 2	2339
	21st to end	Night	58% 3 8% 17% 12% 3	1980
March	1st to 10th	Night	52% 2 5% 23% 13% 4	5290
	11th to 20th	Night	37% 3 8% 28% 20% 4	4980
	21st to end	Night	50% 4 8% 25% 13% 1	5467
April	1st to 10th	Night	41% 5% 11% 23% 18% 3	2090
	11th to 20th	Night	42% 4 9% 21% 19% 6%	1938
	21st to end	Night	43% 5% 11% 26% 14% 2	1800
May	1st to 10th	Night	54% 6% 11% 20% 8% 1	4060
	11th to 20th	Night	51% 7% 12% 22% 8% 1	4033
	21st to end	Night	42% 7% 14% 22% 13% 2	4429
June	1st to 10th	Night	32% 5% 16% 27% 16% 4	1800
	11th to 20th	Night	31% 7% 18% 23% 19% 3	1800
	21st to end	Night	31% 8% 16% 23% 17% 5%	1800
July	1st to 10th	Night	33% 9% 14% 26% 16% 2	2885
	11th to 20th	Night	31% 7% 15% 31% 14% 2	2876
	21st to end	Night	48% 8% 13% 20% 12% 1	2988
August	1st to 10th	Night	33% 7% 15% 29% 16% 1	2814
	11th to 20th	Night	34% 8% 12% 30% 14% 1	2835
	21st to end	Night	20% 6% 14% 34% 22% 4	3476
September	1st to 10th	Night	35% 5% 12% 21% 20% 6%	2019
	11th to 20th	Night	35% 5% 10% 25% 23% 3	2090
	21st to end	Night	34% 4 8% 24% 25% 4	2090
October	1st to 10th	Night	24% 4 7% 22% 39% 3	3480
	11th to 20th	Night	25% 3 7% 25% 34% 6%	3712
	21st to end	Night	29% 2 7% 23% 30% 9%	4272
November	1st to 10th	Night	37% 3 6% 26% 23% 5%	2536
	11th to 20th	Night	42% 3 6% 21% 22% 6%	2600
	21st to end	Night	46% 2 8% 25% 17% 2	2600
December	1st to 10th	Night	53% 3 9% 21% 15% 1	4250
	11th to 20th	Night	55% 3 8% 20% 12% 2	4824
	21st to end	Night	63% 4 9% 16% 7%	6490

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	24% 18% 33% 18% 6% 1	300
	11th to 20th	Day	22% 25% 34% 14% 5%	300
	21st to end	Day	22% 17% 41% 16% 4	330
February	1st to 10th	Day	22% 18% 40% 15% 6%	300
	11th to 20th	Day	32% 19% 34% 11% 3 1	300
	21st to end	Day	31% 15% 36% 15% 4	249
March	1st to 10th	Day	25% 18% 34% 17% 6% 1	330
	11th to 20th	Day	22% 16% 35% 22% 5% 1	500
	21st to end	Day	24% 16% 30% 22% 7% 1	550
April	1st to 10th	Day	20% 15% 29% 24% 10% 2	500
	11th to 20th	Day	20% 14% 27% 23% 12% 4	492
	21st to end	Day	20% 10% 28% 25% 12% 4	500
May	1st to 10th	Day	21% 11% 27% 23% 13% 5%	503
	11th to 20th	Day	17% 10% 19% 28% 18% 8%	527
	21st to end	Day	15% 5% 18% 26% 23% 14%	573
June	1st to 10th	Day	11% 6% 19% 25% 26% 14%	530
	11th to 20th	Day	9% 4 16% 28% 29% 14%	530
	21st to end	Day	8% 4 17% 30% 23% 18%	530
July	1st to 10th	Day	13% 4 12% 20% 25% 28%	525
	11th to 20th	Day	9% 4 14% 24% 29% 21%	524
	21st to end	Day	15% 4 8% 26% 29% 18%	532
August	1st to 10th	Day	12% 2 8% 27% 29% 21%	486
	11th to 20th	Day	12% 2 13% 30% 29% 14%	479
	21st to end	Day	11% 2 6% 25% 32% 25%	539
September	1st to 10th	Day	14% 2 10% 22% 29% 21%	490
	11th to 20th	Day	14% 2 9% 20% 34% 21%	490
	21st to end	Day	18% 6% 16% 24% 26% 11%	490
October	1st to 10th	Day	12% 4 15% 30% 28% 12%	490
	11th to 20th	Day	11% 10% 24% 29% 19% 7%	490
	21st to end	Day	21% 10% 27% 26% 15% 2	444
November	1st to 10th	Day	18% 4 31% 26% 15% 5%	300
	11th to 20th	Day	21% 14% 29% 26% 8% 2	300
	21st to end	Day	22% 14% 31% 26% 7% 1	300
December	1st to 10th	Day	21% 21% 29% 23% 7% 1	300
	11th to 20th	Day	22% 11% 36% 20% 9% 1	300
	21st to end	Day	25% 17% 32% 20% 5% 1	330
January	1st to 10th	Night	24% 16% 32% 24% 5%	800
	11th to 20th	Night	30% 13% 31% 23% 3	800
	21st to end	Night	23% 11% 36% 28% 2	880
February	1st to 10th	Night	24% 12% 35% 28% 2	800
	11th to 20th	Night	29% 12% 35% 22% 2	783
	21st to end	Night	29% 13% 35% 21% 2	660
March	1st to 10th	Night	24% 13% 30% 29% 5%	800
	11th to 20th	Night	23% 10% 31% 27% 8% 1	800
	21st to end	Night	20% 11% 28% 33% 7%	880
April	1st to 10th	Night	18% 10% 32% 31% 9%	690
	11th to 20th	Night	15% 10% 29% 32% 13% 1	593
	21st to end	Night	16% 7% 33% 33% 11% 1	600
May	1st to 10th	Night	18% 6% 27% 32% 15% 2	600
	11th to 20th	Night	17% 5% 18% 37% 21% 3	591
	21st to end	Night	8% 3 15% 38% 29% 6%	650
June	1st to 10th	Night	10% 3 14% 43% 27% 3	600
	11th to 20th	Night	6% 2 14% 40% 35% 4	600
	21st to end	Night	7% 2 11% 42% 33% 5%	600
July	1st to 10th	Night	9% 2 7% 33% 39% 9%	585
	11th to 20th	Night	6% 2 8% 32% 46% 5%	582
	21st to end	Night	12% 2 8% 33% 39% 7%	624
August	1st to 10th	Night	9% 3 5% 34% 43% 6%	567
	11th to 20th	Night	8% 3 9% 34% 41% 6%	564
	21st to end	Night	7% 2 6% 34% 43% 9%	627
September	1st to 10th	Night	13% 2 7% 32% 40% 6%	689
	11th to 20th	Night	13% 3 10% 28% 40% 6%	760
	21st to end	Night	14% 4 14% 34% 32% 3	760
October	1st to 10th	Night	11% 4 16% 33% 31% 5%	760
	11th to 20th	Night	18% 6% 23% 33% 19% 2	760
	21st to end	Night	19% 6% 27% 32% 15% 1	836
November	1st to 10th	Night	16% 6% 23% 35% 18% 2	780
	11th to 20th	Night	20% 8% 30% 31% 10% 1	800
	21st to end	Night	22% 9% 24% 33% 10% 1	800
December	1st to 10th	Night	26% 11% 28% 30% 6%	800
	11th to 20th	Night	24% 9% 31% 28% 7% 1	800
	21st to end	Night	25% 14% 28% 28% 5%	880

Page generated at 2025-07-18 01:10:02. Version ac07

Copyright © 2025 Attilla Danko.