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 2024-11-04.

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	14% 8% 3 7% 5% 4 5% 4 6% 7% 37%	101005
	Night	14% 9% 3 7% 4 4 4 4 5% 7% 39%	97976
Transparency	Day	44% 4 10% 24% 15% 2	104749
	Night	43% 4 10% 23% 17% 3	102370
Seeing	Day	17% 9% 22% 24% 18% 10%	14742
	Night	18% 7% 22% 31% 19% 2	24775

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% 14% 6% 10% 6% 3 4 4 5% 6% 25%	2836
	11th to 20th	Day	17% 14% 6% 16% 5% 5% 3 2 3 4 24%	2840
	21st to end	Day	25% 14% 7% 7% 4 3 3 3 4 4 24%	3124
February	1st to 10th	Day	17% 12% 5% 10% 5% 4 4 3 4 5% 30%	1847
	11th to 20th	Day	19% 11% 4 8% 5% 5% 3 3 5% 6% 32%	1848
	21st to end	Day	20% 11% 3 9% 5% 4 4 3 4 5% 31%	1570
March	1st to 10th	Day	24% 12% 3 6% 4 4 4 3 4 5% 31%	3174
	11th to 20th	Day	14% 7% 2 5% 4 4 3 3 5% 6% 45%	3480
	21st to end	Day	15% 9% 3 6% 4 4 4 4 5% 7% 38%	3815
April	1st to 10th	Day	13% 8% 2 8% 5% 4 4 4 6% 7% 39%	2209
	11th to 20th	Day	15% 9% 3 7% 4 3 4 3 4 7% 41%	2286
	21st to end	Day	15% 6% 3 7% 5% 5% 5% 4 6% 7% 37%	2470
May	1st to 10th	Day	17% 10% 3 9% 5% 5% 6% 4 6% 7% 29%	4082
	11th to 20th	Day	16% 9% 3 7% 5% 4 4 3 5% 6% 36%	4081
	21st to end	Day	10% 6% 3 7% 5% 5% 6% 5% 7% 10% 37%	4476
June	1st to 10th	Day	10% 5% 2 7% 6% 6% 6% 5% 6% 8% 40%	2470
	11th to 20th	Day	7% 5% 2 6% 4 6% 6% 6% 8% 10% 42%	2470
	21st to end	Day	8% 5% 2 6% 5% 6% 5% 5% 7% 11% 41%	2470
July	1st to 10th	Day	9% 6% 3 7% 6% 5% 6% 4 7% 10% 38%	4120
	11th to 20th	Day	8% 5% 2 7% 4 5% 6% 4 7% 11% 42%	4120
	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 5% 4 3 3 3 5% 5% 54%	1699
	11th to 20th	Day	16% 9% 2 5% 4 3 4 3 4 5% 44%	1710
	21st to end	Day	16% 10% 3 7% 4 3 3 3 5% 6% 40%	1710
December	1st to 10th	Day	21% 13% 4 8% 4 4 4 3 5% 4 31%	1710
	11th to 20th	Day	15% 12% 4 8% 4 4 5% 4 5% 6% 33%	2153
	21st to end	Day	17% 16% 4 9% 6% 5% 5% 4 5% 6% 22%	3124
January	1st to 10th	Night	16% 16% 6% 11% 5% 4 4 3 4 5% 26%	4109
	11th to 20th	Night	19% 13% 4 11% 6% 3 3 2 4 4 29%	4120
	21st to end	Night	23% 13% 4 8% 5% 4 3 3 3 5% 30%	4532
February	1st to 10th	Night	19% 11% 4 10% 5% 5% 4 3 4 6% 30%	2330
	11th to 20th	Night	21% 11% 3 8% 4 4 3 3 5% 6% 32%	2218
	21st to end	Night	23% 12% 4 8% 5% 4 4 3 4 4 29%	1884
March	1st to 10th	Night	21% 14% 3 6% 4 4 4 3 5% 5% 32%	3678
	11th to 20th	Night	14% 6% 2 5% 3 3 3 3 5% 6% 49%	3480
	21st to end	Night	13% 8% 3 7% 5% 5% 4 4 6% 6% 39%	3817
April	1st to 10th	Night	12% 6% 3 6% 4 4 5% 4 6% 6% 44%	1986
	11th to 20th	Night	13% 8% 2 6% 4 5% 4 4 5% 6% 43%	1840
	21st to end	Night	15% 7% 3 7% 5% 3 4 4 6% 7% 39%	1710
May	1st to 10th	Night	17% 10% 3 9% 5% 5% 6% 4 6% 7% 29%	2813
	11th to 20th	Night	18% 8% 3 5% 4 4 3 4 7% 8% 37%	2812
	21st to end	Night	7% 5% 3 7% 5% 5% 5% 5% 6% 10% 42%	3079
June	1st to 10th	Night	9% 5% 2 5% 4 4 4 4 7% 10% 46%	1710
	11th to 20th	Night	4 4 1 6% 4 4 6% 5% 8% 12% 47%	1710
	21st to end	Night	7% 4 1 6% 3 5% 5% 5% 8% 13% 43%	1710
July	1st to 10th	Night	6% 5% 2 7% 5% 5% 6% 4 7% 10% 44%	2840
	11th to 20th	Night	6% 5% 2 6% 5% 4 5% 3 6% 10% 48%	2840
	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	11% 7% 3 6% 3 4 3 3 4 5% 52%	2452
	11th to 20th	Night	15% 9% 2 5% 4 3 4 2 5% 6% 46%	2470
	21st to end	Night	17% 10% 3 5% 3 4 3 3 4 5% 42%	2470
December	1st to 10th	Night	19% 12% 4 8% 4 4 4 3 4 6% 32%	2470
	11th to 20th	Night	18% 10% 5% 9% 4 4 3 2 5% 5% 35%	3042
	21st to end	Night	16% 15% 4 11% 7% 6% 6% 4 5% 6% 21%	4532

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	60% 3 9% 19% 9% 1	2837
	11th to 20th	Day	65% 3 6% 19% 7% 1	2840
	21st to end	Day	63% 2 6% 22% 6% 1	3124
February	1st to 10th	Day	57% 3 7% 20% 12% 2	1847
	11th to 20th	Day	55% 3 6% 22% 11% 3	1858
	21st to end	Day	56% 3 8% 17% 11% 4	1570
March	1st to 10th	Day	57% 2 7% 19% 12% 4	3174
	11th to 20th	Day	40% 3 11% 25% 19% 2	3480
	21st to end	Day	46% 3 10% 29% 11% 1	3815
April	1st to 10th	Day	43% 5% 11% 24% 14% 3	2209
	11th to 20th	Day	45% 3 8% 24% 17% 4	2286
	21st to end	Day	46% 4 11% 26% 11% 2	2470
May	1st to 10th	Day	54% 4 11% 22% 8% 1	4120
	11th to 20th	Day	50% 3 11% 23% 12% 1	4085
	21st to end	Day	41% 5% 13% 21% 17% 2	4489
June	1st to 10th	Day	41% 5% 11% 27% 15% 2	2470
	11th to 20th	Day	35% 7% 16% 23% 17% 2	2470
	21st to end	Day	35% 8% 16% 21% 17% 4	2470
July	1st to 10th	Day	40% 7% 16% 22% 13% 1	4055
	11th to 20th	Day	35% 6% 14% 34% 10% 2	4051
	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	33% 3 9% 26% 25% 4	1699
	11th to 20th	Day	43% 2 7% 24% 19% 4	1710
	21st to end	Day	46% 3 8% 23% 18% 3	1710
December	1st to 10th	Day	57% 3 9% 22% 9% 1	1710
	11th to 20th	Day	52% 3 9% 19% 14% 2	2150
	21st to end	Day	62% 3 8% 20% 6% 1	3124
January	1st to 10th	Night	62% 2 7% 18% 9% 1	4109
	11th to 20th	Night	61% 2 5% 19% 11% 2	4120
	21st to end	Night	59% 2 8% 20% 10% 1	4532
February	1st to 10th	Night	56% 3 10% 19% 10% 1	2330
	11th to 20th	Night	55% 3 7% 20% 13% 2	2219
	21st to end	Night	60% 3 8% 15% 11% 3	1884
March	1st to 10th	Night	55% 2 6% 22% 12% 2	3678
	11th to 20th	Night	37% 3 9% 32% 14% 5%	3480
	21st to end	Night	46% 3 9% 26% 15% 1	3817
April	1st to 10th	Night	40% 4 11% 23% 19% 3	1986
	11th to 20th	Night	42% 4 9% 20% 19% 6%	1840
	21st to end	Night	44% 5% 11% 26% 14% 2	1710
May	1st to 10th	Night	54% 4 11% 21% 8% 2	2840
	11th to 20th	Night	44% 6% 13% 26% 10% 1	2813
	21st to end	Night	36% 6% 12% 25% 18% 3	3087
June	1st to 10th	Night	32% 5% 17% 27% 15% 4	1710
	11th to 20th	Night	29% 7% 18% 24% 20% 3	1710
	21st to end	Night	32% 8% 16% 23% 16% 5%	1710
July	1st to 10th	Night	34% 9% 14% 26% 15% 2	2795
	11th to 20th	Night	30% 7% 15% 31% 15% 2	2786
	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	36% 3 6% 26% 24% 5%	2452
	11th to 20th	Night	41% 3 6% 20% 23% 6%	2470
	21st to end	Night	45% 2 8% 26% 17% 2	2470
December	1st to 10th	Night	54% 3 10% 19% 12% 1	2470
	11th to 20th	Night	53% 2 6% 23% 13% 2	3044
	21st to end	Night	63% 4 9% 16% 8%	4532

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% 16% 33% 19% 7% 1	270
	11th to 20th	Day	24% 23% 34% 14% 5%	270
	21st to end	Day	24% 17% 39% 16% 4	297
February	1st to 10th	Day	22% 17% 38% 16% 6%	270
	11th to 20th	Day	30% 19% 35% 12% 4 1	270
	21st to end	Day	34% 14% 34% 14% 4	225
March	1st to 10th	Day	26% 16% 36% 17% 4	298
	11th to 20th	Day	22% 15% 34% 22% 5% 1	460
	21st to end	Day	25% 15% 29% 23% 7% 2	506
April	1st to 10th	Day	18% 16% 28% 25% 10% 3	460
	11th to 20th	Day	20% 13% 25% 23% 13% 5%	452
	21st to end	Day	20% 11% 29% 25% 12% 4	460
May	1st to 10th	Day	20% 11% 26% 24% 13% 5%	463
	11th to 20th	Day	17% 8% 18% 28% 19% 9%	487
	21st to end	Day	14% 4 17% 26% 24% 15%	529
June	1st to 10th	Day	11% 5% 18% 25% 27% 14%	490
	11th to 20th	Day	9% 3 15% 29% 29% 15%	490
	21st to end	Day	9% 4 18% 32% 21% 17%	490
July	1st to 10th	Day	13% 4 12% 20% 23% 29%	485
	11th to 20th	Day	9% 3 14% 23% 30% 21%	484
	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	16% 5% 29% 29% 16% 5%	279
	11th to 20th	Day	20% 14% 28% 27% 9% 2	270
	21st to end	Day	21% 12% 31% 27% 8% 1	270
December	1st to 10th	Day	21% 21% 26% 24% 7% 1	270
	11th to 20th	Day	21% 11% 37% 20% 10% 1	270
	21st to end	Day	25% 17% 33% 19% 5% 1	297
January	1st to 10th	Night	24% 16% 32% 23% 5%	760
	11th to 20th	Night	30% 13% 30% 23% 4	760
	21st to end	Night	23% 11% 35% 28% 3	836
February	1st to 10th	Night	24% 11% 34% 28% 2	760
	11th to 20th	Night	29% 12% 35% 23% 2	743
	21st to end	Night	30% 13% 35% 20% 2	628
March	1st to 10th	Night	24% 12% 30% 29% 5%	760
	11th to 20th	Night	24% 10% 31% 27% 8% 1	760
	21st to end	Night	20% 11% 27% 34% 7%	836
April	1st to 10th	Night	16% 10% 32% 32% 9%	656
	11th to 20th	Night	16% 10% 28% 32% 13% 1	563
	21st to end	Night	17% 6% 33% 32% 10% 1	570
May	1st to 10th	Night	18% 5% 28% 31% 16% 2	570
	11th to 20th	Night	17% 5% 17% 37% 21% 2	561
	21st to end	Night	8% 3 14% 39% 31% 7%	617
June	1st to 10th	Night	9% 4 14% 43% 27% 3	570
	11th to 20th	Night	5% 2 14% 39% 36% 4	570
	21st to end	Night	8% 2 12% 43% 31% 4	570
July	1st to 10th	Night	10% 2 7% 33% 39% 10%	555
	11th to 20th	Night	6% 2 8% 33% 45% 5%	552
	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% 22% 35% 19% 2	755
	11th to 20th	Night	20% 8% 30% 31% 10% 1	760
	21st to end	Night	22% 9% 24% 34% 11% 1	760
December	1st to 10th	Night	26% 11% 27% 30% 6%	760
	11th to 20th	Night	24% 9% 31% 28% 8% 1	760
	21st to end	Night	25% 14% 27% 28% 5%	836

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