Eureka Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eureka Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	17% 13% 5% 10% 5% 4 4 3 5% 6% 28%	131621
	Night	16% 15% 6% 11% 5% 4 4 3 4 5% 26%	130516
Transparency	Day	56% 5% 11% 18% 9% 1	135220
	Night	59% 5% 10% 16% 9% 1	134987
Seeing	Day	25% 17% 29% 19% 8% 2	16336
	Night	25% 16% 27% 22% 9% 1	26058

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% 24% 10% 29% 5% 3 2 2 2 3 5%	4998
	11th to 20th	Day	10% 23% 10% 23% 9% 8% 3 2 2 3 8%	5280
	21st to end	Day	16% 18% 7% 17% 10% 7% 5% 3 3 3 11%	5780
February	1st to 10th	Day	16% 20% 6% 16% 9% 4 4 3 3 3 15%	1890
	11th to 20th	Day	15% 18% 10% 16% 9% 4 3 3 3 3 15%	1843
	21st to end	Day	18% 19% 4 15% 5% 3 4 3 4 6% 19%	1687
March	1st to 10th	Day	16% 13% 9% 12% 6% 3 3 2 4 6% 26%	5045
	11th to 20th	Day	13% 17% 7% 10% 4 4 3 3 4 5% 30%	5090
	21st to end	Day	20% 16% 6% 12% 4 3 3 2 3 5% 25%	5586
April	1st to 10th	Day	18% 15% 4 8% 4 4 3 3 4 7% 29%	2427
	11th to 20th	Day	19% 14% 3 6% 4 4 4 4 5% 5% 33%	2529
	21st to end	Day	20% 12% 3 7% 5% 4 3 2 5% 6% 34%	2600
May	1st to 10th	Day	21% 10% 3 8% 5% 4 4 3 5% 6% 30%	5862
	11th to 20th	Day	19% 9% 3 7% 4 5% 5% 4 6% 7% 31%	5924
	21st to end	Day	20% 9% 3 6% 5% 4 4 4 5% 6% 35%	6934
June	1st to 10th	Day	17% 7% 2 6% 5% 4 4 3 6% 8% 38%	2800
	11th to 20th	Day	15% 6% 3 7% 4 4 5% 4 7% 8% 37%	2800
	21st to end	Day	15% 6% 2 5% 5% 5% 5% 5% 7% 8% 40%	2800
July	1st to 10th	Day	10% 5% 2 7% 4 4 5% 4 7% 10% 42%	6360
	11th to 20th	Day	15% 6% 2 6% 4 4 4 4 6% 9% 39%	5956
	21st to end	Day	13% 6% 2 6% 4 4 4 4 6% 7% 45%	6424
August	1st to 10th	Day	15% 6% 2 7% 4 4 5% 4 7% 8% 35%	4234
	11th to 20th	Day	15% 7% 3 8% 5% 5% 5% 4 7% 9% 31%	4174
	21st to end	Day	13% 5% 2 6% 4 4 5% 5% 7% 9% 40%	4510
September	1st to 10th	Day	17% 8% 2 6% 3 3 4 4 5% 9% 38%	2400
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 7% 45%	2210
	21st to end	Day	20% 8% 3 8% 5% 4 4 3 4 6% 34%	2200
October	1st to 10th	Day	19% 6% 2 7% 4 4 4 3 6% 8% 36%	2000
	11th to 20th	Day	23% 10% 3 8% 5% 4 5% 3 5% 6% 29%	2000
	21st to end	Day	27% 14% 3 8% 5% 4 4 3 5% 6% 21%	2200
November	1st to 10th	Day	17% 16% 3 8% 5% 3 4 3 5% 6% 30%	1912
	11th to 20th	Day	17% 24% 4 8% 5% 4 3 3 5% 6% 21%	1688
	21st to end	Day	20% 29% 6% 8% 4 4 4 3 4 5% 13%	1680
December	1st to 10th	Day	18% 33% 14% 12% 4 3 2 2 3 2 7%	2680
	11th to 20th	Day	20% 26% 8% 14% 6% 3 3 2 4 3 10%	3070
	21st to end	Day	22% 25% 8% 15% 7% 3 3 2 2 3 9%	4048
January	1st to 10th	Night	14% 23% 12% 30% 5% 3 2 2 2 2 6%	7891
	11th to 20th	Night	10% 22% 12% 23% 9% 5% 3 2 3 3 8%	7680
	21st to end	Night	16% 18% 8% 16% 8% 5% 5% 3 4 4 13%	8401
February	1st to 10th	Night	18% 20% 6% 15% 8% 4 3 3 3 3 17%	2730
	11th to 20th	Night	16% 18% 8% 15% 8% 5% 4 3 4 4 16%	2663
	21st to end	Night	18% 16% 6% 15% 6% 4 4 2 3 4 20%	2116
March	1st to 10th	Night	18% 17% 5% 9% 6% 3 3 2 4 5% 26%	5028
	11th to 20th	Night	13% 15% 7% 8% 4 3 3 3 5% 6% 34%	5090
	21st to end	Night	16% 15% 7% 11% 5% 4 3 3 4 4 30%	5588
April	1st to 10th	Night	19% 18% 4 8% 4 5% 4 3 4 5% 27%	2191
	11th to 20th	Night	19% 12% 3 7% 5% 4 4 3 6% 6% 31%	1969
	21st to end	Night	16% 12% 3 6% 5% 4 4 4 5% 7% 34%	1800
May	1st to 10th	Night	24% 10% 3 7% 5% 4 4 3 5% 5% 29%	4033
	11th to 20th	Night	16% 9% 3 9% 5% 5% 3 3 5% 7% 36%	3951
	21st to end	Night	21% 7% 2 6% 3 3 3 3 5% 9% 38%	3920
June	1st to 10th	Night	11% 7% 3 6% 5% 5% 4 4 7% 7% 40%	1600
	11th to 20th	Night	11% 8% 3 8% 5% 4 4 3 6% 8% 40%	1600
	21st to end	Night	13% 6% 2 6% 5% 4 5% 4 7% 8% 41%	1600
July	1st to 10th	Night	12% 7% 3 7% 4 4 4 3 6% 9% 41%	3598
	11th to 20th	Night	12% 6% 3 7% 4 5% 4 4 6% 9% 41%	3983
	21st to end	Night	10% 5% 2 6% 4 4 5% 3 6% 8% 47%	4427
August	1st to 10th	Night	11% 7% 3 8% 5% 4 6% 3 6% 7% 40%	2915
	11th to 20th	Night	16% 7% 4 8% 4 5% 5% 3 4 5% 38%	2875
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 4 7% 9% 42%	3355
September	1st to 10th	Night	14% 6% 2 5% 5% 3 4 3 6% 7% 45%	2000
	11th to 20th	Night	13% 6% 2 5% 3 3 4 3 5% 6% 49%	2186
	21st to end	Night	19% 7% 3 8% 5% 4 5% 4 5% 6% 35%	2200
October	1st to 10th	Night	19% 6% 2 6% 4 3 4 3 6% 7% 40%	2400
	11th to 20th	Night	23% 10% 3 7% 5% 4 4 4 5% 6% 30%	2400
	21st to end	Night	28% 12% 3 6% 3 4 4 3 4 5% 27%	2640
November	1st to 10th	Night	17% 16% 4 6% 4 4 3 3 5% 5% 31%	2601
	11th to 20th	Night	19% 22% 4 8% 5% 5% 4 3 5% 5% 20%	2930
	21st to end	Night	20% 26% 6% 8% 4 4 3 3 4 4 18%	2940
December	1st to 10th	Night	16% 27% 13% 12% 5% 3 4 3 3 3 11%	4720
	11th to 20th	Night	18% 26% 9% 14% 5% 4 3 2 3 4 12%	5345
	21st to end	Night	20% 24% 7% 13% 7% 4 4 3 4 4 11%	7150

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	87% 2 5% 5% 1	4999
	11th to 20th	Day	85% 2 6% 5% 2	5280
	21st to end	Day	80% 2 6% 7% 4	5780
February	1st to 10th	Day	75% 4 8% 8% 4	1890
	11th to 20th	Day	74% 4 7% 10% 5%	1852
	21st to end	Day	66% 4 11% 13% 4 2	1687
March	1st to 10th	Day	60% 5% 12% 18% 4	5045
	11th to 20th	Day	56% 5% 15% 17% 5% 2	5090
	21st to end	Day	63% 4 7% 17% 9% 1	5586
April	1st to 10th	Day	56% 5% 12% 16% 11%	2427
	11th to 20th	Day	52% 6% 10% 20% 11% 2	2529
	21st to end	Day	52% 5% 10% 19% 12% 2	2600
May	1st to 10th	Day	54% 6% 8% 19% 9% 4	5897
	11th to 20th	Day	50% 7% 11% 21% 10% 1	5928
	21st to end	Day	48% 6% 12% 19% 12% 3	6949
June	1st to 10th	Day	44% 6% 13% 23% 13% 1	2800
	11th to 20th	Day	43% 7% 15% 22% 13% 1	2800
	21st to end	Day	39% 8% 13% 24% 15% 1	2800
July	1st to 10th	Day	37% 5% 14% 25% 18% 1	6290
	11th to 20th	Day	40% 5% 14% 25% 16%	5885
	21st to end	Day	37% 5% 13% 26% 18% 1	6240
August	1st to 10th	Day	44% 7% 16% 22% 12%	4221
	11th to 20th	Day	46% 7% 14% 25% 6% 1	4213
	21st to end	Day	38% 6% 13% 29% 12% 1	4510
September	1st to 10th	Day	42% 7% 14% 23% 13% 2	2400
	11th to 20th	Day	37% 7% 13% 27% 15%	2210
	21st to end	Day	50% 6% 12% 21% 10% 1	2200
October	1st to 10th	Day	40% 6% 14% 23% 16% 1	3260
	11th to 20th	Day	49% 5% 13% 19% 12% 2	3240
	21st to end	Day	50% 4 15% 23% 8% 1	3538
November	1st to 10th	Day	55% 5% 10% 18% 10% 2	1912
	11th to 20th	Day	63% 6% 10% 12% 6% 2	1688
	21st to end	Day	74% 5% 8% 8% 4	1680
December	1st to 10th	Day	85% 4 5% 5% 1	2680
	11th to 20th	Day	80% 3 6% 9% 1	3066
	21st to end	Day	83% 3 8% 6% 1	4048
January	1st to 10th	Night	87% 2 5% 5% 1	7892
	11th to 20th	Night	84% 3 6% 5% 2	7680
	21st to end	Night	76% 3 7% 10% 3	8401
February	1st to 10th	Night	73% 4 9% 10% 4	2730
	11th to 20th	Night	72% 5% 9% 10% 4	2665
	21st to end	Night	68% 4 9% 11% 7% 1	2116
March	1st to 10th	Night	61% 4 11% 17% 7%	5028
	11th to 20th	Night	52% 4 13% 21% 9% 1	5090
	21st to end	Night	57% 6% 8% 15% 12% 1	5588
April	1st to 10th	Night	59% 5% 9% 16% 10% 1	2191
	11th to 20th	Night	53% 6% 10% 17% 12% 2	1969
	21st to end	Night	49% 7% 11% 20% 11% 3	1800
May	1st to 10th	Night	56% 5% 9% 14% 11% 5%	4060
	11th to 20th	Night	48% 6% 11% 23% 11% 1	3952
	21st to end	Night	43% 7% 12% 23% 13% 2	3927
June	1st to 10th	Night	41% 7% 12% 22% 15% 3	1600
	11th to 20th	Night	41% 6% 13% 22% 17% 2	1600
	21st to end	Night	40% 8% 13% 23% 16% 1	1600
July	1st to 10th	Night	40% 5% 12% 24% 17% 3	3560
	11th to 20th	Night	40% 5% 12% 25% 16% 2	3934
	21st to end	Night	35% 6% 12% 22% 24% 1	4297
August	1st to 10th	Night	43% 5% 12% 26% 13% 1	2904
	11th to 20th	Night	46% 7% 13% 25% 9% 1	2902
	21st to end	Night	37% 6% 15% 29% 11% 1	3355
September	1st to 10th	Night	37% 8% 12% 27% 14% 2	2000
	11th to 20th	Night	34% 6% 14% 26% 19% 1	2186
	21st to end	Night	48% 6% 12% 20% 14% 1	2200
October	1st to 10th	Night	40% 6% 9% 23% 20% 2	3920
	11th to 20th	Night	50% 5% 12% 16% 14% 4	3899
	21st to end	Night	46% 3 14% 22% 13% 2	4252
November	1st to 10th	Night	54% 4 12% 19% 9% 2	2601
	11th to 20th	Night	65% 5% 10% 12% 5% 2	2930
	21st to end	Night	71% 4 10% 11% 3 1	2940
December	1st to 10th	Night	79% 4 9% 6% 2	4720
	11th to 20th	Night	77% 4 8% 9% 2	5348
	21st to end	Night	77% 4 9% 9% 1	7150

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	35% 28% 28% 7% 2	281
	11th to 20th	Day	34% 35% 24% 5% 2	330
	21st to end	Day	32% 32% 27% 8% 1	363
February	1st to 10th	Day	32% 26% 34% 8%	330
	11th to 20th	Day	34% 30% 29% 7% 1	330
	21st to end	Day	35% 28% 29% 8%	278
March	1st to 10th	Day	30% 26% 31% 9% 4	540
	11th to 20th	Day	31% 24% 32% 11% 2 1	540
	21st to end	Day	35% 22% 27% 13% 2	594
April	1st to 10th	Day	29% 24% 31% 12% 4	540
	11th to 20th	Day	33% 18% 27% 13% 7% 1	516
	21st to end	Day	27% 19% 33% 15% 4 1	500
May	1st to 10th	Day	28% 18% 29% 18% 5% 1	503
	11th to 20th	Day	25% 15% 26% 22% 10% 2	527
	21st to end	Day	19% 9% 31% 26% 11% 4	573
June	1st to 10th	Day	19% 14% 32% 24% 8% 2	530
	11th to 20th	Day	17% 13% 29% 28% 12% 2	530
	21st to end	Day	18% 10% 33% 27% 11% 2	530
July	1st to 10th	Day	11% 7% 29% 30% 18% 5%	525
	11th to 20th	Day	16% 8% 30% 26% 15% 4	524
	21st to end	Day	12% 5% 37% 26% 16% 4	576
August	1st to 10th	Day	14% 6% 27% 30% 16% 6%	526
	11th to 20th	Day	17% 7% 24% 31% 14% 7%	519
	21st to end	Day	14% 8% 28% 27% 17% 5%	583
September	1st to 10th	Day	19% 10% 23% 26% 17% 5%	530
	11th to 20th	Day	12% 5% 24% 31% 21% 7%	530
	21st to end	Day	21% 14% 27% 22% 11% 4	530
October	1st to 10th	Day	20% 13% 26% 24% 13% 3	530
	11th to 20th	Day	30% 17% 31% 15% 6% 1	530
	21st to end	Day	37% 18% 29% 14% 1	583
November	1st to 10th	Day	34% 19% 28% 14% 4 1	390
	11th to 20th	Day	40% 22% 26% 9% 3	223
	21st to end	Day	37% 25% 30% 7%	220
December	1st to 10th	Day	35% 30% 25% 7% 1	220
	11th to 20th	Day	36% 28% 29% 7%	220
	21st to end	Day	36% 27% 29% 7% 1	242
January	1st to 10th	Night	31% 30% 28% 9% 3	840
	11th to 20th	Night	34% 25% 27% 12% 1	840
	21st to end	Night	28% 25% 31% 14% 2	924
February	1st to 10th	Night	32% 22% 29% 16% 1	840
	11th to 20th	Night	28% 25% 32% 13% 2	823
	21st to end	Night	29% 25% 31% 13% 1	692
March	1st to 10th	Night	27% 21% 32% 16% 4	840
	11th to 20th	Night	27% 21% 31% 18% 3	840
	21st to end	Night	27% 18% 31% 20% 5% 1	924
April	1st to 10th	Night	29% 16% 30% 19% 5%	840
	11th to 20th	Night	26% 14% 31% 21% 7% 1	749
	21st to end	Night	21% 15% 31% 27% 6% 1	600
May	1st to 10th	Night	26% 8% 26% 30% 9% 1	600
	11th to 20th	Night	18% 10% 30% 33% 8% 1	562
	21st to end	Night	16% 6% 27% 30% 18% 2	433
June	1st to 10th	Night	14% 4 34% 33% 14% 1	400
	11th to 20th	Night	14% 8% 27% 37% 14% 1	400
	21st to end	Night	14% 8% 27% 35% 16% 1	400
July	1st to 10th	Night	11% 4 21% 40% 20% 4	390
	11th to 20th	Night	13% 4 22% 34% 24% 3	549
	21st to end	Night	13% 4 19% 39% 25% 1	657
August	1st to 10th	Night	11% 3 21% 38% 24% 3	597
	11th to 20th	Night	14% 4 18% 34% 27% 4	594
	21st to end	Night	16% 5% 22% 30% 24% 3	740
September	1st to 10th	Night	17% 7% 19% 33% 21% 4	800
	11th to 20th	Night	13% 5% 20% 33% 25% 4	800
	21st to end	Night	21% 11% 23% 27% 16% 3	800
October	1st to 10th	Night	22% 8% 27% 26% 16% 2	800
	11th to 20th	Night	28% 16% 28% 20% 7% 1	800
	21st to end	Night	37% 15% 29% 16% 2	880
November	1st to 10th	Night	28% 16% 28% 21% 6%	820
	11th to 20th	Night	36% 19% 28% 12% 4 1	840
	21st to end	Night	36% 22% 29% 10% 3	840
December	1st to 10th	Night	37% 25% 27% 9% 3	840
	11th to 20th	Night	33% 29% 25% 11% 2	840
	21st to end	Night	34% 24% 30% 11% 1	924

Page generated at 2026-04-14 02:44:26. Version ac07

Copyright © 2026 Attilla Danko.