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 2024-12-24.

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% 4 9% 5% 4 4 3 5% 7% 30%	101658
	Night	17% 15% 5% 10% 5% 4 4 3 4 6% 28%	98404
Transparency	Day	53% 6% 12% 19% 10% 1	105301
	Night	57% 5% 11% 17% 9% 1	102900
Seeing	Day	24% 17% 29% 20% 9% 2	14671
	Night	25% 16% 27% 22% 9% 1	24332

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	17% 28% 12% 14% 5% 3 3 3 3 3 9%	2686
	11th to 20th	Day	12% 26% 9% 20% 7% 4 2 2 3 4 12%	2840
	21st to end	Day	24% 21% 10% 18% 7% 5% 3 2 2 2 6%	3124
February	1st to 10th	Day	17% 21% 6% 16% 9% 4 4 3 3 3 14%	1710
	11th to 20th	Day	15% 19% 10% 16% 9% 3 4 3 3 3 15%	1663
	21st to end	Day	19% 18% 4 14% 5% 3 4 3 4 6% 20%	1531
March	1st to 10th	Day	19% 15% 8% 7% 4 3 4 3 5% 7% 26%	3435
	11th to 20th	Day	11% 22% 8% 9% 4 3 3 3 5% 5% 27%	3480
	21st to end	Day	21% 15% 5% 11% 3 3 4 3 3 5% 27%	3815
April	1st to 10th	Day	17% 15% 4 7% 5% 4 3 3 4 7% 30%	2195
	11th to 20th	Day	20% 14% 2 6% 4 4 4 4 5% 5% 33%	2333
	21st to end	Day	20% 12% 3 7% 5% 4 3 2 5% 5% 33%	2470
May	1st to 10th	Day	19% 10% 3 8% 5% 4 4 3 5% 6% 31%	4082
	11th to 20th	Day	19% 10% 3 7% 4 4 4 4 6% 7% 32%	4130
	21st to end	Day	13% 6% 2 5% 5% 4 5% 4 6% 7% 42%	4823
June	1st to 10th	Day	17% 7% 2 6% 5% 4 4 3 6% 8% 37%	2660
	11th to 20th	Day	15% 6% 3 7% 4 4 5% 4 6% 8% 38%	2660
	21st to end	Day	15% 6% 2 5% 5% 5% 4 5% 6% 8% 39%	2660
July	1st to 10th	Day	10% 5% 2 7% 4 4 5% 4 7% 10% 40%	4440
	11th to 20th	Day	14% 6% 2 6% 4 4 4 4 6% 10% 42%	4162
	21st to end	Day	11% 6% 2 6% 5% 5% 5% 4 7% 7% 42%	4466
August	1st to 10th	Day	16% 6% 2 7% 4 4 5% 4 7% 8% 35%	4104
	11th to 20th	Day	15% 7% 3 8% 5% 5% 5% 5% 7% 9% 31%	4044
	21st to end	Day	13% 5% 2 6% 4 5% 5% 5% 7% 8% 40%	4372
September	1st to 10th	Day	18% 8% 2 6% 4 3 4 4 5% 9% 38%	2280
	11th to 20th	Day	13% 6% 2 6% 5% 4 4 4 6% 7% 42%	2100
	21st to end	Day	20% 8% 3 8% 5% 4 4 3 4 6% 33%	2090
October	1st to 10th	Day	20% 7% 2 8% 5% 4 4 3 6% 8% 33%	1900
	11th to 20th	Day	23% 10% 3 8% 5% 4 5% 3 5% 6% 28%	1900
	21st to end	Day	27% 14% 3 8% 5% 4 4 3 5% 6% 21%	2090
November	1st to 10th	Day	16% 16% 3 8% 5% 3 4 3 5% 6% 30%	1819
	11th to 20th	Day	18% 24% 4 8% 5% 4 3 3 5% 6% 22%	1608
	21st to end	Day	20% 29% 5% 8% 4 4 4 3 3 5% 14%	1600
December	1st to 10th	Day	19% 33% 9% 10% 4 3 3 2 4 3 11%	1600
	11th to 20th	Day	18% 26% 9% 13% 4 3 3 3 4 3 13%	1990
	21st to end	Day	24% 25% 8% 13% 5% 3 3 2 2 3 10%	2796
January	1st to 10th	Night	17% 27% 12% 17% 5% 4 2 2 2 3 9%	4255
	11th to 20th	Night	13% 24% 11% 18% 5% 4 4 3 3 4 10%	4120
	21st to end	Night	24% 23% 9% 17% 5% 3 3 1 2 2 10%	4532
February	1st to 10th	Night	19% 20% 6% 14% 9% 4 3 3 3 3 16%	2470
	11th to 20th	Night	16% 19% 7% 15% 7% 5% 4 3 4 4 16%	2403
	21st to end	Night	19% 16% 6% 15% 6% 4 3 2 3 4 21%	1921
March	1st to 10th	Night	21% 17% 4 9% 4 3 3 2 4 5% 28%	3418
	11th to 20th	Night	17% 18% 8% 9% 3 3 3 2 4 5% 28%	3480
	21st to end	Night	16% 13% 6% 10% 5% 3 3 3 3 3 34%	3817
April	1st to 10th	Night	18% 18% 4 7% 4 4 3 3 4 5% 29%	1983
	11th to 20th	Night	19% 12% 3 7% 5% 4 4 3 6% 6% 32%	1812
	21st to end	Night	16% 12% 3 6% 4 4 4 4 5% 7% 33%	1710
May	1st to 10th	Night	24% 10% 3 6% 4 4 4 3 5% 6% 31%	2813
	11th to 20th	Night	16% 9% 3 8% 5% 4 4 3 6% 8% 35%	2759
	21st to end	Night	14% 5% 2 5% 3 3 4 3 6% 11% 45%	2732
June	1st to 10th	Night	11% 7% 3 7% 5% 5% 4 4 7% 7% 40%	1520
	11th to 20th	Night	10% 7% 3 8% 6% 4 4 3 6% 8% 42%	1520
	21st to end	Night	14% 6% 2 7% 5% 4 5% 3 7% 8% 40%	1520
July	1st to 10th	Night	12% 7% 3 7% 4 4 4 3 6% 9% 41%	2520
	11th to 20th	Night	11% 6% 3 6% 4 5% 4 3 6% 9% 43%	2794
	21st to end	Night	9% 6% 2 6% 5% 5% 5% 4 6% 8% 44%	3087
August	1st to 10th	Night	12% 7% 3 8% 5% 4 6% 3 6% 7% 39%	2825
	11th to 20th	Night	16% 7% 4 8% 4 5% 5% 3 5% 5% 38%	2785
	21st to end	Night	10% 6% 2 6% 5% 5% 5% 4 6% 9% 42%	3252
September	1st to 10th	Night	15% 6% 2 5% 5% 3 4 3 6% 7% 44%	1900
	11th to 20th	Night	13% 6% 3 5% 4 3 4 3 5% 6% 48%	2076
	21st to end	Night	19% 7% 3 8% 5% 4 5% 4 5% 7% 34%	2090
October	1st to 10th	Night	20% 6% 3 6% 4 3 4 4 6% 7% 38%	2280
	11th to 20th	Night	24% 10% 3 7% 5% 4 4 3 5% 6% 29%	2280
	21st to end	Night	28% 12% 3 6% 4 5% 4 3 4 5% 26%	2508
November	1st to 10th	Night	17% 16% 4 6% 4 4 3 3 5% 5% 32%	2474
	11th to 20th	Night	20% 22% 3 8% 5% 4 4 3 5% 5% 21%	2790
	21st to end	Night	20% 26% 6% 9% 4 4 3 3 4 4 18%	2800
December	1st to 10th	Night	20% 30% 9% 10% 5% 3 3 2 3 3 12%	2800
	11th to 20th	Night	17% 27% 9% 14% 5% 3 3 2 3 4 13%	3425
	21st to end	Night	23% 26% 7% 12% 6% 4 4 3 4 3 8%	4933

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	80% 4 7% 7% 1	2687
	11th to 20th	Day	78% 3 9% 8% 2	2840
	21st to end	Day	86% 3 6% 3 1	3124
February	1st to 10th	Day	76% 4 8% 7% 4	1710
	11th to 20th	Day	74% 4 7% 10% 5%	1672
	21st to end	Day	65% 5% 11% 13% 5% 2	1531
March	1st to 10th	Day	58% 7% 10% 22% 4	3435
	11th to 20th	Day	57% 7% 16% 14% 5% 1	3480
	21st to end	Day	60% 5% 8% 17% 10% 1	3815
April	1st to 10th	Day	54% 6% 12% 17% 11%	2195
	11th to 20th	Day	51% 6% 10% 19% 11% 2	2333
	21st to end	Day	52% 5% 10% 18% 11% 2	2470
May	1st to 10th	Day	51% 7% 9% 23% 9% 1	4120
	11th to 20th	Day	49% 8% 12% 21% 8% 1	4134
	21st to end	Day	38% 7% 14% 21% 16% 4	4837
June	1st to 10th	Day	44% 6% 13% 23% 13% 1	2660
	11th to 20th	Day	41% 7% 15% 22% 13% 1	2660
	21st to end	Day	40% 8% 13% 25% 14% 1	2660
July	1st to 10th	Day	36% 6% 15% 24% 17% 2	4370
	11th to 20th	Day	37% 6% 14% 26% 17% 1	4091
	21st to end	Day	39% 6% 15% 24% 16% 1	4324
August	1st to 10th	Day	44% 7% 15% 22% 11%	4091
	11th to 20th	Day	46% 8% 15% 25% 6% 1	4083
	21st to end	Day	38% 6% 13% 29% 12% 1	4372
September	1st to 10th	Day	43% 7% 14% 22% 13% 2	2280
	11th to 20th	Day	39% 8% 13% 25% 16%	2100
	21st to end	Day	51% 6% 12% 21% 10% 1	2090
October	1st to 10th	Day	40% 6% 15% 22% 16% 1	3160
	11th to 20th	Day	49% 5% 13% 18% 12% 2	3140
	21st to end	Day	50% 4 15% 24% 7%	3428
November	1st to 10th	Day	55% 5% 10% 18% 10% 2	1819
	11th to 20th	Day	63% 6% 10% 12% 7% 2	1608
	21st to end	Day	74% 5% 9% 8% 4	1600
December	1st to 10th	Day	79% 5% 7% 8% 1	1600
	11th to 20th	Day	77% 4 8% 9% 1	1986
	21st to end	Day	82% 3 8% 6% 1	2796
January	1st to 10th	Night	82% 3 7% 6% 2	4255
	11th to 20th	Night	79% 5% 9% 6% 2	4120
	21st to end	Night	83% 2 7% 6% 1	4532
February	1st to 10th	Night	73% 5% 9% 10% 3	2470
	11th to 20th	Night	71% 5% 9% 11% 4	2405
	21st to end	Night	68% 4 10% 11% 7% 1	1921
March	1st to 10th	Night	58% 5% 11% 20% 6%	3418
	11th to 20th	Night	59% 5% 13% 16% 6% 1	3480
	21st to end	Night	53% 7% 9% 18% 12% 2	3817
April	1st to 10th	Night	57% 6% 9% 17% 11% 1	1983
	11th to 20th	Night	51% 7% 11% 17% 12% 2	1812
	21st to end	Night	49% 7% 11% 19% 10% 3	1710
May	1st to 10th	Night	52% 6% 10% 18% 11% 3	2840
	11th to 20th	Night	47% 7% 13% 22% 10% 2	2760
	21st to end	Night	32% 9% 14% 26% 16% 3	2739
June	1st to 10th	Night	41% 8% 12% 22% 15% 2	1520
	11th to 20th	Night	40% 6% 13% 22% 17% 2	1520
	21st to end	Night	41% 8% 12% 23% 15% 1	1520
July	1st to 10th	Night	40% 5% 13% 20% 18% 4	2480
	11th to 20th	Night	37% 5% 12% 26% 16% 3	2742
	21st to end	Night	37% 7% 14% 22% 20% 1	2988
August	1st to 10th	Night	43% 5% 12% 26% 12% 1	2814
	11th to 20th	Night	47% 7% 13% 24% 8% 1	2812
	21st to end	Night	37% 6% 15% 29% 12% 2	3252
September	1st to 10th	Night	38% 7% 12% 26% 14% 2	1900
	11th to 20th	Night	35% 6% 14% 25% 19% 1	2076
	21st to end	Night	48% 6% 12% 20% 13% 1	2090
October	1st to 10th	Night	40% 6% 9% 23% 20% 1	3800
	11th to 20th	Night	51% 5% 12% 16% 14% 3	3779
	21st to end	Night	46% 3 15% 22% 13% 1	4120
November	1st to 10th	Night	54% 5% 12% 18% 9% 2	2474
	11th to 20th	Night	65% 5% 10% 12% 6% 3	2790
	21st to end	Night	70% 5% 10% 11% 3 1	2800
December	1st to 10th	Night	79% 4 8% 7% 2	2800
	11th to 20th	Night	76% 5% 9% 8% 2	3428
	21st to end	Night	81% 5% 8% 5%	4933

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	29% 28% 31% 9% 3	229
	11th to 20th	Day	29% 36% 26% 6% 3	270
	21st to end	Day	27% 34% 29% 8% 1	297
February	1st to 10th	Day	32% 25% 36% 7%	270
	11th to 20th	Day	31% 32% 29% 8% 1	270
	21st to end	Day	37% 28% 28% 8%	230
March	1st to 10th	Day	29% 25% 33% 9% 4	460
	11th to 20th	Day	27% 24% 34% 12% 2 1	460
	21st to end	Day	34% 22% 26% 15% 2	506
April	1st to 10th	Day	25% 25% 32% 13% 5%	460
	11th to 20th	Day	32% 19% 27% 14% 8% 1	452
	21st to end	Day	27% 19% 33% 15% 5% 1	460
May	1st to 10th	Day	27% 19% 28% 20% 5% 1	463
	11th to 20th	Day	25% 14% 26% 23% 10% 2	487
	21st to end	Day	15% 9% 33% 28% 12% 4	529
June	1st to 10th	Day	19% 13% 31% 26% 9% 3	490
	11th to 20th	Day	16% 12% 29% 28% 13% 2	490
	21st to end	Day	19% 10% 33% 27% 10% 2	490
July	1st to 10th	Day	10% 7% 28% 31% 19% 5%	485
	11th to 20th	Day	16% 8% 30% 26% 16% 4	484
	21st to end	Day	11% 5% 37% 26% 16% 4	532
August	1st to 10th	Day	15% 6% 27% 30% 16% 6%	486
	11th to 20th	Day	17% 7% 24% 31% 14% 7%	479
	21st to end	Day	14% 8% 27% 28% 18% 5%	539
September	1st to 10th	Day	19% 10% 23% 26% 17% 5%	490
	11th to 20th	Day	13% 5% 25% 30% 21% 7%	490
	21st to end	Day	22% 15% 26% 22% 11% 4	490
October	1st to 10th	Day	21% 14% 27% 22% 12% 3	490
	11th to 20th	Day	30% 17% 29% 16% 6% 1	490
	21st to end	Day	37% 19% 29% 15% 1	539
November	1st to 10th	Day	32% 18% 29% 15% 4 1	357
	11th to 20th	Day	37% 24% 27% 9% 3	203
	21st to end	Day	35% 26% 32% 8%	200
December	1st to 10th	Day	35% 32% 24% 7% 2 1	200
	11th to 20th	Day	35% 29% 30% 8%	200
	21st to end	Day	32% 27% 33% 6% 1	204
January	1st to 10th	Night	30% 29% 28% 9% 3	760
	11th to 20th	Night	33% 24% 28% 13% 1	760
	21st to end	Night	26% 25% 33% 14% 2	836
February	1st to 10th	Night	32% 23% 28% 16% 1	760
	11th to 20th	Night	27% 25% 33% 14% 2	743
	21st to end	Night	29% 24% 32% 14% 1	628
March	1st to 10th	Night	26% 20% 33% 17% 4	760
	11th to 20th	Night	26% 21% 31% 19% 4	760
	21st to end	Night	26% 17% 31% 21% 5% 1	836
April	1st to 10th	Night	27% 16% 31% 20% 5%	760
	11th to 20th	Night	26% 13% 32% 21% 7% 1	688
	21st to end	Night	21% 14% 31% 28% 6% 1	570
May	1st to 10th	Night	26% 8% 25% 30% 10% 1	570
	11th to 20th	Night	17% 9% 29% 34% 8% 1	534
	21st to end	Night	14% 7% 27% 31% 19% 2	411
June	1st to 10th	Night	14% 3 34% 33% 15% 1	380
	11th to 20th	Night	13% 8% 26% 38% 14% 1	380
	21st to end	Night	14% 8% 27% 35% 15% 1	380
July	1st to 10th	Night	11% 4 21% 39% 21% 4	370
	11th to 20th	Night	12% 4 22% 33% 25% 3	521
	21st to end	Night	12% 4 20% 38% 25% 1	624
August	1st to 10th	Night	12% 2 20% 38% 25% 3	567
	11th to 20th	Night	14% 4 18% 33% 27% 4	564
	21st to end	Night	16% 5% 23% 29% 24% 3	703
September	1st to 10th	Night	18% 7% 19% 32% 21% 4	760
	11th to 20th	Night	13% 5% 21% 32% 24% 4	760
	21st to end	Night	21% 11% 22% 26% 16% 3	760
October	1st to 10th	Night	22% 8% 27% 24% 16% 2	760
	11th to 20th	Night	29% 16% 27% 20% 8% 1	760
	21st to end	Night	37% 15% 29% 17% 2	836
November	1st to 10th	Night	27% 16% 28% 21% 6%	780
	11th to 20th	Night	36% 20% 28% 12% 4 1	800
	21st to end	Night	35% 22% 29% 11% 3	800
December	1st to 10th	Night	37% 25% 27% 8% 3	800
	11th to 20th	Night	33% 28% 25% 12% 2	800
	21st to end	Night	33% 24% 31% 11% 1	851

Page generated at 2024-12-21 04:13:48. Version ac07

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