Eel Bay Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eel Bay Observatory clear sky chart from 2005-11-06 to 2024-11-06.

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	22% 10% 3 8% 4 4 4 3 5% 6% 30%	101994
	Night	22% 11% 4 8% 5% 4 4 3 5% 6% 28%	97105
Transparency	Day	54% 6% 11% 19% 10% 1	105584
	Night	56% 6% 10% 18% 9% 1	101593
Seeing	Day	27% 18% 32% 17% 6% 1	17108
	Night	26% 15% 29% 22% 7% 1	24700

By Month

Percentage Cloud Cover NE (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	20% 13% 5% 10% 6% 5% 4 4 5% 6% 21%	2564
	11th to 20th	Day	20% 10% 5% 16% 7% 5% 5% 5% 6% 6% 16%	2840
	21st to end	Day	22% 11% 4 10% 6% 5% 5% 4 5% 7% 22%	3124
February	1st to 10th	Day	21% 15% 4 10% 6% 6% 5% 4 5% 6% 18%	1710
	11th to 20th	Day	18% 11% 3 11% 7% 5% 6% 4 6% 6% 22%	1663
	21st to end	Day	16% 12% 4 10% 6% 5% 4 3 5% 5% 28%	1508
March	1st to 10th	Day	28% 13% 3 8% 5% 3 3 3 5% 4 24%	3395
	11th to 20th	Day	17% 14% 4 10% 5% 4 4 4 5% 7% 28%	3480
	21st to end	Day	31% 10% 4 11% 4 3 3 2 3 5% 23%	3816
April	1st to 10th	Day	24% 13% 3 8% 5% 4 3 3 5% 5% 29%	2199
	11th to 20th	Day	20% 10% 2 7% 5% 4 3 3 5% 6% 36%	2383
	21st to end	Day	23% 8% 2 7% 4 4 4 3 5% 6% 35%	2470
May	1st to 10th	Day	23% 11% 4 9% 5% 5% 4 3 5% 5% 27%	4106
	11th to 20th	Day	25% 12% 3 7% 4 3 4 3 5% 6% 29%	4238
	21st to end	Day	20% 9% 3 7% 4 4 4 4 6% 8% 31%	4836
June	1st to 10th	Day	25% 9% 2 6% 4 3 4 3 5% 6% 32%	2660
	11th to 20th	Day	21% 10% 3 5% 4 3 4 4 4 7% 36%	2797
	21st to end	Day	27% 9% 3 7% 4 4 4 4 5% 7% 28%	2850
July	1st to 10th	Day	26% 12% 3 6% 3 3 4 3 5% 6% 29%	4711
	11th to 20th	Day	31% 11% 2 6% 4 4 4 3 5% 7% 24%	4323
	21st to end	Day	21% 10% 3 6% 4 3 3 3 5% 7% 34%	4467
August	1st to 10th	Day	30% 13% 3 6% 3 3 3 3 4 6% 26%	4105
	11th to 20th	Day	18% 9% 3 8% 4 4 4 4 5% 6% 36%	4045
	21st to end	Day	12% 6% 2 5% 3 3 4 3 5% 7% 49%	4419
September	1st to 10th	Day	16% 6% 3 7% 4 4 3 3 5% 6% 44%	2280
	11th to 20th	Day	19% 7% 2 5% 4 3 4 2 5% 6% 45%	2100
	21st to end	Day	20% 7% 3 5% 4 3 4 3 4 6% 40%	2090
October	1st to 10th	Day	20% 7% 2 6% 4 4 4 3 5% 7% 39%	1904
	11th to 20th	Day	20% 6% 3 6% 5% 4 5% 4 6% 7% 36%	1900
	21st to end	Day	21% 7% 3 8% 5% 5% 6% 4 5% 6% 31%	2087
November	1st to 10th	Day	21% 8% 3 6% 5% 4 4 3 4 7% 34%	1675
	11th to 20th	Day	22% 9% 2 8% 5% 6% 5% 4 5% 7% 27%	1520
	21st to end	Day	20% 12% 3 10% 6% 5% 5% 4 7% 5% 23%	1520
December	1st to 10th	Day	24% 14% 4 9% 6% 4 5% 4 5% 5% 21%	1520
	11th to 20th	Day	30% 13% 4 8% 5% 4 5% 3 5% 5% 19%	1906
	21st to end	Day	23% 14% 4 7% 4 5% 5% 3 5% 6% 25%	2783
January	1st to 10th	Night	17% 14% 6% 12% 7% 5% 5% 3 5% 6% 21%	4388
	11th to 20th	Night	18% 13% 5% 14% 6% 6% 5% 4 5% 6% 19%	4120
	21st to end	Night	20% 13% 5% 12% 7% 5% 4 4 5% 6% 19%	4532
February	1st to 10th	Night	20% 14% 5% 12% 7% 5% 4 4 4 6% 20%	2470
	11th to 20th	Night	19% 10% 4 10% 7% 6% 5% 5% 5% 7% 22%	2403
	21st to end	Night	14% 11% 4 8% 6% 6% 6% 4 5% 8% 29%	1941
March	1st to 10th	Night	26% 12% 3 9% 6% 4 4 3 4 3 27%	3445
	11th to 20th	Night	16% 12% 5% 11% 5% 4 4 4 5% 7% 27%	3479
	21st to end	Night	33% 10% 5% 8% 3 3 2 2 3 4 28%	3816
April	1st to 10th	Night	23% 13% 3 7% 4 3 3 3 4 6% 31%	1981
	11th to 20th	Night	24% 8% 3 6% 3 3 3 3 4 6% 35%	1744
	21st to end	Night	23% 9% 3 6% 5% 3 4 3 5% 6% 33%	1710
May	1st to 10th	Night	21% 15% 4 6% 4 3 4 4 6% 6% 27%	2834
	11th to 20th	Night	25% 14% 3 7% 3 4 3 3 4 5% 29%	2661
	21st to end	Night	22% 13% 3 8% 4 3 3 2 5% 5% 29%	2749
June	1st to 10th	Night	26% 10% 2 8% 4 3 3 3 5% 7% 29%	1520
	11th to 20th	Night	19% 11% 4 7% 4 4 3 3 3 6% 35%	1396
	21st to end	Night	30% 12% 3 6% 4 3 3 3 6% 6% 24%	1330
July	1st to 10th	Night	31% 14% 4 6% 4 3 4 3 5% 6% 21%	2210
	11th to 20th	Night	35% 15% 3 6% 4 3 4 3 4 5% 19%	2638
	21st to end	Night	24% 11% 3 7% 4 4 4 4 5% 6% 28%	3087
August	1st to 10th	Night	26% 16% 3 7% 4 3 3 3 4 6% 26%	2825
	11th to 20th	Night	22% 11% 3 8% 5% 4 4 3 4 6% 31%	2782
	21st to end	Night	12% 10% 3 6% 3 4 4 3 5% 7% 43%	3236
September	1st to 10th	Night	18% 8% 2 4 4 4 4 3 5% 7% 40%	1900
	11th to 20th	Night	20% 6% 3 5% 3 3 4 3 5% 6% 43%	2076
	21st to end	Night	21% 9% 3 6% 4 3 3 3 4 6% 38%	2090
October	1st to 10th	Night	21% 7% 2 6% 4 3 4 3 5% 6% 39%	2275
	11th to 20th	Night	19% 6% 2 5% 3 3 3 3 5% 7% 42%	2280
	21st to end	Night	22% 8% 3 7% 4 4 3 4 4 6% 36%	2508
November	1st to 10th	Night	19% 8% 2 6% 4 4 4 3 5% 7% 38%	2522
	11th to 20th	Night	19% 10% 3 8% 5% 5% 4 4 6% 7% 29%	2660
	21st to end	Night	19% 11% 4 9% 5% 4 5% 5% 6% 6% 26%	2660
December	1st to 10th	Night	23% 14% 4 9% 5% 4 5% 3 6% 7% 21%	2660
	11th to 20th	Night	27% 13% 4 9% 4 4 4 3 4 5% 21%	3293
	21st to end	Night	24% 11% 4 8% 4 4 4 3 4 5% 27%	4884

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% 11% 12% 13% 4	2565
	11th to 20th	Day	66% 9% 10% 11% 4	2840
	21st to end	Day	62% 7% 10% 13% 7%	3124
February	1st to 10th	Day	67% 7% 11% 10% 6%	1710
	11th to 20th	Day	62% 8% 10% 13% 7%	1671
	21st to end	Day	55% 7% 12% 17% 8% 2	1508
March	1st to 10th	Day	62% 5% 10% 15% 7% 1	3399
	11th to 20th	Day	54% 7% 13% 16% 9% 1	3480
	21st to end	Day	66% 3 7% 14% 9% 2	3816
April	1st to 10th	Day	58% 4 9% 15% 12% 1	2199
	11th to 20th	Day	49% 5% 11% 21% 13% 1	2383
	21st to end	Day	49% 5% 11% 18% 15% 2	2470
May	1st to 10th	Day	57% 7% 11% 18% 6% 1	4120
	11th to 20th	Day	55% 5% 12% 19% 8% 1	4242
	21st to end	Day	48% 12% 12% 15% 11% 1	4836
June	1st to 10th	Day	52% 6% 10% 21% 10% 1	2660
	11th to 20th	Day	48% 7% 10% 19% 13% 2	2797
	21st to end	Day	56% 7% 12% 15% 9% 1	2850
July	1st to 10th	Day	56% 6% 8% 16% 12% 1	4637
	11th to 20th	Day	61% 5% 7% 20% 7% 1	4247
	21st to end	Day	50% 6% 11% 23% 11% 1	4325
August	1st to 10th	Day	60% 5% 8% 17% 9% 1	4092
	11th to 20th	Day	47% 6% 13% 25% 8%	4084
	21st to end	Day	34% 4 11% 29% 19% 2	4419
September	1st to 10th	Day	42% 5% 10% 24% 17% 2	2280
	11th to 20th	Day	41% 5% 11% 23% 17% 2	2100
	21st to end	Day	45% 5% 7% 23% 18% 2	2090
October	1st to 10th	Day	46% 4 10% 27% 12% 1	3164
	11th to 20th	Day	39% 5% 9% 26% 21% 1	3140
	21st to end	Day	44% 5% 11% 25% 11% 4	3413
November	1st to 10th	Day	50% 5% 12% 21% 10% 1	1675
	11th to 20th	Day	56% 7% 12% 17% 8%	1520
	21st to end	Day	60% 9% 13% 15% 3	1520
December	1st to 10th	Day	64% 7% 10% 13% 5%	1520
	11th to 20th	Day	68% 5% 10% 12% 5%	1905
	21st to end	Day	59% 7% 16% 12% 6%	2783
January	1st to 10th	Night	63% 10% 10% 14% 3 1	4388
	11th to 20th	Night	63% 9% 10% 12% 5%	4120
	21st to end	Night	65% 7% 10% 12% 5%	4532
February	1st to 10th	Night	66% 6% 11% 10% 6%	2470
	11th to 20th	Night	60% 7% 10% 15% 7% 1	2406
	21st to end	Night	53% 6% 15% 17% 9% 1	1941
March	1st to 10th	Night	62% 6% 9% 12% 10% 1	3454
	11th to 20th	Night	56% 6% 14% 14% 9% 1	3480
	21st to end	Night	62% 3 7% 15% 10% 2	3816
April	1st to 10th	Night	56% 4 9% 16% 12% 3	1981
	11th to 20th	Night	50% 5% 9% 22% 13% 1	1744
	21st to end	Night	50% 4 12% 19% 12% 2	1710
May	1st to 10th	Night	56% 7% 12% 18% 6%	2840
	11th to 20th	Night	57% 6% 9% 20% 8%	2662
	21st to end	Night	55% 8% 9% 22% 6%	2750
June	1st to 10th	Night	54% 6% 10% 22% 8% 1	1520
	11th to 20th	Night	51% 5% 10% 20% 12% 2	1396
	21st to end	Night	62% 6% 8% 13% 10% 1	1330
July	1st to 10th	Night	65% 5% 7% 14% 9%	2175
	11th to 20th	Night	67% 5% 6% 12% 8% 2	2591
	21st to end	Night	56% 7% 9% 18% 10% 1	2989
August	1st to 10th	Night	62% 4 8% 18% 8%	2813
	11th to 20th	Night	55% 5% 11% 23% 6%	2809
	21st to end	Night	40% 5% 9% 26% 18% 1	3235
September	1st to 10th	Night	43% 6% 9% 24% 16% 3	1900
	11th to 20th	Night	42% 5% 8% 25% 18% 2	2076
	21st to end	Night	48% 4 6% 23% 17% 1	2090
October	1st to 10th	Night	48% 4 11% 23% 13% 1	3795
	11th to 20th	Night	34% 4 10% 25% 25% 1	3779
	21st to end	Night	42% 4 10% 27% 16% 1	4120
November	1st to 10th	Night	47% 6% 13% 23% 10%	2522
	11th to 20th	Night	54% 7% 13% 17% 8%	2660
	21st to end	Night	55% 10% 15% 15% 5%	2660
December	1st to 10th	Night	63% 6% 12% 14% 4	2660
	11th to 20th	Night	65% 6% 10% 14% 5%	3295
	21st to end	Night	59% 7% 12% 18% 4 1	4884

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% 30% 28% 11% 2	370
	11th to 20th	Day	31% 23% 30% 14% 2	370
	21st to end	Day	22% 28% 33% 12% 4	407
February	1st to 10th	Day	29% 24% 32% 12% 2	370
	11th to 20th	Day	30% 30% 27% 12% 2	366
	21st to end	Day	26% 26% 32% 14% 2 1	352
March	1st to 10th	Day	30% 25% 31% 11% 2	460
	11th to 20th	Day	30% 24% 31% 11% 4	460
	21st to end	Day	28% 23% 32% 11% 5% 1	506
April	1st to 10th	Day	33% 25% 28% 11% 2	460
	11th to 20th	Day	27% 23% 36% 11% 3	452
	21st to end	Day	26% 22% 33% 15% 5% 1	460
May	1st to 10th	Day	28% 22% 33% 13% 2 1	463
	11th to 20th	Day	26% 19% 33% 17% 5% 1	579
	21st to end	Day	28% 14% 32% 19% 6% 1	735
June	1st to 10th	Day	29% 13% 33% 20% 5%	680
	11th to 20th	Day	23% 11% 35% 22% 8% 1	680
	21st to end	Day	33% 11% 29% 20% 5% 1	680
July	1st to 10th	Day	29% 6% 32% 23% 10% 1	670
	11th to 20th	Day	28% 6% 33% 22% 9% 1	603
	21st to end	Day	24% 7% 35% 25% 8% 1	532
August	1st to 10th	Day	21% 8% 35% 24% 9% 2	486
	11th to 20th	Day	26% 10% 31% 21% 10% 3	479
	21st to end	Day	21% 10% 31% 24% 11% 3	539
September	1st to 10th	Day	19% 11% 32% 28% 9% 2	490
	11th to 20th	Day	24% 11% 31% 23% 9% 3	490
	21st to end	Day	21% 12% 37% 20% 7% 3	490
October	1st to 10th	Day	20% 13% 33% 22% 10% 2	392
	11th to 20th	Day	19% 19% 30% 22% 7% 2	390
	21st to end	Day	25% 20% 30% 19% 6%	429
November	1st to 10th	Day	24% 19% 32% 15% 8% 1	381
	11th to 20th	Day	27% 28% 26% 14% 5% 1	370
	21st to end	Day	28% 24% 31% 14% 4 1	370
December	1st to 10th	Day	33% 23% 29% 13% 3	370
	11th to 20th	Day	37% 27% 23% 11% 1	370
	21st to end	Day	29% 26% 28% 14% 2 1	407
January	1st to 10th	Night	28% 25% 29% 15% 3	837
	11th to 20th	Night	29% 19% 31% 19% 3	760
	21st to end	Night	22% 24% 34% 17% 3	836
February	1st to 10th	Night	28% 22% 31% 17% 2	760
	11th to 20th	Night	25% 26% 31% 15% 2	743
	21st to end	Night	21% 20% 35% 18% 5% 1	628
March	1st to 10th	Night	28% 19% 33% 17% 3	760
	11th to 20th	Night	24% 21% 30% 19% 6%	760
	21st to end	Night	26% 19% 31% 19% 5% 1	836
April	1st to 10th	Night	29% 18% 33% 18% 3	651
	11th to 20th	Night	27% 14% 39% 15% 4	564
	21st to end	Night	28% 12% 31% 21% 8% 1	570
May	1st to 10th	Night	29% 14% 30% 21% 5% 1	570
	11th to 20th	Night	28% 10% 31% 24% 6% 1	562
	21st to end	Night	26% 8% 31% 27% 7% 1	616
June	1st to 10th	Night	28% 9% 31% 26% 6%	570
	11th to 20th	Night	23% 6% 30% 33% 8%	446
	21st to end	Night	34% 5% 23% 27% 11%	380
July	1st to 10th	Night	30% 4 25% 31% 10% 1	370
	11th to 20th	Night	29% 5% 28% 28% 10%	551
	21st to end	Night	29% 5% 26% 31% 9%	623
August	1st to 10th	Night	23% 4 27% 34% 11% 1	568
	11th to 20th	Night	25% 5% 25% 29% 14% 1	562
	21st to end	Night	22% 4 26% 27% 18% 2	627
September	1st to 10th	Night	21% 7% 27% 30% 13% 1	570
	11th to 20th	Night	22% 7% 26% 27% 15% 3	746
	21st to end	Night	25% 8% 27% 24% 12% 3	760
October	1st to 10th	Night	25% 10% 27% 26% 12% 1	760
	11th to 20th	Night	22% 16% 28% 23% 10% 2	760
	21st to end	Night	27% 16% 27% 23% 7%	836
November	1st to 10th	Night	22% 16% 28% 24% 8% 1	783
	11th to 20th	Night	25% 21% 27% 20% 7%	850
	21st to end	Night	24% 26% 26% 18% 6%	850
December	1st to 10th	Night	30% 20% 30% 16% 5%	850
	11th to 20th	Night	30% 26% 28% 14% 3	850
	21st to end	Night	28% 24% 27% 18% 3	935

Page generated at 2024-11-03 14:18:59. Version ac07

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