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 2025-11-03.

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% 11% 4 8% 5% 4 4 3 5% 6% 28%	127600
	Night	21% 12% 4 9% 5% 4 4 3 5% 6% 28%	122509
Transparency	Day	56% 5% 10% 18% 10% 1	131179
	Night	56% 5% 10% 18% 10% 1	127006
Seeing	Day	27% 17% 32% 17% 6% 1	18474
	Night	26% 15% 29% 22% 7% 1	26020

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	17% 17% 6% 17% 5% 4 4 4 5% 5% 16%	3680
	11th to 20th	Day	17% 13% 6% 19% 7% 6% 5% 4 5% 5% 13%	4027
	21st to end	Day	18% 14% 4 13% 8% 6% 5% 4 5% 6% 19%	4466
February	1st to 10th	Day	21% 15% 5% 11% 7% 6% 5% 4 5% 6% 18%	1800
	11th to 20th	Day	18% 11% 3 13% 7% 5% 6% 4 6% 6% 22%	1753
	21st to end	Day	15% 12% 4 11% 6% 5% 4 3 5% 5% 28%	1585
March	1st to 10th	Day	25% 13% 5% 11% 6% 4 4 3 5% 5% 20%	4881
	11th to 20th	Day	17% 17% 4 9% 5% 4 4 3 5% 6% 29%	4980
	21st to end	Day	29% 10% 4 11% 5% 4 4 3 4 5% 22%	5466
April	1st to 10th	Day	25% 13% 3 7% 5% 4 3 3 5% 5% 28%	2315
	11th to 20th	Day	20% 10% 2 7% 5% 5% 3 3 5% 6% 35%	2510
	21st to end	Day	23% 8% 2 7% 4 4 4 3 5% 6% 36%	2600
May	1st to 10th	Day	27% 15% 4 10% 4 4 3 3 4 4 22%	5886
	11th to 20th	Day	24% 12% 3 7% 4 4 4 3 5% 6% 28%	6088
	21st to end	Day	21% 11% 3 8% 5% 4 4 3 5% 7% 29%	6948
June	1st to 10th	Day	25% 9% 2 6% 4 3 4 3 5% 6% 32%	2800
	11th to 20th	Day	21% 10% 3 5% 4 3 4 3 4 7% 37%	2944
	21st to end	Day	27% 9% 3 6% 4 4 4 4 5% 7% 29%	3000
July	1st to 10th	Day	29% 13% 3 6% 3 3 4 3 4 6% 26%	6757
	11th to 20th	Day	29% 13% 3 6% 4 4 4 3 5% 8% 22%	6187
	21st to end	Day	16% 9% 3 6% 4 4 3 4 5% 8% 38%	6425
August	1st to 10th	Day	29% 13% 3 6% 3 3 3 3 4 6% 27%	4235
	11th to 20th	Day	18% 9% 3 7% 4 4 4 4 5% 6% 37%	4175
	21st to end	Day	12% 6% 2 5% 3 3 4 3 5% 7% 49%	4559
September	1st to 10th	Day	16% 7% 3 7% 4 4 3 3 5% 6% 43%	2400
	11th to 20th	Day	18% 6% 2 5% 4 3 4 2 5% 6% 46%	2210
	21st to end	Day	19% 8% 3 6% 4 3 4 3 4 6% 41%	2200
October	1st to 10th	Day	19% 6% 2 6% 4 4 4 3 5% 7% 40%	2004
	11th to 20th	Day	19% 7% 3 6% 5% 4 5% 4 6% 7% 35%	2000
	21st to end	Day	21% 7% 3 8% 5% 5% 5% 4 5% 6% 32%	2197
November	1st to 10th	Day	21% 8% 3 7% 4 4 4 4 4 7% 34%	1734
	11th to 20th	Day	22% 10% 2 8% 5% 6% 5% 4 6% 7% 26%	1600
	21st to end	Day	20% 13% 3 10% 6% 5% 5% 4 6% 4 23%	1600
December	1st to 10th	Day	25% 12% 4 11% 6% 4 4 4 5% 5% 20%	2610
	11th to 20th	Day	29% 11% 4 7% 5% 4 5% 3 5% 6% 22%	2996
	21st to end	Day	21% 15% 5% 8% 3 4 4 2 4 5% 29%	3982
January	1st to 10th	Night	14% 18% 7% 16% 7% 5% 5% 4 4 5% 16%	6280
	11th to 20th	Night	16% 12% 6% 15% 6% 6% 6% 4 5% 6% 19%	5858
	21st to end	Night	15% 13% 5% 15% 7% 5% 5% 4 5% 6% 19%	6490
February	1st to 10th	Night	20% 14% 5% 12% 7% 5% 4 4 4 6% 20%	2600
	11th to 20th	Night	19% 10% 5% 11% 7% 6% 5% 5% 6% 7% 22%	2533
	21st to end	Night	15% 11% 4 8% 6% 6% 5% 4 5% 8% 29%	2040
March	1st to 10th	Night	24% 13% 4 10% 7% 5% 4 3 4 4 22%	4959
	11th to 20th	Night	18% 13% 6% 10% 4 4 5% 3 4 6% 27%	4979
	21st to end	Night	30% 11% 4 7% 3 3 2 2 4 5% 28%	5466
April	1st to 10th	Night	24% 13% 3 7% 4 3 3 3 4 6% 30%	2085
	11th to 20th	Night	25% 8% 3 6% 3 3 3 3 5% 7% 35%	1837
	21st to end	Night	23% 9% 3 6% 5% 3 4 3 5% 6% 33%	1800
May	1st to 10th	Night	29% 15% 3 7% 4 3 3 3 4 5% 23%	4054
	11th to 20th	Night	25% 13% 3 6% 3 3 3 2 4 5% 31%	3816
	21st to end	Night	22% 15% 3 8% 4 3 3 2 4 5% 30%	3948
June	1st to 10th	Night	25% 10% 2 7% 4 3 3 3 5% 7% 30%	1600
	11th to 20th	Night	20% 11% 3 7% 4 4 3 4 3 6% 35%	1469
	21st to end	Night	29% 12% 3 7% 4 4 3 3 5% 6% 25%	1400
July	1st to 10th	Night	32% 14% 4 6% 3 3 3 2 4 6% 21%	3160
	11th to 20th	Night	33% 15% 3 6% 4 3 3 3 4 5% 20%	3767
	21st to end	Night	20% 13% 3 6% 4 4 3 3 5% 6% 32%	4428
August	1st to 10th	Night	26% 15% 3 7% 4 3 3 3 4 6% 27%	2915
	11th to 20th	Night	22% 11% 3 8% 5% 4 4 3 4 6% 31%	2872
	21st to end	Night	12% 10% 3 6% 3 4 4 3 5% 7% 43%	3338
September	1st to 10th	Night	19% 9% 2 4 4 4 4 3 5% 7% 40%	2000
	11th to 20th	Night	19% 6% 2 5% 3 3 4 3 5% 6% 44%	2185
	21st to end	Night	21% 9% 3 5% 4 3 4 3 4 7% 38%	2200
October	1st to 10th	Night	20% 7% 2 5% 4 3 4 3 4 6% 41%	2395
	11th to 20th	Night	19% 7% 2 5% 3 4 3 3 5% 7% 41%	2400
	21st to end	Night	22% 8% 3 7% 4 4 3 4 4 6% 36%	2640
November	1st to 10th	Night	18% 8% 2 6% 4 4 4 3 5% 7% 38%	2606
	11th to 20th	Night	19% 10% 3 8% 6% 5% 4 4 6% 7% 28%	2800
	21st to end	Night	20% 11% 4 8% 5% 4 5% 5% 6% 6% 26%	2800
December	1st to 10th	Night	16% 12% 4 10% 6% 5% 6% 4 6% 7% 22%	4580
	11th to 20th	Night	23% 12% 3 7% 4 4 4 3 5% 7% 26%	5213
	21st to end	Night	23% 11% 4 8% 5% 4 4 3 4 5% 29%	6996

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	69% 7% 9% 12% 3	3681
	11th to 20th	Day	71% 7% 8% 11% 3	4060
	21st to end	Day	65% 6% 8% 10% 10% 1	4466
February	1st to 10th	Day	67% 6% 10% 10% 6%	1800
	11th to 20th	Day	63% 7% 10% 13% 7%	1761
	21st to end	Day	55% 6% 12% 17% 8% 2	1585
March	1st to 10th	Day	65% 5% 8% 12% 9% 1	4885
	11th to 20th	Day	56% 5% 15% 16% 8% 1	4980
	21st to end	Day	65% 3 6% 15% 9% 1	5466
April	1st to 10th	Day	58% 4 9% 15% 12% 1	2315
	11th to 20th	Day	49% 5% 11% 21% 13% 1	2510
	21st to end	Day	48% 5% 11% 19% 15% 2	2600
May	1st to 10th	Day	66% 5% 8% 15% 5%	5897
	11th to 20th	Day	57% 5% 9% 18% 8% 1	6092
	21st to end	Day	53% 9% 10% 16% 10% 2	6948
June	1st to 10th	Day	52% 6% 10% 21% 10% 1	2800
	11th to 20th	Day	48% 7% 10% 20% 13% 2	2944
	21st to end	Day	55% 7% 12% 16% 9% 1	3000
July	1st to 10th	Day	60% 5% 9% 15% 10% 1	6683
	11th to 20th	Day	61% 5% 6% 21% 8%	6111
	21st to end	Day	45% 4 12% 26% 13% 1	6242
August	1st to 10th	Day	59% 5% 8% 18% 9% 1	4222
	11th to 20th	Day	47% 6% 13% 26% 8% 1	4214
	21st to end	Day	35% 4 11% 29% 19% 2	4559
September	1st to 10th	Day	42% 5% 10% 24% 17% 2	2400
	11th to 20th	Day	40% 5% 11% 23% 18% 2	2210
	21st to end	Day	45% 5% 8% 23% 18% 2	2200
October	1st to 10th	Day	45% 4 10% 27% 12% 1	3264
	11th to 20th	Day	40% 5% 9% 26% 20% 1	3240
	21st to end	Day	44% 5% 11% 26% 11% 4	3523
November	1st to 10th	Day	50% 5% 13% 21% 10% 1	1734
	11th to 20th	Day	57% 7% 12% 16% 8% 1	1600
	21st to end	Day	61% 9% 13% 14% 3	1600
December	1st to 10th	Day	65% 5% 11% 17% 3	2610
	11th to 20th	Day	63% 4 12% 12% 8% 1	2995
	21st to end	Day	57% 5% 13% 16% 9%	3982
January	1st to 10th	Night	69% 7% 9% 13% 3	6280
	11th to 20th	Night	64% 7% 9% 15% 5%	5900
	21st to end	Night	63% 6% 10% 12% 9%	6490
February	1st to 10th	Night	66% 6% 11% 10% 6%	2600
	11th to 20th	Night	61% 7% 10% 15% 6% 1	2536
	21st to end	Night	52% 5% 15% 18% 9% 1	2040
March	1st to 10th	Night	65% 4 7% 10% 12% 2	4968
	11th to 20th	Night	59% 5% 14% 16% 7% 1	4980
	21st to end	Night	60% 3 7% 18% 11% 1	5466
April	1st to 10th	Night	57% 3 9% 16% 12% 3	2085
	11th to 20th	Night	50% 5% 9% 22% 13% 1	1837
	21st to end	Night	51% 4 11% 20% 12% 2	1800
May	1st to 10th	Night	63% 5% 9% 14% 8%	4060
	11th to 20th	Night	56% 4 7% 21% 9% 2	3817
	21st to end	Night	56% 6% 8% 20% 9%	3949
June	1st to 10th	Night	53% 6% 9% 22% 9% 1	1600
	11th to 20th	Night	51% 5% 9% 20% 13% 2	1469
	21st to end	Night	61% 6% 8% 13% 11% 1	1400
July	1st to 10th	Night	66% 4 7% 16% 8%	3125
	11th to 20th	Night	66% 4 6% 13% 9% 2	3720
	21st to end	Night	53% 5% 8% 20% 12% 2	4297
August	1st to 10th	Night	60% 4 8% 18% 9%	2903
	11th to 20th	Night	55% 5% 11% 23% 6%	2899
	21st to end	Night	41% 5% 9% 26% 18% 1	3337
September	1st to 10th	Night	44% 5% 9% 24% 15% 3	2000
	11th to 20th	Night	41% 5% 8% 25% 19% 2	2185
	21st to end	Night	48% 4 6% 23% 17% 1	2200
October	1st to 10th	Night	47% 4 11% 24% 14% 1	3915
	11th to 20th	Night	35% 4 11% 25% 24% 1	3899
	21st to end	Night	42% 4 10% 28% 16% 1	4252
November	1st to 10th	Night	46% 6% 14% 24% 10%	2606
	11th to 20th	Night	54% 7% 13% 17% 9%	2800
	21st to end	Night	56% 9% 15% 15% 5%	2800
December	1st to 10th	Night	59% 5% 11% 22% 4	4580
	11th to 20th	Night	57% 5% 13% 14% 11%	5215
	21st to end	Night	58% 5% 10% 16% 10% 2	6996

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	30% 30% 28% 10% 2	400
	11th to 20th	Day	31% 22% 30% 14% 3	400
	21st to end	Day	23% 29% 32% 12% 4	440
February	1st to 10th	Day	30% 26% 30% 12% 2	400
	11th to 20th	Day	31% 29% 27% 11% 2	396
	21st to end	Day	26% 25% 33% 14% 2 1	381
March	1st to 10th	Day	32% 24% 31% 10% 2	500
	11th to 20th	Day	30% 23% 32% 11% 4	500
	21st to end	Day	29% 23% 31% 11% 5% 1	550
April	1st to 10th	Day	35% 26% 27% 10% 2	500
	11th to 20th	Day	29% 22% 36% 11% 2	492
	21st to end	Day	25% 22% 34% 14% 4 1	500
May	1st to 10th	Day	31% 21% 33% 12% 2 1	503
	11th to 20th	Day	26% 18% 32% 18% 5% 1	624
	21st to end	Day	28% 13% 33% 19% 6% 1	790
June	1st to 10th	Day	29% 13% 32% 20% 5%	730
	11th to 20th	Day	24% 12% 33% 22% 8% 1	730
	21st to end	Day	32% 11% 30% 20% 5% 1	730
July	1st to 10th	Day	28% 7% 33% 22% 10% 1	720
	11th to 20th	Day	28% 7% 32% 22% 9% 2	649
	21st to end	Day	23% 7% 36% 25% 8% 1	576
August	1st to 10th	Day	21% 8% 35% 25% 10% 2	526
	11th to 20th	Day	25% 9% 30% 22% 10% 3	519
	21st to end	Day	21% 12% 32% 22% 10% 3	583
September	1st to 10th	Day	19% 11% 33% 27% 9% 2	530
	11th to 20th	Day	23% 11% 32% 22% 9% 3	530
	21st to end	Day	20% 12% 38% 20% 7% 3	530
October	1st to 10th	Day	20% 14% 33% 22% 10% 2	422
	11th to 20th	Day	20% 18% 30% 24% 6% 2	420
	21st to end	Day	26% 20% 29% 18% 6%	462
November	1st to 10th	Day	25% 20% 32% 14% 7% 1	401
	11th to 20th	Day	28% 28% 26% 13% 4 1	400
	21st to end	Day	29% 24% 29% 14% 3 1	400
December	1st to 10th	Day	33% 22% 29% 14% 3	400
	11th to 20th	Day	38% 26% 23% 12% 2 1	400
	21st to end	Day	29% 25% 27% 15% 2 1	440
January	1st to 10th	Night	28% 25% 29% 14% 3	885
	11th to 20th	Night	28% 18% 31% 19% 3	800
	21st to end	Night	22% 25% 33% 17% 3	880
February	1st to 10th	Night	29% 22% 30% 17% 2	800
	11th to 20th	Night	27% 26% 31% 14% 2	783
	21st to end	Night	21% 20% 35% 18% 5% 1	660
March	1st to 10th	Night	29% 20% 32% 17% 3	800
	11th to 20th	Night	24% 21% 30% 19% 6%	800
	21st to end	Night	26% 19% 31% 19% 5% 1	880
April	1st to 10th	Night	29% 18% 32% 18% 2	685
	11th to 20th	Night	27% 14% 39% 15% 4	594
	21st to end	Night	28% 11% 32% 21% 8% 1	600
May	1st to 10th	Night	31% 13% 30% 20% 5% 1	600
	11th to 20th	Night	29% 11% 31% 23% 6% 1	592
	21st to end	Night	27% 8% 31% 27% 6% 1	649
June	1st to 10th	Night	28% 9% 30% 26% 6%	600
	11th to 20th	Night	24% 6% 30% 32% 8%	469
	21st to end	Night	34% 5% 23% 27% 11%	400
July	1st to 10th	Night	31% 4 25% 30% 9% 1	390
	11th to 20th	Night	30% 4 27% 28% 10%	581
	21st to end	Night	29% 5% 26% 31% 8%	656
August	1st to 10th	Night	22% 4 27% 34% 12% 1	598
	11th to 20th	Night	24% 5% 24% 30% 15% 2	592
	21st to end	Night	22% 4 28% 26% 17% 2	660
September	1st to 10th	Night	22% 7% 27% 30% 13% 1	600
	11th to 20th	Night	21% 7% 26% 27% 16% 3	785
	21st to end	Night	25% 8% 28% 24% 13% 3	800
October	1st to 10th	Night	24% 11% 27% 26% 12% 1	800
	11th to 20th	Night	22% 15% 28% 23% 10% 2	800
	21st to end	Night	27% 16% 27% 23% 7%	880
November	1st to 10th	Night	22% 17% 28% 23% 8% 1	811
	11th to 20th	Night	25% 22% 27% 19% 6%	900
	21st to end	Night	25% 26% 26% 17% 5%	900
December	1st to 10th	Night	29% 20% 30% 16% 5%	900
	11th to 20th	Night	30% 26% 28% 14% 3	900
	21st to end	Night	28% 23% 27% 18% 3	990

Page generated at 2025-10-31 03:02:39. Version ac07

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