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 2024-01-16.

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	21% 9% 3 6% 5% 3 4 4 5% 6% 31%	75139
	Night	22% 10% 4 7% 6% 3 4 4 5% 7% 29%	74655
Transparency	Day	53% 7% 11% 18% 10% 1	74850
	Night	55% 7% 11% 18% 9% 1	74480
Seeing	Day	26% 18% 32% 17% 6% 1	15941
	Night	26% 16% 29% 22% 7% 1	23703

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% 16% 6% 8% 7% 4 4 5% 5% 6% 19%	1542
	11th to 20th	Day	25% 11% 4 9% 7% 4 6% 6% 5% 6% 18%	1665
	21st to end	Day	15% 10% 5% 10% 9% 5% 6% 6% 5% 8% 21%	1782
February	1st to 10th	Day	21% 12% 6% 10% 7% 4 6% 5% 5% 6% 18%	1620
	11th to 20th	Day	19% 10% 5% 10% 8% 4 6% 5% 5% 7% 21%	1573
	21st to end	Day	16% 11% 5% 9% 7% 4 5% 4 5% 5% 29%	1422
March	1st to 10th	Day	19% 12% 4 9% 7% 3 4 3 5% 6% 29%	1953
	11th to 20th	Day	18% 11% 4 6% 6% 3 5% 4 4 8% 32%	1980
	21st to end	Day	19% 10% 4 8% 5% 2 4 3 4 6% 34%	2178
April	1st to 10th	Day	25% 11% 3 7% 6% 3 4 3 5% 6% 28%	2083
	11th to 20th	Day	20% 9% 2 6% 5% 4 4 3 5% 6% 36%	2256
	21st to end	Day	24% 8% 2 6% 5% 2 4 4 4 6% 34%	2340
May	1st to 10th	Day	26% 10% 4 7% 5% 3 5% 3 5% 4 28%	2340
	11th to 20th	Day	24% 7% 3 5% 4 2 4 4 5% 6% 36%	2388
	21st to end	Day	24% 8% 3 6% 5% 3 4 5% 4 8% 31%	2724
June	1st to 10th	Day	27% 7% 3 4 5% 2 4 4 5% 6% 32%	2520
	11th to 20th	Day	22% 9% 3 5% 4 3 3 5% 4 7% 34%	2649
	21st to end	Day	26% 9% 3 6% 5% 3 4 5% 5% 7% 29%	2700
July	1st to 10th	Day	21% 8% 3 5% 4 2 4 4 5% 7% 35%	2679
	11th to 20th	Day	21% 9% 3 6% 5% 3 6% 5% 6% 9% 27%	2459
	21st to end	Day	21% 9% 4 7% 5% 3 4 5% 5% 7% 31%	2548
August	1st to 10th	Day	18% 9% 3 5% 5% 2 4 5% 6% 8% 35%	2327
	11th to 20th	Day	19% 9% 3 6% 5% 2 5% 4 5% 7% 35%	2304
	21st to end	Day	18% 6% 3 5% 4 3 3 4 5% 7% 42%	2517
September	1st to 10th	Day	15% 6% 3 6% 5% 2 4 3 5% 7% 44%	2160
	11th to 20th	Day	18% 7% 2 4 4 3 4 3 5% 6% 44%	1990
	21st to end	Day	20% 7% 3 5% 4 2 4 4 3 6% 42%	1980
October	1st to 10th	Day	20% 6% 2 5% 5% 3 5% 4 4 7% 40%	1804
	11th to 20th	Day	21% 6% 3 5% 5% 3 4 5% 5% 7% 36%	1800
	21st to end	Day	22% 6% 3 7% 6% 4 5% 6% 5% 6% 30%	1978
November	1st to 10th	Day	22% 7% 3 6% 5% 4 4 4 4 7% 34%	1629
	11th to 20th	Day	23% 8% 3 7% 6% 4 6% 5% 5% 7% 27%	1520
	21st to end	Day	20% 11% 4 8% 7% 5% 5% 5% 6% 5% 24%	1520
December	1st to 10th	Day	24% 13% 4 7% 7% 4 5% 5% 5% 5% 21%	1520
	11th to 20th	Day	31% 13% 4 7% 6% 3 5% 4 5% 5% 19%	1906
	21st to end	Day	24% 13% 5% 7% 4 3 5% 4 4 6% 25%	2783
January	1st to 10th	Night	21% 14% 6% 9% 7% 3 5% 5% 5% 7% 18%	2621
	11th to 20th	Night	21% 11% 5% 8% 6% 4 6% 5% 5% 8% 21%	2406
	21st to end	Night	16% 11% 5% 10% 9% 4 6% 6% 6% 8% 20%	2574
February	1st to 10th	Night	20% 12% 5% 11% 8% 4 6% 5% 4 6% 20%	2340
	11th to 20th	Night	20% 9% 4 8% 8% 4 6% 6% 5% 7% 21%	2273
	21st to end	Night	14% 10% 5% 7% 6% 5% 6% 5% 6% 8% 28%	1830
March	1st to 10th	Night	20% 9% 4 7% 6% 3 6% 5% 5% 6% 29%	1994
	11th to 20th	Night	19% 9% 4 7% 7% 3 5% 5% 5% 6% 30%	1979
	21st to end	Night	18% 9% 4 7% 5% 2 4 5% 4 6% 37%	2178
April	1st to 10th	Night	24% 12% 4 6% 5% 2 4 4 4 6% 30%	1877
	11th to 20th	Night	24% 8% 3 5% 4 3 3 4 4 7% 35%	1652
	21st to end	Night	24% 9% 3 5% 5% 2 4 4 5% 7% 32%	1620
May	1st to 10th	Night	24% 11% 3 6% 5% 3 5% 5% 5% 6% 27%	1620
	11th to 20th	Night	25% 10% 3 6% 4 3 4 3 4 6% 32%	1506
	21st to end	Night	24% 10% 3 7% 5% 3 4 4 6% 6% 29%	1550
June	1st to 10th	Night	26% 9% 3 7% 5% 3 3 5% 5% 7% 29%	1440
	11th to 20th	Night	21% 10% 5% 7% 4 3 3 4 3 7% 33%	1323
	21st to end	Night	30% 11% 4 6% 5% 2 3 3 5% 6% 25%	1260
July	1st to 10th	Night	24% 10% 4 6% 4 2 4 4 6% 9% 28%	1260
	11th to 20th	Night	24% 10% 4 7% 6% 4 5% 5% 5% 7% 22%	1496
	21st to end	Night	27% 10% 4 6% 4 3 5% 4 5% 6% 26%	1770
August	1st to 10th	Night	19% 10% 4 6% 6% 3 3 4 4 8% 35%	1612
	11th to 20th	Night	21% 11% 3 7% 6% 2 5% 4 3 6% 31%	1596
	21st to end	Night	19% 8% 3 5% 5% 3 4 4 5% 7% 36%	1838
September	1st to 10th	Night	18% 8% 3 4 4 3 4 4 5% 7% 40%	1800
	11th to 20th	Night	20% 6% 3 4 4 2 3 4 5% 6% 43%	1966
	21st to end	Night	21% 9% 3 5% 5% 2 3 3 4 7% 38%	1980
October	1st to 10th	Night	21% 7% 3 5% 4 3 4 4 5% 6% 39%	2155
	11th to 20th	Night	19% 6% 2 5% 4 3 3 4 5% 7% 41%	2160
	21st to end	Night	23% 7% 3 6% 5% 3 4 4 4 6% 35%	2376
November	1st to 10th	Night	19% 7% 3 6% 5% 3 4 4 4 7% 37%	2446
	11th to 20th	Night	19% 9% 3 7% 7% 4 5% 5% 6% 7% 29%	2660
	21st to end	Night	19% 10% 4 8% 6% 3 5% 5% 7% 7% 26%	2660
December	1st to 10th	Night	23% 13% 4 8% 6% 3 5% 4 6% 7% 21%	2660
	11th to 20th	Night	27% 12% 4 8% 5% 3 5% 4 4 5% 21%	3293
	21st to end	Night	24% 11% 4 7% 5% 3 4 4 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	65% 9% 10% 11% 5%	1543
	11th to 20th	Day	64% 9% 11% 13% 3	1665
	21st to end	Day	60% 8% 10% 16% 6%	1782
February	1st to 10th	Day	66% 7% 11% 10% 5%	1620
	11th to 20th	Day	62% 8% 10% 13% 6%	1581
	21st to end	Day	55% 7% 12% 17% 8% 1	1422
March	1st to 10th	Day	57% 5% 10% 17% 9% 2	1957
	11th to 20th	Day	51% 5% 12% 17% 13% 2	1980
	21st to end	Day	52% 4 10% 19% 12% 2	2178
April	1st to 10th	Day	58% 4 10% 14% 13% 1	2083
	11th to 20th	Day	49% 5% 11% 21% 13% 1	2256
	21st to end	Day	51% 5% 11% 18% 14% 2	2340
May	1st to 10th	Day	60% 5% 9% 17% 9% 1	2340
	11th to 20th	Day	49% 6% 9% 21% 14% 2	2392
	21st to end	Day	52% 7% 12% 18% 9% 1	2724
June	1st to 10th	Day	52% 6% 9% 22% 10% 1	2520
	11th to 20th	Day	49% 7% 10% 18% 13% 3	2649
	21st to end	Day	55% 7% 12% 16% 9% 1	2700
July	1st to 10th	Day	48% 9% 12% 19% 11% 2	2605
	11th to 20th	Day	54% 9% 10% 21% 6% 1	2383
	21st to end	Day	52% 9% 11% 21% 7% 1	2406
August	1st to 10th	Day	46% 8% 13% 21% 11% 1	2314
	11th to 20th	Day	48% 9% 13% 19% 10% 1	2304
	21st to end	Day	41% 6% 11% 24% 16% 1	2517
September	1st to 10th	Day	41% 5% 11% 24% 17% 2	2160
	11th to 20th	Day	42% 5% 10% 23% 17% 3	1990
	21st to end	Day	45% 5% 7% 23% 18% 2	1980
October	1st to 10th	Day	45% 6% 11% 25% 12% 2	1804
	11th to 20th	Day	47% 6% 12% 24% 10% 1	1800
	21st to end	Day	53% 7% 11% 18% 10% 1	1978
November	1st to 10th	Day	50% 5% 12% 21% 10% 1	1629
	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	66% 7% 10% 13% 4	2621
	11th to 20th	Night	61% 9% 13% 12% 5%	2406
	21st to end	Night	61% 8% 13% 14% 4	2574
February	1st to 10th	Night	66% 6% 12% 11% 5%	2340
	11th to 20th	Night	62% 7% 10% 14% 6% 1	2276
	21st to end	Night	53% 6% 16% 17% 8% 1	1830
March	1st to 10th	Night	55% 8% 10% 16% 9% 2	2003
	11th to 20th	Night	55% 5% 12% 13% 13% 1	1980
	21st to end	Night	48% 6% 9% 21% 14% 2	2178
April	1st to 10th	Night	57% 4 10% 15% 12% 3	1877
	11th to 20th	Night	50% 5% 9% 22% 13% 1	1652
	21st to end	Night	51% 5% 12% 20% 11% 2	1620
May	1st to 10th	Night	56% 6% 9% 18% 10% 1	1620
	11th to 20th	Night	54% 5% 6% 21% 13% 1	1507
	21st to end	Night	56% 6% 10% 20% 8% 1	1551
June	1st to 10th	Night	55% 6% 9% 22% 8% 1	1440
	11th to 20th	Night	53% 6% 9% 19% 12% 2	1323
	21st to end	Night	61% 6% 8% 13% 11% 1	1260
July	1st to 10th	Night	53% 8% 11% 18% 9%	1225
	11th to 20th	Night	59% 8% 9% 15% 7% 1	1449
	21st to end	Night	57% 9% 8% 17% 7% 1	1672
August	1st to 10th	Night	50% 6% 10% 23% 11%	1600
	11th to 20th	Night	55% 8% 9% 18% 9% 1	1596
	21st to end	Night	46% 9% 9% 22% 14%	1838
September	1st to 10th	Night	43% 6% 9% 25% 14% 3	1800
	11th to 20th	Night	42% 6% 8% 25% 17% 2	1966
	21st to end	Night	48% 5% 6% 23% 17% 1	1980
October	1st to 10th	Night	46% 6% 10% 22% 14% 2	2155
	11th to 20th	Night	43% 6% 12% 27% 12% 1	2160
	21st to end	Night	51% 6% 10% 20% 12% 1	2376
November	1st to 10th	Night	47% 6% 14% 22% 10%	2446
	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	30% 23% 31% 14% 3	355
	21st to end	Day	20% 29% 35% 12% 4	374
February	1st to 10th	Day	29% 26% 34% 10% 1	340
	11th to 20th	Day	30% 29% 26% 12% 3	336
	21st to end	Day	25% 27% 33% 13% 2 1	320
March	1st to 10th	Day	28% 26% 31% 11% 3	420
	11th to 20th	Day	28% 25% 32% 12% 4	420
	21st to end	Day	25% 23% 34% 12% 6% 1	462
April	1st to 10th	Day	34% 25% 28% 11% 2	420
	11th to 20th	Day	27% 23% 36% 12% 2	412
	21st to end	Day	28% 20% 31% 15% 5% 1	420
May	1st to 10th	Day	29% 23% 32% 12% 2 1	423
	11th to 20th	Day	26% 19% 33% 16% 5% 1	534
	21st to end	Day	30% 13% 32% 19% 6% 1	680
June	1st to 10th	Day	30% 11% 32% 21% 5%	630
	11th to 20th	Day	24% 12% 34% 21% 8% 1	630
	21st to end	Day	31% 11% 30% 21% 6% 1	630
July	1st to 10th	Day	27% 7% 32% 23% 10%	620
	11th to 20th	Day	26% 7% 34% 23% 9% 1	557
	21st to end	Day	24% 7% 34% 25% 9% 1	488
August	1st to 10th	Day	17% 9% 37% 24% 10% 2	446
	11th to 20th	Day	27% 10% 31% 20% 10% 3	439
	21st to end	Day	22% 11% 29% 24% 11% 3	495
September	1st to 10th	Day	18% 10% 33% 29% 9% 2	450
	11th to 20th	Day	25% 11% 32% 21% 8% 3	450
	21st to end	Day	21% 12% 37% 19% 7% 4	450
October	1st to 10th	Day	21% 13% 33% 21% 10% 2	362
	11th to 20th	Day	19% 19% 30% 22% 7% 3	360
	21st to end	Day	25% 21% 29% 19% 6% 1	396
November	1st to 10th	Day	24% 19% 32% 15% 8% 1	365
	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	28% 18% 31% 19% 3	741
	21st to end	Night	22% 24% 34% 17% 3	792
February	1st to 10th	Night	28% 23% 31% 17% 2	720
	11th to 20th	Night	26% 25% 31% 15% 3	703
	21st to end	Night	20% 21% 36% 18% 5% 1	592
March	1st to 10th	Night	27% 20% 34% 17% 3	720
	11th to 20th	Night	24% 20% 30% 19% 7%	720
	21st to end	Night	23% 19% 32% 20% 5% 1	792
April	1st to 10th	Night	29% 18% 32% 18% 3	617
	11th to 20th	Night	27% 14% 40% 14% 4	534
	21st to end	Night	29% 11% 31% 21% 8% 1	540
May	1st to 10th	Night	30% 13% 30% 20% 5% 1	540
	11th to 20th	Night	29% 10% 31% 23% 6% 1	532
	21st to end	Night	27% 8% 30% 27% 7% 1	583
June	1st to 10th	Night	28% 9% 32% 26% 6%	540
	11th to 20th	Night	24% 6% 30% 31% 9%	423
	21st to end	Night	33% 5% 23% 27% 12%	360
July	1st to 10th	Night	29% 4 25% 31% 10%	350
	11th to 20th	Night	27% 5% 29% 29% 10%	521
	21st to end	Night	29% 5% 26% 31% 9%	590
August	1st to 10th	Night	21% 4 27% 34% 12% 1	538
	11th to 20th	Night	25% 5% 25% 30% 14% 1	532
	21st to end	Night	23% 5% 26% 26% 18% 2	594
September	1st to 10th	Night	21% 7% 26% 31% 13% 1	540
	11th to 20th	Night	22% 7% 27% 26% 14% 3	706
	21st to end	Night	25% 9% 27% 24% 13% 3	720
October	1st to 10th	Night	25% 10% 27% 25% 12% 1	720
	11th to 20th	Night	22% 15% 29% 23% 10% 2	720
	21st to end	Night	28% 16% 27% 23% 7%	792
November	1st to 10th	Night	23% 16% 27% 25% 9% 1	759
	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 2024-01-13 02:38:09. Version ac07

Copyright 2024 Attilla Danko