Eustis Ridge Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Eustis Ridge Observatory clear sky chart from 2005-11-06 to 2026-06-01.

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	15% 10% 4 9% 6% 5% 5% 4 6% 7% 29%	131321
	Night	13% 10% 4 10% 6% 6% 5% 4 6% 7% 29%	131846
Transparency	Day	52% 7% 11% 18% 10% 1	134802
	Night	53% 7% 12% 18% 9% 1	136453
Seeing	Day	24% 17% 30% 19% 8% 2	17154
	Night	22% 15% 26% 27% 9% 1	25898

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	11% 12% 5% 12% 8% 7% 6% 4 6% 6% 25%	4661
	11th to 20th	Day	7% 12% 6% 16% 9% 6% 6% 4 5% 5% 22%	4650
	21st to end	Day	5% 9% 4 12% 7% 7% 8% 5% 6% 6% 30%	5124
February	1st to 10th	Day	9% 12% 4 11% 10% 6% 6% 4 7% 6% 25%	1779
	11th to 20th	Day	8% 12% 5% 15% 9% 7% 5% 4 5% 7% 25%	2038
	21st to end	Day	10% 14% 3 11% 7% 6% 6% 5% 6% 6% 27%	1730
March	1st to 10th	Day	17% 16% 5% 11% 6% 5% 4 3 5% 4 24%	4576
	11th to 20th	Day	13% 13% 4 10% 5% 5% 5% 3 4 6% 31%	4944
	21st to end	Day	17% 13% 4 9% 5% 4 4 3 5% 6% 28%	5812
April	1st to 10th	Day	16% 17% 4 9% 5% 4 4 3 4 6% 27%	2520
	11th to 20th	Day	15% 12% 4 9% 5% 5% 4 3 4 6% 33%	2602
	21st to end	Day	18% 14% 4 10% 5% 5% 4 3 4 4 30%	2730
May	1st to 10th	Day	26% 13% 3 9% 5% 5% 5% 3 5% 5% 20%	6016
	11th to 20th	Day	22% 10% 4 9% 6% 5% 5% 3 5% 6% 26%	6306
	21st to end	Day	19% 9% 3 7% 5% 5% 5% 4 7% 8% 28%	7103
June	1st to 10th	Day	22% 9% 3 8% 5% 5% 6% 4 6% 7% 25%	2912
	11th to 20th	Day	16% 7% 3 8% 5% 5% 5% 4 7% 7% 33%	3000
	21st to end	Day	20% 8% 4 9% 5% 5% 5% 4 7% 7% 27%	2902
July	1st to 10th	Day	15% 6% 2 7% 6% 6% 6% 6% 8% 10% 27%	6360
	11th to 20th	Day	12% 5% 3 6% 5% 5% 5% 4 7% 9% 36%	6360
	21st to end	Day	12% 6% 2 7% 5% 5% 5% 4 6% 9% 40%	6926
August	1st to 10th	Day	14% 6% 2 8% 6% 6% 6% 5% 8% 8% 30%	4377
	11th to 20th	Day	14% 8% 3 8% 6% 6% 6% 5% 7% 9% 27%	4175
	21st to end	Day	13% 6% 2 7% 5% 5% 5% 4 7% 8% 38%	4486
September	1st to 10th	Day	11% 5% 2 7% 5% 4 5% 4 5% 8% 42%	2380
	11th to 20th	Day	12% 7% 3 6% 4 4 4 4 5% 6% 46%	2200
	21st to end	Day	14% 8% 3 8% 4 4 5% 4 6% 7% 37%	2200
October	1st to 10th	Day	15% 10% 4 8% 4 4 4 4 6% 7% 34%	2200
	11th to 20th	Day	13% 13% 4 8% 6% 4 5% 3 6% 7% 31%	1884
	21st to end	Day	19% 13% 5% 9% 5% 5% 5% 4 5% 5% 25%	1980
November	1st to 10th	Day	13% 12% 5% 9% 6% 4 5% 4 5% 6% 31%	1851
	11th to 20th	Day	14% 11% 4 11% 6% 5% 6% 4 5% 6% 28%	1890
	21st to end	Day	14% 13% 4 9% 6% 5% 4 4 7% 7% 26%	1715
December	1st to 10th	Day	9% 15% 5% 15% 10% 7% 6% 4 5% 5% 19%	2350
	11th to 20th	Day	14% 11% 6% 10% 6% 4 5% 3 5% 6% 30%	2688
	21st to end	Day	15% 14% 4 10% 6% 5% 4 4 5% 6% 27%	3894
January	1st to 10th	Night	8% 12% 5% 14% 10% 8% 5% 4 6% 5% 24%	8234
	11th to 20th	Night	8% 14% 8% 15% 10% 7% 7% 4 5% 6% 15%	8235
	21st to end	Night	7% 8% 5% 12% 7% 10% 8% 5% 7% 7% 24%	9057
February	1st to 10th	Night	8% 11% 4 11% 8% 6% 6% 5% 7% 8% 26%	2840
	11th to 20th	Night	6% 11% 5% 12% 8% 7% 6% 5% 7% 8% 25%	2463
	21st to end	Night	10% 12% 4 9% 6% 6% 5% 5% 6% 6% 31%	2076
March	1st to 10th	Night	17% 15% 5% 12% 6% 5% 5% 3 5% 5% 23%	5484
	11th to 20th	Night	12% 11% 5% 12% 6% 6% 5% 4 6% 6% 27%	5235
	21st to end	Night	18% 14% 3 8% 6% 4 4 4 5% 7% 27%	5359
April	1st to 10th	Night	16% 14% 4 9% 5% 4 5% 5% 5% 5% 29%	2100
	11th to 20th	Night	17% 10% 3 7% 5% 4 4 4 6% 7% 32%	1966
	21st to end	Night	18% 11% 3 9% 6% 5% 4 2 5% 7% 31%	1890
May	1st to 10th	Night	26% 11% 3 8% 4 5% 4 4 6% 6% 24%	4144
	11th to 20th	Night	19% 11% 3 7% 5% 5% 5% 4 5% 7% 29%	3849
	21st to end	Night	18% 8% 2 6% 4 4 5% 4 6% 7% 36%	4014
June	1st to 10th	Night	21% 8% 3 7% 6% 4 4 3 6% 7% 32%	1480
	11th to 20th	Night	14% 9% 2 5% 5% 4 5% 5% 6% 8% 37%	1400
	21st to end	Night	20% 7% 2 6% 5% 5% 3 5% 7% 8% 32%	1489
July	1st to 10th	Night	11% 7% 3 7% 5% 6% 6% 4 7% 9% 35%	3600
	11th to 20th	Night	14% 5% 2 6% 5% 5% 5% 5% 6% 7% 40%	3600
	21st to end	Night	9% 5% 2 6% 4 5% 5% 5% 7% 9% 43%	3926
August	1st to 10th	Night	17% 7% 4 7% 4 5% 4 4 6% 6% 35%	2741
	11th to 20th	Night	12% 6% 3 8% 5% 5% 6% 4 7% 8% 34%	2872
	21st to end	Night	13% 5% 2 6% 4 4 5% 3 5% 7% 45%	3410
September	1st to 10th	Night	11% 6% 2 5% 4 5% 4 3 6% 7% 48%	2020
	11th to 20th	Night	12% 6% 2 5% 5% 3 4 3 5% 6% 48%	2200
	21st to end	Night	13% 8% 2 5% 4 4 5% 5% 6% 7% 42%	2200
October	1st to 10th	Night	14% 9% 2 7% 5% 5% 4 4 5% 7% 38%	2200
	11th to 20th	Night	15% 11% 3 8% 5% 5% 5% 3 6% 7% 33%	2515
	21st to end	Night	19% 11% 3 7% 4 5% 6% 5% 7% 7% 27%	2860
November	1st to 10th	Night	13% 11% 5% 10% 6% 5% 5% 5% 5% 6% 30%	2667
	11th to 20th	Night	12% 9% 5% 12% 7% 6% 5% 4 5% 8% 28%	2730
	21st to end	Night	13% 10% 4 11% 6% 6% 6% 5% 5% 7% 27%	2898
December	1st to 10th	Night	10% 13% 7% 17% 9% 6% 6% 4 6% 5% 18%	5060
	11th to 20th	Night	17% 14% 3 10% 6% 6% 5% 4 5% 6% 24%	5741
	21st to end	Night	15% 12% 4 11% 6% 5% 4 4 5% 5% 30%	7291

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	61% 5% 10% 16% 7% 1	4662
	11th to 20th	Day	62% 6% 8% 14% 8% 2	4680
	21st to end	Day	51% 7% 10% 14% 15% 3	5124
February	1st to 10th	Day	57% 6% 8% 16% 11% 2	1779
	11th to 20th	Day	58% 6% 10% 14% 10% 2	2047
	21st to end	Day	56% 5% 10% 16% 10% 3	1730
March	1st to 10th	Day	62% 5% 7% 11% 10% 5%	4579
	11th to 20th	Day	52% 7% 13% 18% 9% 2	4944
	21st to end	Day	56% 5% 7% 16% 14% 2	5812
April	1st to 10th	Day	58% 5% 8% 14% 11% 3	2520
	11th to 20th	Day	52% 6% 9% 19% 12% 1	2602
	21st to end	Day	58% 5% 8% 16% 10% 3	2730
May	1st to 10th	Day	64% 8% 8% 14% 6% 1	6027
	11th to 20th	Day	58% 8% 11% 15% 6% 2	6310
	21st to end	Day	50% 9% 12% 19% 10% 1	7103
June	1st to 10th	Day	55% 8% 12% 19% 6%	2912
	11th to 20th	Day	48% 7% 12% 21% 11% 1	3000
	21st to end	Day	54% 8% 11% 19% 7% 1	2902
July	1st to 10th	Day	49% 7% 18% 19% 8%	6290
	11th to 20th	Day	41% 8% 15% 28% 7%	6285
	21st to end	Day	41% 7% 13% 30% 9%	6728
August	1st to 10th	Day	50% 8% 16% 21% 5%	4363
	11th to 20th	Day	51% 8% 16% 19% 5%	4214
	21st to end	Day	42% 7% 14% 22% 13% 2	4486
September	1st to 10th	Day	38% 8% 14% 28% 12% 1	2380
	11th to 20th	Day	38% 4 12% 30% 15% 2	2200
	21st to end	Day	43% 8% 12% 23% 12% 1	2200
October	1st to 10th	Day	51% 6% 12% 20% 11%	3590
	11th to 20th	Day	50% 5% 11% 24% 10%	3035
	21st to end	Day	47% 5% 15% 19% 13% 1	3180
November	1st to 10th	Day	53% 6% 10% 18% 11% 1	1851
	11th to 20th	Day	56% 8% 10% 14% 10% 2	1890
	21st to end	Day	56% 7% 15% 16% 6% 1	1715
December	1st to 10th	Day	68% 5% 8% 11% 7% 1	2350
	11th to 20th	Day	56% 5% 9% 13% 16% 1	2688
	21st to end	Day	56% 8% 9% 11% 12% 4	3894
January	1st to 10th	Night	62% 6% 10% 14% 8% 1	8234
	11th to 20th	Night	67% 7% 9% 10% 5% 1	8280
	21st to end	Night	57% 6% 8% 15% 12% 2	9057
February	1st to 10th	Night	54% 7% 11% 17% 9% 2	2840
	11th to 20th	Night	55% 7% 10% 16% 10% 1	2465
	21st to end	Night	52% 6% 9% 18% 12% 3	2076
March	1st to 10th	Night	62% 5% 8% 11% 10% 3	5494
	11th to 20th	Night	53% 8% 16% 15% 7% 1	5236
	21st to end	Night	55% 6% 7% 16% 15% 1	5359
April	1st to 10th	Night	55% 6% 9% 14% 14% 2	2100
	11th to 20th	Night	50% 6% 11% 20% 13% 1	1966
	21st to end	Night	52% 6% 12% 17% 11% 2	1890
May	1st to 10th	Night	58% 9% 13% 13% 6%	4150
	11th to 20th	Night	53% 10% 13% 18% 6%	3850
	21st to end	Night	45% 9% 15% 19% 11%	4015
June	1st to 10th	Night	51% 7% 14% 18% 9%	1480
	11th to 20th	Night	44% 6% 15% 21% 12% 2	1400
	21st to end	Night	49% 7% 16% 19% 9%	1489
July	1st to 10th	Night	46% 7% 18% 22% 7%	3560
	11th to 20th	Night	42% 9% 14% 29% 6%	3552
	21st to end	Night	38% 8% 19% 27% 8%	3809
August	1st to 10th	Night	50% 7% 15% 23% 5%	2731
	11th to 20th	Night	47% 9% 19% 17% 7%	2899
	21st to end	Night	39% 6% 15% 23% 15% 2	3410
September	1st to 10th	Night	33% 7% 17% 32% 11%	2020
	11th to 20th	Night	36% 6% 13% 29% 15% 1	2200
	21st to end	Night	41% 7% 13% 26% 12% 1	2200
October	1st to 10th	Night	45% 7% 13% 23% 12%	3590
	11th to 20th	Night	49% 5% 11% 23% 11%	4103
	21st to end	Night	49% 5% 15% 21% 8% 2	4610
November	1st to 10th	Night	54% 6% 11% 18% 9% 1	2667
	11th to 20th	Night	55% 7% 10% 16% 10% 2	2730
	21st to end	Night	56% 8% 13% 16% 7% 1	2898
December	1st to 10th	Night	71% 4 8% 12% 4	5060
	11th to 20th	Night	58% 6% 9% 14% 12% 1	5742
	21st to end	Night	55% 6% 11% 15% 12% 2	7291

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% 23% 31% 13% 2	220
	11th to 20th	Day	27% 23% 31% 15% 3 1	220
	21st to end	Day	21% 20% 35% 20% 3	242
February	1st to 10th	Day	24% 24% 29% 19% 3 1	319
	11th to 20th	Day	26% 27% 31% 13% 2	531
	21st to end	Day	28% 22% 29% 17% 3	446
March	1st to 10th	Day	28% 21% 32% 15% 3	540
	11th to 20th	Day	29% 23% 29% 12% 5% 1	540
	21st to end	Day	29% 21% 27% 16% 6% 2	594
April	1st to 10th	Day	35% 18% 28% 12% 6% 1	540
	11th to 20th	Day	31% 23% 26% 13% 6% 2	532
	21st to end	Day	29% 19% 30% 14% 4 3	540
May	1st to 10th	Day	35% 16% 30% 13% 5% 2	543
	11th to 20th	Day	26% 18% 30% 18% 7% 2	567
	21st to end	Day	22% 20% 34% 17% 7% 1	617
June	1st to 10th	Day	25% 14% 32% 20% 7% 1	642
	11th to 20th	Day	19% 13% 29% 24% 13% 2	730
	21st to end	Day	22% 10% 33% 22% 9% 3	632
July	1st to 10th	Day	17% 13% 37% 22% 9% 2	525
	11th to 20th	Day	15% 11% 36% 26% 9% 2	524
	21st to end	Day	13% 11% 37% 29% 8% 1	576
August	1st to 10th	Day	15% 8% 29% 31% 11% 6%	526
	11th to 20th	Day	15% 10% 31% 24% 13% 7%	519
	21st to end	Day	16% 15% 27% 24% 14% 5%	583
September	1st to 10th	Day	13% 14% 28% 24% 16% 5%	530
	11th to 20th	Day	18% 9% 26% 19% 17% 10%	530
	21st to end	Day	18% 14% 24% 23% 15% 6%	530
October	1st to 10th	Day	20% 15% 26% 20% 15% 4	530
	11th to 20th	Day	26% 17% 24% 19% 11% 3	428
	21st to end	Day	29% 18% 25% 22% 6%	462
November	1st to 10th	Day	31% 18% 24% 20% 7% 1	425
	11th to 20th	Day	27% 23% 25% 16% 7% 2	430
	21st to end	Day	33% 21% 25% 16% 5% 1	359
December	1st to 10th	Day	21% 22% 34% 18% 5%	220
	11th to 20th	Day	34% 21% 31% 8% 5% 1	220
	21st to end	Day	34% 20% 26% 12% 5% 2	242
January	1st to 10th	Night	22% 21% 33% 21% 3	840
	11th to 20th	Night	22% 22% 28% 24% 4	840
	21st to end	Night	17% 22% 34% 22% 5% 1	924
February	1st to 10th	Night	19% 22% 33% 22% 4	840
	11th to 20th	Night	19% 25% 31% 22% 3	823
	21st to end	Night	22% 17% 35% 22% 3	692
March	1st to 10th	Night	26% 17% 31% 23% 2	840
	11th to 20th	Night	26% 18% 27% 23% 6%	840
	21st to end	Night	23% 19% 27% 24% 7%	924
April	1st to 10th	Night	29% 17% 27% 22% 5%	840
	11th to 20th	Night	24% 17% 26% 26% 7%	722
	21st to end	Night	25% 14% 30% 24% 7%	630
May	1st to 10th	Night	24% 11% 28% 28% 8% 1	630
	11th to 20th	Night	22% 11% 28% 30% 8% 1	504
	21st to end	Night	18% 9% 25% 36% 11% 1	455
June	1st to 10th	Night	25% 7% 25% 34% 10% 1	400
	11th to 20th	Night	19% 9% 22% 32% 18%	400
	21st to end	Night	21% 7% 21% 39% 12%	400
July	1st to 10th	Night	14% 5% 26% 38% 15% 1	390
	11th to 20th	Night	17% 4 21% 42% 16% 1	388
	21st to end	Night	15% 3 19% 42% 21%	438
August	1st to 10th	Night	15% 4 17% 43% 22% 1	488
	11th to 20th	Night	17% 4 21% 37% 21% 1	592
	21st to end	Night	15% 7% 24% 33% 20% 2	769
September	1st to 10th	Night	13% 7% 23% 37% 18% 2	800
	11th to 20th	Night	16% 9% 19% 31% 23% 2	800
	21st to end	Night	18% 8% 24% 31% 17% 2	800
October	1st to 10th	Night	21% 11% 19% 32% 16% 1	800
	11th to 20th	Night	23% 16% 23% 25% 12% 2	800
	21st to end	Night	28% 15% 26% 25% 5%	880
November	1st to 10th	Night	24% 18% 25% 24% 8% 1	820
	11th to 20th	Night	20% 21% 27% 22% 8% 2	840
	21st to end	Night	26% 20% 25% 22% 6% 1	892
December	1st to 10th	Night	25% 20% 28% 21% 6%	950
	11th to 20th	Night	27% 23% 23% 21% 4 1	950
	21st to end	Night	29% 19% 28% 20% 3	957

Page generated at 2026-05-29 01:53:26. Version ac07

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