Edward D Finan Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Edward D Finan Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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	8% 6% 2 7% 5% 5% 6% 5% 7% 9% 39%	131589
	Night	9% 6% 2 6% 4 4 5% 4 7% 9% 44%	130588
Transparency	Day	37% 8% 16% 20% 13% 5%	135286
	Night	36% 8% 16% 20% 14% 6%	134961
Seeing	Day	10% 8% 24% 26% 20% 13%	17479
	Night	10% 6% 21% 32% 25% 6%	27615

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	8% 5% 2 6% 3 3 3 3 5% 8% 53%	5258
	11th to 20th	Day	19% 10% 4 8% 5% 4 4 4 4 5% 34%	5280
	21st to end	Day	11% 6% 2 6% 4 4 4 3 5% 7% 48%	5798
February	1st to 10th	Day	14% 6% 2 7% 4 4 4 4 5% 7% 44%	2100
	11th to 20th	Day	9% 6% 2 5% 4 5% 4 3 5% 7% 49%	2048
	21st to end	Day	10% 6% 2 6% 5% 4 5% 3 6% 7% 46%	1846
March	1st to 10th	Day	9% 7% 3 8% 4 5% 4 4 6% 9% 41%	5045
	11th to 20th	Day	6% 4 2 4 3 3 3 3 6% 9% 58%	5090
	21st to end	Day	10% 5% 2 7% 3 4 4 4 7% 8% 46%	5586
April	1st to 10th	Day	8% 4 2 6% 4 5% 5% 4 6% 9% 47%	2310
	11th to 20th	Day	8% 5% 2 6% 4 5% 5% 4 7% 7% 44%	2312
	21st to end	Day	5% 4 2 5% 4 4 5% 4 6% 10% 51%	2400
May	1st to 10th	Day	4 3 2 6% 4 5% 6% 5% 8% 11% 47%	5804
	11th to 20th	Day	10% 5% 2 7% 5% 4 5% 4 6% 8% 45%	5861
	21st to end	Day	4 3 2 5% 5% 5% 6% 5% 9% 12% 44%	6433
June	1st to 10th	Day	9% 7% 3 9% 6% 7% 8% 6% 9% 10% 26%	2600
	11th to 20th	Day	5% 5% 2 8% 8% 7% 7% 8% 9% 11% 30%	2600
	21st to end	Day	8% 8% 3 9% 7% 7% 7% 7% 7% 10% 28%	2600
July	1st to 10th	Day	7% 6% 2 9% 7% 8% 9% 8% 11% 13% 20%	5900
	11th to 20th	Day	4 5% 2 7% 7% 7% 7% 7% 10% 14% 30%	5900
	21st to end	Day	5% 4 2 7% 6% 6% 7% 6% 10% 12% 33%	6424
August	1st to 10th	Day	7% 6% 3 9% 7% 7% 8% 6% 8% 12% 26%	4234
	11th to 20th	Day	5% 4 3 7% 6% 7% 7% 6% 8% 12% 35%	4085
	21st to end	Day	9% 6% 2 8% 6% 6% 7% 7% 10% 12% 25%	3959
September	1st to 10th	Day	9% 7% 3 10% 8% 8% 9% 6% 8% 11% 20%	2200
	11th to 20th	Day	10% 8% 3 10% 7% 7% 8% 6% 8% 9% 22%	2200
	21st to end	Day	11% 7% 3 9% 7% 5% 7% 6% 8% 9% 28%	2200
October	1st to 10th	Day	13% 7% 3 8% 7% 6% 6% 5% 6% 8% 31%	2151
	11th to 20th	Day	7% 5% 2 7% 4 5% 4 5% 6% 9% 46%	2000
	21st to end	Day	8% 5% 2 6% 4 4 5% 4 7% 7% 48%	2200
November	1st to 10th	Day	8% 7% 3 6% 4 5% 5% 5% 7% 8% 42%	2057
	11th to 20th	Day	9% 7% 3 7% 4 6% 5% 5% 6% 7% 43%	2100
	21st to end	Day	10% 7% 3 6% 5% 4 5% 4 5% 7% 45%	1960
December	1st to 10th	Day	6% 4 2 5% 3 3 5% 3 6% 8% 54%	3020
	11th to 20th	Day	11% 7% 3 7% 5% 5% 6% 4 7% 9% 37%	3463
	21st to end	Day	12% 7% 3 7% 5% 4 4 4 5% 7% 42%	4565
January	1st to 10th	Night	7% 6% 2 5% 3 3 3 3 5% 7% 57%	7635
	11th to 20th	Night	17% 9% 3 7% 4 4 4 3 5% 7% 36%	7680
	21st to end	Night	14% 6% 2 6% 4 4 4 3 6% 8% 43%	8364
February	1st to 10th	Night	11% 6% 2 7% 4 4 4 4 5% 6% 47%	2520
	11th to 20th	Night	10% 6% 2 6% 4 4 5% 4 5% 7% 47%	2458
	21st to end	Night	10% 5% 2 6% 4 4 3 3 6% 7% 51%	1957
March	1st to 10th	Night	14% 8% 3 7% 4 4 4 3 5% 6% 43%	5028
	11th to 20th	Night	6% 4 2 4 3 3 4 3 6% 10% 55%	5090
	21st to end	Night	9% 6% 2 5% 4 4 4 3 5% 7% 49%	5588
April	1st to 10th	Night	8% 5% 2 5% 4 4 4 4 6% 8% 50%	2310
	11th to 20th	Night	7% 5% 2 6% 5% 3 4 3 6% 8% 52%	2167
	21st to end	Night	6% 4 1 4 3 3 5% 4 6% 10% 53%	2000
May	1st to 10th	Night	5% 4 2 5% 4 4 4 4 7% 9% 52%	4132
	11th to 20th	Night	9% 6% 2 6% 4 4 4 4 7% 10% 44%	4032
	21st to end	Night	4 3 2 4 4 4 5% 5% 8% 12% 51%	4421
June	1st to 10th	Night	9% 7% 2 7% 4 6% 5% 5% 9% 12% 34%	1800
	11th to 20th	Night	6% 5% 2 7% 6% 6% 7% 6% 8% 12% 35%	1800
	21st to end	Night	7% 6% 3 6% 6% 6% 6% 6% 8% 11% 35%	1800
July	1st to 10th	Night	7% 7% 4 9% 7% 7% 7% 7% 10% 12% 26%	4058
	11th to 20th	Night	3 4 2 6% 5% 6% 6% 6% 10% 14% 38%	4057
	21st to end	Night	5% 6% 3 8% 6% 5% 6% 5% 9% 12% 36%	4427
August	1st to 10th	Night	6% 7% 3 7% 6% 6% 6% 5% 7% 13% 32%	2915
	11th to 20th	Night	3 3 2 6% 5% 6% 6% 6% 10% 14% 40%	2929
	21st to end	Night	8% 5% 3 7% 5% 5% 6% 5% 8% 12% 35%	3932
September	1st to 10th	Night	9% 7% 3 9% 6% 7% 7% 6% 8% 10% 28%	2200
	11th to 20th	Night	9% 9% 4 8% 6% 6% 6% 5% 8% 10% 28%	2200
	21st to end	Night	10% 6% 3 8% 5% 6% 6% 5% 8% 10% 35%	2200
October	1st to 10th	Night	11% 7% 3 7% 4 4 5% 5% 7% 8% 38%	2246
	11th to 20th	Night	6% 5% 2 5% 5% 4 4 4 6% 8% 51%	2400
	21st to end	Night	9% 5% 2 5% 4 4 3 4 6% 7% 50%	2640
November	1st to 10th	Night	10% 5% 2 5% 4 3 4 4 7% 8% 47%	2461
	11th to 20th	Night	8% 6% 2 7% 5% 4 3 5% 6% 7% 46%	2520
	21st to end	Night	9% 5% 2 6% 5% 6% 4 5% 6% 8% 44%	2656
December	1st to 10th	Night	7% 5% 2 5% 4 4 4 3 4 7% 56%	4380
	11th to 20th	Night	12% 7% 2 6% 5% 4 4 4 5% 6% 45%	4952
	21st to end	Night	13% 8% 3 7% 4 5% 5% 3 5% 6% 40%	6633

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	30% 4 14% 17% 24% 11%	5259
	11th to 20th	Day	52% 3 6% 13% 16% 11%	5280
	21st to end	Day	36% 4 9% 24% 15% 11%	5798
February	1st to 10th	Day	39% 6% 9% 14% 21% 11%	2100
	11th to 20th	Day	35% 4 10% 20% 20% 11%	2058
	21st to end	Day	36% 7% 9% 16% 22% 10%	1846
March	1st to 10th	Day	39% 7% 11% 17% 20% 6%	5045
	11th to 20th	Day	24% 4 10% 22% 23% 16%	5090
	21st to end	Day	33% 6% 11% 25% 16% 9%	5586
April	1st to 10th	Day	32% 7% 13% 21% 18% 8%	2310
	11th to 20th	Day	35% 7% 9% 20% 21% 7%	2312
	21st to end	Day	26% 7% 13% 24% 23% 6%	2400
May	1st to 10th	Day	26% 9% 21% 27% 12% 3	5838
	11th to 20th	Day	34% 9% 13% 32% 10% 1	5865
	21st to end	Day	27% 8% 20% 31% 13% 1	6447
June	1st to 10th	Day	45% 12% 18% 18% 6% 1	2600
	11th to 20th	Day	42% 14% 22% 20% 3	2600
	21st to end	Day	46% 14% 22% 15% 4	2600
July	1st to 10th	Day	48% 14% 28% 10% 1	5835
	11th to 20th	Day	40% 12% 35% 13%	5831
	21st to end	Day	37% 12% 31% 19% 2	6240
August	1st to 10th	Day	45% 13% 29% 11% 1	4221
	11th to 20th	Day	38% 12% 25% 20% 5%	4124
	21st to end	Day	45% 12% 23% 18% 1	3959
September	1st to 10th	Day	53% 16% 17% 10% 4 1	2200
	11th to 20th	Day	51% 14% 18% 12% 5%	2200
	21st to end	Day	47% 11% 15% 15% 10% 3	2200
October	1st to 10th	Day	55% 6% 13% 13% 9% 4	3502
	11th to 20th	Day	29% 6% 6% 22% 29% 9%	3240
	21st to end	Day	26% 5% 7% 25% 31% 6%	3538
November	1st to 10th	Day	36% 7% 11% 18% 18% 9%	2057
	11th to 20th	Day	38% 6% 9% 17% 20% 9%	2100
	21st to end	Day	37% 7% 8% 22% 20% 6%	1960
December	1st to 10th	Day	27% 5% 15% 25% 19% 8%	3020
	11th to 20th	Day	42% 6% 10% 18% 17% 7%	3460
	21st to end	Day	40% 5% 10% 21% 18% 6%	4565
January	1st to 10th	Night	27% 4 12% 20% 26% 11%	7636
	11th to 20th	Night	48% 3 7% 16% 16% 9%	7680
	21st to end	Night	38% 6% 12% 20% 14% 11%	8364
February	1st to 10th	Night	36% 6% 10% 16% 19% 13%	2520
	11th to 20th	Night	35% 6% 10% 20% 18% 10%	2459
	21st to end	Night	31% 6% 13% 19% 20% 10%	1957
March	1st to 10th	Night	41% 6% 11% 15% 18% 9%	5028
	11th to 20th	Night	24% 6% 13% 22% 18% 17%	5090
	21st to end	Night	33% 6% 12% 22% 19% 8%	5588
April	1st to 10th	Night	29% 7% 18% 23% 14% 9%	2310
	11th to 20th	Night	30% 6% 11% 24% 21% 8%	2167
	21st to end	Night	25% 7% 16% 27% 20% 6%	2000
May	1st to 10th	Night	25% 11% 21% 28% 13% 3	4160
	11th to 20th	Night	32% 10% 18% 28% 11% 1	4033
	21st to end	Night	24% 8% 22% 30% 15% 1	4429
June	1st to 10th	Night	38% 12% 25% 20% 5%	1800
	11th to 20th	Night	37% 16% 24% 20% 3	1800
	21st to end	Night	38% 14% 25% 17% 5% 1	1800
July	1st to 10th	Night	47% 14% 28% 11% 1	4015
	11th to 20th	Night	33% 16% 38% 12% 1	4006
	21st to end	Night	37% 11% 29% 21% 2	4297
August	1st to 10th	Night	42% 11% 34% 12% 1	2904
	11th to 20th	Night	30% 11% 30% 23% 5%	2956
	21st to end	Night	40% 13% 27% 18% 2	3932
September	1st to 10th	Night	46% 15% 21% 13% 5%	2200
	11th to 20th	Night	47% 14% 19% 16% 5%	2200
	21st to end	Night	41% 10% 18% 20% 10% 2	2200
October	1st to 10th	Night	54% 8% 11% 14% 9% 4	3675
	11th to 20th	Night	30% 5% 6% 20% 32% 7%	3899
	21st to end	Night	28% 5% 8% 23% 29% 7%	4252
November	1st to 10th	Night	32% 7% 14% 19% 18% 9%	2461
	11th to 20th	Night	36% 6% 10% 19% 20% 10%	2520
	21st to end	Night	36% 7% 11% 20% 19% 8%	2656
December	1st to 10th	Night	29% 4 14% 27% 12% 13%	4380
	11th to 20th	Night	40% 5% 12% 20% 15% 9%	4954
	21st to end	Night	42% 6% 12% 19% 15% 5%	6633

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	13% 15% 45% 20% 5% 2	330
	11th to 20th	Day	11% 15% 43% 22% 7% 2	330
	21st to end	Day	18% 20% 49% 11% 2	363
February	1st to 10th	Day	17% 16% 33% 26% 7% 1	330
	11th to 20th	Day	16% 14% 41% 22% 6% 2	330
	21st to end	Day	12% 16% 39% 24% 9% 1	389
March	1st to 10th	Day	9% 16% 37% 26% 11% 1	540
	11th to 20th	Day	14% 13% 34% 23% 14% 2	540
	21st to end	Day	9% 13% 34% 27% 15% 2	594
April	1st to 10th	Day	9% 16% 32% 26% 14% 3	540
	11th to 20th	Day	9% 12% 35% 25% 13% 6%	516
	21st to end	Day	7% 6% 31% 29% 16% 11%	500
May	1st to 10th	Day	7% 7% 28% 29% 18% 11%	503
	11th to 20th	Day	10% 4 24% 30% 21% 12%	527
	21st to end	Day	8% 4 14% 29% 27% 17%	573
June	1st to 10th	Day	11% 3 18% 26% 24% 17%	530
	11th to 20th	Day	6% 4 15% 25% 30% 21%	530
	21st to end	Day	6% 3 8% 26% 29% 28%	530
July	1st to 10th	Day	9% 2 5% 20% 31% 33%	525
	11th to 20th	Day	6% 2 3 20% 35% 34%	524
	21st to end	Day	6% 2 6% 21% 34% 30%	576
August	1st to 10th	Day	6% 3 9% 24% 31% 28%	526
	11th to 20th	Day	4 2 6% 16% 36% 37%	519
	21st to end	Day	11% 2 8% 24% 26% 28%	583
September	1st to 10th	Day	11% 5% 9% 21% 31% 24%	530
	11th to 20th	Day	10% 2 15% 31% 25% 17%	530
	21st to end	Day	11% 4 12% 29% 26% 18%	530
October	1st to 10th	Day	16% 4 27% 29% 18% 6%	530
	11th to 20th	Day	7% 7% 21% 37% 22% 6%	530
	21st to end	Day	9% 6% 28% 33% 18% 5%	583
November	1st to 10th	Day	11% 9% 24% 32% 21% 3	535
	11th to 20th	Day	13% 8% 29% 30% 16% 4	540
	21st to end	Day	13% 13% 33% 29% 9% 4	400
December	1st to 10th	Day	9% 16% 40% 22% 11% 2	330
	11th to 20th	Day	14% 14% 36% 27% 9% 1	330
	21st to end	Day	12% 12% 33% 35% 7% 1	363
January	1st to 10th	Night	13% 11% 33% 31% 11%	840
	11th to 20th	Night	12% 10% 31% 33% 13% 1	840
	21st to end	Night	17% 12% 34% 31% 6%	924
February	1st to 10th	Night	14% 8% 34% 32% 12%	840
	11th to 20th	Night	12% 9% 35% 35% 8% 1	823
	21st to end	Night	11% 11% 33% 37% 7%	692
March	1st to 10th	Night	7% 10% 35% 36% 11%	840
	11th to 20th	Night	11% 10% 34% 32% 13% 1	840
	21st to end	Night	9% 8% 33% 37% 13% 1	924
April	1st to 10th	Night	10% 8% 33% 36% 13%	840
	11th to 20th	Night	9% 9% 30% 35% 17% 1	816
	21st to end	Night	6% 5% 28% 43% 18% 1	800
May	1st to 10th	Night	6% 3 22% 45% 23% 2	648
	11th to 20th	Night	8% 2 17% 41% 29% 4	591
	21st to end	Night	7% 2 8% 37% 40% 6%	650
June	1st to 10th	Night	12% 2 9% 34% 38% 6%	600
	11th to 20th	Night	7% 2 9% 25% 41% 17%	600
	21st to end	Night	7% 2 4 27% 44% 16%	600
July	1st to 10th	Night	8% 1 5% 17% 46% 22%	585
	11th to 20th	Night	5% 1 4 15% 52% 23%	582
	21st to end	Night	6% 1 4 19% 52% 17%	657
August	1st to 10th	Night	10% 1 4 17% 49% 19%	597
	11th to 20th	Night	4 1 3 18% 51% 23%	622
	21st to end	Night	12% 2 7% 22% 39% 18%	880
September	1st to 10th	Night	12% 3 7% 21% 41% 16%	800
	11th to 20th	Night	12% 3 10% 30% 36% 11%	800
	21st to end	Night	12% 2 10% 27% 39% 10%	800
October	1st to 10th	Night	15% 3 16% 35% 27% 4	800
	11th to 20th	Night	7% 5% 17% 38% 31% 4	800
	21st to end	Night	11% 4 17% 40% 25% 3	880
November	1st to 10th	Night	11% 5% 22% 37% 23% 2	820
	11th to 20th	Night	11% 7% 24% 38% 19% 2	840
	21st to end	Night	10% 6% 30% 36% 18% 1	840
December	1st to 10th	Night	10% 7% 27% 34% 20% 2	840
	11th to 20th	Night	12% 10% 25% 35% 16% 1	840
	21st to end	Night	14% 8% 28% 33% 17% 1	924

Page generated at 2026-04-14 02:44:26. Version ac07

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