Walter F. Barber, Jr. Memorial Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Walter F. Barber, Jr. Memorial Observatory clear sky chart from 2005-11-06 to 2025-12-18.

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	12% 7% 3 7% 5% 5% 5% 4 6% 7% 40%	126872
	Night	12% 7% 2 6% 4 4 4 4 5% 7% 44%	124156
Transparency	Day	40% 6% 11% 19% 18% 5%	130659
	Night	37% 5% 10% 20% 20% 7%	128437
Seeing	Day	16% 11% 24% 24% 17% 8%	16255
	Night	15% 11% 23% 27% 19% 4	26268

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	15% 10% 4 6% 3 3 3 3 4 4 46%	4056
	11th to 20th	Day	10% 8% 4 5% 3 3 3 3 4 5% 53%	4060
	21st to end	Day	20% 11% 3 6% 4 3 3 3 4 5% 38%	4788
February	1st to 10th	Day	16% 9% 3 6% 5% 5% 4 3 5% 6% 38%	2000
	11th to 20th	Day	20% 8% 2 6% 5% 3 4 3 4 5% 40%	1948
	21st to end	Day	18% 8% 2 7% 4 4 4 4 4 5% 40%	1650
March	1st to 10th	Day	19% 11% 3 8% 4 4 4 3 5% 5% 34%	4525
	11th to 20th	Day	10% 7% 3 6% 4 4 4 3 5% 6% 49%	4980
	21st to end	Day	15% 7% 3 7% 4 4 4 4 5% 5% 43%	5465
April	1st to 10th	Day	12% 6% 2 7% 5% 4 4 4 5% 6% 45%	2323
	11th to 20th	Day	11% 6% 2 6% 5% 4 3 4 5% 6% 47%	2374
	21st to end	Day	10% 5% 2 5% 5% 5% 4 4 6% 9% 48%	2508
May	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 4 7% 9% 36%	5862
	11th to 20th	Day	16% 8% 3 7% 5% 6% 6% 4 7% 8% 29%	5861
	21st to end	Day	11% 7% 3 7% 5% 4 5% 4 6% 8% 40%	6433
June	1st to 10th	Day	10% 8% 3 9% 6% 6% 7% 5% 7% 9% 32%	2600
	11th to 20th	Day	5% 4 2 6% 6% 6% 6% 6% 9% 11% 39%	2600
	21st to end	Day	6% 5% 2 6% 6% 7% 7% 6% 8% 10% 36%	2600
July	1st to 10th	Day	6% 5% 3 9% 7% 8% 8% 6% 9% 11% 30%	5900
	11th to 20th	Day	5% 5% 2 8% 6% 7% 8% 6% 10% 13% 32%	5900
	21st to end	Day	8% 7% 3 10% 8% 8% 8% 7% 9% 10% 22%	6424
August	1st to 10th	Day	6% 4 2 6% 5% 6% 7% 6% 9% 11% 38%	4155
	11th to 20th	Day	7% 5% 2 5% 5% 5% 6% 4 7% 11% 43%	3849
	21st to end	Day	8% 4 1 5% 5% 5% 6% 4 6% 8% 47%	4312
September	1st to 10th	Day	10% 6% 2 6% 5% 6% 5% 5% 6% 9% 39%	2245
	11th to 20th	Day	15% 6% 2 6% 4 4 5% 4 6% 7% 41%	2200
	21st to end	Day	13% 6% 2 6% 4 4 4 5% 5% 6% 46%	2200
October	1st to 10th	Day	12% 5% 2 6% 3 4 4 3 4 6% 52%	2200
	11th to 20th	Day	10% 5% 1 4 4 3 3 3 4 6% 57%	2135
	21st to end	Day	14% 5% 1 6% 3 3 3 3 4 5% 54%	2165
November	1st to 10th	Day	11% 7% 3 5% 4 3 2 2 5% 4 54%	1852
	11th to 20th	Day	13% 5% 2 4 4 2 3 3 5% 5% 52%	1890
	21st to end	Day	12% 7% 2 6% 4 4 4 4 5% 6% 47%	1890
December	1st to 10th	Day	16% 8% 2 6% 4 4 3 3 5% 6% 45%	3020
	11th to 20th	Day	13% 9% 4 8% 5% 4 4 2 4 5% 42%	3436
	21st to end	Day	19% 7% 3 5% 4 4 4 3 5% 6% 40%	4466
January	1st to 10th	Night	14% 9% 3 6% 4 3 3 3 4 5% 46%	5889
	11th to 20th	Night	12% 8% 3 7% 4 3 4 3 4 5% 47%	5900
	21st to end	Night	18% 10% 3 5% 3 3 3 3 4 4 43%	6150
February	1st to 10th	Night	14% 7% 2 7% 5% 4 4 4 5% 6% 42%	2400
	11th to 20th	Night	21% 8% 3 6% 4 3 4 2 4 5% 41%	2338
	21st to end	Night	17% 8% 2 7% 4 4 4 3 5% 6% 40%	1980
March	1st to 10th	Night	19% 11% 3 7% 5% 4 4 3 5% 6% 33%	5327
	11th to 20th	Night	10% 6% 2 5% 3 3 4 3 6% 7% 51%	4980
	21st to end	Night	13% 6% 2 7% 5% 4 4 3 4 6% 45%	5467
April	1st to 10th	Night	10% 5% 2 5% 5% 4 5% 4 6% 7% 46%	2092
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 6% 7% 50%	1975
	21st to end	Night	8% 5% 3 6% 3 4 4 4 6% 7% 51%	1890
May	1st to 10th	Night	9% 7% 3 7% 4 6% 6% 4 8% 9% 37%	4033
	11th to 20th	Night	16% 8% 3 8% 5% 5% 5% 4 6% 8% 31%	4032
	21st to end	Night	11% 5% 2 6% 4 4 5% 4 8% 11% 41%	4421
June	1st to 10th	Night	5% 6% 2 6% 6% 6% 6% 4 8% 13% 38%	1800
	11th to 20th	Night	3 4 2 5% 5% 5% 5% 6% 9% 12% 45%	1800
	21st to end	Night	6% 3 2 6% 5% 4 6% 4 6% 14% 43%	1800
July	1st to 10th	Night	6% 6% 3 9% 6% 6% 7% 5% 8% 11% 34%	4058
	11th to 20th	Night	5% 4 2 6% 6% 6% 6% 6% 9% 13% 38%	4057
	21st to end	Night	8% 8% 3 10% 6% 6% 6% 5% 8% 11% 29%	4427
August	1st to 10th	Night	7% 5% 2 6% 5% 5% 5% 5% 7% 12% 42%	2976
	11th to 20th	Night	6% 5% 2 6% 5% 4 4 4 6% 9% 48%	3200
	21st to end	Night	8% 5% 2 4 3 4 4 3 6% 8% 51%	3586
September	1st to 10th	Night	8% 6% 2 7% 5% 4 6% 5% 7% 8% 43%	2135
	11th to 20th	Night	12% 7% 2 5% 4 4 4 3 5% 8% 45%	2200
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 7% 49%	2200
October	1st to 10th	Night	10% 6% 2 4 3 3 3 3 4 7% 56%	2200
	11th to 20th	Night	10% 5% 2 5% 4 3 3 2 3 5% 59%	2263
	21st to end	Night	12% 5% 2 5% 3 3 3 3 3 4 56%	2670
November	1st to 10th	Night	12% 6% 2 5% 4 3 3 2 4 5% 55%	2666
	11th to 20th	Night	11% 6% 2 5% 4 2 3 3 4 6% 55%	2730
	21st to end	Night	12% 6% 2 5% 4 4 4 4 5% 6% 47%	2730
December	1st to 10th	Night	15% 9% 2 5% 3 3 3 3 4 5% 48%	4380
	11th to 20th	Night	15% 8% 3 6% 3 4 3 3 4 5% 45%	4914
	21st to end	Night	18% 8% 3 6% 4 4 4 3 5% 6% 39%	6490

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	42% 3 4 23% 22% 6%	4057
	11th to 20th	Day	35% 2 6% 22% 29% 7%	4060
	21st to end	Day	50% 2 5% 15% 22% 7%	4788
February	1st to 10th	Day	46% 4 7% 19% 21% 4	2000
	11th to 20th	Day	46% 4 5% 18% 20% 6%	1958
	21st to end	Day	46% 4 6% 14% 21% 8%	1650
March	1st to 10th	Day	52% 3 8% 11% 19% 8%	4525
	11th to 20th	Day	36% 4 6% 14% 24% 17%	4980
	21st to end	Day	43% 3 8% 20% 21% 5%	5465
April	1st to 10th	Day	40% 4 9% 21% 19% 7%	2323
	11th to 20th	Day	37% 5% 6% 19% 25% 8%	2374
	21st to end	Day	33% 5% 10% 26% 22% 3	2508
May	1st to 10th	Day	43% 5% 15% 20% 15% 2	5897
	11th to 20th	Day	50% 7% 15% 18% 9% 1	5865
	21st to end	Day	41% 5% 11% 23% 18% 2	6447
June	1st to 10th	Day	46% 7% 14% 20% 11% 2	2600
	11th to 20th	Day	34% 8% 19% 23% 14% 1	2600
	21st to end	Day	38% 9% 16% 23% 11% 2	2600
July	1st to 10th	Day	42% 11% 19% 21% 7% 1	5835
	11th to 20th	Day	38% 13% 27% 17% 4 1	5831
	21st to end	Day	51% 12% 17% 16% 4	6240
August	1st to 10th	Day	34% 9% 15% 32% 11%	4142
	11th to 20th	Day	32% 8% 16% 28% 14% 3	3885
	21st to end	Day	33% 8% 14% 21% 21% 3	4311
September	1st to 10th	Day	40% 6% 15% 17% 17% 5%	2245
	11th to 20th	Day	41% 6% 11% 21% 19% 2	2200
	21st to end	Day	39% 5% 10% 18% 22% 6%	2200
October	1st to 10th	Day	26% 4 6% 13% 37% 15%	3590
	11th to 20th	Day	20% 3 4 16% 30% 27%	3453
	21st to end	Day	26% 3 6% 20% 33% 11%	3479
November	1st to 10th	Day	34% 3 7% 17% 29% 10%	1852
	11th to 20th	Day	33% 3 7% 18% 28% 11%	1890
	21st to end	Day	37% 5% 7% 19% 26% 7%	1890
December	1st to 10th	Day	42% 3 5% 18% 26% 7%	3020
	11th to 20th	Day	47% 3 7% 17% 20% 6%	3433
	21st to end	Day	44% 3 7% 24% 19% 2	4466
January	1st to 10th	Night	40% 3 5% 20% 27% 5%	5889
	11th to 20th	Night	40% 3 4 19% 26% 7%	5900
	21st to end	Night	45% 2 4 15% 28% 6%	6150
February	1st to 10th	Night	43% 4 6% 20% 19% 8%	2400
	11th to 20th	Night	47% 4 5% 17% 20% 8%	2339
	21st to end	Night	45% 3 7% 16% 18% 10%	1980
March	1st to 10th	Night	52% 3 7% 14% 18% 6%	5327
	11th to 20th	Night	31% 4 9% 15% 24% 17%	4980
	21st to end	Night	40% 3 6% 22% 20% 8%	5467
April	1st to 10th	Night	36% 4 10% 20% 22% 8%	2092
	11th to 20th	Night	34% 3 7% 22% 24% 10%	1975
	21st to end	Night	32% 5% 11% 27% 20% 5%	1890
May	1st to 10th	Night	40% 7% 17% 21% 14% 2	4060
	11th to 20th	Night	49% 7% 15% 21% 8%	4033
	21st to end	Night	35% 6% 16% 25% 17% 1	4429
June	1st to 10th	Night	35% 9% 18% 21% 15% 1	1800
	11th to 20th	Night	27% 9% 22% 25% 14% 2	1800
	21st to end	Night	31% 9% 20% 20% 16% 4	1800
July	1st to 10th	Night	41% 11% 19% 23% 7% 1	4015
	11th to 20th	Night	33% 14% 29% 18% 5% 1	4006
	21st to end	Night	46% 11% 21% 18% 4 1	4297
August	1st to 10th	Night	32% 8% 16% 33% 10% 1	2965
	11th to 20th	Night	31% 7% 15% 30% 15% 1	3230
	21st to end	Night	30% 7% 14% 18% 27% 3	3586
September	1st to 10th	Night	36% 8% 15% 19% 16% 6%	2135
	11th to 20th	Night	38% 6% 11% 23% 19% 3	2200
	21st to end	Night	34% 6% 11% 20% 21% 8%	2200
October	1st to 10th	Night	25% 3 7% 15% 35% 14%	3590
	11th to 20th	Night	18% 2 4 17% 31% 28%	3684
	21st to end	Night	23% 2 4 23% 31% 17%	4306
November	1st to 10th	Night	32% 4 8% 17% 27% 12%	2666
	11th to 20th	Night	32% 3 6% 19% 28% 12%	2730
	21st to end	Night	37% 4 7% 18% 26% 8%	2730
December	1st to 10th	Night	39% 3 5% 19% 27% 7%	4380
	11th to 20th	Night	42% 3 10% 14% 25% 6%	4916
	21st to end	Night	45% 4 6% 25% 18% 2	6490

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	25% 29% 33% 11% 1	300
	11th to 20th	Day	26% 24% 34% 12% 3 1	300
	21st to end	Day	25% 26% 37% 9% 2 1	330
February	1st to 10th	Day	22% 25% 37% 12% 3	300
	11th to 20th	Day	36% 20% 34% 9% 1	300
	21st to end	Day	26% 24% 38% 10% 2	249
March	1st to 10th	Day	27% 24% 31% 14% 3 1	319
	11th to 20th	Day	23% 22% 32% 16% 6% 1	500
	21st to end	Day	21% 20% 35% 19% 5% 1	550
April	1st to 10th	Day	19% 19% 34% 18% 9% 1	500
	11th to 20th	Day	14% 17% 35% 23% 9% 2	492
	21st to end	Day	14% 13% 29% 27% 14% 2	500
May	1st to 10th	Day	14% 13% 30% 26% 13% 4	503
	11th to 20th	Day	13% 9% 25% 28% 17% 7%	527
	21st to end	Day	14% 6% 24% 26% 19% 10%	573
June	1st to 10th	Day	13% 5% 18% 31% 22% 11%	530
	11th to 20th	Day	6% 4 15% 30% 25% 20%	530
	21st to end	Day	8% 3 16% 30% 27% 16%	530
July	1st to 10th	Day	8% 1 10% 30% 30% 21%	525
	11th to 20th	Day	7% 2 8% 23% 32% 29%	524
	21st to end	Day	5% 2 9% 29% 36% 19%	576
August	1st to 10th	Day	12% 3 13% 27% 25% 20%	526
	11th to 20th	Day	10% 3 10% 29% 31% 17%	519
	21st to end	Day	15% 3 10% 28% 28% 17%	583
September	1st to 10th	Day	12% 4 15% 27% 28% 14%	530
	11th to 20th	Day	18% 3 14% 28% 27% 10%	530
	21st to end	Day	14% 4 21% 25% 27% 8%	530
October	1st to 10th	Day	15% 4 28% 29% 17% 8%	530
	11th to 20th	Day	13% 8% 24% 31% 21% 3	530
	21st to end	Day	19% 10% 26% 26% 16% 3	548
November	1st to 10th	Day	22% 13% 26% 25% 11% 2	330
	11th to 20th	Day	14% 18% 39% 21% 7% 1	330
	21st to end	Day	13% 21% 42% 17% 6% 1	330
December	1st to 10th	Day	28% 23% 30% 17% 2	330
	11th to 20th	Day	23% 17% 35% 19% 4 1	321
	21st to end	Day	29% 17% 33% 16% 4 1	330
January	1st to 10th	Night	23% 22% 30% 21% 4 1	800
	11th to 20th	Night	22% 18% 31% 23% 6%	800
	21st to end	Night	20% 21% 35% 22% 3	880
February	1st to 10th	Night	19% 16% 34% 26% 5%	800
	11th to 20th	Night	25% 20% 33% 21% 2	783
	21st to end	Night	21% 20% 35% 20% 3	660
March	1st to 10th	Night	19% 19% 34% 23% 5%	800
	11th to 20th	Night	19% 16% 33% 23% 9% 1	800
	21st to end	Night	16% 17% 31% 27% 8% 1	880
April	1st to 10th	Night	13% 16% 33% 26% 11% 1	692
	11th to 20th	Night	17% 14% 29% 27% 12% 1	593
	21st to end	Night	12% 9% 26% 36% 16% 2	600
May	1st to 10th	Night	11% 9% 28% 32% 20% 1	600
	11th to 20th	Night	12% 7% 23% 31% 23% 4	591
	21st to end	Night	13% 3 15% 31% 32% 6%	650
June	1st to 10th	Night	7% 4 14% 35% 33% 7%	600
	11th to 20th	Night	3 2 17% 30% 34% 15%	600
	21st to end	Night	7% 2 9% 33% 42% 7%	600
July	1st to 10th	Night	8% 3 9% 25% 41% 14%	585
	11th to 20th	Night	8% 2 6% 21% 47% 16%	582
	21st to end	Night	7% 2 7% 28% 45% 10%	657
August	1st to 10th	Night	10% 2 6% 30% 39% 13%	597
	11th to 20th	Night	10% 3 9% 28% 40% 11%	594
	21st to end	Night	16% 3 8% 26% 35% 12%	660
September	1st to 10th	Night	10% 2 11% 27% 38% 11%	735
	11th to 20th	Night	14% 3 12% 28% 37% 6%	800
	21st to end	Night	14% 3 14% 34% 31% 3	800
October	1st to 10th	Night	12% 5% 23% 33% 22% 6%	800
	11th to 20th	Night	12% 8% 24% 33% 20% 3	800
	21st to end	Night	15% 10% 23% 32% 17% 3	880
November	1st to 10th	Night	15% 8% 26% 31% 19% 2	820
	11th to 20th	Night	15% 15% 29% 28% 13% 1	840
	21st to end	Night	17% 16% 27% 28% 12%	840
December	1st to 10th	Night	22% 16% 29% 25% 8% 1	840
	11th to 20th	Night	22% 12% 31% 26% 7% 1	829
	21st to end	Night	26% 15% 28% 24% 6%	880

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