Phillips Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Phillips Observatory clear sky chart from 2005-11-06 to 2026-02-16.

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	6% 10% 3 7% 4 4 4 3 5% 6% 49%	139689
	Night	7% 10% 3 7% 4 4 4 3 5% 6% 46%	138548
Transparency	Day	34% 6% 10% 20% 19% 11%	134400
	Night	37% 7% 10% 21% 17% 9%	134840
Seeing	Day	13% 17% 29% 26% 13% 3	16851
	Night	12% 9% 22% 37% 18% 2	26656

By Month

Percentage Cloud Cover SW (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% 28% 8% 11% 4 3 3 3 4 5% 24%	4722
	11th to 20th	Day	4 13% 5% 13% 5% 3 3 2 3 5% 43%	5072
	21st to end	Day	7% 16% 4 8% 4 4 4 3 5% 6% 40%	5844
February	1st to 10th	Day	10% 18% 7% 13% 5% 4 4 3 4 4 27%	1890
	11th to 20th	Day	9% 21% 5% 11% 6% 4 4 3 5% 6% 27%	1842
	21st to end	Day	8% 18% 7% 13% 5% 5% 4 4 4 5% 28%	1650
March	1st to 10th	Day	7% 20% 6% 11% 4 4 3 2 3 4 35%	4752
	11th to 20th	Day	7% 17% 6% 8% 4 3 3 3 4 5% 40%	5000
	21st to end	Day	9% 21% 6% 10% 5% 6% 5% 4 5% 6% 23%	5486
April	1st to 10th	Day	11% 14% 4 8% 4 5% 5% 4 5% 7% 34%	2254
	11th to 20th	Day	8% 12% 4 7% 6% 4 5% 3 5% 7% 39%	2567
	21st to end	Day	9% 11% 3 7% 5% 4 4 5% 6% 7% 38%	2600
May	1st to 10th	Day	6% 11% 3 5% 4 4 4 3 5% 7% 48%	5880
	11th to 20th	Day	9% 8% 2 6% 4 4 4 4 6% 7% 45%	5841
	21st to end	Day	6% 7% 2 5% 3 4 4 3 4 6% 55%	6671
June	1st to 10th	Day	7% 6% 2 5% 4 4 5% 4 6% 8% 50%	2876
	11th to 20th	Day	7% 6% 2 5% 3 4 4 4 6% 7% 53%	3000
	21st to end	Day	3 3 1 3 3 3 3 3 5% 6% 69%	3000
July	1st to 10th	Day	2 2 1 3 2 3 3 2 4 5% 74%	6487
	11th to 20th	Day	1 3 1 4 3 3 3 3 4 6% 70%	6190
	21st to end	Day	3 3 1 3 3 3 3 3 5% 7% 66%	6401
August	1st to 10th	Day	3 3 1 4 3 3 4 3 5% 7% 64%	5779
	11th to 20th	Day	3 2 1 3 3 4 4 3 5% 6% 67%	5798
	21st to end	Day	5% 4 1 4 3 3 3 3 4 6% 65%	6468
September	1st to 10th	Day	4 3 1 4 3 3 4 3 4 7% 65%	2295
	11th to 20th	Day	5% 4 1 4 3 3 4 3 5% 7% 61%	2200
	21st to end	Day	7% 4 2 3 3 3 3 2 4 5% 63%	2200
October	1st to 10th	Day	7% 5% 1 3 2 2 3 3 4 5% 65%	3440
	11th to 20th	Day	6% 7% 2 6% 3 3 4 2 4 5% 58%	3250
	21st to end	Day	6% 11% 3 6% 4 3 4 4 6% 8% 45%	3401
November	1st to 10th	Day	12% 13% 4 7% 4 4 4 3 5% 6% 39%	1854
	11th to 20th	Day	11% 12% 5% 7% 4 3 4 3 6% 7% 38%	1819
	21st to end	Day	7% 13% 4 9% 6% 4 3 3 5% 6% 39%	1680
December	1st to 10th	Day	7% 11% 4 7% 4 2 3 2 3 5% 52%	2700
	11th to 20th	Day	10% 19% 5% 11% 6% 5% 6% 4 6% 6% 22%	3106
	21st to end	Day	12% 13% 7% 15% 5% 5% 5% 4 5% 6% 22%	3674
January	1st to 10th	Night	9% 19% 8% 15% 5% 4 4 3 4 5% 24%	8172
	11th to 20th	Night	5% 13% 5% 12% 6% 4 3 3 4 6% 41%	7900
	21st to end	Night	5% 15% 6% 9% 4 4 4 3 4 5% 42%	8370
February	1st to 10th	Night	10% 17% 5% 11% 6% 4 4 4 6% 6% 28%	2730
	11th to 20th	Night	8% 16% 5% 10% 5% 4 4 2 6% 6% 34%	2552
	21st to end	Night	6% 16% 6% 12% 5% 5% 4 4 5% 6% 31%	1980
March	1st to 10th	Night	6% 19% 5% 8% 4 4 4 3 5% 7% 36%	5141
	11th to 20th	Night	5% 14% 4 6% 4 4 5% 4 6% 8% 39%	5000
	21st to end	Night	7% 11% 4 8% 4 5% 6% 5% 6% 8% 36%	5489
April	1st to 10th	Night	8% 13% 4 6% 5% 4 4 4 6% 6% 42%	2141
	11th to 20th	Night	8% 9% 4 8% 5% 5% 4 3 6% 6% 42%	1781
	21st to end	Night	10% 7% 3 6% 5% 4 4 4 5% 5% 49%	1800
May	1st to 10th	Night	7% 10% 2 6% 3 3 4 3 4 6% 51%	4100
	11th to 20th	Night	9% 6% 2 5% 4 5% 4 4 6% 7% 48%	4074
	21st to end	Night	6% 5% 2 4 3 4 5% 4 6% 8% 55%	4209
June	1st to 10th	Night	6% 5% 2 6% 4 4 4 5% 6% 8% 51%	1518
	11th to 20th	Night	9% 4 1 4 4 3 3 4 4 7% 57%	1400
	21st to end	Night	4 3 1 4 3 2 3 3 4 6% 68%	1400
July	1st to 10th	Night	2 2 1 4 3 3 3 3 4 5% 70%	3453
	11th to 20th	Night	1 1 3 3 3 3 3 5% 6% 70%	3773
	21st to end	Night	1 2 1 3 3 3 3 3 5% 7% 68%	4472
August	1st to 10th	Night	3 2 1 4 3 3 5% 4 7% 8% 61%	4033
	11th to 20th	Night	2 3 1 4 3 4 4 3 5% 6% 65%	4051
	21st to end	Night	5% 4 2 4 3 3 3 3 4 5% 65%	4509
September	1st to 10th	Night	3 3 1 4 2 2 3 3 4 5% 69%	2102
	11th to 20th	Night	6% 4 2 4 3 3 3 3 4 6% 62%	2200
	21st to end	Night	8% 3 1 4 3 2 3 2 4 5% 64%	2200
October	1st to 10th	Night	6% 4 1 3 2 2 2 2 4 6% 69%	3755
	11th to 20th	Night	10% 7% 2 5% 3 4 3 2 4 5% 54%	3898
	21st to end	Night	6% 10% 3 6% 3 3 4 3 5% 8% 48%	4398
November	1st to 10th	Night	10% 11% 3 7% 4 4 5% 3 5% 7% 42%	2665
	11th to 20th	Night	9% 11% 4 7% 5% 4 4 4 6% 7% 40%	2777
	21st to end	Night	6% 12% 4 9% 6% 5% 5% 3 5% 6% 38%	2940
December	1st to 10th	Night	7% 11% 4 9% 4 3 3 3 4 5% 48%	4710
	11th to 20th	Night	10% 17% 5% 10% 5% 4 4 3 6% 6% 29%	5321
	21st to end	Night	12% 18% 6% 11% 5% 5% 5% 4 5% 6% 23%	7534

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	62% 7% 15% 10% 5% 1	4723
	11th to 20th	Day	43% 6% 14% 23% 10% 4	5072
	21st to end	Day	45% 7% 12% 20% 14% 2	5844
February	1st to 10th	Day	58% 7% 12% 16% 6% 1	1890
	11th to 20th	Day	56% 8% 13% 15% 7% 2	1851
	21st to end	Day	58% 7% 12% 14% 8% 2	1650
March	1st to 10th	Day	52% 6% 10% 21% 8% 4	4752
	11th to 20th	Day	42% 7% 10% 29% 11% 1	5000
	21st to end	Day	56% 11% 13% 16% 4	5486
April	1st to 10th	Day	49% 7% 14% 17% 11% 2	2254
	11th to 20th	Day	43% 6% 10% 24% 15% 2	2570
	21st to end	Day	42% 8% 11% 25% 14%	2600
May	1st to 10th	Day	32% 7% 10% 27% 20% 4	5880
	11th to 20th	Day	34% 7% 8% 29% 19% 3	5844
	21st to end	Day	28% 6% 7% 33% 23% 2	6672
June	1st to 10th	Day	30% 7% 14% 29% 19% 2	2876
	11th to 20th	Day	29% 6% 10% 23% 24% 9%	3000
	21st to end	Day	15% 4 6% 21% 38% 16%	3000
July	1st to 10th	Day	13% 3 4 12% 24% 44%	6419
	11th to 20th	Day	15% 3 3 10% 28% 41%	6115
	21st to end	Day	17% 3 5% 10% 34% 30%	6217
August	1st to 10th	Day	22% 4 6% 13% 32% 23%	4208
	11th to 20th	Day	17% 4 6% 17% 36% 21%	4197
	21st to end	Day	15% 3 5% 18% 31% 27%	4663
September	1st to 10th	Day	19% 4 7% 18% 31% 21%	2295
	11th to 20th	Day	21% 5% 8% 23% 29% 13%	2200
	21st to end	Day	24% 5% 7% 24% 33% 8%	2200
October	1st to 10th	Day	23% 3 6% 19% 34% 15%	3440
	11th to 20th	Day	29% 4 7% 34% 18% 8%	3250
	21st to end	Day	35% 6% 16% 27% 16% 1	3401
November	1st to 10th	Day	44% 8% 14% 22% 11% 2	1854
	11th to 20th	Day	43% 7% 18% 22% 8% 2	1819
	21st to end	Day	44% 7% 16% 21% 10% 1	1680
December	1st to 10th	Day	37% 6% 15% 32% 8% 2	2700
	11th to 20th	Day	57% 9% 13% 17% 3	3104
	21st to end	Day	56% 11% 13% 12% 7%	3674
January	1st to 10th	Night	60% 7% 9% 17% 6%	8172
	11th to 20th	Night	43% 7% 13% 28% 7% 2	7900
	21st to end	Night	44% 6% 11% 19% 17% 3	8370
February	1st to 10th	Night	55% 8% 14% 17% 6%	2730
	11th to 20th	Night	50% 7% 14% 19% 10% 1	2553
	21st to end	Night	51% 7% 13% 19% 7% 3	1980
March	1st to 10th	Night	46% 7% 9% 22% 9% 7%	5141
	11th to 20th	Night	37% 9% 12% 25% 16% 1	5000
	21st to end	Night	40% 11% 18% 22% 7%	5489
April	1st to 10th	Night	41% 7% 14% 19% 15% 4	2141
	11th to 20th	Night	40% 7% 13% 23% 15% 2	1781
	21st to end	Night	35% 7% 14% 26% 18%	1800
May	1st to 10th	Night	31% 7% 10% 26% 22% 4	4097
	11th to 20th	Night	29% 8% 11% 26% 22% 4	4075
	21st to end	Night	23% 7% 9% 28% 27% 5%	4209
June	1st to 10th	Night	29% 8% 13% 24% 24% 4	1518
	11th to 20th	Night	26% 5% 9% 23% 24% 12%	1400
	21st to end	Night	17% 4 4 19% 31% 26%	1400
July	1st to 10th	Night	18% 3 3 10% 19% 47%	3413
	11th to 20th	Night	13% 3 3 8% 27% 45%	3725
	21st to end	Night	15% 3 4 9% 34% 35%	4344
August	1st to 10th	Night	19% 5% 6% 12% 24% 35%	2928
	11th to 20th	Night	22% 4 4 14% 35% 22%	2928
	21st to end	Night	15% 3 5% 17% 31% 28%	3245
September	1st to 10th	Night	17% 3 8% 13% 29% 29%	2102
	11th to 20th	Night	23% 6% 6% 22% 27% 17%	2200
	21st to end	Night	22% 4 6% 23% 33% 11%	2200
October	1st to 10th	Night	18% 3 6% 19% 37% 16%	3755
	11th to 20th	Night	33% 3 7% 30% 18% 8%	3898
	21st to end	Night	33% 6% 11% 30% 18% 2	4398
November	1st to 10th	Night	40% 8% 12% 23% 15% 2	2665
	11th to 20th	Night	41% 7% 17% 24% 10% 1	2777
	21st to end	Night	44% 8% 13% 23% 10% 2	2940
December	1st to 10th	Night	37% 6% 14% 34% 7% 2	4710
	11th to 20th	Night	53% 9% 16% 18% 4	5322
	21st to end	Night	55% 10% 13% 15% 7%	7534

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% 21% 23% 20% 7% 1	220
	11th to 20th	Day	15% 14% 33% 26% 11% 1	279
	21st to end	Day	13% 17% 32% 27% 10%	363
February	1st to 10th	Day	23% 23% 28% 22% 4	330
	11th to 20th	Day	22% 23% 31% 17% 5% 2	357
	21st to end	Day	18% 26% 32% 17% 7%	414
March	1st to 10th	Day	19% 29% 28% 20% 4	500
	11th to 20th	Day	16% 23% 33% 21% 7% 1	500
	21st to end	Day	23% 29% 27% 17% 3	550
April	1st to 10th	Day	23% 23% 26% 22% 5% 1	500
	11th to 20th	Day	21% 29% 29% 13% 8%	492
	21st to end	Day	19% 25% 26% 21% 6% 2	500
May	1st to 10th	Day	12% 22% 35% 22% 7% 1	503
	11th to 20th	Day	18% 19% 30% 23% 9% 2	528
	21st to end	Day	15% 11% 28% 25% 16% 5%	678
June	1st to 10th	Day	9% 14% 25% 29% 19% 4	730
	11th to 20th	Day	12% 16% 28% 30% 12% 1	730
	21st to end	Day	5% 11% 28% 33% 20% 3	730
July	1st to 10th	Day	4 9% 30% 35% 20% 3	603
	11th to 20th	Day	3 7% 37% 37% 14% 2	524
	21st to end	Day	3 6% 29% 37% 21% 4	575
August	1st to 10th	Day	7% 9% 33% 30% 18% 3	527
	11th to 20th	Day	3 9% 33% 33% 17% 5%	517
	21st to end	Day	5% 12% 27% 31% 21% 4	583
September	1st to 10th	Day	4 10% 29% 30% 21% 6%	530
	11th to 20th	Day	7% 17% 27% 24% 21% 5%	530
	21st to end	Day	9% 13% 26% 30% 17% 5%	530
October	1st to 10th	Day	14% 14% 26% 23% 17% 5%	435
	11th to 20th	Day	12% 24% 18% 24% 18% 5%	330
	21st to end	Day	16% 16% 29% 26% 11% 1	363
November	1st to 10th	Day	18% 18% 24% 25% 10% 5%	330
	11th to 20th	Day	13% 19% 33% 25% 8% 2	330
	21st to end	Day	15% 16% 29% 30% 10% 1	330
December	1st to 10th	Day	19% 13% 30% 25% 11% 3	330
	11th to 20th	Day	20% 18% 34% 24% 5%	330
	21st to end	Day	29% 19% 27% 19% 5% 1	250
January	1st to 10th	Night	17% 11% 25% 32% 13% 1	840
	11th to 20th	Night	17% 10% 24% 33% 14% 2	840
	21st to end	Night	17% 9% 23% 37% 13% 1	924
February	1st to 10th	Night	17% 14% 26% 31% 10% 1	840
	11th to 20th	Night	13% 14% 28% 33% 11% 2	804
	21st to end	Night	12% 14% 30% 32% 11% 1	660
March	1st to 10th	Night	12% 12% 29% 36% 10% 1	800
	11th to 20th	Night	12% 10% 26% 37% 15% 1	800
	21st to end	Night	15% 12% 26% 35% 11% 1	880
April	1st to 10th	Night	14% 13% 23% 35% 14% 1	770
	11th to 20th	Night	14% 13% 28% 33% 11% 1	593
	21st to end	Night	13% 11% 25% 33% 18% 1	600
May	1st to 10th	Night	12% 9% 23% 36% 18% 2	600
	11th to 20th	Night	11% 8% 18% 37% 23% 3	591
	21st to end	Night	14% 7% 18% 35% 22% 4	650
June	1st to 10th	Night	8% 6% 20% 38% 27% 2	518
	11th to 20th	Night	9% 6% 22% 44% 18% 1	400
	21st to end	Night	5% 3 15% 45% 27% 6%	400
July	1st to 10th	Night	3 1 12% 50% 30% 5%	390
	11th to 20th	Night	2 2 14% 54% 26% 1	443
	21st to end	Night	3 1 10% 49% 34% 3	657
August	1st to 10th	Night	5% 2 13% 49% 30% 1	597
	11th to 20th	Night	2 2 15% 46% 32% 3	594
	21st to end	Night	5% 4 16% 42% 28% 3	660
September	1st to 10th	Night	5% 4 15% 43% 30% 4	727
	11th to 20th	Night	7% 5% 16% 41% 26% 6%	800
	21st to end	Night	9% 5% 14% 40% 25% 6%	800
October	1st to 10th	Night	12% 6% 17% 37% 25% 3	800
	11th to 20th	Night	12% 9% 15% 37% 25% 3	800
	21st to end	Night	11% 10% 18% 42% 17% 3	880
November	1st to 10th	Night	15% 9% 19% 40% 15% 2	820
	11th to 20th	Night	13% 10% 28% 36% 11% 1	887
	21st to end	Night	10% 13% 23% 36% 14% 3	1050
December	1st to 10th	Night	18% 12% 24% 34% 12%	1050
	11th to 20th	Night	18% 13% 29% 30% 10% 1	1050
	21st to end	Night	20% 17% 27% 28% 8% 1	1141

Page generated at 2026-02-12 23:42:16. Version ac07

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