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 2024-01-15.

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% 9% 4 6% 5% 3 4 4 5% 7% 45%	74778
	Night	8% 9% 4 7% 5% 3 4 4 5% 7% 44%	74985
Transparency	Day	38% 6% 11% 20% 17% 8%	74491
	Night	39% 7% 12% 20% 15% 7%	74792
Seeing	Day	12% 16% 29% 26% 13% 3	14074
	Night	12% 9% 21% 37% 18% 2	23898

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	11% 20% 7% 8% 5% 3 4 3 3 6% 28%	1519
	11th to 20th	Day	9% 15% 6% 10% 7% 3 4 4 4 6% 31%	1566
	21st to end	Day	11% 18% 7% 9% 7% 4 4 3 5% 5% 27%	1782
February	1st to 10th	Day	10% 17% 7% 11% 6% 3 4 4 4 5% 28%	1620
	11th to 20th	Day	10% 18% 6% 10% 6% 3 4 4 4 7% 28%	1612
	21st to end	Day	9% 17% 9% 12% 7% 3 4 5% 4 5% 26%	1480
March	1st to 10th	Day	9% 17% 7% 9% 6% 3 4 4 5% 5% 31%	1891
	11th to 20th	Day	9% 13% 5% 6% 6% 3 5% 5% 7% 7% 34%	1980
	21st to end	Day	11% 18% 7% 9% 6% 3 4 4 5% 6% 26%	2178
April	1st to 10th	Day	12% 13% 5% 8% 6% 3 5% 5% 5% 7% 32%	2027
	11th to 20th	Day	10% 12% 5% 6% 7% 3 6% 4 5% 6% 37%	2307
	21st to end	Day	9% 11% 4 6% 5% 3 4 5% 6% 7% 39%	2340
May	1st to 10th	Day	8% 9% 3 5% 6% 4 6% 5% 6% 7% 41%	2340
	11th to 20th	Day	7% 8% 3 5% 6% 4 5% 6% 8% 9% 39%	2301
	21st to end	Day	12% 10% 4 7% 6% 4 6% 5% 5% 7% 35%	2624
June	1st to 10th	Day	8% 6% 2 4 4 3 5% 5% 6% 8% 48%	2588
	11th to 20th	Day	8% 6% 2 4 4 2 4 5% 6% 7% 52%	2700
	21st to end	Day	3 3 1 2 3 2 3 3 5% 7% 69%	2700
July	1st to 10th	Day	3 2 1 3 3 2 3 4 5% 8% 66%	2590
	11th to 20th	Day	2 3 1 3 3 2 3 3 6% 6% 67%	2468
	21st to end	Day	4 4 2 2 4 3 4 4 6% 8% 61%	2545
August	1st to 10th	Day	6% 4 1 3 5% 3 4 5% 5% 7% 58%	2328
	11th to 20th	Day	3 2 1 3 4 2 3 3 4 6% 69%	2299
	21st to end	Day	4 3 2 4 3 3 4 4 4 6% 63%	2574
September	1st to 10th	Day	4 4 1 3 4 2 3 4 3 7% 66%	2067
	11th to 20th	Day	5% 4 1 3 3 2 4 4 5% 7% 62%	1980
	21st to end	Day	8% 4 2 3 3 3 4 3 4 5% 62%	1980
October	1st to 10th	Day	12% 7% 2 4 4 3 5% 5% 4 7% 47%	1895
	11th to 20th	Day	7% 9% 3 5% 5% 3 4 3 3 6% 51%	1800
	21st to end	Day	8% 9% 3 5% 4 3 3 4 5% 6% 49%	1900
November	1st to 10th	Day	13% 13% 4 6% 5% 2 4 4 5% 6% 38%	1674
	11th to 20th	Day	12% 11% 5% 5% 4 2 4 4 6% 7% 41%	1647
	21st to end	Day	8% 12% 4 8% 7% 3 3 4 5% 6% 40%	1520
December	1st to 10th	Day	11% 16% 7% 9% 7% 3 5% 4 4 5% 30%	1520
	11th to 20th	Day	10% 16% 5% 11% 9% 4 6% 5% 6% 6% 22%	1926
	21st to end	Day	8% 10% 7% 11% 7% 5% 5% 6% 5% 7% 30%	2510
January	1st to 10th	Night	10% 17% 5% 11% 5% 3 4 4 5% 7% 28%	2646
	11th to 20th	Night	11% 16% 5% 8% 6% 3 4 4 5% 7% 30%	2477
	21st to end	Night	10% 17% 7% 9% 7% 3 5% 4 4 6% 28%	2574
February	1st to 10th	Night	9% 15% 6% 10% 7% 3 5% 5% 6% 6% 28%	2340
	11th to 20th	Night	8% 14% 5% 9% 5% 3 4 3 6% 6% 37%	2232
	21st to end	Night	7% 15% 8% 11% 6% 4 4 5% 5% 6% 29%	1776
March	1st to 10th	Night	8% 12% 6% 8% 7% 4 6% 6% 5% 7% 32%	2068
	11th to 20th	Night	6% 10% 4 7% 5% 4 4 4 6% 8% 41%	1980
	21st to end	Night	10% 11% 4 8% 6% 3 6% 6% 6% 7% 32%	2178
April	1st to 10th	Night	9% 12% 5% 6% 5% 3 4 5% 6% 5% 41%	1929
	11th to 20th	Night	9% 8% 4 7% 6% 3 5% 4 5% 6% 41%	1601
	21st to end	Night	10% 6% 2 5% 5% 3 5% 4 5% 5% 51%	1620
May	1st to 10th	Night	7% 6% 3 6% 5% 3 5% 5% 5% 7% 48%	1620
	11th to 20th	Night	9% 5% 2 6% 5% 4 5% 6% 6% 8% 45%	1594
	21st to end	Night	13% 7% 3 4 6% 4 6% 6% 5% 6% 40%	1649
June	1st to 10th	Night	7% 5% 2 4 5% 3 4 5% 6% 9% 50%	1367
	11th to 20th	Night	9% 3 2 4 5% 2 4 4 4 7% 57%	1260
	21st to end	Night	3 3 1 3 3 1 3 3 4 6% 68%	1260
July	1st to 10th	Night	2 2 2 3 4 3 4 5% 7% 10% 59%	1365
	11th to 20th	Night	3 2 1 3 2 2 3 4 4 7% 69%	1489
	21st to end	Night	3 3 1 4 3 3 3 4 5% 7% 64%	1773
August	1st to 10th	Night	4 3 1 4 5% 2 4 6% 8% 8% 55%	1611
	11th to 20th	Night	3 3 1 3 4 2 3 4 5% 7% 65%	1602
	21st to end	Night	4 4 2 3 3 2 2 3 5% 7% 67%	1782
September	1st to 10th	Night	4 3 1 3 3 2 3 3 4 4 70%	1890
	11th to 20th	Night	6% 4 2 3 4 2 3 4 4 6% 63%	1980
	21st to end	Night	9% 3 2 3 3 1 3 3 4 5% 63%	1980
October	1st to 10th	Night	9% 6% 2 4 3 3 3 3 5% 6% 55%	2065
	11th to 20th	Night	10% 6% 2 4 4 2 3 4 4 5% 56%	2160
	21st to end	Night	7% 8% 2 4 4 3 4 4 6% 8% 51%	2455
November	1st to 10th	Night	11% 11% 4 6% 4 3 4 4 5% 6% 41%	2405
	11th to 20th	Night	9% 11% 4 6% 5% 3 5% 4 6% 7% 40%	2512
	21st to end	Night	6% 11% 5% 8% 6% 4 5% 5% 5% 6% 40%	2660
December	1st to 10th	Night	10% 15% 5% 9% 6% 3 4 5% 6% 7% 31%	2660
	11th to 20th	Night	11% 14% 5% 10% 8% 4 5% 5% 6% 6% 26%	3271
	21st to end	Night	8% 12% 6% 10% 7% 4 5% 6% 6% 7% 29%	5154

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	58% 7% 17% 13% 4 1	1519
	11th to 20th	Day	53% 7% 13% 18% 7% 2	1566
	21st to end	Day	60% 6% 13% 15% 6% 1	1782
February	1st to 10th	Day	57% 8% 11% 17% 6% 1	1620
	11th to 20th	Day	55% 7% 14% 16% 6% 2	1621
	21st to end	Day	60% 7% 11% 14% 7% 1	1480
March	1st to 10th	Day	53% 7% 11% 19% 8% 1	1891
	11th to 20th	Day	45% 9% 14% 21% 9% 1	1980
	21st to end	Day	57% 9% 13% 15% 6%	2178
April	1st to 10th	Day	51% 7% 12% 16% 11% 3	2027
	11th to 20th	Day	46% 6% 10% 22% 13% 2	2310
	21st to end	Day	42% 7% 11% 26% 14%	2340
May	1st to 10th	Day	38% 7% 12% 24% 18% 1	2340
	11th to 20th	Day	37% 9% 11% 28% 13% 2	2304
	21st to end	Day	47% 7% 11% 24% 10% 1	2624
June	1st to 10th	Day	32% 6% 14% 28% 17% 2	2588
	11th to 20th	Day	30% 7% 10% 22% 22% 9%	2700
	21st to end	Day	15% 4 6% 21% 38% 15%	2700
July	1st to 10th	Day	16% 4 5% 19% 34% 21%	2521
	11th to 20th	Day	17% 3 5% 16% 29% 29%	2393
	21st to end	Day	20% 4 7% 15% 27% 27%	2404
August	1st to 10th	Day	24% 5% 8% 17% 25% 21%	2313
	11th to 20th	Day	17% 4 5% 18% 36% 20%	2299
	21st to end	Day	21% 4 6% 17% 33% 19%	2574
September	1st to 10th	Day	20% 4 7% 16% 31% 22%	2067
	11th to 20th	Day	21% 5% 9% 23% 28% 15%	1980
	21st to end	Day	25% 5% 7% 25% 32% 7%	1980
October	1st to 10th	Day	37% 6% 10% 23% 20% 5%	1895
	11th to 20th	Day	35% 6% 12% 29% 15% 4	1800
	21st to end	Day	34% 6% 17% 26% 17% 1	1900
November	1st to 10th	Day	46% 8% 14% 20% 11% 2	1674
	11th to 20th	Day	42% 7% 18% 22% 9% 2	1647
	21st to end	Day	44% 7% 16% 20% 12% 2	1520
December	1st to 10th	Day	56% 8% 12% 16% 5% 4	1520
	11th to 20th	Day	58% 10% 13% 16% 2	1924
	21st to end	Day	48% 11% 15% 15% 10%	2510
January	1st to 10th	Night	56% 8% 16% 14% 6% 1	2646
	11th to 20th	Night	53% 8% 13% 19% 5% 2	2477
	21st to end	Night	58% 7% 13% 14% 6% 3	2574
February	1st to 10th	Night	54% 8% 14% 18% 6%	2340
	11th to 20th	Night	48% 6% 15% 21% 9% 1	2233
	21st to end	Night	53% 7% 13% 19% 6% 2	1776
March	1st to 10th	Night	49% 9% 11% 22% 8% 1	2068
	11th to 20th	Night	39% 8% 12% 26% 13% 1	1980
	21st to end	Night	47% 10% 16% 19% 7% 1	2178
April	1st to 10th	Night	42% 8% 14% 19% 14% 4	1929
	11th to 20th	Night	41% 7% 13% 22% 14% 2	1601
	21st to end	Night	34% 6% 14% 26% 19%	1620
May	1st to 10th	Night	34% 7% 10% 29% 18% 2	1620
	11th to 20th	Night	33% 8% 12% 25% 19% 4	1595
	21st to end	Night	41% 8% 13% 26% 10% 1	1649
June	1st to 10th	Night	29% 8% 13% 24% 23% 3	1367
	11th to 20th	Night	27% 6% 10% 23% 22% 12%	1260
	21st to end	Night	17% 4 4 19% 31% 25%	1260
July	1st to 10th	Night	19% 5% 5% 18% 30% 23%	1325
	11th to 20th	Night	15% 4 4 13% 29% 36%	1441
	21st to end	Night	19% 4 6% 13% 26% 32%	1674
August	1st to 10th	Night	22% 6% 8% 18% 21% 25%	1602
	11th to 20th	Night	19% 4 4 14% 34% 25%	1602
	21st to end	Night	18% 4 6% 18% 31% 24%	1782
September	1st to 10th	Night	18% 3 8% 12% 27% 30%	1890
	11th to 20th	Night	21% 6% 6% 21% 27% 19%	1980
	21st to end	Night	23% 4 6% 24% 32% 10%	1980
October	1st to 10th	Night	30% 5% 11% 24% 23% 7%	2065
	11th to 20th	Night	30% 5% 11% 31% 19% 4	2160
	21st to end	Night	30% 7% 14% 29% 19% 2	2455
November	1st to 10th	Night	41% 8% 12% 22% 14% 3	2405
	11th to 20th	Night	42% 7% 15% 23% 11% 1	2512
	21st to end	Night	44% 8% 13% 23% 10% 2	2660
December	1st to 10th	Night	51% 8% 16% 15% 7% 3	2660
	11th to 20th	Night	56% 8% 14% 16% 6%	3272
	21st to end	Night	49% 10% 13% 18% 10%	5154

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% 20% 22% 23% 8% 1	180
	11th to 20th	Day	17% 12% 33% 23% 12% 2	210
	21st to end	Day	15% 17% 31% 25% 11%	264
February	1st to 10th	Day	20% 24% 30% 24% 3	240
	11th to 20th	Day	18% 22% 32% 19% 5% 2	277
	21st to end	Day	18% 26% 34% 16% 6% 1	346
March	1st to 10th	Day	17% 31% 27% 21% 3	420
	11th to 20th	Day	15% 23% 33% 22% 6%	420
	21st to end	Day	23% 29% 28% 16% 3	462
April	1st to 10th	Day	25% 23% 26% 20% 4 1	420
	11th to 20th	Day	22% 31% 29% 11% 7%	412
	21st to end	Day	19% 24% 25% 22% 7% 2	420
May	1st to 10th	Day	11% 22% 38% 22% 6% 1	423
	11th to 20th	Day	16% 18% 30% 25% 10% 2	448
	21st to end	Day	16% 11% 26% 26% 17% 5%	579
June	1st to 10th	Day	10% 13% 25% 28% 20% 4	630
	11th to 20th	Day	12% 17% 27% 30% 13% 2	630
	21st to end	Day	4 9% 27% 34% 23% 4	630
July	1st to 10th	Day	3 7% 29% 36% 22% 3	516
	11th to 20th	Day	4 7% 37% 37% 14% 1	444
	21st to end	Day	3 5% 26% 37% 24% 5%	487
August	1st to 10th	Day	7% 8% 33% 31% 18% 3	447
	11th to 20th	Day	2 8% 30% 35% 19% 6%	437
	21st to end	Day	4 11% 26% 34% 22% 4	495
September	1st to 10th	Day	4 11% 30% 28% 21% 6%	450
	11th to 20th	Day	7% 17% 26% 24% 22% 5%	450
	21st to end	Day	9% 13% 26% 30% 16% 5%	450
October	1st to 10th	Day	14% 15% 24% 24% 18% 5%	365
	11th to 20th	Day	10% 23% 17% 25% 19% 6%	270
	21st to end	Day	16% 16% 30% 26% 12%	297
November	1st to 10th	Day	19% 17% 25% 23% 10% 6%	270
	11th to 20th	Day	11% 17% 34% 26% 10% 2	270
	21st to end	Day	16% 14% 30% 30% 9% 1	270
December	1st to 10th	Day	18% 12% 31% 27% 10% 1	270
	11th to 20th	Day	17% 17% 37% 24% 6%	270
	21st to end	Day	24% 20% 30% 19% 6% 1	205
January	1st to 10th	Night	18% 8% 24% 34% 15% 1	760
	11th to 20th	Night	18% 10% 23% 32% 15% 2	736
	21st to end	Night	18% 10% 22% 35% 14% 2	792
February	1st to 10th	Night	16% 13% 27% 34% 10% 1	720
	11th to 20th	Night	12% 13% 27% 34% 11% 2	704
	21st to end	Night	12% 15% 28% 32% 12% 2	592
March	1st to 10th	Night	12% 11% 29% 37% 10% 2	720
	11th to 20th	Night	11% 10% 26% 38% 15% 1	720
	21st to end	Night	15% 11% 26% 34% 11% 1	792
April	1st to 10th	Night	14% 13% 22% 35% 13% 2	694
	11th to 20th	Night	14% 14% 28% 32% 11% 1	533
	21st to end	Night	12% 10% 24% 33% 19% 1	540
May	1st to 10th	Night	11% 9% 24% 36% 18% 3	540
	11th to 20th	Night	11% 7% 17% 37% 25% 3	531
	21st to end	Night	15% 7% 18% 34% 22% 4	584
June	1st to 10th	Night	8% 6% 19% 35% 29% 2	467
	11th to 20th	Night	10% 5% 22% 42% 20% 1	360
	21st to end	Night	5% 2 14% 44% 29% 6%	360
July	1st to 10th	Night	2 1 12% 49% 31% 5%	350
	11th to 20th	Night	3 2 14% 54% 26% 1	397
	21st to end	Night	4 1 9% 47% 37% 3	591
August	1st to 10th	Night	5% 1 12% 47% 33% 1	537
	11th to 20th	Night	2 2 12% 45% 35% 3	534
	21st to end	Night	5% 4 15% 43% 30% 3	594
September	1st to 10th	Night	5% 4 15% 42% 29% 5%	653
	11th to 20th	Night	7% 4 15% 41% 27% 7%	720
	21st to end	Night	10% 5% 14% 39% 25% 7%	720
October	1st to 10th	Night	12% 6% 17% 37% 25% 3	720
	11th to 20th	Night	12% 8% 15% 37% 26% 3	720
	21st to end	Night	11% 10% 17% 42% 17% 3	792
November	1st to 10th	Night	16% 9% 18% 39% 16% 3	740
	11th to 20th	Night	14% 10% 26% 37% 11% 1	802
	21st to end	Night	11% 12% 23% 37% 14% 4	950
December	1st to 10th	Night	18% 12% 24% 36% 10%	950
	11th to 20th	Night	19% 12% 28% 30% 10% 1	950
	21st to end	Night	19% 16% 27% 28% 8% 1	1033

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