Princeton University
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Princeton University clear sky chart from 2005-11-06 to 2025-09-04.

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	16% 7% 2 6% 4 4 4 4 5% 7% 40%	126179
	Night	16% 7% 2 6% 4 4 4 3 5% 6% 43%	122529
Transparency	Day	42% 5% 11% 22% 17% 3	129706
	Night	41% 5% 11% 23% 17% 3	127089
Seeing	Day	22% 15% 29% 22% 10% 3	16067
	Night	20% 13% 31% 27% 9% 1	25233

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	13% 6% 2 4 3 3 4 3 4 6% 51%	3597
	11th to 20th	Day	14% 8% 3 8% 5% 4 4 3 4 6% 42%	3570
	21st to end	Day	18% 9% 3 7% 4 4 4 2 3 5% 40%	4443
February	1st to 10th	Day	16% 7% 3 7% 4 4 4 3 5% 7% 40%	1800
	11th to 20th	Day	17% 8% 3 6% 5% 5% 3 3 5% 6% 40%	1933
	21st to end	Day	19% 9% 3 6% 4 3 4 4 5% 5% 39%	1650
March	1st to 10th	Day	22% 5% 2 4 4 4 3 3 4 5% 44%	4476
	11th to 20th	Day	12% 7% 2 6% 5% 5% 5% 4 5% 7% 43%	4879
	21st to end	Day	18% 5% 2 6% 4 5% 4 3 4 6% 43%	5868
April	1st to 10th	Day	18% 8% 3 6% 4 4 4 3 5% 6% 39%	2400
	11th to 20th	Day	16% 6% 2 5% 5% 4 4 4 6% 9% 40%	2374
	21st to end	Day	17% 6% 2 6% 4 4 4 4 5% 7% 41%	2400
May	1st to 10th	Day	26% 8% 2 6% 4 4 5% 4 6% 8% 29%	5847
	11th to 20th	Day	24% 9% 3 6% 4 4 3 3 4 6% 34%	6318
	21st to end	Day	20% 6% 2 6% 4 4 4 4 6% 8% 35%	6949
June	1st to 10th	Day	14% 6% 2 5% 4 4 6% 5% 7% 9% 38%	2800
	11th to 20th	Day	15% 6% 2 6% 5% 5% 5% 4 6% 8% 37%	2800
	21st to end	Day	11% 5% 2 6% 5% 5% 6% 5% 6% 8% 41%	2800
July	1st to 10th	Day	9% 6% 2 8% 5% 5% 6% 5% 8% 10% 36%	6360
	11th to 20th	Day	11% 7% 3 8% 5% 6% 6% 6% 8% 10% 31%	6360
	21st to end	Day	11% 7% 2 7% 5% 4 5% 4 6% 9% 39%	6926
August	1st to 10th	Day	19% 6% 2 7% 5% 4 5% 5% 7% 8% 32%	4204
	11th to 20th	Day	13% 6% 2 6% 4 5% 5% 4 6% 9% 40%	3850
	21st to end	Day	11% 5% 2 5% 4 3 4 4 6% 8% 48%	4312
September	1st to 10th	Day	14% 6% 2 6% 5% 3 4 3 5% 7% 45%	2316
	11th to 20th	Day	13% 5% 2 5% 3 3 3 3 5% 8% 50%	2223
	21st to end	Day	19% 6% 2 6% 5% 4 4 4 4 6% 39%	2090
October	1st to 10th	Day	19% 7% 2 5% 4 4 3 3 5% 6% 43%	1946
	11th to 20th	Day	14% 6% 2 4 3 3 3 3 5% 6% 49%	1900
	21st to end	Day	19% 6% 2 5% 3 3 3 3 4 6% 46%	1981
November	1st to 10th	Day	12% 6% 2 5% 3 3 3 3 5% 6% 52%	1761
	11th to 20th	Day	16% 6% 2 4 3 2 4 3 6% 6% 48%	1800
	21st to end	Day	15% 6% 2 4 4 3 3 3 5% 7% 45%	1700
December	1st to 10th	Day	15% 8% 2 5% 3 3 4 3 5% 7% 45%	2600
	11th to 20th	Day	22% 8% 2 4 3 3 4 2 4 6% 43%	2986
	21st to end	Day	17% 6% 2 6% 4 4 4 4 6% 8% 38%	3960
January	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 4 6% 51%	6349
	11th to 20th	Night	14% 7% 2 5% 4 3 3 3 4 6% 48%	6315
	21st to end	Night	18% 9% 2 5% 3 3 4 3 5% 6% 41%	6508
February	1st to 10th	Night	17% 9% 2 5% 4 3 4 3 5% 6% 41%	2600
	11th to 20th	Night	18% 8% 3 6% 4 3 3 3 5% 5% 42%	2352
	21st to end	Night	18% 8% 2 6% 4 3 4 3 4 6% 42%	1980
March	1st to 10th	Night	19% 8% 2 5% 3 3 3 2 4 6% 44%	5364
	11th to 20th	Night	13% 9% 2 5% 4 3 3 3 4 5% 49%	5076
	21st to end	Night	17% 7% 2 7% 5% 4 3 3 4 6% 42%	5064
April	1st to 10th	Night	18% 7% 3 6% 4 5% 4 4 6% 7% 38%	2000
	11th to 20th	Night	17% 7% 3 5% 5% 4 4 3 5% 6% 41%	1975
	21st to end	Night	18% 6% 1 5% 3 3 4 4 5% 8% 44%	2000
May	1st to 10th	Night	23% 10% 3 7% 4 4 4 4 5% 8% 28%	4106
	11th to 20th	Night	23% 10% 3 6% 4 3 3 3 5% 6% 34%	3588
	21st to end	Night	22% 7% 3 5% 4 4 4 3 5% 8% 35%	3926
June	1st to 10th	Night	15% 6% 2 6% 4 4 4 4 5% 7% 41%	1600
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 5% 5% 9% 41%	1600
	21st to end	Night	10% 5% 2 8% 4 5% 4 3 7% 10% 41%	1600
July	1st to 10th	Night	8% 6% 2 8% 6% 6% 6% 5% 6% 9% 38%	3600
	11th to 20th	Night	14% 9% 3 7% 4 5% 5% 4 7% 10% 32%	3600
	21st to end	Night	12% 6% 2 6% 4 4 4 4 6% 8% 44%	3926
August	1st to 10th	Night	23% 6% 2 6% 4 4 4 4 7% 7% 32%	2896
	11th to 20th	Night	13% 6% 3 7% 4 4 5% 4 6% 8% 40%	3197
	21st to end	Night	10% 5% 2 5% 4 4 4 3 5% 7% 51%	3586
September	1st to 10th	Night	14% 6% 3 5% 5% 4 5% 3 4 6% 45%	1931
	11th to 20th	Night	13% 6% 2 4 3 3 4 3 4 8% 51%	1957
	21st to end	Night	19% 7% 2 6% 4 3 3 4 5% 6% 41%	2090
October	1st to 10th	Night	18% 7% 2 5% 3 3 3 4 4 5% 44%	2232
	11th to 20th	Night	14% 5% 1 4 4 3 4 3 5% 6% 51%	2280
	21st to end	Night	18% 7% 3 5% 3 3 3 3 5% 5% 46%	2615
November	1st to 10th	Night	11% 5% 2 5% 4 3 3 3 5% 5% 54%	2537
	11th to 20th	Night	18% 6% 2 6% 3 3 3 3 4 5% 48%	2600
	21st to end	Night	14% 6% 2 5% 4 4 4 3 6% 7% 45%	2695
December	1st to 10th	Night	14% 8% 2 5% 3 3 3 3 5% 6% 47%	4580
	11th to 20th	Night	23% 7% 2 5% 3 3 3 3 4 5% 42%	5208
	21st to end	Night	20% 8% 3 6% 4 3 3 3 4 5% 41%	6996

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	35% 4 8% 28% 22% 3	3598
	11th to 20th	Day	44% 4 8% 22% 16% 6%	3600
	21st to end	Day	48% 4 10% 22% 15% 1	4443
February	1st to 10th	Day	45% 5% 13% 24% 12% 2	1800
	11th to 20th	Day	46% 4 8% 23% 16% 3	1942
	21st to end	Day	46% 6% 9% 21% 14% 4	1650
March	1st to 10th	Day	42% 4 9% 22% 16% 8%	4479
	11th to 20th	Day	38% 5% 12% 27% 13% 5%	4879
	21st to end	Day	43% 4 9% 15% 24% 6%	5868
April	1st to 10th	Day	45% 6% 10% 19% 15% 5%	2400
	11th to 20th	Day	40% 6% 13% 20% 17% 3	2374
	21st to end	Day	41% 7% 10% 19% 19% 4	2400
May	1st to 10th	Day	53% 6% 13% 16% 10% 1	5858
	11th to 20th	Day	51% 5% 9% 21% 13% 1	6322
	21st to end	Day	46% 6% 10% 23% 15% 2	6949
June	1st to 10th	Day	40% 8% 13% 22% 15% 1	2800
	11th to 20th	Day	42% 9% 13% 21% 12% 2	2800
	21st to end	Day	39% 8% 13% 22% 16% 1	2800
July	1st to 10th	Day	43% 7% 14% 20% 15% 2	6290
	11th to 20th	Day	46% 5% 18% 19% 11% 1	6285
	21st to end	Day	41% 4 11% 20% 22% 1	6728
August	1st to 10th	Day	48% 7% 13% 22% 10% 1	4190
	11th to 20th	Day	41% 4 13% 27% 14% 1	3886
	21st to end	Day	34% 5% 10% 27% 23% 2	4312
September	1st to 10th	Day	39% 7% 11% 21% 20% 3	2316
	11th to 20th	Day	34% 5% 11% 23% 24% 3	2223
	21st to end	Day	46% 6% 8% 17% 19% 3	2090
October	1st to 10th	Day	39% 4 10% 17% 25% 5%	3232
	11th to 20th	Day	25% 3 10% 25% 31% 6%	3140
	21st to end	Day	27% 3 13% 25% 27% 5%	3245
November	1st to 10th	Day	34% 4 14% 25% 22% 3	1761
	11th to 20th	Day	37% 5% 9% 24% 22% 3	1800
	21st to end	Day	37% 5% 14% 29% 12% 3	1700
December	1st to 10th	Day	40% 3 14% 27% 13% 2	2600
	11th to 20th	Day	44% 3 11% 27% 12% 2	2986
	21st to end	Day	43% 6% 11% 22% 16% 2	3960
January	1st to 10th	Night	35% 4 8% 26% 22% 6%	6349
	11th to 20th	Night	40% 3 12% 23% 16% 7%	6360
	21st to end	Night	44% 4 9% 21% 18% 4	6508
February	1st to 10th	Night	44% 5% 13% 24% 12% 2	2600
	11th to 20th	Night	44% 4 9% 19% 19% 4	2354
	21st to end	Night	43% 5% 7% 23% 18% 4	1980
March	1st to 10th	Night	43% 4 9% 22% 13% 10%	5374
	11th to 20th	Night	38% 5% 14% 24% 15% 4	5077
	21st to end	Night	45% 4 6% 16% 23% 6%	5064
April	1st to 10th	Night	44% 7% 10% 19% 17% 4	2000
	11th to 20th	Night	44% 6% 10% 18% 20% 3	1975
	21st to end	Night	38% 6% 10% 21% 20% 5%	2000
May	1st to 10th	Night	55% 5% 12% 16% 10% 2	4112
	11th to 20th	Night	52% 6% 12% 16% 13% 2	3589
	21st to end	Night	47% 6% 11% 22% 12% 2	3927
June	1st to 10th	Night	40% 7% 11% 20% 20% 2	1600
	11th to 20th	Night	41% 8% 13% 25% 12% 1	1600
	21st to end	Night	36% 8% 16% 24% 14% 2	1600
July	1st to 10th	Night	43% 7% 16% 19% 13% 2	3560
	11th to 20th	Night	48% 5% 16% 20% 10% 1	3552
	21st to end	Night	38% 5% 14% 22% 20% 1	3809
August	1st to 10th	Night	49% 6% 18% 17% 9% 1	2886
	11th to 20th	Night	41% 6% 11% 29% 12% 1	3227
	21st to end	Night	33% 5% 9% 31% 22% 1	3586
September	1st to 10th	Night	40% 7% 10% 21% 20% 2	1931
	11th to 20th	Night	32% 6% 10% 22% 25% 4	1957
	21st to end	Night	42% 6% 8% 21% 21% 2	2090
October	1st to 10th	Night	40% 3 10% 23% 22% 2	3726
	11th to 20th	Night	25% 4 9% 29% 29% 5%	3779
	21st to end	Night	29% 3 11% 31% 24% 3	4301
November	1st to 10th	Night	33% 4 12% 24% 23% 4	2537
	11th to 20th	Night	40% 4 9% 23% 20% 5%	2600
	21st to end	Night	38% 5% 13% 26% 16% 2	2695
December	1st to 10th	Night	39% 3 12% 31% 13% 2	4580
	11th to 20th	Night	45% 4 9% 24% 14% 4	5208
	21st to end	Night	46% 4 10% 20% 17% 3	6996

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% 24% 26% 17% 3 1	200
	11th to 20th	Day	35% 26% 30% 9% 2	200
	21st to end	Day	23% 27% 33% 13% 4	325
February	1st to 10th	Day	29% 25% 33% 10% 3	300
	11th to 20th	Day	32% 21% 34% 11% 2	481
	21st to end	Day	30% 22% 30% 16% 3	414
March	1st to 10th	Day	29% 21% 30% 17% 3	500
	11th to 20th	Day	25% 26% 31% 14% 3 1	500
	21st to end	Day	27% 24% 31% 13% 5% 1	550
April	1st to 10th	Day	30% 21% 33% 10% 6% 1	500
	11th to 20th	Day	25% 22% 31% 13% 8% 2	492
	21st to end	Day	27% 20% 31% 16% 5% 2	500
May	1st to 10th	Day	31% 18% 28% 14% 7% 2	503
	11th to 20th	Day	24% 16% 31% 20% 8% 1	527
	21st to end	Day	22% 11% 24% 26% 14% 3	573
June	1st to 10th	Day	18% 11% 30% 25% 13% 2	530
	11th to 20th	Day	19% 13% 28% 27% 10% 3	530
	21st to end	Day	16% 10% 31% 29% 11% 2	530
July	1st to 10th	Day	13% 6% 23% 31% 21% 5%	525
	11th to 20th	Day	10% 5% 26% 33% 20% 6%	524
	21st to end	Day	13% 7% 30% 31% 15% 4	576
August	1st to 10th	Day	12% 6% 25% 34% 16% 7%	526
	11th to 20th	Day	15% 5% 20% 34% 21% 6%	519
	21st to end	Day	14% 6% 27% 29% 15% 9%	583
September	1st to 10th	Day	16% 9% 25% 24% 16% 9%	502
	11th to 20th	Day	14% 7% 27% 28% 16% 9%	490
	21st to end	Day	24% 12% 25% 23% 10% 6%	490
October	1st to 10th	Day	26% 8% 25% 23% 13% 6%	490
	11th to 20th	Day	18% 16% 27% 26% 10% 3	490
	21st to end	Day	21% 17% 28% 21% 11% 2	482
November	1st to 10th	Day	18% 13% 31% 23% 12% 3	395
	11th to 20th	Day	25% 16% 34% 18% 7% 1	400
	21st to end	Day	23% 17% 35% 18% 7% 1	300
December	1st to 10th	Day	29% 24% 31% 10% 7%	200
	11th to 20th	Day	39% 19% 25% 12% 6%	200
	21st to end	Day	27% 16% 34% 15% 7% 1	220
January	1st to 10th	Night	20% 20% 38% 19% 3	800
	11th to 20th	Night	24% 18% 33% 23% 3	800
	21st to end	Night	20% 20% 38% 20% 1	880
February	1st to 10th	Night	22% 18% 36% 23% 2	800
	11th to 20th	Night	26% 22% 35% 16% 1	783
	21st to end	Night	24% 18% 38% 17% 2	660
March	1st to 10th	Night	24% 20% 33% 20% 3	800
	11th to 20th	Night	24% 18% 36% 19% 4	800
	21st to end	Night	21% 14% 40% 19% 4	880
April	1st to 10th	Night	23% 19% 37% 18% 4	800
	11th to 20th	Night	21% 15% 36% 23% 5%	791
	21st to end	Night	20% 14% 36% 25% 5%	800
May	1st to 10th	Night	24% 16% 32% 23% 5%	626
	11th to 20th	Night	22% 10% 36% 26% 5%	407
	21st to end	Night	18% 7% 25% 37% 12% 1	433
June	1st to 10th	Night	17% 6% 29% 37% 12% 1	400
	11th to 20th	Night	14% 6% 29% 40% 11%	400
	21st to end	Night	11% 3 28% 45% 13%	400
July	1st to 10th	Night	12% 3 24% 42% 18% 1	390
	11th to 20th	Night	13% 3 21% 41% 21% 1	388
	21st to end	Night	11% 2 25% 44% 17%	438
August	1st to 10th	Night	14% 2 17% 44% 20% 2	578
	11th to 20th	Night	13% 3 19% 38% 24% 3	790
	21st to end	Night	14% 4 19% 41% 20% 1	880
September	1st to 10th	Night	15% 5% 26% 34% 18% 2	773
	11th to 20th	Night	16% 5% 21% 33% 22% 4	760
	21st to end	Night	22% 6% 20% 34% 16% 1	760
October	1st to 10th	Night	22% 8% 25% 27% 16% 3	760
	11th to 20th	Night	16% 11% 30% 32% 11%	760
	21st to end	Night	22% 11% 31% 27% 8%	836
November	1st to 10th	Night	15% 11% 33% 28% 12% 1	780
	11th to 20th	Night	21% 16% 30% 26% 7%	800
	21st to end	Night	18% 17% 34% 24% 7% 1	800
December	1st to 10th	Night	25% 17% 31% 20% 6% 1	800
	11th to 20th	Night	22% 15% 34% 24% 4	800
	21st to end	Night	22% 16% 32% 26% 3	880

Page generated at 2025-09-01 01:51:06. Version ac07

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