Princeton Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Princeton Observatory clear sky chart from 2005-11-06 to 2024-11-30.

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	13% 6% 2 6% 5% 4 5% 4 6% 8% 42%	100670
	Night	14% 7% 2 6% 4 4 4 4 6% 7% 43%	98818
Transparency	Day	39% 5% 12% 21% 18% 5%	104316
	Night	40% 5% 10% 20% 19% 6%	103311
Seeing	Day	15% 8% 23% 25% 20% 9%	15476
	Night	16% 8% 25% 33% 18% 2	25804

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	16% 5% 2 5% 4 3 3 3 3 4 51%	2836
	11th to 20th	Day	12% 7% 2 5% 4 3 4 2 4 4 52%	2840
	21st to end	Day	16% 9% 3 6% 4 4 4 3 4 4 43%	3124
February	1st to 10th	Day	15% 6% 2 6% 3 4 4 4 4 7% 45%	1710
	11th to 20th	Day	20% 6% 2 6% 4 3 3 3 4 5% 46%	1727
	21st to end	Day	17% 7% 3 6% 4 4 4 3 4 6% 42%	1665
March	1st to 10th	Day	24% 8% 2 5% 3 4 4 4 5% 6% 35%	3435
	11th to 20th	Day	10% 4 2 5% 4 3 3 3 5% 6% 54%	3480
	21st to end	Day	17% 6% 2 6% 4 3 4 3 5% 6% 44%	3815
April	1st to 10th	Day	10% 4 2 4 4 3 3 3 5% 8% 53%	2242
	11th to 20th	Day	9% 5% 2 6% 3 4 4 4 6% 7% 50%	2254
	21st to end	Day	13% 5% 2 5% 4 4 4 4 5% 7% 49%	2280
May	1st to 10th	Day	9% 7% 3 8% 5% 6% 5% 5% 7% 9% 36%	3915
	11th to 20th	Day	16% 7% 3 7% 5% 5% 5% 4 6% 8% 33%	4081
	21st to end	Day	7% 4 2 5% 5% 5% 5% 4 7% 9% 47%	4476
June	1st to 10th	Day	9% 6% 2 7% 5% 5% 6% 5% 7% 8% 39%	2470
	11th to 20th	Day	9% 5% 1 5% 5% 5% 5% 5% 7% 10% 42%	2470
	21st to end	Day	8% 5% 2 6% 6% 4 7% 5% 9% 9% 38%	2470
July	1st to 10th	Day	6% 5% 2 7% 6% 6% 7% 5% 9% 13% 35%	4120
	11th to 20th	Day	10% 7% 3 8% 6% 6% 7% 6% 7% 11% 28%	4120
	21st to end	Day	10% 8% 3 9% 6% 6% 6% 5% 8% 10% 28%	4466
August	1st to 10th	Day	19% 9% 3 10% 6% 6% 6% 5% 7% 9% 21%	4104
	11th to 20th	Day	9% 7% 3 9% 6% 6% 6% 5% 7% 10% 33%	3826
	21st to end	Day	7% 5% 2 5% 4 4 5% 4 6% 11% 46%	4180
September	1st to 10th	Day	9% 6% 2 6% 4 5% 5% 4 7% 9% 43%	2280
	11th to 20th	Day	15% 6% 2 6% 4 5% 4 3 5% 7% 41%	2218
	21st to end	Day	18% 8% 3 8% 5% 5% 4 3 5% 5% 37%	2012
October	1st to 10th	Day	15% 7% 1 5% 3 3 4 3 5% 5% 48%	1900
	11th to 20th	Day	12% 5% 2 5% 3 3 4 2 5% 5% 54%	1900
	21st to end	Day	12% 4 2 6% 4 3 4 3 3 5% 55%	2090
November	1st to 10th	Day	14% 6% 2 5% 3 3 3 3 5% 5% 52%	1957
	11th to 20th	Day	13% 6% 2 5% 4 3 3 3 5% 7% 49%	1990
	21st to end	Day	14% 5% 2 4 4 4 3 2 5% 6% 52%	1663
December	1st to 10th	Day	17% 6% 3 5% 4 4 4 3 5% 7% 40%	1520
	11th to 20th	Day	16% 5% 2 4 3 3 2 2 3 5% 55%	1910
	21st to end	Day	15% 7% 3 7% 3 3 4 3 5% 8% 42%	3124
January	1st to 10th	Night	14% 7% 2 5% 4 4 3 3 5% 6% 47%	4109
	11th to 20th	Night	14% 8% 3 7% 5% 4 4 3 5% 5% 43%	4120
	21st to end	Night	16% 7% 3 6% 4 4 3 3 5% 5% 44%	4532
February	1st to 10th	Night	16% 6% 2 5% 4 3 5% 3 5% 5% 44%	2470
	11th to 20th	Night	18% 7% 2 6% 3 4 4 3 5% 6% 41%	2337
	21st to end	Night	16% 6% 2 4 3 4 3 4 5% 6% 47%	1784
March	1st to 10th	Night	25% 10% 3 7% 5% 4 3 3 4 4 33%	3418
	11th to 20th	Night	13% 6% 1 5% 4 3 3 3 5% 6% 51%	3480
	21st to end	Night	16% 5% 2 4 3 3 3 3 5% 6% 49%	3817
April	1st to 10th	Night	11% 7% 2 6% 4 4 3 4 5% 7% 48%	1936
	11th to 20th	Night	11% 5% 2 5% 4 4 4 3 6% 7% 49%	1875
	21st to end	Night	10% 5% 1 5% 4 3 5% 4 6% 7% 49%	1900
May	1st to 10th	Night	9% 8% 2 9% 5% 5% 4 4 6% 8% 39%	3008
	11th to 20th	Night	17% 8% 3 8% 5% 5% 4 3 5% 6% 37%	2812
	21st to end	Night	8% 4 2 6% 5% 4 5% 4 6% 10% 45%	3079
June	1st to 10th	Night	10% 6% 2 5% 6% 5% 5% 5% 8% 11% 39%	1710
	11th to 20th	Night	8% 5% 2 7% 7% 4 5% 5% 7% 8% 40%	1646
	21st to end	Night	7% 5% 2 7% 6% 5% 5% 6% 9% 11% 38%	1710
July	1st to 10th	Night	8% 6% 2 8% 5% 5% 5% 5% 8% 12% 34%	2840
	11th to 20th	Night	12% 8% 3 7% 5% 5% 5% 5% 9% 12% 29%	2840
	21st to end	Night	16% 10% 3 10% 6% 5% 5% 5% 7% 7% 28%	3087
August	1st to 10th	Night	19% 9% 4 9% 5% 5% 5% 5% 6% 9% 24%	2825
	11th to 20th	Night	9% 6% 3 9% 6% 6% 7% 5% 8% 8% 33%	2969
	21st to end	Night	6% 4 2 6% 4 4 6% 5% 7% 10% 45%	3476
September	1st to 10th	Night	11% 5% 3 5% 4 5% 5% 4 7% 8% 44%	1900
	11th to 20th	Night	14% 6% 2 6% 5% 4 5% 4 6% 7% 41%	1957
	21st to end	Night	19% 7% 3 7% 5% 4 5% 2 6% 6% 36%	2166
October	1st to 10th	Night	13% 6% 2 5% 3 3 3 3 5% 7% 50%	2280
	11th to 20th	Night	12% 6% 2 4 4 3 2 3 4 5% 55%	2280
	21st to end	Night	11% 5% 1 5% 3 3 2 3 4 6% 57%	2508
November	1st to 10th	Night	13% 5% 2 6% 4 3 3 3 4 5% 52%	2341
	11th to 20th	Night	14% 6% 2 5% 4 3 3 3 4 6% 49%	2405
	21st to end	Night	12% 5% 2 5% 4 3 3 3 5% 7% 51%	2728
December	1st to 10th	Night	17% 8% 2 6% 4 4 4 3 5% 6% 40%	2660
	11th to 20th	Night	17% 6% 2 5% 3 3 3 3 5% 5% 48%	3281
	21st to end	Night	15% 7% 3 5% 3 3 4 3 5% 6% 46%	4532

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	39% 2 5% 24% 24% 6%	2837
	11th to 20th	Day	38% 3 4 17% 31% 7%	2840
	21st to end	Day	45% 4 5% 17% 21% 9%	3124
February	1st to 10th	Day	40% 3 10% 20% 21% 6%	1710
	11th to 20th	Day	42% 3 6% 19% 24% 6%	1736
	21st to end	Day	45% 3 8% 18% 19% 6%	1665
March	1st to 10th	Day	50% 3 8% 17% 14% 8%	3435
	11th to 20th	Day	32% 3 8% 18% 26% 13%	3480
	21st to end	Day	43% 3 7% 23% 19% 5%	3815
April	1st to 10th	Day	31% 3 10% 24% 25% 7%	2242
	11th to 20th	Day	33% 4 11% 23% 22% 7%	2254
	21st to end	Day	35% 5% 12% 24% 19% 4	2280
May	1st to 10th	Day	44% 4 18% 20% 12% 2	3950
	11th to 20th	Day	48% 5% 11% 24% 12% 1	4085
	21st to end	Day	32% 6% 15% 25% 19% 3	4489
June	1st to 10th	Day	40% 6% 14% 23% 16% 1	2470
	11th to 20th	Day	35% 6% 16% 26% 15% 2	2470
	21st to end	Day	36% 9% 17% 22% 13% 2	2470
July	1st to 10th	Day	37% 10% 19% 21% 11% 2	4055
	11th to 20th	Day	47% 9% 23% 15% 5% 1	4051
	21st to end	Day	48% 9% 17% 16% 11%	4324
August	1st to 10th	Day	58% 7% 16% 13% 5%	4091
	11th to 20th	Day	43% 8% 14% 24% 10% 1	3865
	21st to end	Day	32% 5% 15% 32% 15% 2	4180
September	1st to 10th	Day	36% 7% 16% 18% 18% 4	2280
	11th to 20th	Day	43% 4 10% 20% 20% 3	2218
	21st to end	Day	48% 4 9% 19% 14% 5%	2012
October	1st to 10th	Day	34% 3 10% 23% 24% 6%	3160
	11th to 20th	Day	22% 2 5% 20% 26% 25%	3140
	21st to end	Day	27% 2 5% 24% 27% 16%	3428
November	1st to 10th	Day	34% 3 6% 24% 26% 7%	1957
	11th to 20th	Day	36% 3 8% 21% 26% 7%	1990
	21st to end	Day	34% 3 7% 24% 25% 7%	1663
December	1st to 10th	Day	43% 4 11% 21% 17% 4	1520
	11th to 20th	Day	35% 1 7% 25% 23% 9%	1906
	21st to end	Day	42% 2 8% 23% 22% 2	3124
January	1st to 10th	Night	38% 3 6% 21% 27% 5%	4109
	11th to 20th	Night	44% 3 7% 16% 24% 6%	4120
	21st to end	Night	43% 3 6% 18% 23% 7%	4532
February	1st to 10th	Night	41% 3 8% 21% 21% 6%	2470
	11th to 20th	Night	45% 3 8% 16% 22% 7%	2339
	21st to end	Night	39% 3 9% 20% 21% 8%	1784
March	1st to 10th	Night	57% 2 7% 14% 15% 6%	3418
	11th to 20th	Night	36% 3 8% 12% 25% 17%	3480
	21st to end	Night	37% 3 9% 24% 21% 6%	3817
April	1st to 10th	Night	37% 4 9% 17% 24% 9%	1936
	11th to 20th	Night	35% 4 10% 23% 21% 7%	1875
	21st to end	Night	32% 5% 12% 28% 18% 5%	1900
May	1st to 10th	Night	43% 4 16% 23% 12% 2	3038
	11th to 20th	Night	49% 4 10% 21% 16% 1	2813
	21st to end	Night	32% 7% 17% 24% 18% 2	3087
June	1st to 10th	Night	38% 8% 18% 22% 13% 2	1710
	11th to 20th	Night	39% 7% 16% 22% 14% 1	1646
	21st to end	Night	37% 10% 16% 22% 12% 2	1710
July	1st to 10th	Night	40% 13% 19% 17% 10% 1	2795
	11th to 20th	Night	45% 12% 24% 13% 5% 1	2786
	21st to end	Night	54% 8% 13% 15% 10%	2988
August	1st to 10th	Night	56% 7% 18% 15% 4	2814
	11th to 20th	Night	44% 9% 18% 21% 7% 1	2996
	21st to end	Night	32% 6% 14% 31% 16% 2	3476
September	1st to 10th	Night	36% 9% 14% 20% 16% 5%	1900
	11th to 20th	Night	41% 7% 11% 20% 19% 3	1957
	21st to end	Night	49% 5% 11% 17% 16% 3	2166
October	1st to 10th	Night	38% 3 7% 21% 26% 5%	3800
	11th to 20th	Night	24% 2 6% 17% 26% 25%	3779
	21st to end	Night	22% 2 7% 28% 24% 16%	4120
November	1st to 10th	Night	35% 3 6% 20% 28% 8%	2341
	11th to 20th	Night	37% 3 8% 16% 25% 11%	2405
	21st to end	Night	33% 3 7% 23% 26% 8%	2728
December	1st to 10th	Night	45% 4 10% 19% 18% 5%	2660
	11th to 20th	Night	38% 3 7% 20% 24% 8%	3284
	21st to end	Night	40% 3 6% 24% 25% 3	4532

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	18% 18% 40% 19% 5%	270
	11th to 20th	Day	20% 22% 36% 16% 7%	270
	21st to end	Day	21% 18% 39% 17% 5%	297
February	1st to 10th	Day	19% 14% 41% 21% 4 1	270
	11th to 20th	Day	31% 13% 36% 16% 3	334
	21st to end	Day	24% 17% 32% 21% 6% 1	382
March	1st to 10th	Day	19% 15% 35% 24% 6% 1	460
	11th to 20th	Day	18% 17% 37% 20% 7% 1	460
	21st to end	Day	20% 14% 34% 22% 9% 1	506
April	1st to 10th	Day	16% 17% 31% 23% 12% 2	460
	11th to 20th	Day	12% 13% 29% 26% 16% 4	452
	21st to end	Day	17% 9% 27% 26% 19% 3	460
May	1st to 10th	Day	13% 11% 32% 23% 14% 7%	463
	11th to 20th	Day	15% 6% 21% 28% 22% 9%	487
	21st to end	Day	18% 4 13% 27% 24% 14%	529
June	1st to 10th	Day	11% 4 17% 30% 27% 12%	490
	11th to 20th	Day	12% 2 15% 25% 27% 19%	490
	21st to end	Day	9% 2 14% 29% 33% 14%	490
July	1st to 10th	Day	9% 1 9% 23% 35% 23%	485
	11th to 20th	Day	10% 3 7% 24% 36% 21%	484
	21st to end	Day	8% 2 10% 31% 35% 14%	532
August	1st to 10th	Day	13% 4 12% 24% 29% 18%	486
	11th to 20th	Day	11% 3 10% 24% 30% 21%	479
	21st to end	Day	10% 2 10% 26% 29% 23%	539
September	1st to 10th	Day	13% 3 14% 23% 26% 21%	490
	11th to 20th	Day	17% 3 14% 26% 26% 14%	490
	21st to end	Day	22% 5% 18% 26% 22% 8%	490
October	1st to 10th	Day	18% 5% 19% 22% 23% 12%	490
	11th to 20th	Day	13% 7% 23% 29% 22% 6%	490
	21st to end	Day	13% 7% 22% 32% 19% 7%	539
November	1st to 10th	Day	18% 6% 25% 31% 15% 5%	495
	11th to 20th	Day	16% 12% 31% 28% 12% 1	495
	21st to end	Day	14% 12% 27% 26% 18% 2	265
December	1st to 10th	Day	22% 13% 33% 22% 9% 1	180
	11th to 20th	Day	18% 10% 35% 22% 12% 3	180
	21st to end	Day	21% 12% 32% 21% 12% 2	297
January	1st to 10th	Night	19% 13% 36% 26% 6%	760
	11th to 20th	Night	19% 14% 31% 30% 7%	760
	21st to end	Night	21% 10% 38% 28% 3	836
February	1st to 10th	Night	19% 10% 37% 29% 5%	760
	11th to 20th	Night	22% 16% 40% 21% 2	743
	21st to end	Night	19% 13% 35% 28% 4	628
March	1st to 10th	Night	17% 12% 35% 30% 5%	760
	11th to 20th	Night	19% 13% 37% 24% 7%	760
	21st to end	Night	17% 10% 38% 30% 5%	836
April	1st to 10th	Night	16% 13% 38% 27% 6%	760
	11th to 20th	Night	13% 12% 33% 30% 10% 1	751
	21st to end	Night	14% 8% 31% 37% 10% 1	760
May	1st to 10th	Night	11% 10% 33% 34% 12%	690
	11th to 20th	Night	15% 6% 21% 40% 17% 1	561
	21st to end	Night	12% 2 14% 40% 30% 2	617
June	1st to 10th	Night	12% 2 19% 41% 25% 2	570
	11th to 20th	Night	10% 3 15% 39% 28% 4	506
	21st to end	Night	9% 3 14% 41% 31% 2	570
July	1st to 10th	Night	9% 3 8% 37% 38% 4	555
	11th to 20th	Night	9% 2 7% 35% 42% 5%	552
	21st to end	Night	8% 3 9% 38% 40% 3	624
August	1st to 10th	Night	14% 3 6% 40% 33% 4	567
	11th to 20th	Night	14% 2 10% 33% 35% 6%	670
	21st to end	Night	12% 2 9% 33% 38% 6%	836
September	1st to 10th	Night	12% 3 11% 33% 36% 6%	760
	11th to 20th	Night	17% 3 14% 33% 31% 3	760
	21st to end	Night	22% 4 18% 30% 24% 2	760
October	1st to 10th	Night	16% 3 16% 35% 27% 5%	760
	11th to 20th	Night	15% 6% 24% 34% 20% 2	760
	21st to end	Night	13% 5% 26% 32% 22% 3	836
November	1st to 10th	Night	15% 7% 24% 35% 17% 2	780
	11th to 20th	Night	17% 11% 26% 32% 13% 1	800
	21st to end	Night	14% 8% 30% 32% 15% 1	800
December	1st to 10th	Night	21% 10% 31% 33% 6%	760
	11th to 20th	Night	19% 8% 29% 35% 8% 1	760
	21st to end	Night	18% 11% 31% 30% 9%	836

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