Union College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Union College 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	15% 8% 3 7% 5% 5% 5% 4 6% 8% 34%	101626
	Night	16% 8% 3 8% 5% 5% 5% 4 5% 7% 34%	97973
Transparency	Day	46% 8% 13% 21% 11% 1	105120
	Night	48% 8% 12% 21% 11% 1	102555
Seeing	Day	21% 15% 29% 22% 11% 3	14879
	Night	20% 13% 31% 27% 8% 1	23955

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	12% 10% 5% 11% 6% 6% 6% 4 6% 7% 28%	2517
	11th to 20th	Day	15% 11% 5% 12% 6% 5% 6% 4 6% 6% 22%	2520
	21st to end	Day	19% 12% 4 9% 6% 6% 5% 5% 5% 6% 23%	2872
February	1st to 10th	Day	15% 10% 3 7% 8% 6% 6% 5% 7% 6% 26%	1710
	11th to 20th	Day	13% 10% 5% 8% 6% 6% 7% 5% 6% 7% 29%	1811
	21st to end	Day	18% 11% 3 9% 6% 5% 4 4 4 5% 30%	1570
March	1st to 10th	Day	23% 11% 4 8% 4 4 4 3 4 5% 29%	3116
	11th to 20th	Day	12% 9% 3 8% 6% 6% 5% 4 6% 7% 33%	3310
	21st to end	Day	18% 8% 2 7% 7% 5% 4 3 4 6% 35%	4153
April	1st to 10th	Day	17% 9% 3 8% 6% 4 5% 4 5% 6% 34%	2280
	11th to 20th	Day	17% 7% 2 7% 5% 5% 6% 5% 6% 6% 36%	2254
	21st to end	Day	19% 7% 3 7% 4 5% 5% 5% 5% 7% 33%	2361
May	1st to 10th	Day	22% 9% 3 8% 4 4 4 3 5% 7% 30%	4202
	11th to 20th	Day	16% 7% 2 8% 5% 5% 5% 4 6% 7% 34%	4398
	21st to end	Day	10% 5% 2 6% 5% 5% 5% 4 7% 10% 41%	4837
June	1st to 10th	Day	14% 6% 3 7% 6% 5% 6% 5% 7% 8% 34%	2660
	11th to 20th	Day	14% 6% 2 6% 4 5% 6% 5% 8% 10% 34%	2660
	21st to end	Day	14% 7% 3 7% 5% 5% 5% 4 6% 10% 35%	2660
July	1st to 10th	Day	8% 6% 3 8% 5% 6% 6% 6% 7% 10% 36%	4440
	11th to 20th	Day	8% 6% 2 7% 5% 6% 6% 6% 8% 10% 38%	4440
	21st to end	Day	12% 6% 2 7% 5% 6% 6% 5% 7% 8% 35%	4814
August	1st to 10th	Day	15% 6% 2 7% 5% 5% 6% 4 7% 8% 35%	4400
	11th to 20th	Day	12% 6% 3 8% 5% 6% 6% 4 7% 10% 33%	3737
	21st to end	Day	10% 4 2 6% 5% 5% 5% 5% 7% 10% 40%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 4 6% 4 7% 8% 43%	2280
	11th to 20th	Day	12% 6% 2 5% 4 3 4 3 6% 8% 48%	2199
	21st to end	Day	15% 5% 2 6% 4 4 4 5% 6% 8% 41%	2090
October	1st to 10th	Day	18% 8% 2 6% 5% 5% 5% 4 5% 6% 35%	1973
	11th to 20th	Day	14% 7% 3 8% 4 5% 5% 4 6% 7% 39%	1900
	21st to end	Day	22% 7% 3 6% 5% 5% 4 4 5% 7% 32%	1895
November	1st to 10th	Day	13% 7% 2 6% 4 3 4 4 7% 7% 43%	1761
	11th to 20th	Day	14% 7% 3 7% 5% 5% 5% 4 5% 7% 38%	1800
	21st to end	Day	18% 8% 2 6% 6% 5% 5% 5% 7% 7% 33%	1628
December	1st to 10th	Day	18% 12% 4 9% 7% 5% 5% 4 5% 6% 24%	1520
	11th to 20th	Day	21% 10% 3 6% 5% 4 4 3 5% 6% 31%	1906
	21st to end	Day	24% 14% 5% 10% 5% 5% 5% 3 3 5% 21%	2772
January	1st to 10th	Night	15% 11% 4 9% 6% 5% 4 4 5% 6% 31%	4429
	11th to 20th	Night	13% 12% 5% 11% 6% 5% 5% 4 5% 6% 27%	4440
	21st to end	Night	22% 12% 4 10% 6% 5% 5% 3 5% 5% 24%	4781
February	1st to 10th	Night	14% 12% 3 11% 7% 6% 5% 4 6% 7% 27%	2470
	11th to 20th	Night	11% 10% 3 10% 6% 6% 6% 4 6% 6% 31%	2253
	21st to end	Night	16% 10% 3 9% 6% 4 4 4 5% 6% 31%	1884
March	1st to 10th	Night	23% 11% 3 8% 4 5% 5% 4 6% 5% 27%	3724
	11th to 20th	Night	16% 10% 3 7% 5% 5% 5% 4 5% 6% 34%	3645
	21st to end	Night	15% 10% 3 10% 6% 5% 5% 3 5% 4 34%	3478
April	1st to 10th	Night	19% 8% 3 7% 5% 5% 4 5% 5% 6% 33%	1900
	11th to 20th	Night	19% 8% 3 7% 5% 4 5% 3 5% 6% 36%	1875
	21st to end	Night	20% 5% 2 7% 4 4 5% 3 5% 6% 39%	1819
May	1st to 10th	Night	21% 8% 3 8% 5% 4 4 3 5% 7% 31%	2752
	11th to 20th	Night	16% 9% 3 8% 5% 6% 5% 5% 5% 8% 30%	2495
	21st to end	Night	11% 5% 3 8% 5% 5% 5% 5% 7% 9% 38%	2738
June	1st to 10th	Night	16% 6% 2 6% 4 5% 4 5% 6% 8% 39%	1520
	11th to 20th	Night	14% 5% 2 5% 4 4 4 5% 7% 8% 41%	1520
	21st to end	Night	12% 5% 2 6% 5% 5% 6% 4 6% 8% 43%	1520
July	1st to 10th	Night	10% 5% 2 8% 6% 5% 6% 5% 6% 7% 39%	2520
	11th to 20th	Night	12% 6% 3 6% 4 5% 5% 4 7% 8% 41%	2520
	21st to end	Night	12% 7% 2 7% 5% 5% 5% 4 6% 8% 40%	2739
August	1st to 10th	Night	18% 7% 2 6% 4 4 5% 5% 6% 7% 36%	2526
	11th to 20th	Night	15% 7% 2 7% 4 5% 5% 4 6% 7% 38%	3058
	21st to end	Night	12% 6% 2 5% 4 4 4 4 6% 9% 45%	3476
September	1st to 10th	Night	12% 6% 3 6% 4 4 4 4 5% 8% 45%	1900
	11th to 20th	Night	14% 4 2 4 4 3 3 3 4 8% 51%	1981
	21st to end	Night	14% 6% 2 6% 4 4 4 4 5% 7% 44%	2090
October	1st to 10th	Night	19% 7% 3 8% 4 4 5% 4 5% 6% 37%	2204
	11th to 20th	Night	15% 5% 2 7% 5% 4 5% 4 5% 8% 40%	2280
	21st to end	Night	22% 7% 2 7% 4 4 4 4 5% 7% 33%	2703
November	1st to 10th	Night	13% 8% 2 7% 4 4 3 3 5% 6% 43%	2537
	11th to 20th	Night	16% 7% 2 6% 6% 4 6% 3 5% 7% 38%	2600
	21st to end	Night	15% 9% 2 7% 5% 5% 5% 3 6% 7% 36%	2764
December	1st to 10th	Night	18% 13% 3 8% 5% 5% 4 5% 5% 7% 29%	2660
	11th to 20th	Night	21% 8% 3 7% 5% 4 5% 4 6% 7% 29%	3288
	21st to end	Night	26% 12% 5% 8% 4 4 4 3 5% 5% 24%	4884

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	52% 8% 13% 20% 6% 1	2518
	11th to 20th	Day	59% 9% 12% 14% 5% 1	2520
	21st to end	Day	60% 8% 13% 13% 5% 1	2872
February	1st to 10th	Day	56% 7% 13% 15% 8% 1	1710
	11th to 20th	Day	54% 7% 11% 18% 9% 2	1820
	21st to end	Day	55% 7% 11% 16% 11% 1	1570
March	1st to 10th	Day	57% 6% 13% 15% 8% 1	3119
	11th to 20th	Day	48% 8% 17% 16% 9% 2	3310
	21st to end	Day	50% 6% 7% 17% 17% 3	4153
April	1st to 10th	Day	51% 6% 9% 21% 12% 2	2280
	11th to 20th	Day	47% 7% 10% 20% 14% 2	2254
	21st to end	Day	49% 7% 10% 17% 14% 3	2361
May	1st to 10th	Day	53% 7% 9% 18% 11% 1	4216
	11th to 20th	Day	47% 10% 13% 20% 9% 1	4402
	21st to end	Day	36% 10% 13% 26% 14% 1	4837
June	1st to 10th	Day	46% 9% 14% 19% 11% 1	2660
	11th to 20th	Day	42% 10% 14% 19% 13% 1	2660
	21st to end	Day	44% 9% 14% 21% 12% 1	2660
July	1st to 10th	Day	41% 9% 13% 23% 14% 1	4370
	11th to 20th	Day	38% 10% 15% 26% 11% 1	4365
	21st to end	Day	43% 8% 15% 22% 12%	4661
August	1st to 10th	Day	45% 9% 15% 21% 10% 1	4386
	11th to 20th	Day	44% 8% 16% 25% 6%	3773
	21st to end	Day	36% 9% 15% 28% 11% 1	4180
September	1st to 10th	Day	36% 8% 13% 27% 14% 1	2280
	11th to 20th	Day	35% 7% 11% 26% 19% 1	2199
	21st to end	Day	39% 9% 11% 25% 13% 2	2090
October	1st to 10th	Day	43% 7% 8% 24% 16% 1	3272
	11th to 20th	Day	37% 6% 12% 29% 16% 1	3140
	21st to end	Day	36% 7% 14% 28% 13% 2	3095
November	1st to 10th	Day	39% 8% 15% 25% 13% 1	1761
	11th to 20th	Day	46% 7% 11% 20% 14% 2	1800
	21st to end	Day	50% 7% 17% 18% 7% 1	1628
December	1st to 10th	Day	60% 7% 12% 14% 7%	1520
	11th to 20th	Day	55% 6% 15% 16% 7% 1	1906
	21st to end	Day	66% 5% 10% 13% 6%	2772
January	1st to 10th	Night	52% 10% 12% 16% 9% 1	4429
	11th to 20th	Night	56% 9% 10% 16% 8% 1	4440
	21st to end	Night	61% 7% 11% 14% 6% 1	4781
February	1st to 10th	Night	55% 8% 11% 17% 7% 1	2470
	11th to 20th	Night	51% 8% 12% 18% 9% 1	2255
	21st to end	Night	52% 8% 13% 17% 8% 2	1884
March	1st to 10th	Night	57% 7% 9% 17% 9% 1	3734
	11th to 20th	Night	50% 8% 11% 22% 7% 1	3646
	21st to end	Night	52% 6% 7% 15% 17% 3	3478
April	1st to 10th	Night	50% 6% 11% 20% 11% 2	1900
	11th to 20th	Night	49% 6% 9% 21% 14% 1	1875
	21st to end	Night	46% 7% 9% 18% 16% 3	1819
May	1st to 10th	Night	53% 8% 11% 15% 12% 2	2758
	11th to 20th	Night	50% 10% 12% 18% 8% 2	2496
	21st to end	Night	40% 9% 15% 19% 15% 2	2739
June	1st to 10th	Night	42% 8% 12% 23% 13% 1	1520
	11th to 20th	Night	39% 8% 16% 23% 13% 1	1520
	21st to end	Night	38% 9% 13% 25% 14% 1	1520
July	1st to 10th	Night	41% 9% 13% 21% 16% 1	2480
	11th to 20th	Night	40% 8% 13% 23% 15%	2472
	21st to end	Night	41% 7% 11% 29% 11% 1	2651
August	1st to 10th	Night	46% 9% 14% 23% 8%	2516
	11th to 20th	Night	44% 8% 17% 21% 10%	3088
	21st to end	Night	35% 7% 14% 32% 11% 1	3476
September	1st to 10th	Night	38% 8% 11% 27% 16% 1	1900
	11th to 20th	Night	33% 6% 12% 27% 21% 1	1981
	21st to end	Night	39% 8% 10% 24% 17% 1	2090
October	1st to 10th	Night	47% 6% 11% 22% 14% 1	3672
	11th to 20th	Night	33% 6% 12% 28% 21%	3779
	21st to end	Night	41% 7% 13% 29% 10% 1	4453
November	1st to 10th	Night	42% 8% 10% 22% 16% 2	2537
	11th to 20th	Night	46% 8% 10% 21% 13% 2	2600
	21st to end	Night	46% 8% 16% 21% 8% 1	2764
December	1st to 10th	Night	55% 8% 12% 18% 7%	2660
	11th to 20th	Night	53% 9% 15% 15% 9% 1	3288
	21st to end	Night	62% 7% 11% 15% 4	4884

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	23% 27% 24% 20% 5% 1	180
	11th to 20th	Day	30% 23% 28% 16% 3 1	180
	21st to end	Day	21% 24% 32% 16% 7%	227
February	1st to 10th	Day	30% 27% 33% 10% 1	270
	11th to 20th	Day	25% 26% 35% 12% 2	419
	21st to end	Day	28% 20% 30% 20% 3	382
March	1st to 10th	Day	27% 19% 32% 18% 4	460
	11th to 20th	Day	27% 22% 30% 17% 4	460
	21st to end	Day	31% 21% 26% 15% 6% 2	506
April	1st to 10th	Day	28% 22% 25% 17% 7% 1	460
	11th to 20th	Day	25% 24% 25% 17% 8% 2	452
	21st to end	Day	28% 17% 31% 17% 5% 2	460
May	1st to 10th	Day	28% 15% 30% 16% 9% 2	463
	11th to 20th	Day	18% 13% 35% 24% 8% 2	487
	21st to end	Day	16% 9% 31% 26% 13% 3	529
June	1st to 10th	Day	16% 16% 29% 25% 12% 2	490
	11th to 20th	Day	15% 14% 27% 26% 17% 2	490
	21st to end	Day	17% 12% 33% 22% 13% 2	490
July	1st to 10th	Day	12% 8% 28% 28% 19% 5%	485
	11th to 20th	Day	11% 8% 27% 32% 18% 3	484
	21st to end	Day	13% 6% 31% 30% 16% 4	532
August	1st to 10th	Day	12% 8% 25% 31% 17% 7%	486
	11th to 20th	Day	13% 6% 27% 29% 17% 7%	479
	21st to end	Day	12% 10% 26% 29% 17% 6%	539
September	1st to 10th	Day	15% 8% 24% 24% 18% 10%	490
	11th to 20th	Day	15% 8% 24% 21% 18% 14%	490
	21st to end	Day	19% 10% 23% 26% 14% 8%	490
October	1st to 10th	Day	24% 10% 27% 20% 13% 6%	490
	11th to 20th	Day	16% 18% 28% 23% 12% 2	490
	21st to end	Day	28% 14% 28% 23% 7% 1	436
November	1st to 10th	Day	23% 15% 28% 18% 14% 2	395
	11th to 20th	Day	24% 21% 28% 21% 6% 1	400
	21st to end	Day	26% 22% 28% 21% 4	230
December	1st to 10th	Day	27% 18% 34% 15% 6%	180
	11th to 20th	Day	27% 22% 34% 15% 3	180
	21st to end	Day	24% 25% 30% 18% 2 1	198
January	1st to 10th	Night	21% 16% 38% 22% 3	760
	11th to 20th	Night	24% 20% 31% 22% 3	760
	21st to end	Night	19% 17% 38% 23% 3	836
February	1st to 10th	Night	23% 16% 40% 19% 2	760
	11th to 20th	Night	18% 20% 41% 19% 1	743
	21st to end	Night	23% 20% 34% 19% 3	628
March	1st to 10th	Night	23% 19% 36% 19% 2	760
	11th to 20th	Night	24% 18% 34% 20% 3	760
	21st to end	Night	22% 20% 31% 21% 5%	836
April	1st to 10th	Night	24% 18% 33% 23% 3	760
	11th to 20th	Night	24% 16% 34% 20% 6%	751
	21st to end	Night	22% 17% 32% 24% 5%	679
May	1st to 10th	Night	24% 15% 30% 23% 7% 1	527
	11th to 20th	Night	19% 10% 36% 28% 7% 1	374
	21st to end	Night	12% 8% 30% 39% 10% 1	411
June	1st to 10th	Night	17% 6% 32% 34% 10% 1	380
	11th to 20th	Night	16% 6% 27% 39% 11%	380
	21st to end	Night	13% 6% 29% 39% 12%	380
July	1st to 10th	Night	12% 5% 25% 42% 14% 1	370
	11th to 20th	Night	14% 4 22% 43% 16% 1	368
	21st to end	Night	12% 4 23% 48% 13% 1	416
August	1st to 10th	Night	13% 3 23% 44% 17% 1	385
	11th to 20th	Night	13% 4 24% 37% 19% 3	723
	21st to end	Night	16% 4 23% 36% 19% 2	836
September	1st to 10th	Night	13% 5% 24% 38% 18% 1	760
	11th to 20th	Night	16% 5% 23% 32% 21% 4	760
	21st to end	Night	17% 6% 26% 34% 16% 2	760
October	1st to 10th	Night	21% 9% 26% 29% 13% 2	760
	11th to 20th	Night	16% 11% 29% 34% 9%	760
	21st to end	Night	27% 10% 33% 25% 5%	836
November	1st to 10th	Night	18% 13% 33% 27% 8%	780
	11th to 20th	Night	21% 14% 34% 24% 7% 1	800
	21st to end	Night	20% 13% 35% 26% 5%	800
December	1st to 10th	Night	25% 17% 37% 19% 3	760
	11th to 20th	Night	24% 18% 31% 23% 4	760
	21st to end	Night	30% 15% 34% 19% 2	836

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