Vassar
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Vassar clear sky chart from 2005-11-06 to 2026-10-29.

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% 10% 4 8% 5% 5% 5% 4 5% 7% 35%	148742
	Night	14% 11% 4 9% 5% 4 4 3 5% 6% 34%	143462
Transparency	Day	47% 6% 12% 21% 12% 1	152294
	Night	49% 6% 12% 20% 12% 1	147983
Seeing	Day	19% 14% 29% 22% 12% 4	18146
	Night	19% 11% 31% 27% 10% 1	27723

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	13% 18% 7% 16% 6% 5% 4 3 4 4 20%	4691
	11th to 20th	Day	11% 19% 8% 18% 11% 9% 4 3 2 3 11%	4710
	21st to end	Day	16% 16% 6% 12% 8% 5% 6% 3 4 5% 19%	5479
February	1st to 10th	Day	12% 16% 6% 13% 8% 6% 5% 4 4 6% 21%	1890
	11th to 20th	Day	13% 15% 5% 11% 7% 7% 5% 3 4 4 25%	1977
	21st to end	Day	13% 17% 5% 12% 6% 5% 5% 3 4 6% 25%	1730
March	1st to 10th	Day	15% 11% 4 9% 6% 4 4 4 5% 5% 33%	4663
	11th to 20th	Day	13% 14% 4 9% 5% 4 4 3 5% 7% 31%	5090
	21st to end	Day	18% 15% 5% 9% 5% 4 4 3 4 6% 28%	5586
April	1st to 10th	Day	18% 15% 3 7% 5% 4 4 3 5% 6% 30%	2467
	11th to 20th	Day	18% 10% 3 7% 4 5% 4 3 5% 6% 36%	2669
	21st to end	Day	16% 10% 3 7% 5% 5% 4 3 5% 6% 35%	2730
May	1st to 10th	Day	16% 7% 3 7% 5% 5% 5% 4 6% 8% 35%	7616
	11th to 20th	Day	15% 8% 3 7% 4 4 5% 4 5% 8% 36%	7626
	21st to end	Day	14% 7% 2 6% 4 4 4 3 5% 6% 45%	8621
June	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 4 7% 8% 42%	3122
	11th to 20th	Day	12% 6% 2 6% 5% 4 5% 3 6% 9% 41%	3150
	21st to end	Day	10% 6% 2 7% 5% 5% 5% 5% 7% 9% 40%	3093
July	1st to 10th	Day	10% 6% 3 7% 5% 5% 5% 3 6% 9% 41%	8280
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 6% 9% 51%	8280
	21st to end	Day	8% 5% 2 6% 4 4 4 5% 6% 9% 48%	8677
August	1st to 10th	Day	14% 6% 2 6% 5% 5% 5% 5% 6% 7% 40%	6014
	11th to 20th	Day	11% 5% 2 6% 5% 5% 5% 5% 7% 9% 40%	5951
	21st to end	Day	10% 6% 2 7% 5% 6% 6% 5% 7% 9% 38%	6533
September	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 6% 9% 39%	2392
	11th to 20th	Day	12% 6% 2 5% 3 4 4 4 6% 8% 45%	2310
	21st to end	Day	15% 7% 3 6% 5% 6% 4 3 5% 8% 38%	2310
October	1st to 10th	Day	13% 7% 2 6% 4 4 5% 4 5% 7% 43%	2252
	11th to 20th	Day	19% 11% 3 7% 4 5% 5% 3 5% 7% 32%	2100
	21st to end	Day	21% 10% 4 8% 4 5% 4 3 5% 7% 29%	2078
November	1st to 10th	Day	14% 11% 3 7% 6% 4 4 4 5% 8% 34%	1852
	11th to 20th	Day	14% 15% 4 10% 5% 4 4 4 5% 6% 27%	1890
	21st to end	Day	17% 17% 6% 10% 5% 4 4 4 4 6% 23%	1720
December	1st to 10th	Day	15% 18% 7% 11% 6% 6% 5% 4 6% 6% 17%	2663
	11th to 20th	Day	17% 21% 6% 10% 6% 5% 4 3 4 5% 19%	2695
	21st to end	Day	26% 22% 6% 12% 6% 3 4 2 4 4 12%	3835
January	1st to 10th	Night	14% 14% 6% 13% 6% 5% 4 3 4 4 26%	8232
	11th to 20th	Night	11% 19% 7% 17% 9% 8% 5% 3 4 4 14%	8280
	21st to end	Night	12% 16% 7% 12% 6% 5% 6% 4 5% 6% 22%	8703
February	1st to 10th	Night	14% 15% 5% 10% 7% 6% 6% 3 5% 6% 24%	2730
	11th to 20th	Night	14% 15% 5% 10% 6% 4 5% 4 5% 6% 27%	2525
	21st to end	Night	17% 13% 4 10% 6% 5% 5% 3 5% 5% 26%	2076
March	1st to 10th	Night	14% 12% 4 8% 4 4 3 3 4 7% 38%	5410
	11th to 20th	Night	12% 8% 3 8% 5% 4 4 4 6% 9% 38%	5090
	21st to end	Night	17% 11% 4 8% 5% 5% 4 3 4 5% 34%	5588
April	1st to 10th	Night	18% 13% 4 7% 4 3 3 3 6% 6% 33%	2170
	11th to 20th	Night	18% 8% 3 7% 4 4 4 4 6% 6% 37%	1899
	21st to end	Night	18% 9% 3 6% 4 4 4 4 5% 7% 37%	1890
May	1st to 10th	Night	15% 9% 3 8% 5% 4 4 3 5% 7% 37%	5237
	11th to 20th	Night	16% 9% 3 7% 4 4 3 3 4 7% 40%	5234
	21st to end	Night	16% 6% 2 5% 4 4 4 3 5% 6% 47%	5538
June	1st to 10th	Night	13% 4 2 5% 5% 4 4 3 5% 8% 46%	1491
	11th to 20th	Night	12% 6% 2 5% 4 4 4 4 7% 8% 44%	1470
	21st to end	Night	9% 5% 2 6% 4 4 5% 4 5% 9% 47%	1519
July	1st to 10th	Night	9% 6% 2 6% 4 5% 4 4 6% 8% 46%	4708
	11th to 20th	Night	7% 3 2 4 3 3 3 4 6% 10% 54%	4707
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 7% 47%	5400
August	1st to 10th	Night	15% 6% 2 5% 3 3 4 4 5% 7% 44%	4135
	11th to 20th	Night	11% 5% 2 6% 4 4 5% 4 6% 9% 44%	4095
	21st to end	Night	9% 5% 2 6% 4 5% 5% 4 5% 8% 45%	4576
September	1st to 10th	Night	9% 6% 2 7% 4 5% 4 3 5% 8% 47%	2206
	11th to 20th	Night	10% 5% 1 5% 2 3 3 3 5% 8% 53%	2310
	21st to end	Night	15% 7% 2 7% 4 3 4 3 5% 8% 42%	2310
October	1st to 10th	Night	16% 5% 2 5% 4 4 3 2 5% 7% 46%	2367
	11th to 20th	Night	20% 10% 3 8% 4 4 4 3 5% 5% 34%	2520
	21st to end	Night	21% 10% 3 9% 5% 4 3 4 5% 6% 31%	2935
November	1st to 10th	Night	14% 10% 3 6% 4 4 4 3 6% 6% 40%	2666
	11th to 20th	Night	14% 13% 3 8% 5% 5% 6% 6% 6% 7% 27%	2730
	21st to end	Night	19% 17% 4 8% 5% 5% 4 3 4 6% 25%	2897
December	1st to 10th	Night	14% 16% 6% 11% 6% 5% 4 4 5% 6% 22%	4720
	11th to 20th	Night	13% 21% 6% 11% 6% 4 4 3 4 4 23%	5732
	21st to end	Night	23% 22% 6% 11% 5% 4 4 3 4 5% 17%	7366

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	67% 5% 11% 12% 4	4692
	11th to 20th	Day	78% 4 7% 6% 5%	4710
	21st to end	Day	68% 4 10% 10% 7% 1	5479
February	1st to 10th	Day	62% 7% 11% 16% 5%	1890
	11th to 20th	Day	62% 5% 10% 15% 8% 1	1986
	21st to end	Day	61% 6% 11% 15% 6% 1	1730
March	1st to 10th	Day	51% 5% 15% 18% 9% 2	4663
	11th to 20th	Day	51% 5% 15% 19% 9% 1	5090
	21st to end	Day	57% 5% 12% 15% 9% 1	5586
April	1st to 10th	Day	55% 6% 10% 19% 9% 1	2467
	11th to 20th	Day	48% 5% 11% 20% 14% 2	2669
	21st to end	Day	48% 7% 13% 19% 12% 1	2730
May	1st to 10th	Day	44% 6% 13% 21% 14% 2	7651
	11th to 20th	Day	44% 7% 10% 22% 15% 1	7630
	21st to end	Day	39% 5% 8% 21% 22% 4	8635
June	1st to 10th	Day	37% 7% 12% 25% 16% 2	3122
	11th to 20th	Day	39% 6% 13% 23% 17% 2	3150
	21st to end	Day	39% 8% 13% 25% 14% 1	3093
July	1st to 10th	Day	39% 6% 14% 24% 15% 2	8211
	11th to 20th	Day	29% 6% 11% 35% 19% 1	8205
	21st to end	Day	31% 7% 14% 25% 22% 1	8486
August	1st to 10th	Day	41% 7% 13% 19% 18% 1	6001
	11th to 20th	Day	37% 7% 15% 31% 9% 1	5990
	21st to end	Day	40% 7% 18% 25% 10% 1	6532
September	1st to 10th	Day	41% 8% 13% 25% 10% 2	2392
	11th to 20th	Day	34% 7% 14% 28% 15% 1	2310
	21st to end	Day	45% 6% 12% 23% 13% 1	2310
October	1st to 10th	Day	32% 4 10% 33% 17% 2	3603
	11th to 20th	Day	41% 5% 12% 27% 14%	3340
	21st to end	Day	49% 5% 13% 23% 9% 1	3290
November	1st to 10th	Day	47% 8% 13% 21% 10% 1	1852
	11th to 20th	Day	56% 6% 13% 16% 8%	1890
	21st to end	Day	62% 6% 13% 15% 4 1	1720
December	1st to 10th	Day	65% 8% 16% 9% 2	2663
	11th to 20th	Day	68% 5% 10% 12% 4	2691
	21st to end	Day	77% 5% 7% 7% 4	3835
January	1st to 10th	Night	60% 5% 11% 15% 7% 2	8233
	11th to 20th	Night	72% 4 9% 7% 6% 1	8280
	21st to end	Night	62% 5% 11% 15% 7%	8703
February	1st to 10th	Night	60% 7% 10% 16% 6% 1	2730
	11th to 20th	Night	57% 6% 9% 19% 8% 1	2527
	21st to end	Night	59% 5% 11% 17% 8%	2076
March	1st to 10th	Night	47% 4 15% 23% 10% 1	5410
	11th to 20th	Night	42% 6% 18% 22% 11% 1	5090
	21st to end	Night	51% 6% 12% 18% 12% 1	5588
April	1st to 10th	Night	51% 5% 9% 21% 12% 2	2170
	11th to 20th	Night	46% 7% 10% 19% 17% 2	1899
	21st to end	Night	46% 6% 11% 19% 14% 3	1890
May	1st to 10th	Night	45% 5% 11% 19% 16% 3	5264
	11th to 20th	Night	44% 6% 10% 24% 13% 2	5235
	21st to end	Night	38% 5% 8% 24% 20% 5%	5548
June	1st to 10th	Night	36% 6% 10% 25% 22% 2	1491
	11th to 20th	Night	36% 7% 13% 22% 20% 2	1470
	21st to end	Night	33% 7% 13% 27% 18% 2	1519
July	1st to 10th	Night	35% 6% 13% 23% 18% 4	4670
	11th to 20th	Night	25% 5% 13% 36% 19% 1	4662
	21st to end	Night	35% 7% 13% 24% 21% 1	5278
August	1st to 10th	Night	38% 5% 12% 25% 17% 2	4124
	11th to 20th	Night	36% 7% 19% 28% 9% 1	4122
	21st to end	Night	36% 7% 16% 30% 11% 1	4576
September	1st to 10th	Night	36% 7% 13% 28% 16% 1	2206
	11th to 20th	Night	29% 7% 13% 32% 18% 2	2310
	21st to end	Night	41% 6% 11% 27% 14% 1	2310
October	1st to 10th	Night	27% 5% 12% 33% 22% 1	3796
	11th to 20th	Night	46% 5% 9% 24% 16%	4019
	21st to end	Night	44% 5% 11% 28% 11%	4673
November	1st to 10th	Night	43% 6% 13% 24% 12% 1	2666
	11th to 20th	Night	54% 7% 14% 17% 7% 1	2730
	21st to end	Night	60% 5% 15% 15% 4 1	2897
December	1st to 10th	Night	62% 7% 16% 11% 3	4720
	11th to 20th	Night	63% 4 11% 12% 9% 1	5735
	21st to end	Night	72% 5% 10% 8% 6%	7366

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% 21% 37% 15% 3 1	330
	11th to 20th	Day	25% 19% 42% 12% 3	330
	21st to end	Day	29% 18% 36% 13% 4 1	363
February	1st to 10th	Day	24% 28% 31% 14% 2 1	330
	11th to 20th	Day	24% 23% 37% 15% 2	330
	21st to end	Day	29% 31% 29% 9% 2	273
March	1st to 10th	Day	28% 21% 34% 13% 3 1	361
	11th to 20th	Day	23% 20% 28% 21% 7% 1	540
	21st to end	Day	29% 15% 31% 19% 5% 1	594
April	1st to 10th	Day	25% 20% 31% 18% 5% 1	540
	11th to 20th	Day	28% 18% 29% 14% 8% 3	532
	21st to end	Day	22% 15% 30% 22% 10% 1	540
May	1st to 10th	Day	22% 17% 31% 20% 8% 2	543
	11th to 20th	Day	20% 11% 31% 22% 10% 6%	567
	21st to end	Day	15% 11% 28% 26% 15% 5%	691
June	1st to 10th	Day	12% 14% 30% 29% 11% 4	780
	11th to 20th	Day	16% 12% 32% 24% 13% 3	780
	21st to end	Day	12% 10% 31% 29% 14% 4	780
July	1st to 10th	Day	8% 6% 26% 31% 20% 9%	771
	11th to 20th	Day	11% 8% 28% 30% 16% 7%	768
	21st to end	Day	9% 5% 30% 30% 20% 6%	740
August	1st to 10th	Day	11% 3 25% 27% 23% 12%	566
	11th to 20th	Day	11% 6% 20% 30% 22% 11%	559
	21st to end	Day	14% 7% 23% 26% 22% 9%	627
September	1st to 10th	Day	14% 9% 25% 24% 19% 9%	570
	11th to 20th	Day	11% 7% 19% 24% 25% 14%	570
	21st to end	Day	17% 12% 22% 26% 16% 8%	570
October	1st to 10th	Day	16% 15% 29% 19% 17% 4	570
	11th to 20th	Day	21% 17% 33% 23% 6% 1	570
	21st to end	Day	27% 15% 35% 16% 6% 1	413
November	1st to 10th	Day	23% 22% 32% 17% 6% 1	330
	11th to 20th	Day	26% 22% 35% 11% 5% 1	330
	21st to end	Day	28% 19% 35% 14% 5%	240
December	1st to 10th	Day	26% 20% 31% 17% 5%	220
	11th to 20th	Day	28% 25% 34% 10% 3	220
	21st to end	Day	32% 23% 31% 11% 2	308
January	1st to 10th	Night	22% 16% 37% 20% 5% 1	1050
	11th to 20th	Night	25% 17% 33% 20% 4 1	1050
	21st to end	Night	21% 13% 36% 26% 5%	1032
February	1st to 10th	Night	23% 14% 37% 22% 4	840
	11th to 20th	Night	22% 16% 35% 23% 4	823
	21st to end	Night	24% 19% 36% 19% 2	692
March	1st to 10th	Night	20% 14% 36% 24% 6%	840
	11th to 20th	Night	18% 16% 36% 24% 5% 1	840
	21st to end	Night	23% 13% 31% 26% 5% 1	924
April	1st to 10th	Night	24% 16% 36% 21% 4	700
	11th to 20th	Night	23% 14% 32% 22% 8% 1	623
	21st to end	Night	21% 11% 33% 28% 6%	630
May	1st to 10th	Night	17% 9% 34% 33% 7% 1	630
	11th to 20th	Night	20% 8% 31% 30% 10% 1	621
	21st to end	Night	14% 5% 28% 35% 17% 2	683
June	1st to 10th	Night	12% 6% 32% 40% 9%	441
	11th to 20th	Night	13% 7% 35% 35% 11%	420
	21st to end	Night	9% 6% 28% 42% 14% 1	469
July	1st to 10th	Night	10% 2 19% 44% 24% 2	615
	11th to 20th	Night	9% 4 23% 40% 23% 2	612
	21st to end	Night	8% 4 21% 45% 20% 1	690
August	1st to 10th	Night	9% 3 18% 44% 25% 2	627
	11th to 20th	Night	10% 2 21% 38% 25% 4	624
	21st to end	Night	11% 4 23% 34% 25% 2	693
September	1st to 10th	Night	11% 6% 26% 31% 22% 5%	736
	11th to 20th	Night	12% 4 18% 36% 26% 4	840
	21st to end	Night	16% 8% 22% 33% 18% 4	840
October	1st to 10th	Night	18% 11% 26% 30% 15% 1	840
	11th to 20th	Night	24% 14% 30% 25% 8%	840
	21st to end	Night	26% 11% 34% 24% 4	913
November	1st to 10th	Night	20% 15% 30% 26% 8%	820
	11th to 20th	Night	21% 16% 35% 20% 7%	840
	21st to end	Night	26% 16% 37% 18% 3	840
December	1st to 10th	Night	26% 15% 35% 19% 4 1	840
	11th to 20th	Night	23% 17% 38% 19% 3	1050
	21st to end	Night	33% 12% 34% 18% 2	1155

Page generated at 2026-10-26 04:37:47. Version ac07

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