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 2025-07-06.

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	14% 11% 4 8% 5% 5% 5% 4 5% 7% 33%	119114
	Night	15% 12% 4 9% 5% 4 4 3 5% 6% 33%	119724
Transparency	Day	49% 6% 12% 20% 12% 1	122710
	Night	50% 6% 12% 20% 11% 1	124266
Seeing	Day	19% 14% 29% 22% 12% 4	16325
	Night	19% 11% 31% 27% 10% 1	26000

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	12% 17% 6% 17% 6% 5% 4 3 4 4 22%	3617
	11th to 20th	Day	12% 19% 7% 16% 11% 11% 5% 3 3 3 10%	3620
	21st to end	Day	19% 18% 7% 14% 5% 4 5% 3 4 4 16%	4229
February	1st to 10th	Day	12% 15% 6% 13% 8% 6% 5% 4 5% 6% 21%	1800
	11th to 20th	Day	13% 15% 5% 11% 7% 7% 5% 3 4 4 26%	1880
	21st to end	Day	13% 16% 5% 11% 7% 5% 5% 3 4 6% 25%	1650
March	1st to 10th	Day	14% 11% 4 9% 6% 4 4 4 5% 5% 33%	4562
	11th to 20th	Day	13% 14% 4 9% 5% 4 4 3 5% 7% 31%	4980
	21st to end	Day	18% 15% 5% 8% 5% 4 4 3 4 6% 28%	5465
April	1st to 10th	Day	18% 15% 3 7% 5% 4 4 3 5% 6% 30%	2350
	11th to 20th	Day	18% 9% 3 7% 4 5% 4 3 6% 6% 36%	2541
	21st to end	Day	16% 10% 3 7% 5% 5% 4 3 5% 6% 35%	2600
May	1st to 10th	Day	19% 8% 3 7% 4 4 4 4 5% 7% 36%	5862
	11th to 20th	Day	16% 8% 3 7% 4 4 4 4 5% 8% 36%	5861
	21st to end	Day	15% 7% 3 7% 4 4 4 3 5% 7% 41%	6607
June	1st to 10th	Day	11% 6% 2 6% 5% 4 5% 5% 7% 8% 41%	2973
	11th to 20th	Day	12% 7% 3 6% 5% 4 5% 3 6% 9% 42%	3000
	21st to end	Day	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 40%	2946
July	1st to 10th	Day	11% 6% 3 8% 6% 5% 6% 4 7% 8% 37%	4511
	11th to 20th	Day	8% 4 2 5% 4 4 4 4 6% 9% 50%	4440
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 5% 6% 9% 43%	4639
August	1st to 10th	Day	12% 5% 2 6% 5% 5% 6% 5% 7% 8% 40%	4104
	11th to 20th	Day	13% 5% 2 6% 5% 4 5% 5% 6% 8% 42%	4044
	21st to end	Day	9% 6% 2 6% 5% 5% 5% 4 6% 9% 42%	4486
September	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 6% 9% 38%	2164
	11th to 20th	Day	12% 6% 2 5% 3 4 4 4 6% 9% 46%	2090
	21st to end	Day	16% 7% 3 7% 5% 6% 5% 4 5% 8% 37%	2090
October	1st to 10th	Day	14% 7% 3 7% 4 4 4 4 5% 6% 40%	2038
	11th to 20th	Day	20% 11% 3 7% 4 5% 4 3 5% 7% 31%	1900
	21st to end	Day	22% 11% 4 8% 4 4 4 3 5% 7% 28%	1905
November	1st to 10th	Day	14% 11% 3 7% 6% 4 4 4 5% 7% 34%	1762
	11th to 20th	Day	15% 15% 4 10% 5% 5% 5% 4 5% 6% 27%	1800
	21st to end	Day	17% 17% 6% 10% 5% 4 4 4 4 6% 23%	1638
December	1st to 10th	Day	16% 18% 7% 11% 6% 5% 5% 4 6% 6% 17%	2583
	11th to 20th	Day	17% 21% 6% 10% 6% 5% 4 3 4 5% 19%	2625
	21st to end	Day	26% 22% 6% 12% 6% 3 4 2 4 4 12%	3752
January	1st to 10th	Night	13% 13% 7% 14% 6% 5% 4 3 4 4 27%	6348
	11th to 20th	Night	13% 16% 7% 17% 9% 9% 5% 4 4 4 14%	6360
	21st to end	Night	15% 18% 7% 11% 5% 4 5% 4 5% 6% 19%	6723
February	1st to 10th	Night	14% 16% 5% 10% 7% 6% 6% 3 5% 6% 23%	2600
	11th to 20th	Night	14% 15% 5% 10% 6% 5% 5% 4 5% 6% 26%	2402
	21st to end	Night	17% 13% 4 11% 6% 5% 6% 3 5% 5% 26%	1980
March	1st to 10th	Night	14% 12% 4 8% 4 4 3 3 4 7% 38%	5291
	11th to 20th	Night	11% 8% 3 7% 5% 4 4 4 6% 9% 39%	4980
	21st to end	Night	17% 11% 4 7% 5% 4 4 3 4 5% 34%	5467
April	1st to 10th	Night	19% 12% 4 7% 4 3 3 3 6% 6% 34%	2067
	11th to 20th	Night	18% 8% 3 7% 4 4 3 4 6% 6% 37%	1807
	21st to end	Night	18% 9% 3 7% 4 4 4 4 5% 7% 36%	1800
May	1st to 10th	Night	17% 10% 3 8% 5% 4 4 3 5% 7% 35%	4033
	11th to 20th	Night	15% 10% 3 8% 5% 4 3 3 4 7% 39%	4032
	21st to end	Night	17% 5% 2 5% 3 3 4 3 5% 6% 46%	4250
June	1st to 10th	Night	13% 5% 2 5% 5% 5% 4 4 5% 8% 45%	1420
	11th to 20th	Night	11% 6% 2 5% 4 4 4 4 7% 8% 44%	1400
	21st to end	Night	9% 6% 2 5% 4 5% 5% 4 5% 9% 47%	1447
July	1st to 10th	Night	8% 5% 3 7% 4 5% 4 4 6% 9% 46%	2570
	11th to 20th	Night	9% 3 2 3 4 4 4 4 6% 7% 55%	2530
	21st to end	Night	6% 4 1 6% 5% 6% 5% 5% 8% 8% 47%	2903
August	1st to 10th	Night	11% 5% 2 6% 4 4 5% 4 5% 8% 46%	2825
	11th to 20th	Night	11% 5% 2 5% 4 4 5% 4 6% 8% 47%	2785
	21st to end	Night	8% 5% 2 6% 4 5% 5% 4 5% 8% 47%	3153
September	1st to 10th	Night	9% 6% 2 7% 5% 5% 4 3 5% 7% 47%	1996
	11th to 20th	Night	11% 5% 1 5% 2 3 4 3 5% 7% 54%	2090
	21st to end	Night	16% 7% 2 7% 4 3 4 3 5% 8% 41%	2090
October	1st to 10th	Night	16% 6% 2 5% 4 4 3 2 6% 7% 44%	2141
	11th to 20th	Night	21% 11% 3 8% 4 4 4 3 5% 5% 32%	2280
	21st to end	Night	23% 10% 3 9% 5% 4 3 3 5% 6% 29%	2689
November	1st to 10th	Night	14% 10% 3 6% 4 4 4 3 6% 6% 41%	2536
	11th to 20th	Night	15% 14% 3 8% 5% 5% 6% 6% 5% 6% 27%	2600
	21st to end	Night	19% 16% 4 8% 6% 5% 4 3 4 6% 25%	2759
December	1st to 10th	Night	14% 16% 6% 11% 6% 5% 4 4 5% 6% 21%	4580
	11th to 20th	Night	13% 21% 6% 11% 6% 4 4 3 4 4 23%	5582
	21st to end	Night	23% 22% 6% 11% 5% 4 4 3 4 5% 16%	7208

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	65% 6% 11% 13% 5% 1	3618
	11th to 20th	Day	77% 5% 8% 5% 4	3620
	21st to end	Day	71% 5% 10% 9% 5% 1	4229
February	1st to 10th	Day	63% 7% 11% 15% 5%	1800
	11th to 20th	Day	62% 5% 10% 15% 8% 1	1889
	21st to end	Day	60% 6% 11% 15% 7% 1	1650
March	1st to 10th	Day	51% 5% 15% 18% 9% 2	4562
	11th to 20th	Day	50% 5% 15% 19% 9% 1	4980
	21st to end	Day	56% 6% 12% 15% 9% 1	5465
April	1st to 10th	Day	54% 6% 10% 19% 9% 1	2350
	11th to 20th	Day	48% 5% 11% 20% 14% 2	2541
	21st to end	Day	48% 7% 13% 19% 12% 1	2600
May	1st to 10th	Day	46% 7% 12% 17% 15% 3	5897
	11th to 20th	Day	44% 8% 11% 21% 15% 1	5865
	21st to end	Day	42% 6% 10% 19% 19% 4	6621
June	1st to 10th	Day	38% 8% 12% 26% 15% 2	2973
	11th to 20th	Day	39% 6% 13% 23% 17% 2	3000
	21st to end	Day	39% 8% 13% 24% 15% 2	2946
July	1st to 10th	Day	42% 7% 14% 20% 15% 2	4442
	11th to 20th	Day	29% 7% 14% 33% 16% 1	4365
	21st to end	Day	35% 8% 16% 18% 21% 2	4492
August	1st to 10th	Day	39% 8% 13% 22% 16% 1	4091
	11th to 20th	Day	37% 8% 13% 31% 11% 1	4083
	21st to end	Day	37% 7% 15% 27% 13% 1	4485
September	1st to 10th	Day	41% 9% 14% 24% 11% 2	2164
	11th to 20th	Day	34% 7% 14% 29% 15% 1	2090
	21st to end	Day	46% 6% 13% 23% 12%	2090
October	1st to 10th	Day	33% 5% 10% 32% 18% 2	3389
	11th to 20th	Day	41% 5% 13% 27% 14%	3140
	21st to end	Day	50% 5% 13% 24% 7% 1	3117
November	1st to 10th	Day	48% 8% 13% 21% 10% 1	1762
	11th to 20th	Day	56% 6% 13% 16% 9%	1800
	21st to end	Day	62% 6% 13% 14% 4 1	1638
December	1st to 10th	Day	66% 8% 16% 8% 2	2583
	11th to 20th	Day	68% 5% 10% 13% 4	2621
	21st to end	Day	77% 5% 7% 8% 4	3752
January	1st to 10th	Night	59% 5% 11% 16% 7% 2	6348
	11th to 20th	Night	71% 5% 10% 7% 6% 1	6360
	21st to end	Night	64% 5% 12% 12% 6%	6723
February	1st to 10th	Night	61% 7% 10% 15% 6% 1	2600
	11th to 20th	Night	58% 6% 9% 18% 8% 1	2404
	21st to end	Night	59% 5% 11% 17% 8% 1	1980
March	1st to 10th	Night	47% 4 15% 23% 10% 1	5291
	11th to 20th	Night	41% 6% 18% 23% 11% 1	4980
	21st to end	Night	51% 6% 12% 18% 12% 1	5467
April	1st to 10th	Night	50% 5% 10% 21% 12% 2	2067
	11th to 20th	Night	45% 7% 10% 19% 17% 2	1807
	21st to end	Night	47% 7% 11% 18% 15% 3	1800
May	1st to 10th	Night	48% 6% 11% 16% 14% 4	4060
	11th to 20th	Night	45% 6% 9% 24% 12% 3	4033
	21st to end	Night	38% 6% 9% 25% 17% 5%	4258
June	1st to 10th	Night	36% 6% 11% 25% 20% 2	1420
	11th to 20th	Night	36% 7% 13% 22% 20% 2	1400
	21st to end	Night	33% 7% 14% 26% 18% 2	1447
July	1st to 10th	Night	34% 6% 12% 24% 16% 7%	2530
	11th to 20th	Night	27% 6% 14% 35% 17% 1	2482
	21st to end	Night	31% 10% 18% 20% 20% 1	2810
August	1st to 10th	Night	34% 6% 15% 28% 16% 1	2814
	11th to 20th	Night	34% 7% 15% 29% 13% 2	2812
	21st to end	Night	33% 7% 13% 31% 15% 1	3153
September	1st to 10th	Night	36% 7% 13% 28% 16% 1	1996
	11th to 20th	Night	29% 7% 13% 32% 18% 2	2090
	21st to end	Night	41% 6% 12% 28% 12% 1	2090
October	1st to 10th	Night	27% 5% 12% 32% 22% 1	3570
	11th to 20th	Night	47% 5% 9% 24% 15%	3779
	21st to end	Night	45% 5% 11% 29% 10%	4427
November	1st to 10th	Night	43% 7% 13% 24% 12% 1	2536
	11th to 20th	Night	54% 7% 14% 17% 7% 1	2600
	21st to end	Night	60% 6% 15% 15% 4 1	2759
December	1st to 10th	Night	62% 8% 16% 11% 3	4580
	11th to 20th	Night	64% 4 11% 12% 8% 1	5585
	21st to end	Night	72% 5% 10% 8% 6%	7208

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	22% 21% 36% 17% 4 1	300
	11th to 20th	Day	22% 18% 44% 13% 3	300
	21st to end	Day	29% 17% 35% 14% 5% 1	330
February	1st to 10th	Day	24% 28% 32% 14% 2 1	300
	11th to 20th	Day	23% 23% 38% 14% 1	300
	21st to end	Day	29% 31% 29% 9% 2	249
March	1st to 10th	Day	27% 21% 35% 13% 4 1	330
	11th to 20th	Day	22% 20% 29% 22% 7% 1	500
	21st to end	Day	29% 14% 31% 20% 6% 1	550
April	1st to 10th	Day	24% 19% 32% 19% 6% 1	500
	11th to 20th	Day	27% 18% 29% 14% 7% 3	492
	21st to end	Day	22% 15% 30% 21% 10% 1	500
May	1st to 10th	Day	23% 16% 31% 20% 9% 2	503
	11th to 20th	Day	20% 10% 30% 24% 11% 6%	527
	21st to end	Day	15% 11% 26% 27% 16% 5%	643
June	1st to 10th	Day	12% 14% 30% 29% 11% 4	730
	11th to 20th	Day	15% 11% 32% 25% 14% 3	730
	21st to end	Day	12% 10% 31% 29% 14% 4	730
July	1st to 10th	Day	8% 6% 26% 29% 21% 9%	697
	11th to 20th	Day	11% 8% 27% 30% 16% 8%	668
	21st to end	Day	9% 4 28% 31% 21% 7%	640
August	1st to 10th	Day	11% 3 25% 25% 23% 13%	486
	11th to 20th	Day	11% 5% 19% 29% 24% 12%	479
	21st to end	Day	14% 6% 22% 26% 23% 9%	539
September	1st to 10th	Day	13% 9% 25% 25% 20% 8%	490
	11th to 20th	Day	11% 6% 20% 25% 26% 13%	490
	21st to end	Day	18% 13% 20% 25% 16% 8%	490
October	1st to 10th	Day	17% 15% 28% 19% 17% 4	490
	11th to 20th	Day	22% 17% 32% 22% 6% 1	490
	21st to end	Day	30% 17% 35% 14% 4 1	354
November	1st to 10th	Day	22% 23% 31% 18% 6% 1	300
	11th to 20th	Day	25% 22% 37% 11% 5% 1	300
	21st to end	Day	26% 19% 36% 14% 5%	218
December	1st to 10th	Day	27% 20% 32% 18% 5%	200
	11th to 20th	Day	26% 25% 36% 11% 3 1	200
	21st to end	Day	31% 24% 31% 12% 2	280
January	1st to 10th	Night	21% 15% 37% 21% 5% 1	1000
	11th to 20th	Night	24% 16% 33% 21% 4 1	1000
	21st to end	Night	20% 12% 36% 26% 5%	983
February	1st to 10th	Night	23% 15% 37% 21% 4	800
	11th to 20th	Night	22% 15% 36% 22% 4	783
	21st to end	Night	23% 19% 37% 19% 2	660
March	1st to 10th	Night	20% 14% 35% 25% 6%	800
	11th to 20th	Night	16% 16% 37% 25% 5% 1	800
	21st to end	Night	23% 12% 31% 27% 6% 1	880
April	1st to 10th	Night	23% 15% 36% 21% 4	667
	11th to 20th	Night	23% 13% 33% 22% 8% 2	593
	21st to end	Night	21% 11% 33% 28% 6% 1	600
May	1st to 10th	Night	17% 9% 33% 33% 7% 1	600
	11th to 20th	Night	21% 7% 30% 31% 10% 1	591
	21st to end	Night	14% 4 28% 35% 17% 2	650
June	1st to 10th	Night	13% 7% 32% 40% 9%	420
	11th to 20th	Night	12% 6% 34% 36% 12%	400
	21st to end	Night	9% 6% 28% 42% 14% 1	447
July	1st to 10th	Night	10% 2 19% 44% 24% 2	570
	11th to 20th	Night	9% 3 23% 39% 23% 3	552
	21st to end	Night	8% 3 21% 45% 21% 1	624
August	1st to 10th	Night	8% 3 17% 44% 25% 2	567
	11th to 20th	Night	10% 2 21% 37% 27% 4	564
	21st to end	Night	12% 4 22% 33% 26% 3	627
September	1st to 10th	Night	11% 5% 25% 31% 22% 5%	666
	11th to 20th	Night	13% 4 19% 36% 25% 4	760
	21st to end	Night	17% 8% 21% 33% 18% 4	760
October	1st to 10th	Night	18% 11% 26% 30% 15% 1	760
	11th to 20th	Night	24% 14% 29% 24% 8%	760
	21st to end	Night	28% 11% 35% 22% 4	836
November	1st to 10th	Night	20% 14% 30% 27% 8%	780
	11th to 20th	Night	21% 16% 36% 20% 7%	800
	21st to end	Night	26% 15% 38% 18% 3	800
December	1st to 10th	Night	26% 15% 34% 20% 4 1	800
	11th to 20th	Night	23% 16% 38% 19% 3 1	1000
	21st to end	Night	33% 12% 35% 19% 2	1100

Page generated at 2025-07-03 01:30:14. Version ac07

Copyright © 2025 Attilla Danko.