Geneva College Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Geneva College Observatory clear sky chart from 2005-11-06 to 2025-11-18.

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% 10% 4 9% 5% 5% 5% 4 5% 7% 31%	126583
	Night	17% 11% 4 9% 5% 4 4 3 5% 6% 31%	123772
Transparency	Day	51% 5% 11% 21% 11% 1	130188
	Night	52% 5% 11% 20% 11% 2	128236
Seeing	Day	22% 15% 27% 20% 11% 4	16014
	Night	22% 12% 26% 27% 12% 1	25709

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% 20% 8% 22% 5% 3 3 2 3 4 14%	4056
	11th to 20th	Day	13% 24% 8% 16% 8% 7% 3 1 2 3 14%	4060
	21st to end	Day	22% 19% 6% 12% 7% 5% 4 3 4 4 14%	4466
February	1st to 10th	Day	20% 17% 6% 11% 7% 5% 3 2 3 5% 22%	1800
	11th to 20th	Day	17% 14% 6% 13% 6% 4 5% 3 5% 5% 21%	1753
	21st to end	Day	20% 15% 4 9% 5% 4 5% 4 5% 5% 24%	1648
March	1st to 10th	Day	17% 12% 5% 10% 5% 4 4 3 4 6% 30%	4935
	11th to 20th	Day	13% 8% 4 8% 5% 5% 5% 4 5% 6% 37%	4980
	21st to end	Day	19% 10% 4 8% 5% 5% 4 4 5% 6% 30%	5465
April	1st to 10th	Day	17% 13% 3 9% 5% 4 4 5% 6% 6% 29%	2295
	11th to 20th	Day	17% 11% 3 7% 5% 4 5% 3 5% 6% 33%	2436
	21st to end	Day	21% 9% 3 7% 5% 4 4 3 5% 6% 33%	2600
May	1st to 10th	Day	21% 9% 3 8% 5% 5% 5% 4 5% 7% 27%	5862
	11th to 20th	Day	18% 8% 3 7% 5% 4 5% 4 7% 8% 30%	5861
	21st to end	Day	19% 7% 2 6% 5% 5% 5% 4 7% 8% 33%	6770
June	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 9% 34%	2800
	11th to 20th	Day	15% 7% 3 6% 5% 5% 5% 4 6% 9% 34%	2800
	21st to end	Day	14% 6% 2 6% 5% 4 6% 4 7% 9% 38%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 4 6% 9% 41%	6136
	11th to 20th	Day	11% 7% 3 8% 5% 5% 6% 5% 7% 9% 35%	5900
	21st to end	Day	12% 6% 2 6% 5% 5% 5% 5% 6% 8% 40%	6424
August	1st to 10th	Day	14% 8% 3 8% 6% 6% 6% 5% 6% 8% 32%	4234
	11th to 20th	Day	12% 6% 3 7% 5% 5% 6% 5% 7% 9% 35%	4174
	21st to end	Day	8% 5% 2 7% 5% 6% 6% 5% 7% 9% 40%	4483
September	1st to 10th	Day	16% 6% 2 4 4 4 4 3 5% 7% 45%	2385
	11th to 20th	Day	11% 5% 2 6% 4 5% 4 4 7% 7% 45%	2200
	21st to end	Day	19% 8% 3 8% 5% 4 4 4 5% 6% 35%	2200
October	1st to 10th	Day	15% 6% 2 5% 4 5% 4 4 6% 8% 41%	2010
	11th to 20th	Day	19% 7% 3 8% 5% 5% 4 3 5% 6% 34%	2000
	21st to end	Day	22% 11% 3 7% 4 4 5% 3 4 7% 31%	2200
November	1st to 10th	Day	12% 9% 2 6% 5% 5% 5% 3 5% 6% 42%	1942
	11th to 20th	Day	17% 14% 4 8% 5% 4 4 3 4 6% 30%	1740
	21st to end	Day	18% 16% 6% 9% 6% 3 4 4 5% 6% 24%	1600
December	1st to 10th	Day	20% 15% 7% 14% 6% 4 3 2 4 4 20%	2600
	11th to 20th	Day	24% 18% 7% 10% 4 3 3 2 4 5% 21%	2990
	21st to end	Day	25% 21% 6% 11% 5% 5% 4 2 4 5% 12%	3978
January	1st to 10th	Night	15% 19% 8% 22% 5% 4 4 3 3 4 15%	5889
	11th to 20th	Night	14% 18% 9% 16% 9% 6% 4 3 4 3 14%	5900
	21st to end	Night	23% 17% 6% 12% 6% 4 4 2 3 3 19%	6490
February	1st to 10th	Night	21% 13% 5% 11% 6% 5% 4 4 4 5% 23%	2600
	11th to 20th	Night	19% 13% 5% 12% 6% 5% 4 3 4 4 24%	2533
	21st to end	Night	20% 13% 5% 12% 6% 4 4 3 4 4 25%	1975
March	1st to 10th	Night	22% 12% 4 9% 5% 4 3 3 3 5% 31%	4918
	11th to 20th	Night	15% 10% 4 7% 4 4 4 3 5% 6% 39%	4980
	21st to end	Night	18% 11% 4 8% 4 4 4 3 4 5% 35%	5467
April	1st to 10th	Night	19% 11% 4 6% 5% 4 3 3 5% 6% 33%	2105
	11th to 20th	Night	16% 10% 3 7% 4 5% 5% 4 5% 6% 34%	1930
	21st to end	Night	15% 9% 3 7% 5% 4 4 4 6% 8% 36%	1800
May	1st to 10th	Night	27% 11% 3 7% 4 3 4 4 5% 7% 26%	4033
	11th to 20th	Night	17% 9% 3 7% 4 3 3 4 5% 7% 37%	4032
	21st to end	Night	19% 7% 2 5% 4 4 4 4 6% 9% 36%	4066
June	1st to 10th	Night	10% 6% 3 6% 4 5% 4 5% 7% 8% 43%	1600
	11th to 20th	Night	12% 5% 2 7% 5% 4 4 4 6% 10% 42%	1600
	21st to end	Night	9% 5% 2 7% 6% 5% 5% 4 7% 8% 43%	1600
July	1st to 10th	Night	11% 7% 3 8% 4 4 4 4 5% 8% 42%	3817
	11th to 20th	Night	11% 6% 2 7% 5% 5% 5% 5% 7% 9% 38%	4057
	21st to end	Night	9% 6% 3 7% 5% 5% 4 3 6% 9% 44%	4427
August	1st to 10th	Night	12% 9% 3 8% 5% 4 5% 4 6% 6% 38%	2915
	11th to 20th	Night	13% 7% 2 7% 5% 4 3 3 5% 8% 43%	2875
	21st to end	Night	9% 5% 2 6% 4 5% 6% 5% 7% 8% 44%	3381
September	1st to 10th	Night	13% 5% 2 5% 3 3 4 4 5% 6% 49%	2010
	11th to 20th	Night	9% 5% 2 4 3 3 4 4 6% 8% 53%	2200
	21st to end	Night	17% 8% 3 6% 4 4 4 3 5% 7% 38%	2200
October	1st to 10th	Night	15% 6% 2 6% 4 3 4 3 5% 6% 47%	2390
	11th to 20th	Night	18% 7% 3 7% 4 4 5% 3 5% 7% 39%	2400
	21st to end	Night	24% 9% 2 5% 3 3 4 3 5% 6% 36%	2640
November	1st to 10th	Night	14% 8% 2 5% 5% 4 3 4 5% 5% 44%	2571
	11th to 20th	Night	19% 12% 4 8% 5% 4 4 4 5% 6% 30%	2813
	21st to end	Night	20% 15% 4 8% 5% 4 3 3 5% 5% 29%	2800
December	1st to 10th	Night	18% 17% 10% 13% 5% 4 4 3 4 5% 18%	4580
	11th to 20th	Night	22% 18% 6% 9% 5% 4 3 2 4 5% 21%	5205
	21st to end	Night	24% 18% 6% 11% 5% 4 4 3 4 5% 16%	6973

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	75% 4 8% 9% 4	4057
	11th to 20th	Day	77% 3 6% 10% 3 2	4060
	21st to end	Day	74% 3 8% 10% 4	4466
February	1st to 10th	Day	67% 3 9% 14% 7%	1800
	11th to 20th	Day	65% 5% 9% 14% 7%	1762
	21st to end	Day	60% 4 12% 15% 7% 1	1648
March	1st to 10th	Day	55% 4 10% 22% 7% 1	4935
	11th to 20th	Day	47% 5% 13% 25% 9% 2	4980
	21st to end	Day	53% 4 9% 20% 12% 1	5465
April	1st to 10th	Day	54% 5% 11% 18% 11% 2	2295
	11th to 20th	Day	51% 6% 10% 18% 13% 2	2436
	21st to end	Day	51% 5% 10% 22% 10% 2	2600
May	1st to 10th	Day	55% 6% 10% 17% 8% 3	5897
	11th to 20th	Day	50% 7% 11% 22% 9% 1	5865
	21st to end	Day	46% 7% 13% 22% 10% 3	6785
June	1st to 10th	Day	45% 7% 13% 23% 12% 1	2800
	11th to 20th	Day	45% 7% 14% 21% 12% 1	2800
	21st to end	Day	41% 8% 12% 25% 13% 1	2800
July	1st to 10th	Day	40% 5% 12% 27% 15% 2	6070
	11th to 20th	Day	43% 7% 13% 25% 12%	5831
	21st to end	Day	40% 7% 10% 24% 18% 1	6240
August	1st to 10th	Day	48% 8% 12% 22% 9%	4221
	11th to 20th	Day	42% 6% 11% 29% 10% 1	4213
	21st to end	Day	37% 8% 12% 28% 13% 2	4483
September	1st to 10th	Day	38% 6% 10% 23% 20% 3	2385
	11th to 20th	Day	36% 6% 14% 27% 16% 1	2200
	21st to end	Day	49% 5% 11% 21% 13% 1	2200
October	1st to 10th	Day	32% 5% 10% 32% 20% 1	3270
	11th to 20th	Day	45% 5% 10% 22% 14% 4	3240
	21st to end	Day	40% 4 13% 27% 16% 1	3538
November	1st to 10th	Day	43% 5% 12% 22% 17% 2	1942
	11th to 20th	Day	55% 4 11% 18% 10% 2	1740
	21st to end	Day	61% 5% 12% 16% 6% 1	1600
December	1st to 10th	Day	68% 5% 11% 13% 4	2600
	11th to 20th	Day	68% 3 11% 14% 5%	2986
	21st to end	Day	76% 3 9% 10% 3	3978
January	1st to 10th	Night	75% 4 8% 10% 3	5889
	11th to 20th	Night	75% 4 9% 10% 3	5900
	21st to end	Night	72% 3 8% 11% 5% 1	6490
February	1st to 10th	Night	64% 5% 9% 17% 5%	2600
	11th to 20th	Night	63% 4 10% 16% 7%	2535
	21st to end	Night	62% 5% 9% 14% 9% 1	1975
March	1st to 10th	Night	57% 3 10% 21% 9%	4918
	11th to 20th	Night	45% 4 13% 26% 8% 3	4980
	21st to end	Night	51% 5% 10% 18% 15% 1	5467
April	1st to 10th	Night	53% 4 10% 18% 13% 2	2105
	11th to 20th	Night	49% 5% 12% 17% 14% 3	1930
	21st to end	Night	44% 6% 12% 23% 13% 2	1800
May	1st to 10th	Night	58% 6% 9% 15% 9% 3	4060
	11th to 20th	Night	47% 6% 9% 24% 12% 1	4033
	21st to end	Night	43% 8% 12% 24% 11% 3	4073
June	1st to 10th	Night	37% 8% 13% 23% 17% 2	1600
	11th to 20th	Night	38% 7% 15% 23% 15% 2	1600
	21st to end	Night	37% 8% 13% 24% 17% 2	1600
July	1st to 10th	Night	40% 6% 9% 24% 19% 2	3775
	11th to 20th	Night	40% 7% 17% 24% 12% 1	4006
	21st to end	Night	37% 6% 11% 19% 24% 2	4297
August	1st to 10th	Night	44% 8% 12% 22% 12% 1	2904
	11th to 20th	Night	40% 6% 10% 29% 15% 1	2902
	21st to end	Night	34% 8% 13% 30% 13% 2	3381
September	1st to 10th	Night	33% 7% 11% 24% 20% 5%	2010
	11th to 20th	Night	28% 7% 14% 27% 22% 1	2200
	21st to end	Night	44% 5% 12% 21% 17% 1	2200
October	1st to 10th	Night	35% 5% 8% 27% 24% 1	3910
	11th to 20th	Night	38% 5% 12% 23% 15% 7%	3899
	21st to end	Night	36% 3 9% 30% 19% 2	4252
November	1st to 10th	Night	41% 4 12% 24% 16% 3	2571
	11th to 20th	Night	53% 4 11% 19% 9% 3	2813
	21st to end	Night	58% 4 12% 20% 5% 1	2800
December	1st to 10th	Night	70% 4 10% 13% 3	4580
	11th to 20th	Night	66% 4 12% 15% 3	5208
	21st to end	Night	71% 4 10% 12% 2	6973

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	31% 29% 26% 10% 4 1	300
	11th to 20th	Day	32% 29% 30% 8% 1	300
	21st to end	Day	29% 30% 31% 7% 2 1	330
February	1st to 10th	Day	34% 21% 35% 8% 1	300
	11th to 20th	Day	36% 28% 28% 7%	300
	21st to end	Day	35% 26% 31% 8%	262
March	1st to 10th	Day	30% 21% 30% 16% 3	500
	11th to 20th	Day	32% 21% 29% 12% 5% 1	500
	21st to end	Day	34% 19% 28% 15% 4 1	550
April	1st to 10th	Day	31% 21% 26% 15% 7% 1	500
	11th to 20th	Day	27% 20% 27% 15% 8% 3	492
	21st to end	Day	29% 16% 31% 17% 6% 1	500
May	1st to 10th	Day	26% 20% 26% 18% 7% 3	503
	11th to 20th	Day	22% 16% 30% 20% 9% 3	527
	21st to end	Day	17% 9% 28% 25% 17% 5%	573
June	1st to 10th	Day	13% 14% 31% 25% 12% 4	530
	11th to 20th	Day	18% 13% 24% 28% 14% 4	530
	21st to end	Day	13% 9% 32% 27% 15% 3	530
July	1st to 10th	Day	13% 6% 22% 27% 22% 10%	525
	11th to 20th	Day	12% 7% 26% 28% 19% 8%	524
	21st to end	Day	13% 7% 24% 29% 19% 8%	576
August	1st to 10th	Day	11% 6% 23% 31% 19% 10%	526
	11th to 20th	Day	14% 5% 23% 28% 20% 10%	519
	21st to end	Day	11% 6% 24% 26% 21% 11%	583
September	1st to 10th	Day	17% 8% 20% 26% 20% 8%	530
	11th to 20th	Day	11% 4 18% 31% 25% 12%	530
	21st to end	Day	22% 13% 24% 23% 15% 4	530
October	1st to 10th	Day	15% 9% 27% 25% 18% 5%	530
	11th to 20th	Day	22% 17% 30% 22% 7% 2	530
	21st to end	Day	28% 16% 32% 17% 6%	583
November	1st to 10th	Day	20% 19% 34% 16% 9% 2	420
	11th to 20th	Day	32% 17% 33% 11% 6% 1	258
	21st to end	Day	27% 24% 38% 11% 1	200
December	1st to 10th	Day	31% 27% 29% 9% 3 2	200
	11th to 20th	Day	32% 23% 32% 12% 2	200
	21st to end	Day	35% 22% 29% 13% 1	223
January	1st to 10th	Night	28% 21% 33% 14% 2	800
	11th to 20th	Night	29% 20% 32% 17% 2	800
	21st to end	Night	27% 22% 29% 19% 3	880
February	1st to 10th	Night	29% 18% 28% 22% 3	800
	11th to 20th	Night	31% 19% 33% 16% 1	783
	21st to end	Night	28% 18% 33% 18% 3	660
March	1st to 10th	Night	26% 18% 31% 23% 2	800
	11th to 20th	Night	26% 19% 27% 23% 5% 1	800
	21st to end	Night	25% 16% 33% 21% 5% 1	880
April	1st to 10th	Night	26% 16% 29% 23% 6%	800
	11th to 20th	Night	22% 14% 32% 23% 8% 1	746
	21st to end	Night	21% 10% 35% 29% 6%	600
May	1st to 10th	Night	21% 12% 24% 33% 8% 2	600
	11th to 20th	Night	20% 8% 28% 31% 13% 1	591
	21st to end	Night	13% 3 25% 35% 21% 3	502
June	1st to 10th	Night	11% 5% 25% 40% 19%	400
	11th to 20th	Night	13% 5% 24% 39% 19% 1	400
	21st to end	Night	8% 5% 27% 35% 24% 1	400
July	1st to 10th	Night	7% 3 18% 41% 25% 6%	475
	11th to 20th	Night	8% 2 19% 36% 30% 5%	582
	21st to end	Night	10% 4 14% 42% 27% 2	657
August	1st to 10th	Night	10% 2 18% 39% 29% 3	597
	11th to 20th	Night	11% 4 13% 40% 29% 4	594
	21st to end	Night	12% 4 20% 32% 27% 5%	753
September	1st to 10th	Night	15% 4 18% 34% 25% 4	800
	11th to 20th	Night	11% 3 15% 36% 32% 4	800
	21st to end	Night	18% 9% 23% 27% 19% 4	800
October	1st to 10th	Night	17% 6% 23% 29% 22% 2	800
	11th to 20th	Night	21% 11% 31% 25% 11% 1	800
	21st to end	Night	28% 12% 26% 26% 8%	880
November	1st to 10th	Night	19% 11% 27% 31% 12%	820
	11th to 20th	Night	28% 16% 27% 21% 7% 1	829
	21st to end	Night	28% 17% 27% 22% 6%	800
December	1st to 10th	Night	33% 17% 27% 19% 5%	800
	11th to 20th	Night	30% 18% 28% 20% 4	800
	21st to end	Night	30% 18% 29% 22% 2	880

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