FvH Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the FvH Observatory clear sky chart from 2005-11-06 to 2026-04-17.

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% 9% 3 8% 5% 4 4 4 5% 7% 36%	131884
	Night	16% 10% 3 8% 5% 4 4 4 5% 7% 33%	130250
Transparency	Day	47% 6% 13% 22% 12% 1	135438
	Night	49% 6% 12% 20% 12% 1	134782
Seeing	Day	20% 13% 30% 23% 11% 3	16235
	Night	20% 11% 30% 26% 11% 1	25630

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	16% 12% 4 9% 5% 5% 5% 4 5% 6% 27%	4661
	11th to 20th	Day	11% 19% 8% 15% 7% 6% 5% 3 4 5% 15%	4691
	21st to end	Day	12% 16% 6% 12% 6% 5% 5% 4 5% 6% 22%	5780
February	1st to 10th	Day	13% 14% 5% 11% 7% 5% 4 4 5% 5% 26%	1890
	11th to 20th	Day	14% 11% 4 12% 6% 6% 5% 4 6% 5% 27%	1843
	21st to end	Day	19% 12% 3 11% 6% 4 4 4 5% 4 27%	1667
March	1st to 10th	Day	20% 11% 3 8% 4 4 3 3 5% 5% 33%	4858
	11th to 20th	Day	11% 12% 4 9% 5% 6% 5% 4 5% 7% 33%	5090
	21st to end	Day	19% 9% 3 7% 4 4 3 3 4 5% 39%	5713
April	1st to 10th	Day	18% 11% 3 7% 5% 4 4 4 4 6% 33%	2520
	11th to 20th	Day	21% 8% 3 6% 4 4 4 3 4 5% 38%	2446
	21st to end	Day	19% 9% 2 8% 5% 5% 5% 4 5% 6% 34%	2562
May	1st to 10th	Day	22% 8% 4 8% 4 4 4 4 4 6% 31%	6159
	11th to 20th	Day	16% 9% 3 8% 5% 4 4 3 4 7% 36%	6318
	21st to end	Day	18% 7% 3 6% 5% 4 4 4 5% 8% 38%	6949
June	1st to 10th	Day	16% 7% 2 6% 5% 4 5% 4 7% 9% 35%	2800
	11th to 20th	Day	12% 6% 2 5% 4 4 5% 5% 6% 9% 42%	2800
	21st to end	Day	14% 7% 2 6% 4 4 4 4 5% 9% 40%	2800
July	1st to 10th	Day	9% 6% 2 6% 4 4 4 4 6% 8% 46%	6360
	11th to 20th	Day	10% 4 1 5% 4 4 4 4 6% 9% 49%	6360
	21st to end	Day	7% 4 2 5% 4 5% 5% 4 6% 10% 48%	6849
August	1st to 10th	Day	16% 6% 2 6% 4 4 4 4 5% 7% 41%	4234
	11th to 20th	Day	13% 6% 2 6% 4 4 5% 4 7% 9% 41%	4174
	21st to end	Day	11% 5% 2 5% 4 4 5% 4 7% 8% 45%	4355
September	1st to 10th	Day	12% 5% 2 5% 4 4 4 4 5% 9% 46%	2400
	11th to 20th	Day	9% 5% 2 6% 4 3 3 3 6% 7% 52%	2400
	21st to end	Day	15% 5% 2 7% 5% 4 5% 3 5% 8% 41%	2091
October	1st to 10th	Day	17% 7% 3 7% 4 4 5% 4 6% 7% 38%	2000
	11th to 20th	Day	17% 8% 3 7% 5% 5% 5% 4 5% 7% 34%	2000
	21st to end	Day	24% 9% 3 8% 5% 4 5% 3 5% 6% 28%	2162
November	1st to 10th	Day	15% 8% 3 7% 5% 4 4 3 6% 7% 37%	1798
	11th to 20th	Day	14% 11% 3 7% 6% 5% 3 4 6% 8% 33%	1680
	21st to end	Day	20% 14% 4 7% 6% 5% 4 3 5% 6% 26%	1680
December	1st to 10th	Day	19% 14% 4 10% 6% 5% 5% 4 5% 6% 22%	2680
	11th to 20th	Day	17% 15% 5% 10% 5% 4 4 3 4 5% 28%	3066
	21st to end	Day	25% 12% 4 9% 5% 5% 4 3 4 5% 24%	4048
January	1st to 10th	Night	15% 14% 6% 12% 6% 5% 5% 3 5% 5% 24%	8234
	11th to 20th	Night	14% 15% 6% 15% 8% 6% 5% 4 5% 5% 17%	8193
	21st to end	Night	14% 13% 5% 11% 6% 5% 5% 3 5% 6% 26%	8401
February	1st to 10th	Night	15% 15% 4 10% 6% 5% 5% 4 5% 5% 26%	2730
	11th to 20th	Night	14% 15% 4 9% 5% 5% 4 4 6% 6% 28%	2663
	21st to end	Night	16% 11% 4 8% 5% 6% 5% 4 6% 6% 30%	2138
March	1st to 10th	Night	18% 11% 3 9% 6% 5% 5% 3 5% 5% 31%	5187
	11th to 20th	Night	13% 10% 3 8% 5% 4 5% 4 6% 7% 36%	5089
	21st to end	Night	20% 9% 2 6% 3 3 3 3 4 6% 40%	5460
April	1st to 10th	Night	19% 9% 3 6% 4 4 5% 4 6% 7% 33%	2100
	11th to 20th	Night	18% 8% 3 7% 5% 5% 4 3 5% 6% 37%	2036
	21st to end	Night	19% 7% 3 7% 3 4 3 5% 6% 7% 36%	1855
May	1st to 10th	Night	28% 9% 3 7% 4 4 4 3 4 5% 29%	3782
	11th to 20th	Night	14% 7% 2 7% 5% 5% 4 3 6% 7% 39%	3575
	21st to end	Night	18% 6% 2 6% 4 5% 5% 4 6% 7% 37%	3926
June	1st to 10th	Night	13% 6% 2 8% 6% 4 5% 5% 7% 8% 37%	1600
	11th to 20th	Night	9% 6% 2 7% 4 5% 5% 5% 6% 9% 43%	1600
	21st to end	Night	12% 6% 3 7% 6% 4 5% 4 5% 7% 42%	1600
July	1st to 10th	Night	9% 5% 2 5% 5% 5% 5% 4 7% 9% 42%	3600
	11th to 20th	Night	10% 5% 2 5% 4 4 4 3 5% 7% 49%	3600
	21st to end	Night	9% 5% 2 5% 4 3 3 4 6% 9% 51%	3986
August	1st to 10th	Night	10% 5% 2 6% 4 4 4 3 6% 9% 46%	2915
	11th to 20th	Night	12% 6% 2 6% 3 4 4 3 5% 8% 48%	2875
	21st to end	Night	9% 4 2 4 4 4 5% 4 6% 9% 48%	3510
September	1st to 10th	Night	11% 5% 2 6% 4 3 3 4 7% 7% 47%	2000
	11th to 20th	Night	11% 4 1 4 4 3 4 4 5% 7% 53%	2000
	21st to end	Night	17% 6% 2 5% 5% 4 4 4 4 7% 42%	2304
October	1st to 10th	Night	16% 6% 3 6% 4 4 4 4 6% 6% 40%	2400
	11th to 20th	Night	20% 8% 3 8% 5% 4 4 4 5% 6% 34%	2400
	21st to end	Night	21% 8% 3 7% 5% 4 4 4 5% 6% 32%	2676
November	1st to 10th	Night	17% 8% 2 6% 4 4 4 3 5% 7% 39%	2717
	11th to 20th	Night	16% 9% 3 7% 5% 5% 5% 4 6% 8% 33%	2940
	21st to end	Night	21% 11% 3 7% 4 4 5% 5% 7% 6% 27%	2940
December	1st to 10th	Night	19% 15% 4 7% 5% 5% 5% 4 6% 7% 23%	4720
	11th to 20th	Night	19% 14% 4 9% 4 4 4 3 5% 7% 27%	5348
	21st to end	Night	27% 14% 4 9% 4 4 4 4 4 5% 21%	7150

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	55% 7% 11% 17% 8% 1	4662
	11th to 20th	Day	72% 5% 8% 8% 6% 1	4721
	21st to end	Day	62% 5% 10% 12% 10% 1	5780
February	1st to 10th	Day	60% 7% 11% 16% 5% 1	1890
	11th to 20th	Day	57% 7% 10% 15% 9% 1	1852
	21st to end	Day	59% 5% 11% 15% 9% 1	1667
March	1st to 10th	Day	52% 4 12% 16% 15% 1	4861
	11th to 20th	Day	49% 5% 21% 13% 10% 2	5090
	21st to end	Day	48% 4 8% 21% 17% 2	5713
April	1st to 10th	Day	52% 5% 10% 19% 11% 2	2520
	11th to 20th	Day	49% 4 10% 21% 15% 1	2446
	21st to end	Day	50% 6% 10% 18% 13% 3	2562
May	1st to 10th	Day	53% 6% 9% 18% 11% 3	6168
	11th to 20th	Day	48% 6% 12% 18% 16% 1	6322
	21st to end	Day	44% 6% 13% 23% 11% 2	6949
June	1st to 10th	Day	44% 8% 13% 21% 14% 1	2800
	11th to 20th	Day	37% 7% 14% 26% 14% 2	2800
	21st to end	Day	40% 7% 15% 24% 14% 1	2800
July	1st to 10th	Day	36% 5% 14% 30% 14% 1	6290
	11th to 20th	Day	31% 6% 15% 31% 16% 1	6285
	21st to end	Day	32% 6% 14% 35% 12%	6652
August	1st to 10th	Day	42% 6% 15% 24% 13% 1	4221
	11th to 20th	Day	37% 8% 16% 29% 10%	4213
	21st to end	Day	35% 6% 14% 35% 10% 1	4355
September	1st to 10th	Day	35% 8% 14% 31% 12% 1	2400
	11th to 20th	Day	30% 6% 15% 33% 16% 1	2400
	21st to end	Day	41% 8% 14% 27% 9%	2091
October	1st to 10th	Day	37% 6% 13% 30% 13% 1	3260
	11th to 20th	Day	39% 6% 18% 24% 12%	3240
	21st to end	Day	49% 6% 14% 21% 10%	3476
November	1st to 10th	Day	46% 7% 15% 21% 11% 1	1798
	11th to 20th	Day	49% 8% 15% 17% 10% 1	1680
	21st to end	Day	58% 7% 13% 14% 7%	1680
December	1st to 10th	Day	63% 6% 16% 11% 4	2680
	11th to 20th	Day	60% 6% 10% 15% 9%	3066
	21st to end	Day	63% 7% 9% 15% 7%	4048
January	1st to 10th	Night	61% 6% 11% 15% 7%	8234
	11th to 20th	Night	68% 7% 9% 9% 7%	8238
	21st to end	Night	59% 6% 7% 14% 13% 1	8401
February	1st to 10th	Night	58% 7% 11% 17% 5% 1	2730
	11th to 20th	Night	56% 6% 11% 18% 9% 1	2665
	21st to end	Night	52% 6% 12% 20% 8% 1	2138
March	1st to 10th	Night	54% 5% 7% 17% 13% 5%	5197
	11th to 20th	Night	45% 6% 18% 19% 11% 2	5090
	21st to end	Night	46% 3 10% 21% 18% 2	5460
April	1st to 10th	Night	49% 5% 11% 19% 13% 2	2100
	11th to 20th	Night	48% 5% 10% 19% 17% 1	2036
	21st to end	Night	46% 7% 11% 18% 16% 3	1855
May	1st to 10th	Night	57% 5% 10% 13% 11% 4	3789
	11th to 20th	Night	43% 6% 15% 20% 14% 2	3576
	21st to end	Night	45% 7% 12% 20% 15% 2	3927
June	1st to 10th	Night	43% 8% 10% 23% 15% 2	1600
	11th to 20th	Night	36% 7% 13% 21% 20% 3	1600
	21st to end	Night	42% 6% 13% 23% 14% 2	1600
July	1st to 10th	Night	37% 6% 15% 22% 19% 1	3560
	11th to 20th	Night	34% 5% 12% 32% 16% 1	3552
	21st to end	Night	30% 6% 14% 33% 16% 1	3868
August	1st to 10th	Night	36% 6% 14% 28% 15% 2	2904
	11th to 20th	Night	36% 5% 16% 28% 12% 2	2902
	21st to end	Night	31% 7% 13% 36% 12% 1	3510
September	1st to 10th	Night	34% 8% 15% 28% 14% 1	2000
	11th to 20th	Night	29% 7% 13% 33% 17% 1	2000
	21st to end	Night	43% 6% 11% 26% 14% 1	2304
October	1st to 10th	Night	36% 5% 10% 27% 21% 1	3920
	11th to 20th	Night	42% 5% 14% 30% 9%	3899
	21st to end	Night	44% 5% 13% 26% 10%	4312
November	1st to 10th	Night	44% 6% 14% 22% 13% 1	2717
	11th to 20th	Night	49% 7% 14% 19% 10% 1	2940
	21st to end	Night	55% 9% 15% 15% 5% 1	2940
December	1st to 10th	Night	60% 6% 13% 16% 4	4720
	11th to 20th	Night	58% 6% 11% 13% 11% 1	5348
	21st to end	Night	65% 6% 9% 11% 9%	7150

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% 22% 33% 14% 7% 1	220
	11th to 20th	Day	27% 21% 32% 15% 3 1	228
	21st to end	Day	25% 20% 33% 17% 5% 1	363
February	1st to 10th	Day	27% 24% 30% 15% 4	330
	11th to 20th	Day	27% 22% 38% 12% 1	330
	21st to end	Day	31% 21% 35% 13% 1	383
March	1st to 10th	Day	23% 19% 35% 18% 5%	540
	11th to 20th	Day	26% 20% 31% 19% 3	540
	21st to end	Day	29% 16% 34% 13% 7% 1	594
April	1st to 10th	Day	25% 20% 32% 16% 6% 1	540
	11th to 20th	Day	27% 20% 25% 16% 8% 3	516
	21st to end	Day	24% 16% 35% 17% 6% 2	500
May	1st to 10th	Day	22% 11% 34% 20% 9% 4	503
	11th to 20th	Day	17% 11% 29% 27% 12% 3	527
	21st to end	Day	15% 8% 30% 28% 14% 4	573
June	1st to 10th	Day	18% 11% 30% 28% 11% 2	530
	11th to 20th	Day	12% 10% 32% 27% 16% 3	530
	21st to end	Day	19% 10% 28% 26% 13% 3	530
July	1st to 10th	Day	8% 6% 29% 29% 22% 5%	525
	11th to 20th	Day	13% 6% 27% 28% 19% 6%	524
	21st to end	Day	9% 5% 33% 35% 13% 5%	576
August	1st to 10th	Day	12% 5% 23% 33% 19% 8%	526
	11th to 20th	Day	12% 3 24% 31% 23% 7%	519
	21st to end	Day	12% 6% 29% 31% 15% 7%	583
September	1st to 10th	Day	13% 8% 26% 29% 16% 8%	530
	11th to 20th	Day	9% 6% 21% 28% 22% 13%	530
	21st to end	Day	16% 8% 29% 27% 16% 4	530
October	1st to 10th	Day	20% 12% 26% 20% 17% 5%	530
	11th to 20th	Day	19% 18% 32% 22% 7% 2	530
	21st to end	Day	27% 18% 27% 21% 6%	561
November	1st to 10th	Day	27% 16% 27% 21% 8% 1	372
	11th to 20th	Day	21% 20% 37% 17% 4 1	220
	21st to end	Day	31% 16% 34% 14% 5% 1	220
December	1st to 10th	Day	26% 20% 33% 15% 6%	220
	11th to 20th	Day	30% 24% 31% 11% 2	220
	21st to end	Day	34% 17% 32% 13% 3	242
January	1st to 10th	Night	26% 19% 32% 19% 5%	840
	11th to 20th	Night	29% 17% 30% 20% 5%	840
	21st to end	Night	22% 16% 33% 24% 5%	924
February	1st to 10th	Night	22% 17% 40% 19% 3	840
	11th to 20th	Night	24% 16% 36% 19% 4	823
	21st to end	Night	24% 16% 36% 21% 3	692
March	1st to 10th	Night	22% 13% 35% 25% 4	840
	11th to 20th	Night	24% 15% 34% 23% 4 1	840
	21st to end	Night	21% 12% 36% 24% 7% 1	924
April	1st to 10th	Night	22% 13% 37% 25% 3	840
	11th to 20th	Night	22% 14% 33% 21% 9% 1	816
	21st to end	Night	21% 11% 29% 27% 11% 1	655
May	1st to 10th	Night	23% 6% 28% 28% 14% 1	485
	11th to 20th	Night	15% 4 31% 36% 14%	394
	21st to end	Night	13% 5% 29% 35% 16% 2	433
June	1st to 10th	Night	14% 3 33% 36% 14% 1	400
	11th to 20th	Night	11% 4 29% 39% 17% 1	400
	21st to end	Night	13% 7% 28% 38% 14% 1	400
July	1st to 10th	Night	6% 3 26% 43% 20% 1	390
	11th to 20th	Night	11% 4 21% 40% 23% 2	388
	21st to end	Night	10% 3 21% 45% 19% 2	461
August	1st to 10th	Night	9% 3 18% 40% 26% 4	597
	11th to 20th	Night	9% 4 18% 37% 27% 4	594
	21st to end	Night	10% 5% 24% 37% 23% 2	830
September	1st to 10th	Night	12% 6% 24% 34% 22% 3	800
	11th to 20th	Night	13% 5% 17% 31% 28% 6%	800
	21st to end	Night	19% 7% 22% 29% 19% 4	800
October	1st to 10th	Night	18% 8% 27% 25% 19% 3	800
	11th to 20th	Night	23% 12% 30% 27% 8% 1	800
	21st to end	Night	25% 13% 32% 25% 6%	880
November	1st to 10th	Night	22% 14% 27% 27% 9% 1	820
	11th to 20th	Night	22% 15% 34% 21% 6% 1	840
	21st to end	Night	29% 14% 30% 20% 6%	840
December	1st to 10th	Night	28% 17% 32% 19% 4	840
	11th to 20th	Night	26% 17% 36% 17% 4	840
	21st to end	Night	33% 15% 33% 17% 3	924

Page generated at 2026-04-14 02:44:24. Version ac07

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