Sylva
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Sylva clear sky chart from 2005-11-06 to 2025-09-19.

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% 8% 3 7% 5% 5% 5% 4 6% 7% 38%	126116
	Night	14% 8% 3 7% 4 4 4 4 5% 7% 40%	122945
Transparency	Day	44% 5% 11% 18% 16% 6%	129853
	Night	43% 5% 11% 19% 16% 6%	127277
Seeing	Day	21% 12% 21% 23% 15% 8%	16125
	Night	20% 8% 18% 30% 20% 4	26132

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% 9% 3 5% 3 3 3 2 5% 6% 47%	4056
	11th to 20th	Day	11% 9% 3 7% 4 3 2 2 3 4 51%	4060
	21st to end	Day	16% 10% 3 6% 3 4 4 2 4 5% 44%	4466
February	1st to 10th	Day	16% 8% 3 7% 4 4 4 3 4 6% 39%	1945
	11th to 20th	Day	19% 10% 3 6% 5% 4 3 3 5% 5% 37%	1948
	21st to end	Day	20% 9% 2 7% 5% 4 3 4 4 6% 37%	1650
March	1st to 10th	Day	18% 8% 2 5% 4 5% 4 3 5% 5% 40%	4746
	11th to 20th	Day	12% 7% 2 5% 4 3 3 3 4 6% 52%	4980
	21st to end	Day	15% 8% 3 6% 4 4 4 4 5% 5% 43%	5465
April	1st to 10th	Day	12% 7% 2 6% 5% 4 4 5% 6% 7% 41%	2200
	11th to 20th	Day	14% 8% 3 5% 5% 4 5% 3 5% 7% 41%	2410
	21st to end	Day	14% 7% 2 7% 5% 3 5% 4 5% 7% 42%	2600
May	1st to 10th	Day	14% 9% 3 9% 6% 5% 5% 4 6% 7% 30%	5862
	11th to 20th	Day	16% 9% 3 9% 6% 6% 6% 5% 7% 8% 25%	5861
	21st to end	Day	13% 8% 3 8% 5% 5% 6% 4 6% 7% 35%	6433
June	1st to 10th	Day	11% 8% 3 8% 6% 7% 6% 6% 6% 8% 31%	2600
	11th to 20th	Day	7% 6% 3 8% 7% 7% 7% 5% 8% 10% 32%	2600
	21st to end	Day	8% 7% 2 7% 7% 6% 7% 5% 8% 10% 32%	2600
July	1st to 10th	Day	6% 5% 2 8% 6% 6% 7% 6% 8% 12% 35%	5900
	11th to 20th	Day	8% 6% 3 9% 7% 7% 8% 6% 9% 11% 28%	5900
	21st to end	Day	10% 9% 4 10% 8% 8% 7% 6% 8% 10% 20%	6424
August	1st to 10th	Day	9% 7% 3 9% 6% 7% 7% 5% 8% 10% 29%	4234
	11th to 20th	Day	9% 6% 2 7% 6% 5% 6% 5% 7% 11% 35%	4174
	21st to end	Day	8% 6% 2 7% 6% 5% 6% 4 7% 8% 41%	4259
September	1st to 10th	Day	10% 9% 3 9% 6% 5% 6% 4 7% 8% 35%	2200
	11th to 20th	Day	14% 8% 3 7% 6% 5% 5% 5% 6% 8% 33%	2178
	21st to end	Day	14% 9% 2 5% 4 5% 5% 4 5% 8% 38%	2090
October	1st to 10th	Day	11% 5% 2 6% 4 4 5% 3 4 7% 49%	2090
	11th to 20th	Day	11% 7% 2 5% 3 3 4 2 4 5% 54%	1907
	21st to end	Day	13% 6% 2 4 3 3 3 2 4 6% 54%	2090
November	1st to 10th	Day	10% 6% 1 4 3 4 3 4 4 6% 55%	1819
	11th to 20th	Day	14% 5% 1 5% 3 4 3 4 5% 7% 51%	1800
	21st to end	Day	13% 9% 2 5% 4 4 3 3 5% 6% 46%	1800
December	1st to 10th	Day	18% 7% 1 4 2 3 3 4 5% 5% 48%	2930
	11th to 20th	Day	18% 11% 3 5% 3 4 4 3 5% 6% 37%	3373
	21st to end	Day	16% 10% 4 6% 4 3 3 3 4 6% 41%	4466
January	1st to 10th	Night	13% 12% 2 4 3 3 3 3 4 5% 49%	5889
	11th to 20th	Night	16% 9% 2 5% 3 3 3 3 3 5% 48%	5900
	21st to end	Night	16% 11% 3 5% 3 2 4 3 4 5% 44%	6490
February	1st to 10th	Night	14% 9% 3 6% 4 4 4 5% 5% 6% 41%	2450
	11th to 20th	Night	20% 10% 3 7% 3 4 4 3 4 5% 37%	2338
	21st to end	Night	21% 9% 3 8% 4 4 3 3 5% 5% 35%	1980
March	1st to 10th	Night	18% 11% 3 6% 4 4 5% 4 5% 5% 34%	5107
	11th to 20th	Night	11% 7% 2 4 3 4 3 3 5% 7% 49%	4980
	21st to end	Night	13% 8% 2 7% 4 4 5% 4 5% 6% 42%	5467
April	1st to 10th	Night	14% 8% 2 5% 4 4 4 3 5% 6% 44%	2200
	11th to 20th	Night	11% 8% 3 7% 4 4 3 3 5% 6% 46%	1953
	21st to end	Night	12% 7% 2 6% 4 4 4 4 5% 7% 45%	1800
May	1st to 10th	Night	15% 9% 3 9% 5% 4 5% 4 6% 8% 30%	4033
	11th to 20th	Night	20% 10% 3 9% 5% 5% 5% 4 6% 8% 26%	4032
	21st to end	Night	14% 7% 3 7% 5% 5% 6% 5% 7% 8% 34%	4421
June	1st to 10th	Night	9% 6% 3 8% 5% 5% 5% 5% 7% 11% 35%	1800
	11th to 20th	Night	6% 6% 2 5% 4 5% 6% 5% 8% 12% 41%	1800
	21st to end	Night	9% 4 2 5% 5% 5% 5% 5% 9% 12% 39%	1800
July	1st to 10th	Night	8% 8% 3 9% 6% 5% 5% 5% 7% 10% 35%	4058
	11th to 20th	Night	10% 6% 3 9% 6% 7% 6% 5% 8% 10% 30%	4057
	21st to end	Night	13% 9% 4 10% 6% 6% 6% 4 7% 9% 25%	4427
August	1st to 10th	Night	8% 6% 3 8% 5% 5% 7% 6% 8% 12% 33%	2915
	11th to 20th	Night	12% 8% 3 8% 6% 5% 5% 4 6% 8% 34%	2875
	21st to end	Night	8% 4 2 6% 5% 4 5% 4 7% 9% 44%	3599
September	1st to 10th	Night	10% 7% 3 7% 6% 7% 6% 4 6% 7% 36%	2200
	11th to 20th	Night	13% 7% 3 7% 4 5% 5% 4 7% 8% 36%	2179
	21st to end	Night	13% 7% 2 6% 4 4 5% 4 5% 7% 43%	2090
October	1st to 10th	Night	13% 6% 2 6% 3 3 3 3 4 6% 51%	2090
	11th to 20th	Night	13% 7% 2 5% 4 4 4 3 4 6% 49%	2271
	21st to end	Night	11% 8% 2 5% 4 3 4 3 4 5% 53%	2508
November	1st to 10th	Night	12% 7% 2 6% 3 3 3 3 5% 6% 50%	2474
	11th to 20th	Night	13% 9% 3 6% 4 3 3 3 4 5% 48%	2600
	21st to end	Night	16% 8% 2 5% 3 4 4 4 5% 6% 45%	2600
December	1st to 10th	Night	16% 8% 3 5% 3 3 3 3 4 6% 46%	4250
	11th to 20th	Night	18% 11% 4 7% 3 3 3 2 5% 5% 37%	4822
	21st to end	Night	16% 11% 5% 7% 4 4 4 3 5% 6% 36%	6490

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	40% 2 3 16% 32% 7%	4057
	11th to 20th	Day	39% 2 4 12% 33% 10%	4060
	21st to end	Day	44% 2 6% 16% 22% 10%	4466
February	1st to 10th	Day	47% 3 6% 20% 18% 6%	1945
	11th to 20th	Day	50% 3 5% 17% 19% 6%	1958
	21st to end	Day	50% 2 7% 13% 20% 8%	1650
March	1st to 10th	Day	47% 3 5% 15% 19% 12%	4746
	11th to 20th	Day	36% 2 5% 14% 22% 20%	4980
	21st to end	Day	45% 2 7% 26% 14% 5%	5465
April	1st to 10th	Day	42% 3 11% 22% 18% 5%	2200
	11th to 20th	Day	43% 3 8% 20% 21% 5%	2410
	21st to end	Day	44% 3 11% 26% 15% 2	2600
May	1st to 10th	Day	53% 5% 12% 19% 10% 1	5897
	11th to 20th	Day	58% 5% 11% 18% 7% 1	5865
	21st to end	Day	48% 5% 11% 19% 14% 2	6447
June	1st to 10th	Day	52% 6% 15% 20% 7% 1	2600
	11th to 20th	Day	45% 8% 18% 17% 10% 1	2600
	21st to end	Day	45% 8% 17% 19% 10% 2	2600
July	1st to 10th	Day	39% 11% 19% 21% 9%	5835
	11th to 20th	Day	47% 16% 22% 11% 3 1	5831
	21st to end	Day	58% 11% 16% 11% 4	6240
August	1st to 10th	Day	47% 9% 17% 22% 5%	4221
	11th to 20th	Day	41% 10% 22% 16% 8% 3	4213
	21st to end	Day	41% 7% 15% 25% 11% 1	4259
September	1st to 10th	Day	48% 5% 11% 18% 15% 3	2200
	11th to 20th	Day	50% 5% 9% 20% 15% 2	2178
	21st to end	Day	46% 4 11% 18% 16% 5%	2090
October	1st to 10th	Day	29% 4 7% 20% 30% 11%	3480
	11th to 20th	Day	23% 2 5% 19% 25% 26%	3147
	21st to end	Day	27% 4 6% 25% 23% 15%	3428
November	1st to 10th	Day	32% 3 7% 19% 28% 11%	1819
	11th to 20th	Day	35% 2 5% 21% 27% 10%	1800
	21st to end	Day	40% 3 6% 20% 25% 7%	1800
December	1st to 10th	Day	39% 3 5% 17% 27% 9%	2930
	11th to 20th	Day	49% 3 6% 15% 19% 8%	3370
	21st to end	Day	46% 2 9% 25% 15% 3	4466
January	1st to 10th	Night	40% 2 6% 15% 29% 8%	5889
	11th to 20th	Night	41% 2 4 14% 29% 10%	5900
	21st to end	Night	45% 2 5% 15% 23% 10%	6490
February	1st to 10th	Night	45% 3 9% 20% 20% 4	2450
	11th to 20th	Night	51% 3 6% 18% 17% 5%	2339
	21st to end	Night	52% 3 6% 17% 17% 5%	1980
March	1st to 10th	Night	51% 4 5% 14% 19% 6%	5107
	11th to 20th	Night	36% 3 8% 20% 18% 16%	4980
	21st to end	Night	44% 4 10% 24% 13% 5%	5467
April	1st to 10th	Night	42% 3 12% 21% 19% 3	2200
	11th to 20th	Night	39% 3 10% 23% 21% 4	1953
	21st to end	Night	40% 4 13% 27% 14% 1	1800
May	1st to 10th	Night	53% 6% 13% 22% 6% 1	4060
	11th to 20th	Night	58% 5% 15% 17% 6%	4033
	21st to end	Night	49% 5% 14% 23% 10%	4429
June	1st to 10th	Night	42% 8% 22% 20% 7% 1	1800
	11th to 20th	Night	34% 8% 26% 23% 8% 1	1800
	21st to end	Night	35% 9% 21% 21% 11% 3	1800
July	1st to 10th	Night	40% 10% 22% 23% 5%	4015
	11th to 20th	Night	43% 16% 26% 12% 2 1	4006
	21st to end	Night	51% 12% 20% 12% 5%	4297
August	1st to 10th	Night	39% 11% 27% 18% 4	2904
	11th to 20th	Night	44% 10% 23% 14% 9%	2902
	21st to end	Night	31% 8% 21% 30% 9% 1	3599
September	1st to 10th	Night	44% 9% 14% 19% 13% 2	2200
	11th to 20th	Night	44% 6% 15% 19% 15% 1	2179
	21st to end	Night	40% 6% 12% 22% 16% 4	2090
October	1st to 10th	Night	30% 6% 9% 21% 26% 8%	3480
	11th to 20th	Night	26% 3 5% 20% 24% 22%	3770
	21st to end	Night	30% 3 6% 25% 22% 15%	4120
November	1st to 10th	Night	35% 4 8% 22% 23% 9%	2474
	11th to 20th	Night	40% 2 4 19% 25% 10%	2600
	21st to end	Night	41% 3 5% 23% 23% 6%	2600
December	1st to 10th	Night	41% 4 6% 18% 23% 8%	4250
	11th to 20th	Night	50% 4 6% 14% 20% 6%	4824
	21st to end	Night	48% 4 10% 19% 17% 2	6490

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	35% 22% 27% 12% 4	300
	11th to 20th	Day	39% 21% 21% 15% 4	300
	21st to end	Day	33% 25% 27% 12% 3	330
February	1st to 10th	Day	31% 20% 33% 13% 3	300
	11th to 20th	Day	48% 22% 22% 8%	300
	21st to end	Day	41% 25% 24% 9% 1	249
March	1st to 10th	Day	35% 16% 27% 15% 7% 1	410
	11th to 20th	Day	31% 18% 27% 19% 4 1	500
	21st to end	Day	29% 20% 25% 19% 6% 1	550
April	1st to 10th	Day	27% 17% 24% 21% 10% 1	500
	11th to 20th	Day	23% 20% 21% 22% 12% 2	492
	21st to end	Day	23% 12% 26% 21% 14% 4	500
May	1st to 10th	Day	20% 21% 25% 19% 10% 5%	503
	11th to 20th	Day	21% 12% 21% 25% 13% 8%	527
	21st to end	Day	18% 10% 19% 24% 16% 13%	573
June	1st to 10th	Day	16% 8% 19% 27% 18% 12%	530
	11th to 20th	Day	9% 5% 21% 27% 22% 16%	530
	21st to end	Day	12% 7% 21% 24% 20% 16%	530
July	1st to 10th	Day	12% 3 13% 31% 22% 19%	525
	11th to 20th	Day	10% 6% 11% 29% 24% 20%	524
	21st to end	Day	10% 6% 10% 26% 28% 21%	576
August	1st to 10th	Day	16% 6% 14% 25% 21% 18%	526
	11th to 20th	Day	12% 5% 13% 26% 29% 14%	519
	21st to end	Day	14% 6% 11% 22% 27% 20%	583
September	1st to 10th	Day	11% 5% 12% 28% 31% 13%	530
	11th to 20th	Day	15% 5% 15% 29% 25% 11%	522
	21st to end	Day	19% 5% 16% 29% 23% 8%	490
October	1st to 10th	Day	17% 6% 18% 29% 23% 8%	490
	11th to 20th	Day	15% 12% 22% 30% 17% 4	490
	21st to end	Day	21% 9% 22% 31% 14% 3	539
November	1st to 10th	Day	18% 10% 30% 26% 13% 3	357
	11th to 20th	Day	23% 18% 29% 22% 8%	300
	21st to end	Day	21% 16% 33% 20% 8% 1	300
December	1st to 10th	Day	34% 18% 27% 16% 4 1	300
	11th to 20th	Day	31% 12% 30% 21% 5% 1	300
	21st to end	Day	31% 13% 34% 16% 5% 1	330
January	1st to 10th	Night	30% 9% 30% 26% 5%	800
	11th to 20th	Night	29% 13% 27% 26% 6%	800
	21st to end	Night	24% 12% 35% 26% 4	880
February	1st to 10th	Night	23% 10% 30% 34% 4	800
	11th to 20th	Night	33% 11% 31% 23% 2	783
	21st to end	Night	32% 15% 28% 23% 3	660
March	1st to 10th	Night	25% 12% 29% 29% 5%	800
	11th to 20th	Night	26% 13% 23% 28% 8% 1	800
	21st to end	Night	23% 12% 28% 27% 9%	880
April	1st to 10th	Night	25% 12% 25% 27% 10% 1	800
	11th to 20th	Night	20% 10% 23% 32% 14% 2	629
	21st to end	Night	16% 8% 19% 41% 16% 2	600
May	1st to 10th	Night	16% 9% 19% 36% 18% 2	600
	11th to 20th	Night	16% 8% 14% 35% 23% 4	591
	21st to end	Night	15% 4 7% 33% 32% 8%	650
June	1st to 10th	Night	13% 3 10% 37% 32% 6%	600
	11th to 20th	Night	10% 2 7% 33% 36% 12%	600
	21st to end	Night	12% 3 5% 31% 44% 7%	600
July	1st to 10th	Night	10% 4 5% 24% 49% 8%	585
	11th to 20th	Night	12% 3 3 21% 48% 13%	582
	21st to end	Night	11% 3 3 23% 47% 13%	657
August	1st to 10th	Night	12% 5% 5% 27% 42% 10%	597
	11th to 20th	Night	12% 4 4 28% 42% 9%	594
	21st to end	Night	12% 3 5% 26% 42% 13%	675
September	1st to 10th	Night	14% 4 6% 29% 38% 9%	800
	11th to 20th	Night	16% 4 8% 29% 35% 7%	793
	21st to end	Night	17% 4 10% 34% 32% 4	760
October	1st to 10th	Night	17% 7% 13% 28% 27% 8%	760
	11th to 20th	Night	17% 7% 17% 36% 19% 3	760
	21st to end	Night	17% 8% 17% 38% 16% 3	836
November	1st to 10th	Night	16% 7% 20% 35% 19% 4	780
	11th to 20th	Night	20% 9% 22% 36% 11% 3	800
	21st to end	Night	22% 7% 22% 34% 14% 1	800
December	1st to 10th	Night	28% 10% 24% 30% 8%	800
	11th to 20th	Night	24% 10% 27% 31% 8% 1	800
	21st to end	Night	28% 9% 23% 34% 5%	880

Page generated at 2025-09-16 01:49:11. Version ac07

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