Willow Slough Fish and Wildlife Area
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Willow Slough Fish and Wildlife Area clear sky chart from 2005-11-06 to 2025-08-05.

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% 9% 3 8% 5% 4 5% 4 5% 7% 36%	125579
	Night	15% 10% 4 8% 5% 4 4 3 5% 6% 37%	122712
Transparency	Day	45% 6% 11% 22% 13% 2	129129
	Night	46% 5% 10% 21% 15% 2	127236
Seeing	Day	16% 10% 29% 24% 15% 6%	16776
	Night	18% 10% 30% 30% 12% 1	26581

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	14% 12% 10% 13% 6% 4 5% 3 5% 5% 21%	3617
	11th to 20th	Day	11% 15% 7% 15% 7% 7% 4 5% 5% 3 22%	3620
	21st to end	Day	19% 18% 5% 9% 5% 3 3 3 3 6% 25%	4091
February	1st to 10th	Day	17% 12% 5% 10% 6% 5% 5% 3 5% 6% 26%	1914
	11th to 20th	Day	14% 12% 4 8% 6% 5% 5% 3 5% 6% 32%	1948
	21st to end	Day	16% 14% 4 9% 4 4 3 4 5% 7% 30%	1650
March	1st to 10th	Day	19% 14% 3 8% 4 3 3 2 3 4 37%	4480
	11th to 20th	Day	9% 10% 4 6% 5% 5% 4 4 5% 8% 41%	4653
	21st to end	Day	18% 12% 6% 10% 6% 5% 5% 4 5% 7% 24%	5957
April	1st to 10th	Day	17% 12% 2 7% 4 3 4 4 5% 7% 34%	2400
	11th to 20th	Day	16% 9% 3 5% 5% 4 5% 3 4 6% 37%	2374
	21st to end	Day	20% 7% 3 8% 5% 4 5% 3 5% 7% 33%	2522
May	1st to 10th	Day	19% 8% 3 7% 4 4 5% 4 6% 8% 31%	5862
	11th to 20th	Day	16% 6% 2 6% 5% 5% 5% 3 6% 8% 39%	5952
	21st to end	Day	13% 7% 3 7% 5% 4 4 4 6% 8% 40%	6935
June	1st to 10th	Day	10% 5% 2 5% 5% 4 5% 4 5% 8% 45%	2800
	11th to 20th	Day	9% 5% 2 6% 5% 4 5% 4 6% 8% 45%	2800
	21st to end	Day	10% 5% 2 6% 5% 5% 6% 5% 6% 9% 40%	2800
July	1st to 10th	Day	11% 5% 2 7% 5% 5% 6% 5% 7% 10% 37%	6360
	11th to 20th	Day	9% 5% 2 6% 5% 5% 6% 5% 7% 11% 39%	6360
	21st to end	Day	12% 6% 2 6% 5% 5% 6% 4 7% 9% 38%	6924
August	1st to 10th	Day	8% 4 2 6% 5% 4 5% 4 6% 9% 48%	4268
	11th to 20th	Day	9% 5% 2 6% 4 4 4 4 7% 9% 44%	3767
	21st to end	Day	6% 4 2 5% 4 4 4 4 6% 9% 50%	4180
September	1st to 10th	Day	10% 5% 2 6% 4 4 5% 4 6% 6% 48%	2280
	11th to 20th	Day	11% 5% 2 4 3 4 4 3 6% 8% 50%	2128
	21st to end	Day	11% 5% 3 7% 4 4 5% 4 6% 8% 44%	2090
October	1st to 10th	Day	12% 6% 2 6% 3 4 4 4 5% 7% 46%	2090
	11th to 20th	Day	18% 7% 2 6% 4 4 4 4 5% 6% 39%	1877
	21st to end	Day	19% 10% 2 9% 5% 4 5% 3 6% 6% 32%	1881
November	1st to 10th	Day	12% 7% 1 6% 4 4 4 3 5% 7% 47%	1762
	11th to 20th	Day	17% 11% 4 8% 4 4 3 3 4 6% 36%	1800
	21st to end	Day	17% 12% 3 9% 4 4 3 3 4 5% 35%	1800
December	1st to 10th	Day	10% 12% 4 8% 5% 4 4 3 4 6% 42%	2647
	11th to 20th	Day	14% 15% 5% 12% 5% 5% 4 4 6% 6% 23%	3008
	21st to end	Day	27% 20% 7% 12% 5% 4 4 3 3 4 11%	3982
January	1st to 10th	Night	16% 14% 9% 12% 5% 4 4 3 4 4 24%	6348
	11th to 20th	Night	14% 17% 6% 15% 8% 5% 5% 3 5% 3 20%	6360
	21st to end	Night	20% 15% 6% 8% 4 4 4 3 3 3 30%	6886
February	1st to 10th	Night	16% 14% 5% 8% 7% 4 3 3 4 6% 30%	2485
	11th to 20th	Night	15% 11% 4 9% 5% 4 4 3 5% 5% 34%	2338
	21st to end	Night	18% 13% 3 9% 4 4 4 3 5% 4 33%	1980
March	1st to 10th	Night	17% 12% 4 6% 4 3 3 3 4 5% 39%	5373
	11th to 20th	Night	12% 7% 3 6% 4 4 4 4 5% 7% 45%	5307
	21st to end	Night	19% 10% 5% 7% 5% 5% 5% 4 5% 6% 29%	5014
April	1st to 10th	Night	14% 9% 2 8% 4 4 4 3 5% 7% 39%	2000
	11th to 20th	Night	18% 9% 2 6% 5% 4 4 3 5% 5% 38%	1975
	21st to end	Night	17% 8% 3 7% 5% 5% 4 4 6% 7% 35%	1876
May	1st to 10th	Night	20% 9% 3 7% 5% 4 4 3 5% 6% 33%	4033
	11th to 20th	Night	15% 6% 3 5% 3 4 4 4 5% 8% 42%	3930
	21st to end	Night	15% 8% 3 7% 5% 4 4 3 5% 7% 39%	3942
June	1st to 10th	Night	10% 6% 2 5% 3 4 4 3 6% 7% 50%	1600
	11th to 20th	Night	9% 6% 2 5% 5% 3 4 3 6% 10% 47%	1600
	21st to end	Night	9% 5% 2 6% 6% 5% 5% 4 6% 9% 43%	1600
July	1st to 10th	Night	10% 5% 2 7% 5% 5% 4 4 6% 9% 42%	3618
	11th to 20th	Night	7% 5% 1 6% 4 5% 4 4 6% 9% 48%	3617
	21st to end	Night	9% 7% 2 8% 5% 4 5% 4 6% 7% 43%	3947
August	1st to 10th	Night	8% 5% 2 5% 4 5% 4 4 6% 9% 48%	2751
	11th to 20th	Night	8% 4 2 5% 4 4 5% 4 6% 8% 51%	3059
	21st to end	Night	8% 5% 2 6% 4 3 3 3 5% 8% 53%	3476
September	1st to 10th	Night	10% 5% 1 6% 3 3 3 3 4 8% 53%	1900
	11th to 20th	Night	8% 5% 1 3 3 3 4 3 6% 7% 55%	2033
	21st to end	Night	10% 6% 2 6% 4 4 4 4 5% 8% 47%	2090
October	1st to 10th	Night	12% 5% 2 5% 3 4 3 3 4 6% 53%	2090
	11th to 20th	Night	15% 7% 2 6% 4 3 3 3 4 6% 47%	2299
	21st to end	Night	20% 8% 3 7% 4 3 3 3 4 6% 39%	2717
November	1st to 10th	Night	13% 8% 2 5% 4 4 4 3 5% 7% 46%	2536
	11th to 20th	Night	14% 10% 3 7% 4 4 5% 4 6% 5% 37%	2600
	21st to end	Night	17% 12% 4 7% 5% 4 3 3 4 6% 35%	2600
December	1st to 10th	Night	12% 9% 3 7% 4 4 3 3 5% 5% 44%	4537
	11th to 20th	Night	18% 13% 5% 9% 4 5% 4 3 4 4 30%	5199
	21st to end	Night	26% 18% 5% 11% 6% 4 4 3 4 5% 15%	6996

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	63% 5% 10% 15% 7% 1	3618
	11th to 20th	Day	65% 4 8% 9% 11% 4	3620
	21st to end	Day	62% 3 11% 17% 7%	4091
February	1st to 10th	Day	60% 5% 11% 16% 7% 2	1914
	11th to 20th	Day	53% 5% 10% 22% 9% 1	1958
	21st to end	Day	53% 6% 11% 21% 7% 2	1650
March	1st to 10th	Day	52% 4 7% 22% 13% 3	4480
	11th to 20th	Day	40% 5% 10% 33% 11%	4653
	21st to end	Day	60% 6% 12% 15% 6% 1	5959
April	1st to 10th	Day	50% 6% 11% 20% 14%	2400
	11th to 20th	Day	47% 6% 10% 22% 12% 3	2374
	21st to end	Day	50% 6% 12% 19% 12% 2	2522
May	1st to 10th	Day	49% 8% 11% 20% 11% 1	5897
	11th to 20th	Day	43% 8% 11% 24% 13% 1	5956
	21st to end	Day	40% 6% 10% 24% 17% 3	6949
June	1st to 10th	Day	36% 8% 11% 25% 17% 4	2800
	11th to 20th	Day	36% 6% 11% 24% 20% 4	2800
	21st to end	Day	38% 9% 14% 20% 17% 3	2800
July	1st to 10th	Day	39% 7% 13% 25% 15% 2	6291
	11th to 20th	Day	36% 8% 16% 26% 13% 1	6285
	21st to end	Day	40% 8% 15% 22% 13% 2	6728
August	1st to 10th	Day	32% 7% 16% 28% 15% 1	4253
	11th to 20th	Day	33% 8% 11% 31% 16% 1	3804
	21st to end	Day	30% 7% 15% 26% 19% 4	4179
September	1st to 10th	Day	34% 8% 11% 25% 17% 5%	2280
	11th to 20th	Day	31% 6% 11% 25% 22% 4	2128
	21st to end	Day	37% 6% 9% 25% 21% 2	2090
October	1st to 10th	Day	25% 4 8% 30% 31% 2	3480
	11th to 20th	Day	34% 5% 10% 23% 26% 1	3093
	21st to end	Day	43% 4 10% 25% 16% 2	3081
November	1st to 10th	Day	36% 5% 14% 25% 17% 2	1762
	11th to 20th	Day	51% 4 11% 21% 12% 1	1800
	21st to end	Day	52% 5% 10% 22% 10% 1	1800
December	1st to 10th	Day	45% 4 14% 28% 9%	2647
	11th to 20th	Day	60% 5% 12% 17% 5%	3005
	21st to end	Day	78% 4 7% 8% 3	3982
January	1st to 10th	Night	63% 5% 9% 15% 9%	6348
	11th to 20th	Night	68% 3 9% 9% 9% 2	6360
	21st to end	Night	61% 3 8% 17% 10% 1	6886
February	1st to 10th	Night	56% 5% 10% 18% 9% 2	2485
	11th to 20th	Night	52% 4 10% 20% 12% 2	2339
	21st to end	Night	53% 6% 10% 18% 10% 3	1980
March	1st to 10th	Night	48% 5% 8% 26% 10% 3	5373
	11th to 20th	Night	39% 5% 12% 26% 16% 3	5307
	21st to end	Night	54% 7% 9% 21% 8% 1	5014
April	1st to 10th	Night	45% 5% 12% 23% 14% 2	2000
	11th to 20th	Night	47% 5% 8% 21% 16% 4	1975
	21st to end	Night	47% 6% 10% 21% 13% 3	1876
May	1st to 10th	Night	52% 6% 9% 16% 14% 3	4060
	11th to 20th	Night	39% 8% 12% 26% 13% 2	3931
	21st to end	Night	44% 6% 10% 19% 16% 4	3950
June	1st to 10th	Night	33% 6% 11% 25% 22% 3	1600
	11th to 20th	Night	33% 6% 12% 24% 22% 3	1600
	21st to end	Night	37% 8% 14% 22% 15% 4	1600
July	1st to 10th	Night	37% 7% 13% 27% 13% 4	3580
	11th to 20th	Night	32% 7% 15% 24% 20% 1	3572
	21st to end	Night	38% 7% 14% 21% 15% 4	3832
August	1st to 10th	Night	31% 8% 17% 26% 17% 1	2742
	11th to 20th	Night	30% 7% 15% 28% 19% 1	3088
	21st to end	Night	30% 6% 12% 31% 19% 3	3476
September	1st to 10th	Night	30% 5% 12% 26% 21% 6%	1900
	11th to 20th	Night	26% 6% 9% 28% 27% 5%	2033
	21st to end	Night	35% 6% 9% 25% 20% 3	2090
October	1st to 10th	Night	21% 3 7% 32% 34% 2	3480
	11th to 20th	Night	33% 4 8% 24% 29% 2	3822
	21st to end	Night	39% 4 8% 24% 22% 4	4467
November	1st to 10th	Night	38% 6% 9% 27% 18% 3	2536
	11th to 20th	Night	46% 5% 12% 21% 14% 2	2600
	21st to end	Night	52% 4 10% 23% 10% 1	2600
December	1st to 10th	Night	42% 4 13% 27% 12% 1	4537
	11th to 20th	Night	58% 4 12% 15% 11% 1	5201
	21st to end	Night	73% 5% 8% 10% 5%	6996

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% 13% 37% 20% 7% 1	300
	11th to 20th	Day	22% 17% 39% 18% 3 1	300
	21st to end	Day	23% 18% 34% 18% 6% 1	330
February	1st to 10th	Day	23% 18% 36% 18% 5%	300
	11th to 20th	Day	25% 17% 35% 19% 3 1	300
	21st to end	Day	24% 18% 42% 10% 4	249
March	1st to 10th	Day	21% 21% 38% 19% 2	300
	11th to 20th	Day	24% 16% 34% 20% 6%	355
	21st to end	Day	30% 14% 29% 21% 5% 1	550
April	1st to 10th	Day	26% 12% 31% 23% 7% 1	500
	11th to 20th	Day	21% 15% 33% 16% 12% 3	492
	21st to end	Day	22% 14% 32% 21% 8% 2	500
May	1st to 10th	Day	22% 12% 31% 21% 12% 2	503
	11th to 20th	Day	19% 8% 29% 23% 15% 5%	566
	21st to end	Day	14% 7% 31% 25% 14% 8%	789
June	1st to 10th	Day	12% 9% 28% 28% 17% 6%	730
	11th to 20th	Day	12% 4 28% 32% 18% 5%	730
	21st to end	Day	11% 6% 34% 28% 16% 6%	730
July	1st to 10th	Day	10% 4 18% 29% 23% 15%	721
	11th to 20th	Day	8% 3 23% 31% 21% 13%	718
	21st to end	Day	10% 3 21% 32% 25% 9%	794
August	1st to 10th	Day	8% 3 18% 28% 30% 13%	576
	11th to 20th	Day	10% 3 21% 28% 25% 13%	479
	21st to end	Day	7% 2 17% 33% 29% 12%	539
September	1st to 10th	Day	11% 5% 18% 30% 28% 9%	490
	11th to 20th	Day	11% 3 15% 29% 28% 14%	490
	21st to end	Day	12% 8% 25% 25% 21% 9%	490
October	1st to 10th	Day	13% 7% 27% 27% 20% 6%	490
	11th to 20th	Day	19% 18% 29% 23% 10% 1	305
	21st to end	Day	28% 12% 35% 17% 6% 1	330
November	1st to 10th	Day	16% 11% 39% 20% 10% 4	300
	11th to 20th	Day	22% 17% 36% 15% 8% 1	300
	21st to end	Day	25% 14% 37% 20% 4	300
December	1st to 10th	Day	17% 18% 40% 17% 7% 1	300
	11th to 20th	Day	19% 19% 37% 19% 5%	300
	21st to end	Day	30% 18% 34% 15% 3	330
January	1st to 10th	Night	23% 16% 36% 21% 5%	1000
	11th to 20th	Night	23% 16% 35% 21% 4	1000
	21st to end	Night	24% 13% 35% 25% 3	1100
February	1st to 10th	Night	24% 12% 39% 22% 3	885
	11th to 20th	Night	21% 14% 37% 25% 4	783
	21st to end	Night	23% 14% 39% 21% 3	660
March	1st to 10th	Night	21% 14% 37% 23% 5%	800
	11th to 20th	Night	18% 12% 40% 25% 5% 1	800
	21st to end	Night	23% 12% 36% 22% 6%	686
April	1st to 10th	Night	17% 14% 39% 25% 5% 1	600
	11th to 20th	Night	21% 10% 35% 25% 7% 1	593
	21st to end	Night	20% 11% 34% 29% 7% 1	600
May	1st to 10th	Night	19% 7% 34% 32% 7% 1	600
	11th to 20th	Night	17% 5% 30% 33% 15% 1	591
	21st to end	Night	11% 4 26% 40% 17% 1	650
June	1st to 10th	Night	10% 5% 24% 46% 15%	600
	11th to 20th	Night	11% 4 29% 41% 15% 1	600
	21st to end	Night	10% 3 27% 39% 20% 1	600
July	1st to 10th	Night	9% 2 16% 40% 29% 4	585
	11th to 20th	Night	8% 3 18% 39% 29% 3	582
	21st to end	Night	5% 3 14% 46% 32% 1	657
August	1st to 10th	Night	7% 1 13% 44% 31% 5%	579
	11th to 20th	Night	7% 1 14% 42% 32% 5%	564
	21st to end	Night	8% 1 15% 38% 34% 3	627
September	1st to 10th	Night	11% 2 14% 41% 29% 2	570
	11th to 20th	Night	9% 4 16% 35% 33% 4	703
	21st to end	Night	12% 6% 23% 31% 25% 3	760
October	1st to 10th	Night	13% 5% 24% 38% 19% 2	760
	11th to 20th	Night	18% 13% 30% 27% 11%	760
	21st to end	Night	24% 12% 32% 25% 7%	836
November	1st to 10th	Night	17% 10% 37% 26% 9% 1	780
	11th to 20th	Night	20% 16% 35% 24% 6% 1	800
	21st to end	Night	22% 14% 34% 24% 6%	800
December	1st to 10th	Night	23% 13% 37% 23% 4	970
	11th to 20th	Night	24% 13% 33% 25% 5%	1000
	21st to end	Night	30% 14% 31% 23% 2	1100

Page generated at 2025-08-02 00:32:22. Version ac07

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