Ellwood H. May Environmental Park
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ellwood H. May Environmental Park clear sky chart from 2005-11-06 to 2025-12-03.

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	12% 8% 3 7% 5% 4 5% 4 6% 8% 38%	127083
	Night	13% 9% 3 7% 5% 4 4 4 5% 7% 38%	123871
Transparency	Day	41% 7% 13% 24% 13% 1	130570
	Night	44% 6% 13% 22% 14% 2	128458
Seeing	Day	14% 8% 28% 30% 16% 4	17403
	Night	17% 9% 29% 29% 14% 2	27261

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	9% 10% 4 12% 5% 6% 5% 4 6% 7% 32%	3617
	11th to 20th	Day	10% 10% 4 9% 5% 5% 6% 5% 8% 8% 31%	3620
	21st to end	Day	13% 14% 6% 9% 4 4 4 3 5% 7% 31%	3982
February	1st to 10th	Day	10% 11% 6% 11% 6% 5% 5% 5% 6% 8% 28%	1767
	11th to 20th	Day	11% 12% 4 7% 6% 5% 4 4 6% 6% 35%	1948
	21st to end	Day	12% 11% 4 8% 5% 5% 5% 5% 6% 7% 32%	1650
March	1st to 10th	Day	13% 7% 2 7% 4 4 5% 4 5% 7% 43%	4480
	11th to 20th	Day	11% 7% 3 7% 5% 4 4 4 6% 8% 41%	4649
	21st to end	Day	20% 12% 4 9% 5% 4 4 3 4 5% 29%	5967
April	1st to 10th	Day	15% 11% 4 8% 6% 4 5% 5% 5% 6% 31%	2400
	11th to 20th	Day	16% 9% 3 7% 5% 5% 4 3 6% 6% 35%	2374
	21st to end	Day	17% 9% 3 7% 5% 4 5% 4 6% 6% 35%	2570
May	1st to 10th	Day	13% 7% 3 7% 4 4 4 3 5% 7% 42%	5862
	11th to 20th	Day	11% 8% 3 7% 4 5% 6% 4 6% 8% 38%	6319
	21st to end	Day	14% 6% 3 6% 4 4 4 4 6% 8% 41%	6935
June	1st to 10th	Day	10% 6% 2 6% 4 5% 4 5% 7% 9% 42%	2800
	11th to 20th	Day	12% 5% 2 7% 5% 4 4 4 7% 8% 41%	2800
	21st to end	Day	9% 5% 2 6% 5% 4 4 4 7% 9% 45%	2800
July	1st to 10th	Day	10% 6% 2 6% 4 4 5% 5% 7% 10% 41%	6360
	11th to 20th	Day	6% 4 2 5% 4 4 5% 4 7% 9% 50%	6360
	21st to end	Day	7% 5% 2 6% 4 4 5% 5% 7% 9% 47%	6924
August	1st to 10th	Day	9% 5% 2 5% 4 5% 5% 5% 6% 9% 45%	4540
	11th to 20th	Day	10% 4 2 6% 4 4 5% 4 6% 8% 49%	4019
	21st to end	Day	8% 6% 2 7% 4 5% 5% 4 7% 8% 45%	4312
September	1st to 10th	Day	9% 5% 2 7% 5% 4 5% 4 5% 9% 46%	2400
	11th to 20th	Day	10% 5% 2 6% 4 5% 5% 3 6% 8% 46%	2265
	21st to end	Day	11% 7% 3 6% 4 5% 4 3 6% 9% 42%	2200
October	1st to 10th	Day	13% 5% 2 6% 4 4 4 4 5% 7% 45%	2145
	11th to 20th	Day	20% 8% 3 8% 6% 5% 5% 4 6% 7% 28%	1937
	21st to end	Day	15% 9% 3 8% 5% 5% 4 4 5% 6% 36%	1980
November	1st to 10th	Day	12% 8% 3 6% 4 4 4 4 6% 7% 41%	1852
	11th to 20th	Day	10% 12% 4 10% 6% 5% 5% 5% 7% 6% 31%	1890
	21st to end	Day	17% 11% 3 8% 4 4 4 4 5% 6% 32%	1743
December	1st to 10th	Day	12% 10% 3 8% 6% 6% 6% 5% 6% 7% 30%	2626
	11th to 20th	Day	12% 15% 4 9% 6% 6% 5% 4 9% 6% 24%	3008
	21st to end	Day	20% 20% 6% 11% 6% 4 4 3 4 5% 17%	3982
January	1st to 10th	Night	11% 11% 6% 11% 6% 4 4 3 4 6% 35%	6348
	11th to 20th	Night	11% 13% 4 11% 6% 5% 6% 5% 7% 6% 26%	6360
	21st to end	Night	11% 14% 5% 9% 5% 4 4 3 5% 7% 32%	6996
February	1st to 10th	Night	12% 13% 4 8% 6% 4 5% 4 5% 6% 31%	2629
	11th to 20th	Night	10% 12% 4 8% 5% 5% 4 4 5% 6% 38%	2338
	21st to end	Night	16% 13% 3 8% 4 3 4 3 4 6% 36%	1980
March	1st to 10th	Night	15% 6% 2 5% 4 4 5% 3 5% 6% 43%	5373
	11th to 20th	Night	10% 8% 2 6% 4 5% 5% 4 5% 8% 42%	5348
	21st to end	Night	18% 11% 4 10% 5% 5% 4 4 5% 4 31%	4967
April	1st to 10th	Night	17% 11% 3 6% 5% 4 4 3 5% 6% 36%	2000
	11th to 20th	Night	19% 9% 2 7% 5% 3 4 3 5% 5% 39%	1975
	21st to end	Night	17% 8% 2 6% 5% 4 4 3 5% 6% 40%	1829
May	1st to 10th	Night	14% 6% 2 7% 5% 4 4 3 6% 8% 40%	4016
	11th to 20th	Night	10% 7% 4 7% 5% 5% 5% 5% 6% 8% 38%	3596
	21st to end	Night	18% 7% 2 6% 4 4 4 3 5% 6% 40%	3942
June	1st to 10th	Night	10% 6% 2 6% 4 4 4 4 7% 8% 45%	1600
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 6% 9% 43%	1600
	21st to end	Night	9% 6% 3 6% 5% 4 5% 4 7% 8% 45%	1600
July	1st to 10th	Night	9% 6% 2 6% 4 4 4 4 6% 10% 43%	3618
	11th to 20th	Night	6% 4 1 5% 3 4 5% 4 5% 8% 54%	3617
	21st to end	Night	5% 5% 2 7% 4 4 5% 3 5% 8% 52%	3947
August	1st to 10th	Night	10% 5% 2 5% 4 4 3 4 6% 10% 47%	2619
	11th to 20th	Night	11% 5% 2 6% 4 4 4 3 6% 8% 48%	3027
	21st to end	Night	9% 6% 2 5% 4 4 5% 4 6% 8% 47%	3586
September	1st to 10th	Night	8% 5% 2 6% 4 4 4 4 6% 8% 50%	2000
	11th to 20th	Night	11% 4 2 5% 3 4 4 4 5% 7% 52%	2113
	21st to end	Night	12% 7% 3 6% 4 4 4 4 5% 7% 46%	2200
October	1st to 10th	Night	12% 6% 2 6% 3 4 4 3 5% 6% 49%	2250
	11th to 20th	Night	17% 8% 2 6% 5% 5% 4 4 4 6% 39%	2461
	21st to end	Night	17% 8% 3 6% 4 3 4 4 5% 6% 37%	2860
November	1st to 10th	Night	13% 7% 2 6% 5% 4 4 3 5% 7% 43%	2666
	11th to 20th	Night	13% 10% 3 9% 5% 5% 5% 3 5% 7% 33%	2730
	21st to end	Night	16% 10% 3 8% 5% 5% 4 4 4 5% 36%	2876
December	1st to 10th	Night	12% 9% 3 7% 5% 5% 4 3 6% 7% 39%	4609
	11th to 20th	Night	15% 17% 5% 9% 5% 5% 5% 3 4 5% 26%	5199
	21st to end	Night	21% 17% 5% 10% 5% 5% 5% 3 4 5% 20%	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	50% 8% 17% 18% 7%	3618
	11th to 20th	Day	49% 7% 14% 15% 13% 2	3620
	21st to end	Day	53% 5% 10% 18% 13%	3982
February	1st to 10th	Day	51% 8% 14% 17% 8% 1	1767
	11th to 20th	Day	48% 8% 12% 21% 10% 2	1958
	21st to end	Day	49% 8% 12% 19% 10% 2	1650
March	1st to 10th	Day	40% 6% 14% 21% 15% 5%	4480
	11th to 20th	Day	39% 5% 17% 23% 14% 1	4649
	21st to end	Day	56% 7% 10% 17% 10%	5969
April	1st to 10th	Day	50% 9% 12% 18% 11%	2400
	11th to 20th	Day	48% 6% 11% 20% 13% 2	2374
	21st to end	Day	48% 8% 12% 18% 12% 2	2570
May	1st to 10th	Day	39% 7% 12% 24% 16% 2	5897
	11th to 20th	Day	38% 8% 14% 24% 14% 1	6323
	21st to end	Day	39% 8% 10% 27% 14% 3	6949
June	1st to 10th	Day	35% 8% 12% 28% 16% 2	2800
	11th to 20th	Day	37% 8% 14% 24% 15% 1	2800
	21st to end	Day	33% 9% 15% 25% 15% 2	2800
July	1st to 10th	Day	36% 7% 14% 32% 10% 1	6291
	11th to 20th	Day	28% 8% 15% 33% 14% 1	6285
	21st to end	Day	31% 8% 16% 25% 18% 2	6728
August	1st to 10th	Day	34% 8% 16% 30% 12% 1	4525
	11th to 20th	Day	32% 8% 13% 36% 10% 1	4058
	21st to end	Day	34% 8% 16% 24% 16% 1	4311
September	1st to 10th	Day	35% 9% 14% 27% 14% 1	2400
	11th to 20th	Day	34% 8% 14% 30% 13% 1	2265
	21st to end	Day	38% 8% 14% 30% 11%	2200
October	1st to 10th	Day	24% 5% 8% 38% 23% 2	3496
	11th to 20th	Day	42% 6% 11% 24% 16%	3127
	21st to end	Day	41% 5% 12% 28% 13% 1	3180
November	1st to 10th	Day	41% 7% 15% 22% 13% 2	1852
	11th to 20th	Day	51% 7% 14% 19% 8% 1	1890
	21st to end	Day	52% 7% 14% 20% 6% 1	1743
December	1st to 10th	Day	47% 11% 19% 18% 4	2626
	11th to 20th	Day	53% 7% 17% 14% 8% 1	3005
	21st to end	Day	69% 5% 9% 11% 6%	3982
January	1st to 10th	Night	51% 5% 11% 20% 13% 1	6348
	11th to 20th	Night	57% 5% 13% 9% 14% 2	6360
	21st to end	Night	51% 6% 14% 18% 11%	6996
February	1st to 10th	Night	51% 7% 12% 18% 10% 2	2629
	11th to 20th	Night	47% 7% 11% 23% 11% 2	2339
	21st to end	Night	50% 4 14% 18% 12% 2	1980
March	1st to 10th	Night	40% 5% 15% 25% 12% 3	5373
	11th to 20th	Night	39% 5% 17% 22% 15% 2	5348
	21st to end	Night	55% 5% 10% 17% 12% 1	4967
April	1st to 10th	Night	47% 7% 9% 19% 17% 1	2000
	11th to 20th	Night	46% 5% 9% 20% 16% 3	1975
	21st to end	Night	45% 5% 11% 22% 14% 2	1829
May	1st to 10th	Night	41% 6% 13% 19% 20% 2	4043
	11th to 20th	Night	39% 8% 15% 20% 16% 2	3597
	21st to end	Night	45% 6% 8% 24% 12% 5%	3950
June	1st to 10th	Night	35% 7% 12% 28% 16% 1	1600
	11th to 20th	Night	37% 6% 12% 24% 19% 2	1600
	21st to end	Night	34% 7% 16% 23% 16% 3	1600
July	1st to 10th	Night	35% 6% 14% 30% 11% 3	3580
	11th to 20th	Night	27% 6% 15% 32% 18% 2	3572
	21st to end	Night	29% 6% 15% 26% 21% 2	3832
August	1st to 10th	Night	33% 7% 13% 27% 18% 2	2610
	11th to 20th	Night	34% 7% 12% 30% 17% 1	3054
	21st to end	Night	33% 7% 13% 24% 21% 3	3586
September	1st to 10th	Night	31% 7% 13% 26% 20% 2	2000
	11th to 20th	Night	30% 8% 9% 35% 17% 2	2113
	21st to end	Night	37% 5% 12% 29% 15% 1	2200
October	1st to 10th	Night	27% 4 7% 40% 21% 1	3679
	11th to 20th	Night	35% 6% 14% 26% 17% 2	4010
	21st to end	Night	41% 5% 12% 24% 17%	4610
November	1st to 10th	Night	40% 6% 12% 23% 16% 2	2666
	11th to 20th	Night	49% 6% 13% 19% 11% 2	2730
	21st to end	Night	49% 6% 16% 21% 7% 1	2876
December	1st to 10th	Night	43% 7% 21% 23% 5% 3	4609
	11th to 20th	Night	60% 6% 10% 12% 11% 1	5201
	21st to end	Night	64% 6% 10% 13% 6%	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	17% 10% 38% 26% 7% 1	300
	11th to 20th	Day	17% 15% 38% 24% 6% 1	300
	21st to end	Day	18% 14% 33% 29% 6% 1	330
February	1st to 10th	Day	14% 15% 45% 17% 7% 1	300
	11th to 20th	Day	16% 21% 37% 20% 6%	300
	21st to end	Day	17% 19% 41% 20% 2	249
March	1st to 10th	Day	14% 17% 45% 20% 3	300
	11th to 20th	Day	18% 15% 31% 27% 7% 2	340
	21st to end	Day	24% 12% 31% 25% 7% 1	550
April	1st to 10th	Day	22% 11% 34% 24% 8%	500
	11th to 20th	Day	17% 16% 32% 21% 9% 3	492
	21st to end	Day	19% 8% 33% 26% 12% 1	500
May	1st to 10th	Day	17% 5% 37% 28% 12% 2	503
	11th to 20th	Day	16% 5% 27% 34% 15% 3	725
	21st to end	Day	14% 4 27% 30% 22% 4	789
June	1st to 10th	Day	11% 3 25% 38% 20% 3	730
	11th to 20th	Day	13% 5% 26% 34% 19% 3	730
	21st to end	Day	11% 4 22% 40% 19% 4	730
July	1st to 10th	Day	7% 2 17% 37% 30% 7%	721
	11th to 20th	Day	8% 2 20% 38% 25% 7%	718
	21st to end	Day	5% 2 17% 44% 25% 7%	794
August	1st to 10th	Day	6% 3 18% 38% 26% 9%	716
	11th to 20th	Day	10% 4 16% 29% 31% 9%	519
	21st to end	Day	10% 4 19% 33% 27% 7%	583
September	1st to 10th	Day	11% 6% 21% 33% 20% 9%	530
	11th to 20th	Day	9% 3 17% 35% 26% 10%	530
	21st to end	Day	15% 7% 22% 27% 22% 6%	530
October	1st to 10th	Day	17% 7% 25% 31% 17% 4	475
	11th to 20th	Day	19% 17% 29% 27% 8%	330
	21st to end	Day	20% 10% 38% 23% 7% 1	363
November	1st to 10th	Day	14% 15% 31% 29% 10% 1	330
	11th to 20th	Day	15% 22% 32% 24% 7%	330
	21st to end	Day	23% 11% 38% 23% 5%	330
December	1st to 10th	Day	19% 17% 37% 22% 5%	306
	11th to 20th	Day	22% 18% 38% 15% 7%	300
	21st to end	Day	25% 17% 35% 17% 5% 1	330
January	1st to 10th	Night	17% 15% 39% 24% 5%	1000
	11th to 20th	Night	20% 15% 37% 22% 5% 1	1000
	21st to end	Night	19% 10% 39% 26% 7%	1100
February	1st to 10th	Night	20% 14% 37% 24% 5% 1	994
	11th to 20th	Night	16% 15% 38% 24% 7%	783
	21st to end	Night	22% 12% 40% 22% 4	660
March	1st to 10th	Night	20% 9% 34% 27% 10% 1	800
	11th to 20th	Night	14% 10% 38% 28% 8% 1	800
	21st to end	Night	20% 7% 28% 34% 10% 1	665
April	1st to 10th	Night	20% 10% 37% 26% 7% 1	600
	11th to 20th	Night	23% 8% 28% 30% 10% 2	593
	21st to end	Night	19% 7% 27% 34% 13% 1	600
May	1st to 10th	Night	17% 4 25% 41% 12% 1	600
	11th to 20th	Night	17% 3 26% 35% 17% 2	591
	21st to end	Night	15% 3 18% 39% 22% 4	650
June	1st to 10th	Night	11% 3 22% 40% 22% 2	600
	11th to 20th	Night	14% 3 21% 37% 23% 3	600
	21st to end	Night	10% 3 20% 41% 24% 3	600
July	1st to 10th	Night	9% 2 11% 39% 34% 6%	585
	11th to 20th	Night	7% 1 14% 42% 31% 4	582
	21st to end	Night	5% 2 17% 42% 34% 1	657
August	1st to 10th	Night	9% 2 10% 42% 34% 4	597
	11th to 20th	Night	9% 3 14% 36% 31% 8%	594
	21st to end	Night	10% 2 17% 35% 32% 4	660
September	1st to 10th	Night	9% 4 22% 32% 27% 7%	600
	11th to 20th	Night	12% 4 16% 31% 28% 8%	713
	21st to end	Night	15% 6% 25% 27% 23% 6%	800
October	1st to 10th	Night	15% 6% 26% 29% 21% 2	800
	11th to 20th	Night	22% 10% 29% 27% 11% 1	800
	21st to end	Night	22% 10% 30% 27% 11%	880
November	1st to 10th	Night	16% 10% 35% 29% 9%	820
	11th to 20th	Night	19% 16% 34% 23% 8%	840
	21st to end	Night	23% 14% 35% 22% 6%	986
December	1st to 10th	Night	21% 16% 36% 23% 4	1011
	11th to 20th	Night	24% 12% 37% 20% 6% 1	1000
	21st to end	Night	26% 13% 36% 20% 4	1100

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