Elachee Nature Science Center
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Elachee Nature Science Center clear sky chart from 2005-11-06 to 2025-10-19.

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	14% 6% 2 6% 4 4 5% 4 6% 7% 42%	126373
	Night	15% 6% 2 5% 4 4 4 3 5% 7% 44%	123336
Transparency	Day	38% 7% 12% 19% 18% 6%	130136
	Night	37% 6% 11% 19% 20% 6%	127629
Seeing	Day	18% 10% 23% 23% 18% 8%	16206
	Night	17% 9% 23% 28% 19% 3	26196

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	17% 8% 2 5% 3 3 3 2 4 4 48%	4056
	11th to 20th	Day	13% 6% 2 4 3 3 3 3 4 4 56%	4060
	21st to end	Day	23% 8% 3 6% 3 3 3 2 3 5% 42%	4539
February	1st to 10th	Day	17% 6% 2 6% 4 4 5% 4 4 6% 41%	2000
	11th to 20th	Day	24% 6% 2 5% 3 3 3 3 4 6% 42%	1948
	21st to end	Day	20% 7% 3 6% 4 4 3 3 3 6% 42%	1650
March	1st to 10th	Day	23% 7% 3 7% 4 4 4 3 5% 5% 37%	4680
	11th to 20th	Day	11% 5% 2 6% 3 3 4 3 4 6% 53%	4980
	21st to end	Day	18% 5% 2 7% 4 3 4 3 4 5% 45%	5465
April	1st to 10th	Day	13% 5% 2 5% 4 4 5% 5% 6% 6% 45%	2215
	11th to 20th	Day	11% 6% 2 6% 5% 4 5% 2 5% 6% 48%	2374
	21st to end	Day	12% 4 2 5% 3 4 4 4 6% 7% 49%	2595
May	1st to 10th	Day	15% 6% 2 7% 5% 4 5% 3 6% 8% 38%	5862
	11th to 20th	Day	20% 7% 2 7% 5% 5% 6% 5% 6% 8% 29%	5861
	21st to end	Day	15% 6% 2 6% 5% 5% 5% 4 6% 8% 39%	6433
June	1st to 10th	Day	10% 6% 2 8% 7% 5% 6% 5% 8% 9% 34%	2600
	11th to 20th	Day	6% 4 2 5% 5% 6% 6% 6% 8% 12% 40%	2600
	21st to end	Day	7% 4 2 6% 5% 6% 6% 5% 8% 10% 41%	2600
July	1st to 10th	Day	8% 4 2 6% 5% 6% 6% 6% 8% 11% 38%	5900
	11th to 20th	Day	8% 3 2 6% 5% 6% 7% 6% 9% 12% 36%	5900
	21st to end	Day	12% 6% 2 8% 6% 7% 7% 6% 8% 10% 27%	6424
August	1st to 10th	Day	11% 5% 2 6% 4 5% 6% 5% 8% 11% 36%	4234
	11th to 20th	Day	9% 5% 2 6% 4 5% 6% 4 7% 10% 42%	4011
	21st to end	Day	8% 4 2 5% 4 5% 5% 4 6% 8% 49%	4283
September	1st to 10th	Day	10% 7% 2 6% 5% 4 5% 5% 6% 8% 41%	2200
	11th to 20th	Day	15% 7% 2 6% 4 5% 5% 4 6% 7% 40%	2200
	21st to end	Day	14% 6% 2 5% 3 4 4 4 5% 5% 48%	2200
October	1st to 10th	Day	14% 5% 2 4 3 3 3 3 4 6% 55%	2200
	11th to 20th	Day	11% 5% 2 4 2 3 2 3 4 6% 58%	2037
	21st to end	Day	14% 5% 2 4 3 3 3 3 4 5% 55%	2090
November	1st to 10th	Day	14% 6% 2 5% 3 2 2 3 4 4 55%	1807
	11th to 20th	Day	15% 5% 2 3 4 3 4 3 4 6% 52%	1800
	21st to end	Day	15% 5% 2 5% 4 3 4 3 5% 6% 48%	1800
December	1st to 10th	Day	20% 6% 2 4 3 3 3 3 5% 5% 47%	2930
	11th to 20th	Day	17% 7% 2 7% 5% 4 4 2 5% 5% 43%	3373
	21st to end	Day	23% 6% 2 4 3 3 4 3 5% 6% 41%	4466
January	1st to 10th	Night	16% 5% 2 4 3 3 3 3 4 5% 52%	5889
	11th to 20th	Night	15% 6% 3 5% 4 3 3 3 4 5% 49%	5900
	21st to end	Night	19% 8% 2 4 4 3 3 2 4 5% 46%	6401
February	1st to 10th	Night	16% 6% 2 5% 4 4 4 3 5% 6% 44%	2400
	11th to 20th	Night	22% 7% 2 5% 4 3 3 3 4 5% 41%	2338
	21st to end	Night	20% 6% 2 5% 4 4 3 4 5% 6% 42%	1980
March	1st to 10th	Night	22% 8% 2 6% 3 3 4 3 5% 7% 36%	5171
	11th to 20th	Night	11% 4 1 4 3 3 4 3 6% 8% 53%	4980
	21st to end	Night	15% 6% 2 6% 4 4 4 3 5% 6% 45%	5467
April	1st to 10th	Night	12% 4 2 5% 4 4 4 4 6% 7% 49%	2200
	11th to 20th	Night	13% 5% 2 5% 3 3 3 3 5% 7% 51%	1975
	21st to end	Night	10% 5% 2 5% 4 5% 4 4 5% 6% 50%	1803
May	1st to 10th	Night	12% 6% 3 8% 5% 5% 5% 4 7% 9% 36%	4033
	11th to 20th	Night	21% 7% 2 7% 5% 4 5% 4 6% 8% 31%	4032
	21st to end	Night	14% 5% 2 5% 4 4 4 4 7% 9% 41%	4421
June	1st to 10th	Night	9% 6% 3 7% 6% 5% 6% 5% 8% 11% 36%	1800
	11th to 20th	Night	6% 5% 1 6% 4 5% 5% 5% 8% 12% 43%	1800
	21st to end	Night	7% 4 2 6% 6% 4 6% 4 8% 11% 43%	1800
July	1st to 10th	Night	11% 5% 3 9% 5% 5% 5% 4 8% 9% 34%	4058
	11th to 20th	Night	8% 5% 2 7% 6% 6% 6% 6% 9% 13% 33%	4057
	21st to end	Night	14% 7% 3 7% 6% 6% 6% 5% 8% 9% 29%	4427
August	1st to 10th	Night	12% 5% 2 6% 4 5% 5% 5% 8% 13% 35%	2915
	11th to 20th	Night	12% 5% 2 7% 5% 5% 4 4 5% 8% 43%	3033
	21st to end	Night	9% 5% 2 6% 4 3 4 4 6% 7% 51%	3586
September	1st to 10th	Night	10% 6% 2 7% 5% 5% 5% 4 6% 9% 41%	2195
	11th to 20th	Night	15% 6% 2 6% 5% 4 4 3 6% 8% 41%	2200
	21st to end	Night	14% 5% 2 6% 4 4 4 3 5% 6% 47%	2200
October	1st to 10th	Night	13% 4 2 4 3 3 4 3 4 6% 54%	2200
	11th to 20th	Night	11% 5% 2 4 3 3 3 2 4 4 59%	2317
	21st to end	Night	13% 4 1 4 3 3 3 3 3 5% 57%	2508
November	1st to 10th	Night	14% 6% 2 3 3 3 2 3 4 6% 54%	2488
	11th to 20th	Night	15% 4 2 3 4 2 3 3 4 6% 53%	2600
	21st to end	Night	17% 5% 1 4 3 4 4 3 5% 6% 48%	2600
December	1st to 10th	Night	17% 5% 2 5% 3 3 3 2 3 5% 50%	4250
	11th to 20th	Night	20% 6% 2 5% 3 3 3 3 4 5% 45%	4822
	21st to end	Night	23% 5% 2 5% 3 4 3 3 4 5% 42%	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	39% 5% 5% 19% 24% 8%	4057
	11th to 20th	Day	31% 5% 6% 18% 30% 10%	4060
	21st to end	Day	47% 2 4 16% 21% 9%	4539
February	1st to 10th	Day	43% 4 8% 19% 20% 5%	2000
	11th to 20th	Day	44% 4 7% 19% 20% 6%	1958
	21st to end	Day	45% 5% 7% 14% 20% 9%	1650
March	1st to 10th	Day	48% 5% 7% 12% 18% 10%	4680
	11th to 20th	Day	32% 5% 6% 14% 24% 19%	4980
	21st to end	Day	42% 4 6% 22% 21% 5%	5465
April	1st to 10th	Day	36% 7% 11% 19% 22% 6%	2215
	11th to 20th	Day	36% 6% 7% 19% 23% 9%	2374
	21st to end	Day	32% 7% 12% 26% 19% 4	2595
May	1st to 10th	Day	43% 8% 14% 19% 14% 2	5897
	11th to 20th	Day	50% 9% 14% 18% 9% 1	5865
	21st to end	Day	40% 8% 12% 21% 18% 2	6447
June	1st to 10th	Day	41% 12% 17% 19% 9% 2	2600
	11th to 20th	Day	33% 11% 21% 23% 12% 1	2600
	21st to end	Day	35% 11% 18% 20% 13% 3	2600
July	1st to 10th	Day	36% 12% 19% 26% 7% 1	5835
	11th to 20th	Day	36% 13% 29% 17% 3 1	5831
	21st to end	Day	47% 12% 18% 18% 4	6240
August	1st to 10th	Day	38% 10% 15% 29% 7%	4221
	11th to 20th	Day	34% 10% 20% 20% 13% 3	4050
	21st to end	Day	32% 8% 14% 22% 21% 3	4283
September	1st to 10th	Day	39% 9% 14% 16% 18% 3	2200
	11th to 20th	Day	41% 8% 12% 18% 19% 2	2200
	21st to end	Day	37% 6% 13% 17% 20% 7%	2200
October	1st to 10th	Day	26% 4 7% 14% 37% 12%	3590
	11th to 20th	Day	20% 2 6% 14% 29% 29%	3303
	21st to end	Day	28% 3 5% 21% 29% 13%	3428
November	1st to 10th	Day	32% 4 8% 15% 30% 11%	1807
	11th to 20th	Day	33% 5% 6% 19% 27% 10%	1800
	21st to end	Day	36% 5% 8% 17% 27% 7%	1800
December	1st to 10th	Day	39% 3 5% 18% 26% 8%	2930
	11th to 20th	Day	44% 4 7% 17% 21% 7%	3370
	21st to end	Day	40% 5% 11% 24% 17% 2	4466
January	1st to 10th	Night	34% 5% 6% 19% 29% 7%	5889
	11th to 20th	Night	36% 4 5% 19% 26% 9%	5900
	21st to end	Night	42% 3 5% 15% 28% 7%	6401
February	1st to 10th	Night	40% 5% 9% 19% 21% 7%	2400
	11th to 20th	Night	46% 4 6% 17% 20% 7%	2339
	21st to end	Night	43% 4 8% 16% 19% 9%	1980
March	1st to 10th	Night	46% 5% 7% 15% 20% 7%	5171
	11th to 20th	Night	27% 6% 8% 15% 28% 16%	4980
	21st to end	Night	39% 5% 7% 21% 20% 9%	5467
April	1st to 10th	Night	33% 6% 11% 20% 23% 7%	2200
	11th to 20th	Night	33% 5% 8% 21% 23% 10%	1975
	21st to end	Night	33% 6% 11% 25% 20% 4	1803
May	1st to 10th	Night	41% 10% 16% 20% 12% 1	4060
	11th to 20th	Night	49% 7% 16% 18% 9% 1	4033
	21st to end	Night	36% 8% 14% 25% 16% 2	4429
June	1st to 10th	Night	39% 11% 17% 20% 12% 2	1800
	11th to 20th	Night	31% 12% 20% 24% 13% 1	1800
	21st to end	Night	33% 11% 18% 21% 14% 4	1800
July	1st to 10th	Night	44% 12% 18% 21% 5%	4015
	11th to 20th	Night	39% 15% 26% 16% 4 1	4006
	21st to end	Night	49% 12% 18% 16% 4 1	4297
August	1st to 10th	Night	39% 10% 20% 25% 6%	2904
	11th to 20th	Night	39% 9% 19% 19% 14% 1	3060
	21st to end	Night	31% 9% 13% 23% 22% 2	3586
September	1st to 10th	Night	38% 9% 15% 17% 16% 4	2195
	11th to 20th	Night	40% 8% 12% 20% 18% 2	2200
	21st to end	Night	37% 7% 12% 19% 18% 7%	2200
October	1st to 10th	Night	25% 5% 10% 14% 35% 10%	3590
	11th to 20th	Night	19% 3 5% 17% 32% 25%	3777
	21st to end	Night	25% 3 5% 22% 27% 17%	4120
November	1st to 10th	Night	32% 4 7% 19% 25% 12%	2488
	11th to 20th	Night	33% 4 7% 17% 28% 12%	2600
	21st to end	Night	36% 5% 7% 20% 24% 7%	2600
December	1st to 10th	Night	38% 3 5% 17% 29% 7%	4250
	11th to 20th	Night	41% 5% 9% 17% 21% 7%	4824
	21st to end	Night	43% 5% 8% 27% 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	27% 23% 36% 13% 1	300
	11th to 20th	Day	29% 23% 26% 18% 4	300
	21st to end	Day	28% 24% 34% 11% 2 1	330
February	1st to 10th	Day	23% 20% 36% 19% 2	300
	11th to 20th	Day	39% 20% 29% 10% 1	300
	21st to end	Day	32% 20% 36% 10% 2	249
March	1st to 10th	Day	26% 17% 29% 19% 9%	383
	11th to 20th	Day	23% 19% 32% 19% 6% 1	500
	21st to end	Day	23% 17% 33% 19% 7% 1	550
April	1st to 10th	Day	22% 15% 32% 20% 10% 1	500
	11th to 20th	Day	14% 16% 37% 19% 13% 1	492
	21st to end	Day	17% 11% 27% 27% 14% 4	500
May	1st to 10th	Day	15% 14% 30% 21% 14% 6%	503
	11th to 20th	Day	17% 10% 23% 25% 15% 9%	527
	21st to end	Day	17% 7% 19% 27% 19% 10%	573
June	1st to 10th	Day	12% 5% 20% 29% 23% 10%	530
	11th to 20th	Day	8% 3 18% 30% 25% 15%	530
	21st to end	Day	10% 3 18% 30% 25% 15%	530
July	1st to 10th	Day	10% 2 10% 26% 29% 22%	525
	11th to 20th	Day	10% 2 10% 23% 35% 20%	524
	21st to end	Day	9% 1 8% 27% 36% 19%	576
August	1st to 10th	Day	18% 5% 14% 22% 24% 18%	526
	11th to 20th	Day	12% 4 11% 26% 34% 14%	519
	21st to end	Day	16% 3 11% 24% 29% 17%	583
September	1st to 10th	Day	9% 3 15% 31% 28% 13%	530
	11th to 20th	Day	18% 5% 12% 24% 32% 11%	530
	21st to end	Day	17% 5% 17% 27% 25% 9%	530
October	1st to 10th	Day	17% 6% 19% 33% 18% 9%	530
	11th to 20th	Day	13% 10% 24% 26% 23% 4	522
	21st to end	Day	18% 9% 25% 27% 19% 3	539
November	1st to 10th	Day	23% 8% 25% 26% 15% 2	345
	11th to 20th	Day	19% 18% 33% 22% 6% 1	300
	21st to end	Day	17% 14% 36% 23% 9% 1	300
December	1st to 10th	Day	31% 19% 29% 17% 3	300
	11th to 20th	Day	27% 13% 33% 22% 4	300
	21st to end	Day	33% 16% 28% 16% 5% 2	330
January	1st to 10th	Night	26% 19% 30% 21% 5%	800
	11th to 20th	Night	23% 16% 32% 23% 6%	800
	21st to end	Night	22% 19% 35% 23% 2	880
February	1st to 10th	Night	20% 14% 32% 29% 5%	800
	11th to 20th	Night	25% 17% 33% 22% 3	783
	21st to end	Night	23% 16% 35% 23% 3	660
March	1st to 10th	Night	21% 15% 33% 27% 4	800
	11th to 20th	Night	19% 12% 36% 25% 8% 1	800
	21st to end	Night	18% 15% 30% 29% 8%	880
April	1st to 10th	Night	16% 15% 31% 29% 9% 1	800
	11th to 20th	Night	15% 11% 28% 31% 14% 1	593
	21st to end	Night	13% 7% 26% 32% 20% 1	600
May	1st to 10th	Night	13% 8% 27% 31% 22% 1	600
	11th to 20th	Night	15% 7% 22% 29% 23% 4	591
	21st to end	Night	16% 2 15% 32% 30% 5%	650
June	1st to 10th	Night	10% 3 17% 35% 31% 5%	600
	11th to 20th	Night	7% 3 15% 32% 32% 12%	600
	21st to end	Night	7% 2 12% 38% 33% 8%	600
July	1st to 10th	Night	11% 2 9% 25% 42% 11%	585
	11th to 20th	Night	10% 2 6% 24% 48% 10%	582
	21st to end	Night	8% 2 9% 30% 41% 10%	657
August	1st to 10th	Night	17% 2 7% 29% 35% 11%	597
	11th to 20th	Night	14% 3 10% 25% 39% 9%	594
	21st to end	Night	14% 4 9% 27% 35% 11%	660
September	1st to 10th	Night	11% 4 12% 28% 36% 8%	795
	11th to 20th	Night	17% 3 13% 30% 35% 4	800
	21st to end	Night	17% 4 15% 34% 29% 2	800
October	1st to 10th	Night	15% 5% 18% 32% 25% 6%	800
	11th to 20th	Night	13% 8% 23% 33% 21% 2	793
	21st to end	Night	15% 8% 22% 32% 21% 2	836
November	1st to 10th	Night	16% 9% 22% 32% 19% 1	780
	11th to 20th	Night	18% 12% 29% 28% 13% 1	800
	21st to end	Night	19% 11% 29% 29% 12% 1	800
December	1st to 10th	Night	21% 16% 29% 27% 7%	800
	11th to 20th	Night	22% 13% 29% 27% 10%	800
	21st to end	Night	29% 14% 24% 28% 5%	880

Page generated at 2025-10-16 01:50:27. Version ac07

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