Four Elements Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Four Elements Observatory 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	14% 11% 4 9% 5% 4 4 3 5% 7% 33%	126291
	Night	15% 13% 5% 11% 6% 5% 4 3 5% 6% 27%	122876
Transparency	Day	32% 39% 7% 14% 7% 1	129822
	Night	36% 39% 7% 11% 6% 1	127432
Seeing	Day	19% 9% 25% 28% 15% 3	16392
	Night	22% 11% 26% 26% 13% 2	25562

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	13% 21% 11% 16% 12% 6% 4 3 3 3 9%	3183
	11th to 20th	Day	11% 22% 11% 27% 10% 6% 4 2 2 2 4	3589
	21st to end	Day	15% 18% 10% 20% 6% 6% 4 3 4 4 9%	4420
February	1st to 10th	Day	13% 21% 7% 17% 10% 5% 4 3 3 4 14%	1800
	11th to 20th	Day	12% 16% 7% 13% 8% 6% 5% 4 5% 7% 18%	1753
	21st to end	Day	13% 18% 6% 13% 6% 6% 5% 3 5% 5% 21%	1620
March	1st to 10th	Day	16% 11% 4 11% 4 5% 4 3 5% 6% 31%	4744
	11th to 20th	Day	12% 14% 6% 11% 6% 5% 5% 4 6% 6% 25%	4980
	21st to end	Day	18% 12% 5% 9% 5% 4 4 3 5% 5% 29%	5465
April	1st to 10th	Day	18% 12% 3 8% 5% 4 4 4 5% 6% 31%	2322
	11th to 20th	Day	18% 10% 2 6% 4 4 3 3 4 6% 40%	2572
	21st to end	Day	18% 10% 3 6% 3 4 3 3 5% 7% 36%	2600
May	1st to 10th	Day	15% 8% 2 6% 3 3 4 3 5% 8% 41%	5888
	11th to 20th	Day	13% 7% 3 7% 4 4 4 3 5% 7% 44%	6106
	21st to end	Day	18% 8% 2 6% 4 4 4 3 5% 7% 40%	6985
June	1st to 10th	Day	12% 7% 2 7% 4 4 5% 4 6% 8% 41%	3000
	11th to 20th	Day	11% 6% 2 6% 4 4 5% 4 6% 9% 43%	3000
	21st to end	Day	14% 7% 3 6% 4 4 4 4 6% 8% 41%	3000
July	1st to 10th	Day	12% 7% 3 7% 4 4 4 4 6% 9% 40%	6819
	11th to 20th	Day	10% 5% 2 5% 3 3 3 3 5% 7% 53%	6689
	21st to end	Day	7% 5% 2 5% 4 3 4 3 6% 9% 54%	6525
August	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 7% 9% 45%	4234
	11th to 20th	Day	15% 6% 2 5% 3 4 4 4 6% 9% 42%	4174
	21st to end	Day	9% 6% 2 6% 3 4 5% 4 5% 9% 46%	4675
September	1st to 10th	Day	11% 7% 2 7% 5% 4 5% 4 6% 8% 42%	2360
	11th to 20th	Day	12% 5% 2 5% 3 4 5% 4 6% 8% 46%	2178
	21st to end	Day	16% 8% 3 7% 4 5% 4 4 5% 8% 36%	2090
October	1st to 10th	Day	17% 10% 4 8% 4 4 4 4 6% 6% 33%	1938
	11th to 20th	Day	24% 13% 4 10% 5% 5% 4 3 5% 6% 22%	1900
	21st to end	Day	26% 15% 5% 9% 5% 4 4 3 5% 5% 18%	1986
November	1st to 10th	Day	18% 15% 5% 10% 6% 5% 4 3 5% 6% 21%	1725
	11th to 20th	Day	17% 19% 4 11% 6% 6% 4 4 6% 6% 17%	1600
	21st to end	Day	22% 23% 6% 13% 6% 5% 4 4 4 4 10%	1600
December	1st to 10th	Day	17% 28% 9% 14% 5% 5% 4 3 4 3 7%	2600
	11th to 20th	Day	21% 22% 8% 18% 6% 4 3 2 3 4 9%	2695
	21st to end	Day	26% 23% 8% 17% 5% 6% 3 2 3 3 5%	3476
January	1st to 10th	Night	12% 19% 9% 20% 11% 7% 4 3 3 3 7%	6780
	11th to 20th	Night	13% 19% 9% 23% 9% 6% 4 3 4 4 8%	6316
	21st to end	Night	12% 17% 9% 18% 7% 7% 5% 3 4 5% 12%	6536
February	1st to 10th	Night	11% 21% 6% 18% 8% 5% 4 3 5% 5% 14%	2600
	11th to 20th	Night	13% 17% 6% 14% 7% 5% 5% 3 5% 5% 20%	2533
	21st to end	Night	13% 16% 6% 12% 6% 5% 5% 4 5% 5% 25%	2009
March	1st to 10th	Night	14% 10% 4 9% 6% 5% 4 3 5% 6% 32%	5094
	11th to 20th	Night	12% 11% 4 9% 5% 5% 5% 5% 6% 7% 32%	4979
	21st to end	Night	20% 11% 5% 9% 5% 5% 4 3 4 5% 31%	5467
April	1st to 10th	Night	19% 12% 3 8% 5% 4 3 3 4 6% 33%	2095
	11th to 20th	Night	18% 9% 2 6% 4 4 5% 3 5% 7% 37%	1777
	21st to end	Night	20% 9% 3 7% 5% 4 4 4 5% 7% 32%	1800
May	1st to 10th	Night	15% 7% 3 8% 6% 5% 5% 4 5% 7% 35%	4053
	11th to 20th	Night	11% 7% 3 7% 5% 5% 4 3 5% 8% 42%	3787
	21st to end	Night	18% 7% 3 7% 4 4 4 4 5% 6% 38%	3888
June	1st to 10th	Night	12% 9% 2 6% 6% 5% 4 3 5% 8% 40%	1400
	11th to 20th	Night	10% 7% 2 6% 5% 5% 4 5% 6% 10% 41%	1400
	21st to end	Night	12% 6% 2 6% 5% 5% 4 5% 6% 9% 40%	1400
July	1st to 10th	Night	11% 7% 2 6% 3 3 4 3 6% 9% 44%	3160
	11th to 20th	Night	11% 4 2 5% 4 4 4 3 6% 7% 49%	3287
	21st to end	Night	8% 7% 2 5% 3 3 4 3 5% 9% 50%	4330
August	1st to 10th	Night	9% 7% 2 5% 4 4 5% 4 6% 8% 45%	2915
	11th to 20th	Night	12% 6% 2 6% 4 4 5% 4 6% 8% 42%	2875
	21st to end	Night	10% 6% 2 7% 4 4 4 3 6% 8% 47%	3223
September	1st to 10th	Night	11% 5% 2 5% 3 4 5% 4 6% 8% 46%	2015
	11th to 20th	Night	10% 5% 2 6% 4 3 4 4 7% 7% 48%	2179
	21st to end	Night	16% 9% 3 6% 4 4 4 3 5% 7% 38%	2090
October	1st to 10th	Night	18% 8% 3 9% 5% 4 5% 4 4 6% 34%	2242
	11th to 20th	Night	22% 12% 4 9% 6% 5% 4 3 5% 6% 24%	2280
	21st to end	Night	24% 13% 4 10% 6% 5% 5% 3 4 5% 20%	2608
November	1st to 10th	Night	19% 14% 4 9% 5% 5% 4 3 5% 6% 25%	2571
	11th to 20th	Night	17% 20% 5% 12% 6% 4 5% 4 5% 6% 17%	2800
	21st to end	Night	22% 22% 6% 10% 7% 5% 4 3 5% 5% 11%	2800
December	1st to 10th	Night	18% 24% 7% 16% 7% 5% 5% 4 4 4 8%	4580
	11th to 20th	Night	17% 22% 8% 19% 6% 5% 4 3 4 4 8%	5505
	21st to end	Night	23% 23% 7% 16% 6% 7% 3 2 3 3 6%	7502

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	59% 30% 4 4 2	3184
	11th to 20th	Day	68% 26% 2 3 1	3620
	21st to end	Day	56% 32% 5% 5% 2	4420
February	1st to 10th	Day	28% 64% 3 5% 1	1800
	11th to 20th	Day	25% 61% 6% 4 3	1762
	21st to end	Day	25% 63% 4 6% 3	1620
March	1st to 10th	Day	41% 31% 9% 8% 10% 1	4747
	11th to 20th	Day	43% 32% 8% 12% 4	4980
	21st to end	Day	41% 32% 10% 12% 4 1	5465
April	1st to 10th	Day	23% 62% 4 8% 4	2322
	11th to 20th	Day	19% 62% 3 10% 6% 1	2572
	21st to end	Day	18% 62% 5% 9% 5% 1	2600
May	1st to 10th	Day	31% 30% 9% 14% 13% 3	5897
	11th to 20th	Day	29% 32% 10% 13% 15% 1	6110
	21st to end	Day	35% 30% 8% 13% 12% 3	6985
June	1st to 10th	Day	15% 63% 3 11% 7% 1	3000
	11th to 20th	Day	17% 63% 4 10% 6% 1	3000
	21st to end	Day	17% 61% 5% 10% 7% 1	3000
July	1st to 10th	Day	32% 27% 12% 19% 8% 1	6746
	11th to 20th	Day	22% 26% 9% 32% 10%	6608
	21st to end	Day	20% 26% 10% 29% 14% 1	6339
August	1st to 10th	Day	21% 37% 10% 20% 10% 1	4221
	11th to 20th	Day	28% 38% 10% 18% 6%	4213
	21st to end	Day	21% 38% 10% 25% 6%	4674
September	1st to 10th	Day	18% 59% 7% 11% 5%	2360
	11th to 20th	Day	13% 58% 5% 17% 7%	2178
	21st to end	Day	16% 61% 7% 11% 4	2090
October	1st to 10th	Day	23% 37% 7% 20% 12%	3224
	11th to 20th	Day	31% 38% 8% 18% 6%	3140
	21st to end	Day	36% 38% 9% 13% 3	3249
November	1st to 10th	Day	31% 57% 5% 5% 2	1725
	11th to 20th	Day	29% 58% 6% 5% 1	1600
	21st to end	Day	32% 59% 5% 3 1	1600
December	1st to 10th	Day	51% 38% 7% 3 1	2600
	11th to 20th	Day	55% 34% 6% 4 1	2695
	21st to end	Day	63% 28% 3 4 1	3476
January	1st to 10th	Night	58% 32% 5% 4 1	6780
	11th to 20th	Night	59% 30% 4 5% 2	6360
	21st to end	Night	54% 31% 6% 5% 4 1	6536
February	1st to 10th	Night	27% 63% 3 5% 2	2600
	11th to 20th	Night	22% 61% 4 9% 3	2535
	21st to end	Night	22% 63% 4 6% 4	2009
March	1st to 10th	Night	37% 32% 10% 11% 9% 2	5104
	11th to 20th	Night	35% 31% 13% 14% 5% 1	4980
	21st to end	Night	42% 31% 7% 11% 9% 1	5467
April	1st to 10th	Night	25% 62% 2 7% 4	2095
	11th to 20th	Night	19% 63% 2 8% 6% 1	1777
	21st to end	Night	21% 63% 4 6% 6% 1	1800
May	1st to 10th	Night	35% 31% 6% 11% 12% 5%	4060
	11th to 20th	Night	30% 32% 7% 16% 14% 1	3788
	21st to end	Night	37% 30% 7% 16% 9% 2	3889
June	1st to 10th	Night	13% 62% 3 11% 10% 2	1400
	11th to 20th	Night	14% 62% 5% 11% 8% 1	1400
	21st to end	Night	15% 61% 5% 11% 7% 1	1400
July	1st to 10th	Night	30% 27% 10% 20% 10% 3	3125
	11th to 20th	Night	25% 27% 7% 27% 14%	3245
	21st to end	Night	26% 26% 6% 24% 17% 1	4202
August	1st to 10th	Night	24% 37% 9% 20% 11% 1	2904
	11th to 20th	Night	30% 38% 12% 12% 7%	2902
	21st to end	Night	22% 38% 8% 24% 8%	3223
September	1st to 10th	Night	15% 58% 7% 14% 6%	2015
	11th to 20th	Night	13% 58% 6% 13% 9%	2179
	21st to end	Night	18% 61% 6% 10% 5%	2090
October	1st to 10th	Night	20% 38% 6% 23% 12%	3736
	11th to 20th	Night	29% 38% 10% 17% 6%	3779
	21st to end	Night	31% 39% 12% 15% 3	4295
November	1st to 10th	Night	29% 57% 5% 7% 1	2571
	11th to 20th	Night	28% 58% 8% 4 2	2800
	21st to end	Night	34% 58% 5% 2 1	2800
December	1st to 10th	Night	47% 39% 10% 3 1	4580
	11th to 20th	Night	53% 35% 5% 6% 2	5504
	21st to end	Night	61% 29% 6% 4	7502

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	24% 16% 32% 20% 6% 1	207
	11th to 20th	Day	23% 14% 35% 20% 8%	300
	21st to end	Day	28% 11% 28% 25% 8%	330
February	1st to 10th	Day	25% 22% 30% 17% 6%	300
	11th to 20th	Day	22% 17% 32% 24% 4	300
	21st to end	Day	24% 22% 34% 18% 2	249
March	1st to 10th	Day	20% 11% 36% 25% 7%	411
	11th to 20th	Day	22% 12% 31% 27% 7% 1	500
	21st to end	Day	26% 9% 29% 28% 7% 1	550
April	1st to 10th	Day	22% 10% 30% 28% 8% 1	500
	11th to 20th	Day	27% 10% 26% 21% 12% 4	492
	21st to end	Day	24% 10% 26% 27% 10% 1	500
May	1st to 10th	Day	21% 7% 27% 31% 12% 2	503
	11th to 20th	Day	18% 5% 21% 31% 20% 5%	527
	21st to end	Day	14% 4 21% 36% 20% 6%	583
June	1st to 10th	Day	13% 4 25% 37% 18% 3	730
	11th to 20th	Day	13% 5% 25% 35% 18% 3	730
	21st to end	Day	18% 6% 22% 34% 18% 3	730
July	1st to 10th	Day	10% 4 19% 37% 27% 3	721
	11th to 20th	Day	12% 5% 22% 34% 22% 6%	718
	21st to end	Day	12% 3 23% 33% 25% 4	625
August	1st to 10th	Day	12% 6% 19% 32% 27% 5%	526
	11th to 20th	Day	14% 3 14% 32% 28% 8%	519
	21st to end	Day	16% 6% 21% 28% 22% 6%	583
September	1st to 10th	Day	18% 5% 22% 26% 22% 7%	530
	11th to 20th	Day	11% 5% 17% 29% 29% 9%	522
	21st to end	Day	19% 8% 22% 25% 22% 5%	490
October	1st to 10th	Day	24% 12% 22% 21% 16% 4	490
	11th to 20th	Day	27% 15% 29% 22% 6% 1	490
	21st to end	Day	33% 16% 26% 20% 4 1	435
November	1st to 10th	Day	26% 14% 35% 17% 7%	281
	11th to 20th	Day	27% 18% 34% 18% 3 1	200
	21st to end	Day	28% 21% 35% 15% 2	200
December	1st to 10th	Day	29% 21% 29% 15% 7%	200
	11th to 20th	Day	29% 25% 26% 18% 3	200
	21st to end	Day	32% 20% 35% 10% 2 1	220
January	1st to 10th	Night	24% 19% 30% 20% 6% 1	1000
	11th to 20th	Night	27% 16% 29% 22% 6% 1	1000
	21st to end	Night	22% 12% 31% 25% 9% 1	900
February	1st to 10th	Night	19% 18% 36% 22% 5% 1	800
	11th to 20th	Night	22% 11% 34% 25% 7% 1	783
	21st to end	Night	21% 12% 37% 24% 5%	660
March	1st to 10th	Night	18% 11% 34% 29% 8% 1	800
	11th to 20th	Night	18% 13% 33% 26% 9% 1	800
	21st to end	Night	21% 6% 29% 31% 12% 1	880
April	1st to 10th	Night	25% 10% 29% 28% 8% 1	780
	11th to 20th	Night	22% 10% 25% 28% 12% 3	593
	21st to end	Night	24% 9% 24% 28% 14% 2	600
May	1st to 10th	Night	17% 3 24% 36% 18% 2	600
	11th to 20th	Night	17% 3 16% 38% 23% 2	489
	21st to end	Night	16% 3 18% 38% 21% 4	433
June	1st to 10th	Night	16% 7% 20% 36% 18% 3	400
	11th to 20th	Night	14% 5% 19% 40% 20% 3	400
	21st to end	Night	13% 4 21% 33% 27% 2	400
July	1st to 10th	Night	8% 3 14% 47% 24% 4	390
	11th to 20th	Night	11% 2 19% 35% 30% 4	437
	21st to end	Night	10% 3 15% 40% 29% 3	657
August	1st to 10th	Night	11% 2 17% 36% 28% 6%	597
	11th to 20th	Night	12% 3 17% 29% 30% 9%	594
	21st to end	Night	13% 5% 19% 32% 26% 5%	660
September	1st to 10th	Night	14% 5% 18% 31% 23% 9%	775
	11th to 20th	Night	12% 5% 19% 29% 27% 7%	793
	21st to end	Night	21% 8% 19% 22% 23% 7%	760
October	1st to 10th	Night	22% 9% 24% 24% 18% 3	760
	11th to 20th	Night	28% 13% 23% 25% 8% 2	760
	21st to end	Night	33% 13% 28% 21% 5% 1	836
November	1st to 10th	Night	28% 15% 25% 22% 8% 1	780
	11th to 20th	Night	26% 21% 26% 19% 7%	800
	21st to end	Night	34% 17% 28% 17% 4	800
December	1st to 10th	Night	33% 19% 29% 12% 7%	800
	11th to 20th	Night	28% 22% 33% 13% 4	945
	21st to end	Night	34% 15% 32% 15% 4	1100

Page generated at 2025-09-16 01:49:14. Version ac07

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