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 2024-01-12.

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	15% 11% 4 8% 6% 3 5% 4 5% 7% 33%	74654
	Night	16% 13% 5% 9% 7% 3 5% 4 5% 6% 27%	75054
Transparency	Day	34% 38% 8% 12% 6% 1	74359
	Night	39% 38% 8% 10% 5% 1	74884
Seeing	Day	19% 8% 24% 29% 16% 4	14067
	Night	22% 11% 26% 26% 14% 2	23467

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	16% 22% 11% 13% 10% 4 6% 5% 3 2 7%	1339
	11th to 20th	Day	17% 21% 9% 15% 8% 5% 7% 4 3 3 7%	1448
	21st to end	Day	16% 18% 8% 12% 10% 4 6% 5% 5% 5% 11%	1762
February	1st to 10th	Day	14% 19% 8% 14% 12% 4 4 4 3 4 14%	1620
	11th to 20th	Day	12% 14% 8% 11% 9% 5% 6% 5% 5% 6% 19%	1573
	21st to end	Day	14% 17% 7% 12% 6% 4 6% 4 5% 5% 21%	1453
March	1st to 10th	Day	13% 10% 4 8% 7% 4 5% 5% 5% 6% 34%	1896
	11th to 20th	Day	13% 11% 5% 10% 5% 4 5% 4 6% 6% 32%	1980
	21st to end	Day	16% 8% 4 7% 6% 3 5% 4 4 5% 36%	2178
April	1st to 10th	Day	19% 11% 4 7% 5% 3 4 5% 5% 6% 31%	2089
	11th to 20th	Day	18% 9% 3 6% 4 3 3 3 5% 6% 40%	2312
	21st to end	Day	19% 9% 3 6% 4 3 3 4 5% 7% 36%	2340
May	1st to 10th	Day	17% 7% 3 6% 4 2 5% 5% 5% 8% 39%	2340
	11th to 20th	Day	16% 6% 3 5% 5% 3 4 4 5% 7% 43%	2392
	21st to end	Day	11% 5% 2 5% 5% 3 4 4 6% 9% 46%	2733
June	1st to 10th	Day	12% 6% 2 6% 5% 3 5% 5% 6% 8% 41%	2700
	11th to 20th	Day	11% 6% 2 5% 5% 3 5% 5% 6% 9% 44%	2700
	21st to end	Day	12% 6% 3 5% 5% 3 4 5% 6% 8% 42%	2700
July	1st to 10th	Day	7% 5% 2 4 4 2 4 4 6% 9% 51%	2700
	11th to 20th	Day	10% 5% 2 5% 5% 4 4 5% 6% 9% 47%	2653
	21st to end	Day	9% 5% 2 4 5% 3 5% 4 6% 9% 48%	2592
August	1st to 10th	Day	12% 5% 2 4 4 3 4 3 6% 8% 49%	2327
	11th to 20th	Day	10% 4 2 5% 5% 3 5% 5% 6% 9% 46%	2303
	21st to end	Day	12% 8% 3 6% 4 3 4 5% 5% 9% 42%	2574
September	1st to 10th	Day	12% 6% 2 6% 6% 3 5% 5% 5% 8% 42%	2125
	11th to 20th	Day	13% 5% 2 5% 5% 3 5% 6% 6% 8% 44%	1980
	21st to end	Day	17% 7% 3 6% 6% 4 4 5% 5% 8% 36%	1980
October	1st to 10th	Day	18% 10% 4 6% 5% 3 5% 5% 6% 7% 32%	1836
	11th to 20th	Day	26% 12% 4 8% 6% 4 4 4 5% 6% 21%	1800
	21st to end	Day	27% 14% 5% 8% 7% 3 4 4 4 5% 18%	1882
November	1st to 10th	Day	18% 15% 5% 9% 7% 4 5% 5% 5% 7% 21%	1637
	11th to 20th	Day	18% 17% 5% 9% 8% 5% 4 5% 5% 6% 16%	1520
	21st to end	Day	23% 20% 7% 11% 7% 4 4 5% 4 5% 10%	1520
December	1st to 10th	Day	17% 28% 8% 12% 8% 5% 6% 4 4 3 7%	1520
	11th to 20th	Day	20% 22% 8% 14% 8% 3 4 3 4 4 10%	1719
	21st to end	Day	27% 23% 8% 16% 7% 3 3 3 2 3 4	2431
January	1st to 10th	Night	16% 20% 9% 15% 11% 4 6% 4 3 4 7%	2824
	11th to 20th	Night	18% 21% 10% 13% 10% 4 5% 4 4 4 9%	2541
	21st to end	Night	13% 17% 8% 13% 10% 5% 7% 5% 4 6% 13%	2592
February	1st to 10th	Night	12% 20% 7% 16% 10% 3 4 4 5% 5% 13%	2340
	11th to 20th	Night	14% 16% 8% 11% 8% 4 5% 4 5% 5% 20%	2273
	21st to end	Night	15% 15% 7% 10% 7% 4 5% 4 4 5% 25%	1802
March	1st to 10th	Night	13% 11% 4 7% 8% 4 4 4 4 6% 33%	2049
	11th to 20th	Night	14% 12% 5% 7% 7% 3 5% 5% 5% 6% 33%	1979
	21st to end	Night	16% 8% 4 7% 6% 4 4 4 5% 6% 36%	2178
April	1st to 10th	Night	20% 11% 3 6% 6% 2 4 5% 5% 6% 33%	1886
	11th to 20th	Night	17% 9% 2 6% 4 3 4 4 5% 7% 38%	1597
	21st to end	Night	20% 9% 3 7% 5% 3 5% 4 4 7% 32%	1620
May	1st to 10th	Night	14% 7% 3 6% 6% 3 6% 5% 6% 7% 37%	1620
	11th to 20th	Night	14% 7% 3 5% 5% 4 5% 5% 5% 7% 41%	1501
	21st to end	Night	10% 6% 3 5% 6% 3 5% 5% 5% 7% 45%	1538
June	1st to 10th	Night	12% 8% 2 5% 7% 4 5% 4 4 8% 41%	1260
	11th to 20th	Night	10% 6% 2 4 5% 4 5% 6% 5% 10% 43%	1260
	21st to end	Night	11% 6% 3 5% 5% 4 5% 5% 6% 9% 41%	1260
July	1st to 10th	Night	7% 4 2 4 4 3 4 5% 5% 9% 53%	1260
	11th to 20th	Night	8% 5% 2 5% 6% 3 5% 5% 6% 9% 45%	1303
	21st to end	Night	8% 5% 2 5% 4 3 5% 5% 5% 9% 50%	1725
August	1st to 10th	Night	8% 5% 3 5% 4 2 4 5% 7% 8% 50%	1611
	11th to 20th	Night	9% 5% 2 4 5% 3 5% 6% 6% 8% 48%	1598
	21st to end	Night	11% 6% 3 6% 5% 3 4 4 6% 8% 43%	1782
September	1st to 10th	Night	10% 4 3 5% 4 3 5% 5% 6% 9% 46%	1813
	11th to 20th	Night	12% 4 3 5% 4 3 4 6% 7% 8% 45%	1980
	21st to end	Night	16% 9% 4 6% 5% 4 4 4 5% 7% 37%	1980
October	1st to 10th	Night	19% 8% 4 9% 6% 3 5% 4 4 5% 33%	2124
	11th to 20th	Night	24% 12% 4 8% 7% 4 4 5% 5% 6% 23%	2160
	21st to end	Night	26% 13% 5% 9% 7% 3 5% 4 4 5% 19%	2470
November	1st to 10th	Night	20% 13% 5% 9% 6% 4 5% 4 5% 6% 24%	2439
	11th to 20th	Night	17% 18% 6% 11% 7% 3 5% 5% 5% 6% 17%	2660
	21st to end	Night	23% 20% 7% 9% 8% 4 5% 4 4 5% 11%	2660
December	1st to 10th	Night	21% 23% 10% 14% 9% 3 4 4 3 3 7%	2660
	11th to 20th	Night	18% 21% 8% 16% 7% 3 4 4 4 4 10%	3473
	21st to end	Night	24% 22% 8% 14% 8% 4 4 3 3 3 7%	5236

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	53% 40% 4 3	1340
	11th to 20th	Day	51% 43% 3 2	1448
	21st to end	Day	52% 38% 4 4 1	1762
February	1st to 10th	Day	51% 37% 5% 6% 1	1620
	11th to 20th	Day	47% 40% 6% 6% 2	1582
	21st to end	Day	44% 39% 6% 9% 2	1453
March	1st to 10th	Day	34% 38% 9% 13% 4 1	1899
	11th to 20th	Day	35% 38% 9% 12% 6% 1	1980
	21st to end	Day	33% 38% 8% 12% 7% 1	2178
April	1st to 10th	Day	33% 38% 8% 13% 7% 1	2089
	11th to 20th	Day	30% 37% 6% 15% 11% 1	2312
	21st to end	Day	32% 38% 8% 12% 8% 2	2340
May	1st to 10th	Day	27% 38% 7% 16% 10% 2	2340
	11th to 20th	Day	28% 38% 6% 15% 13% 1	2396
	21st to end	Day	23% 38% 10% 15% 12% 2	2733
June	1st to 10th	Day	26% 39% 9% 16% 9%	2700
	11th to 20th	Day	22% 37% 8% 19% 12%	2700
	21st to end	Day	26% 38% 11% 17% 7% 1	2700
July	1st to 10th	Day	20% 40% 12% 17% 10% 1	2626
	11th to 20th	Day	21% 40% 11% 19% 8% 1	2572
	21st to end	Day	22% 42% 10% 16% 10%	2448
August	1st to 10th	Day	24% 38% 11% 18% 7% 1	2314
	11th to 20th	Day	20% 38% 12% 19% 10%	2303
	21st to end	Day	26% 39% 9% 17% 9% 1	2574
September	1st to 10th	Day	25% 38% 9% 19% 8% 1	2125
	11th to 20th	Day	27% 39% 12% 17% 5%	1980
	21st to end	Day	32% 39% 9% 14% 5%	1980
October	1st to 10th	Day	33% 39% 8% 14% 6%	1836
	11th to 20th	Day	44% 38% 7% 7% 4	1800
	21st to end	Day	45% 39% 7% 7% 2	1882
November	1st to 10th	Day	37% 42% 8% 11% 3	1637
	11th to 20th	Day	42% 41% 8% 5% 4	1520
	21st to end	Day	49% 42% 6% 3	1520
December	1st to 10th	Day	54% 41% 3 2	1520
	11th to 20th	Day	56% 34% 6% 3	1719
	21st to end	Day	67% 26% 5% 1	2431
January	1st to 10th	Night	52% 40% 5% 3	2824
	11th to 20th	Night	51% 42% 5% 2	2541
	21st to end	Night	50% 39% 6% 4 1	2592
February	1st to 10th	Night	49% 37% 6% 6% 1	2340
	11th to 20th	Night	49% 39% 5% 5% 1	2275
	21st to end	Night	42% 38% 10% 8% 2	1802
March	1st to 10th	Night	36% 37% 10% 12% 5%	2059
	11th to 20th	Night	35% 37% 8% 12% 7% 1	1980
	21st to end	Night	32% 37% 8% 14% 7% 1	2178
April	1st to 10th	Night	31% 36% 8% 16% 8% 1	1886
	11th to 20th	Night	29% 36% 6% 15% 12% 1	1597
	21st to end	Night	33% 37% 6% 11% 11% 2	1620
May	1st to 10th	Night	31% 37% 7% 12% 10% 2	1620
	11th to 20th	Night	26% 38% 5% 16% 12% 2	1502
	21st to end	Night	21% 39% 10% 15% 12% 3	1539
June	1st to 10th	Night	32% 38% 6% 13% 10% 1	1260
	11th to 20th	Night	24% 37% 8% 17% 13%	1260
	21st to end	Night	28% 39% 10% 15% 7% 1	1260
July	1st to 10th	Night	19% 39% 8% 19% 14% 2	1225
	11th to 20th	Night	22% 40% 8% 18% 10% 2	1261
	21st to end	Night	21% 42% 9% 19% 9%	1629
August	1st to 10th	Night	24% 39% 11% 18% 8% 1	1600
	11th to 20th	Night	20% 38% 13% 16% 12% 1	1598
	21st to end	Night	25% 38% 11% 17% 9% 2	1782
September	1st to 10th	Night	23% 38% 10% 18% 10% 2	1813
	11th to 20th	Night	26% 38% 12% 18% 6%	1980
	21st to end	Night	32% 38% 9% 15% 6%	1980
October	1st to 10th	Night	34% 37% 7% 15% 7%	2124
	11th to 20th	Night	44% 38% 7% 8% 2	2160
	21st to end	Night	47% 39% 6% 7% 3	2470
November	1st to 10th	Night	34% 43% 8% 12% 3	2439
	11th to 20th	Night	43% 40% 8% 5% 2 1	2660
	21st to end	Night	48% 40% 8% 4	2660
December	1st to 10th	Night	54% 41% 5% 1	2660
	11th to 20th	Night	55% 35% 7% 3	3472
	21st to end	Night	66% 27% 5% 1	5236

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	25% 16% 29% 22% 7% 2	186
	11th to 20th	Day	23% 9% 36% 22% 10%	243
	21st to end	Day	28% 8% 29% 27% 8%	264
February	1st to 10th	Day	25% 22% 30% 17% 5%	240
	11th to 20th	Day	19% 17% 35% 26% 3	240
	21st to end	Day	24% 22% 35% 16% 2	198
March	1st to 10th	Day	19% 9% 37% 27% 7% 1	339
	11th to 20th	Day	20% 12% 31% 28% 7% 2	420
	21st to end	Day	24% 8% 29% 29% 8% 2	462
April	1st to 10th	Day	20% 9% 32% 28% 9% 2	420
	11th to 20th	Day	25% 9% 26% 22% 13% 5%	412
	21st to end	Day	24% 8% 25% 30% 11% 2	420
May	1st to 10th	Day	22% 4 26% 34% 13% 2	423
	11th to 20th	Day	18% 4 21% 32% 21% 5%	447
	21st to end	Day	12% 3 20% 37% 21% 6%	493
June	1st to 10th	Day	12% 3 24% 40% 19% 3	630
	11th to 20th	Day	13% 4 22% 37% 21% 4	630
	21st to end	Day	15% 4 19% 38% 20% 3	630
July	1st to 10th	Day	8% 3 15% 40% 30% 4	621
	11th to 20th	Day	12% 3 18% 36% 24% 7%	618
	21st to end	Day	12% 2 19% 35% 27% 4	533
August	1st to 10th	Day	14% 5% 15% 32% 28% 6%	446
	11th to 20th	Day	12% 3 10% 34% 31% 9%	439
	21st to end	Day	17% 6% 19% 28% 23% 7%	495
September	1st to 10th	Day	16% 5% 20% 27% 24% 8%	450
	11th to 20th	Day	12% 6% 18% 28% 28% 7%	450
	21st to end	Day	19% 8% 21% 25% 22% 6%	450
October	1st to 10th	Day	24% 11% 22% 22% 17% 4	450
	11th to 20th	Day	28% 14% 28% 22% 6% 1	450
	21st to end	Day	33% 15% 26% 21% 4 1	397
November	1st to 10th	Day	24% 13% 36% 18% 8%	253
	11th to 20th	Day	24% 19% 34% 19% 3 1	180
	21st to end	Day	28% 19% 35% 16% 2	180
December	1st to 10th	Day	28% 21% 31% 14% 6%	180
	11th to 20th	Day	26% 26% 27% 18% 3	180
	21st to end	Day	31% 22% 36% 8% 2 1	198
January	1st to 10th	Night	25% 19% 29% 21% 6% 1	950
	11th to 20th	Night	28% 15% 29% 22% 6% 1	908
	21st to end	Night	21% 11% 32% 25% 9% 2	810
February	1st to 10th	Night	20% 18% 36% 21% 5%	720
	11th to 20th	Night	22% 10% 35% 25% 8% 1	703
	21st to end	Night	22% 13% 36% 23% 5%	592
March	1st to 10th	Night	18% 10% 34% 30% 8% 1	720
	11th to 20th	Night	18% 12% 33% 26% 9% 1	720
	21st to end	Night	20% 6% 28% 32% 13% 1	792
April	1st to 10th	Night	25% 9% 28% 29% 8% 1	702
	11th to 20th	Night	21% 9% 25% 29% 13% 3	533
	21st to end	Night	23% 8% 23% 29% 15% 2	540
May	1st to 10th	Night	17% 2 23% 36% 19% 2	540
	11th to 20th	Night	18% 3 16% 38% 24% 2	440
	21st to end	Night	13% 3 19% 39% 22% 4	389
June	1st to 10th	Night	17% 4 19% 38% 19% 3	360
	11th to 20th	Night	13% 3 18% 40% 23% 3	360
	21st to end	Night	11% 3 20% 34% 29% 3	360
July	1st to 10th	Night	8% 3 12% 47% 25% 4	350
	11th to 20th	Night	10% 2 18% 33% 31% 5%	391
	21st to end	Night	9% 3 14% 41% 29% 3	591
August	1st to 10th	Night	10% 2 16% 36% 29% 6%	537
	11th to 20th	Night	11% 3 15% 30% 32% 9%	534
	21st to end	Night	14% 5% 20% 30% 27% 6%	594
September	1st to 10th	Night	13% 5% 18% 32% 24% 9%	697
	11th to 20th	Night	13% 6% 21% 29% 24% 7%	720
	21st to end	Night	20% 8% 19% 22% 23% 8%	720
October	1st to 10th	Night	23% 8% 24% 24% 18% 3	720
	11th to 20th	Night	29% 14% 23% 24% 8% 2	720
	21st to end	Night	34% 12% 27% 22% 5% 1	792
November	1st to 10th	Night	27% 15% 26% 22% 9% 1	740
	11th to 20th	Night	25% 22% 27% 19% 7%	760
	21st to end	Night	34% 17% 28% 17% 4	760
December	1st to 10th	Night	33% 19% 29% 12% 7%	760
	11th to 20th	Night	27% 22% 33% 13% 5%	897
	21st to end	Night	33% 16% 32% 15% 4	1045

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