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 2024-11-05.

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	15% 11% 4 9% 5% 4 4 3 5% 7% 34%	101519
	Night	15% 13% 5% 11% 6% 5% 4 3 5% 6% 28%	97587
Transparency	Day	27% 48% 7% 12% 6% 1	105063
	Night	30% 48% 6% 10% 5%	102131
Seeing	Day	19% 9% 25% 29% 16% 3	15249
	Night	21% 11% 26% 26% 13% 2	24461

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	17% 23% 15% 16% 8% 4 4 2 2 3 7%	2219
	11th to 20th	Day	11% 23% 10% 28% 9% 5% 4 2 2 2 4	2530
	21st to end	Day	20% 21% 10% 15% 6% 5% 4 3 4 4 8%	3091
February	1st to 10th	Day	13% 21% 7% 16% 10% 5% 4 3 3 5% 15%	1710
	11th to 20th	Day	12% 16% 7% 13% 8% 6% 5% 4 5% 7% 18%	1663
	21st to end	Day	13% 18% 6% 12% 6% 6% 5% 3 5% 5% 21%	1541
March	1st to 10th	Day	16% 13% 4 8% 5% 4 4 3 5% 5% 32%	3300
	11th to 20th	Day	11% 15% 6% 12% 5% 5% 5% 3 5% 6% 26%	3480
	21st to end	Day	14% 11% 5% 9% 5% 4 5% 3 5% 6% 32%	3815
April	1st to 10th	Day	19% 12% 3 8% 4 4 4 4 5% 6% 31%	2206
	11th to 20th	Day	19% 10% 2 6% 4 4 3 3 5% 6% 39%	2442
	21st to end	Day	19% 9% 3 6% 3 4 3 3 5% 7% 37%	2470
May	1st to 10th	Day	16% 7% 2 7% 4 3 4 3 5% 7% 42%	4108
	11th to 20th	Day	13% 6% 2 6% 4 4 4 3 6% 8% 44%	4256
	21st to end	Day	14% 6% 2 5% 3 4 4 3 5% 7% 46%	4859
June	1st to 10th	Day	12% 7% 2 7% 4 4 5% 4 6% 8% 40%	2850
	11th to 20th	Day	10% 6% 2 6% 4 4 5% 4 6% 8% 44%	2850
	21st to end	Day	13% 7% 3 6% 4 4 4 4 6% 8% 41%	2850
July	1st to 10th	Day	13% 7% 2 7% 4 4 5% 4 6% 8% 39%	4760
	11th to 20th	Day	10% 4 2 5% 3 4 3 3 5% 8% 52%	4671
	21st to end	Day	9% 5% 2 4 4 3 3 3 6% 8% 53%	4539
August	1st to 10th	Day	11% 5% 2 5% 4 4 4 4 7% 9% 45%	4104
	11th to 20th	Day	15% 6% 2 5% 3 4 4 4 6% 9% 42%	4044
	21st to end	Day	9% 6% 2 6% 3 4 5% 4 5% 9% 46%	4532
September	1st to 10th	Day	11% 7% 2 7% 5% 4 5% 4 5% 8% 43%	2242
	11th to 20th	Day	12% 5% 2 6% 3 4 5% 4 6% 7% 46%	2090
	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% 16% 5% 10% 6% 5% 5% 3 5% 7% 21%	1673
	11th to 20th	Day	17% 19% 4 11% 6% 6% 4 4 6% 6% 16%	1520
	21st to end	Day	23% 22% 6% 12% 6% 5% 4 4 4 4 10%	1520
December	1st to 10th	Day	17% 29% 7% 14% 6% 6% 5% 3 3 3 7%	1520
	11th to 20th	Day	20% 24% 7% 16% 7% 4 4 3 4 4 9%	1719
	21st to end	Day	26% 24% 8% 17% 6% 4 3 3 3 3 4	2431
January	1st to 10th	Night	16% 22% 10% 20% 8% 5% 4 3 3 3 7%	4734
	11th to 20th	Night	14% 17% 10% 25% 10% 6% 4 3 3 3 6%	4440
	21st to end	Night	16% 19% 8% 16% 7% 7% 5% 3 4 4 11%	4564
February	1st to 10th	Night	11% 21% 6% 18% 8% 5% 4 3 5% 5% 14%	2470
	11th to 20th	Night	12% 17% 6% 13% 7% 5% 5% 3 5% 5% 20%	2403
	21st to end	Night	14% 16% 6% 12% 6% 5% 5% 4 4 5% 24%	1912
March	1st to 10th	Night	13% 10% 3 8% 6% 4 4 3 4 6% 38%	3538
	11th to 20th	Night	10% 13% 4 9% 5% 5% 4 4 5% 6% 35%	3479
	21st to end	Night	13% 9% 5% 10% 6% 6% 4 4 6% 6% 32%	3817
April	1st to 10th	Night	19% 12% 3 7% 5% 4 4 4 4 6% 32%	1990
	11th to 20th	Night	18% 9% 2 6% 4 3 5% 3 5% 7% 37%	1687
	21st to end	Night	20% 9% 3 7% 5% 4 4 4 5% 6% 32%	1710
May	1st to 10th	Night	14% 7% 3 7% 5% 5% 5% 4 6% 7% 38%	2833
	11th to 20th	Night	12% 7% 2 7% 5% 5% 4 3 6% 8% 41%	2637
	21st to end	Night	12% 6% 3 6% 4 4 4 4 5% 7% 44%	2713
June	1st to 10th	Night	12% 9% 2 6% 6% 5% 4 3 4 8% 40%	1330
	11th to 20th	Night	9% 6% 2 5% 5% 5% 4 5% 6% 10% 43%	1330
	21st to end	Night	12% 6% 2 6% 5% 5% 4 5% 6% 9% 40%	1330
July	1st to 10th	Night	11% 7% 2 6% 4 3 4 3 5% 8% 46%	2210
	11th to 20th	Night	11% 4 2 5% 4 4 4 4 5% 7% 50%	2295
	21st to end	Night	8% 6% 3 5% 4 3 4 3 5% 9% 51%	3015
August	1st to 10th	Night	10% 7% 2 5% 4 4 5% 4 6% 8% 45%	2825
	11th to 20th	Night	12% 6% 2 6% 4 4 6% 4 6% 8% 42%	2785
	21st to end	Night	10% 6% 2 7% 4 4 4 3 6% 8% 47%	3124
September	1st to 10th	Night	11% 5% 2 5% 4 4 5% 4 6% 8% 46%	1914
	11th to 20th	Night	11% 4 2 6% 4 3 4 4 7% 7% 48%	2090
	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 10% 5% 5% 4 3 5% 6% 24%	2503
	11th to 20th	Night	17% 20% 5% 12% 6% 4 5% 4 5% 6% 17%	2660
	21st to end	Night	23% 22% 6% 10% 7% 5% 4 3 5% 5% 11%	2660
December	1st to 10th	Night	20% 26% 8% 16% 7% 4 4 3 3 3 7%	2660
	11th to 20th	Night	18% 23% 7% 17% 6% 5% 4 3 4 4 10%	3473
	21st to end	Night	24% 23% 8% 16% 6% 5% 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	55% 40% 4 1	2220
	11th to 20th	Day	60% 37% 2 2	2530
	21st to end	Day	49% 44% 4 2 1	3091
February	1st to 10th	Day	25% 66% 3 4 1	1710
	11th to 20th	Day	22% 65% 6% 4 3	1672
	21st to end	Day	22% 65% 4 6% 3	1541
March	1st to 10th	Day	34% 43% 9% 8% 6%	3303
	11th to 20th	Day	36% 44% 6% 11% 2	3480
	21st to end	Day	29% 45% 10% 11% 5% 1	3815
April	1st to 10th	Day	21% 65% 3 7% 4	2206
	11th to 20th	Day	18% 65% 2 9% 4 1	2442
	21st to end	Day	16% 65% 5% 8% 5% 1	2470
May	1st to 10th	Day	25% 42% 7% 12% 13% 1	4120
	11th to 20th	Day	21% 44% 12% 16% 6% 1	4260
	21st to end	Day	23% 41% 8% 10% 14% 3	4859
June	1st to 10th	Day	14% 66% 3 10% 6% 1	2850
	11th to 20th	Day	15% 66% 3 9% 6% 1	2850
	21st to end	Day	15% 64% 4 9% 7% 1	2850
July	1st to 10th	Day	31% 38% 10% 13% 7% 1	4686
	11th to 20th	Day	19% 36% 11% 25% 9%	4590
	21st to end	Day	18% 36% 7% 24% 14% 1	4395
August	1st to 10th	Day	21% 38% 10% 19% 10% 1	4091
	11th to 20th	Day	28% 39% 10% 17% 6%	4083
	21st to end	Day	21% 39% 9% 25% 7%	4531
September	1st to 10th	Day	16% 62% 6% 10% 5%	2242
	11th to 20th	Day	13% 61% 4 15% 7%	2090
	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	29% 59% 5% 4 2	1673
	11th to 20th	Day	28% 61% 6% 4 1	1520
	21st to end	Day	29% 62% 5% 3 1	1520
December	1st to 10th	Day	32% 62% 3 2 1	1520
	11th to 20th	Day	42% 49% 5% 3	1719
	21st to end	Day	55% 39% 5% 1	2431
January	1st to 10th	Night	55% 38% 5% 2	4734
	11th to 20th	Night	55% 39% 3 2	4440
	21st to end	Night	48% 43% 5% 3 1	4564
February	1st to 10th	Night	24% 66% 2 5% 2	2470
	11th to 20th	Night	20% 65% 4 9% 3	2405
	21st to end	Night	21% 66% 4 5% 4	1912
March	1st to 10th	Night	27% 44% 10% 12% 7%	3548
	11th to 20th	Night	27% 42% 10% 14% 5% 1	3480
	21st to end	Night	30% 44% 9% 11% 5% 1	3817
April	1st to 10th	Night	23% 65% 2 6% 4	1990
	11th to 20th	Night	18% 66% 2 7% 6% 1	1687
	21st to end	Night	20% 66% 3 5% 5% 1	1710
May	1st to 10th	Night	25% 43% 8% 11% 12% 1	2840
	11th to 20th	Night	24% 43% 6% 14% 11% 1	2638
	21st to end	Night	25% 41% 7% 13% 11% 2	2714
June	1st to 10th	Night	12% 65% 2 10% 9% 1	1330
	11th to 20th	Night	12% 65% 5% 10% 7% 1	1330
	21st to end	Night	13% 64% 4 11% 7% 1	1330
July	1st to 10th	Night	27% 37% 9% 14% 10% 4	2175
	11th to 20th	Night	21% 37% 6% 27% 9%	2253
	21st to end	Night	22% 36% 6% 15% 20% 1	2919
August	1st to 10th	Night	24% 38% 9% 19% 10% 1	2814
	11th to 20th	Night	29% 39% 12% 12% 7%	2812
	21st to end	Night	22% 39% 7% 24% 9%	3124
September	1st to 10th	Night	14% 61% 6% 13% 6%	1914
	11th to 20th	Night	13% 61% 5% 12% 9%	2090
	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% 59% 5% 6% 1	2503
	11th to 20th	Night	27% 61% 8% 4 1	2660
	21st to end	Night	32% 61% 5% 2 1	2660
December	1st to 10th	Night	31% 62% 4 2 1	2660
	11th to 20th	Night	40% 52% 6% 2	3472
	21st to end	Night	53% 39% 6% 2	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% 15% 30% 22% 7% 2	186
	11th to 20th	Day	22% 11% 37% 21% 9%	270
	21st to end	Day	28% 7% 28% 28% 9%	297
February	1st to 10th	Day	23% 21% 31% 19% 7%	270
	11th to 20th	Day	18% 16% 34% 27% 5%	270
	21st to end	Day	23% 23% 34% 18% 2	225
March	1st to 10th	Day	19% 10% 37% 27% 7% 1	375
	11th to 20th	Day	19% 12% 31% 28% 8% 2	460
	21st to end	Day	23% 9% 30% 29% 8% 1	506
April	1st to 10th	Day	20% 10% 31% 29% 9% 2	460
	11th to 20th	Day	26% 10% 27% 22% 12% 5%	452
	21st to end	Day	23% 10% 25% 30% 11% 2	460
May	1st to 10th	Day	21% 4 26% 34% 13% 2	463
	11th to 20th	Day	17% 4 20% 32% 21% 5%	487
	21st to end	Day	12% 3 20% 37% 21% 6%	538
June	1st to 10th	Day	13% 3 24% 39% 19% 3	680
	11th to 20th	Day	12% 5% 24% 36% 19% 4	680
	21st to end	Day	17% 5% 20% 36% 19% 3	680
July	1st to 10th	Day	10% 3 17% 38% 28% 3	671
	11th to 20th	Day	12% 4 20% 35% 23% 6%	668
	21st to end	Day	12% 2 21% 34% 26% 4	579
August	1st to 10th	Day	13% 6% 17% 31% 27% 6%	486
	11th to 20th	Day	14% 3 13% 33% 29% 9%	479
	21st to end	Day	17% 6% 20% 28% 23% 6%	539
September	1st to 10th	Day	18% 5% 21% 27% 23% 7%	490
	11th to 20th	Day	11% 6% 18% 28% 29% 8%	490
	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	24% 14% 36% 18% 8%	265
	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	27% 16% 29% 21% 6% 1	950
	21st to end	Night	21% 10% 32% 26% 9% 2	855
February	1st to 10th	Night	19% 17% 36% 22% 5% 1	760
	11th to 20th	Night	21% 11% 34% 26% 7% 1	743
	21st to end	Night	21% 12% 36% 24% 5%	628
March	1st to 10th	Night	17% 9% 35% 30% 8% 1	760
	11th to 20th	Night	18% 13% 33% 27% 9% 1	760
	21st to end	Night	19% 6% 29% 32% 13% 1	836
April	1st to 10th	Night	24% 9% 29% 29% 8% 1	741
	11th to 20th	Night	22% 9% 25% 28% 12% 3	563
	21st to end	Night	24% 8% 23% 28% 15% 2	570
May	1st to 10th	Night	17% 2 23% 37% 19% 2	570
	11th to 20th	Night	17% 3 16% 39% 24% 2	464
	21st to end	Night	14% 3 19% 39% 22% 4	411
June	1st to 10th	Night	16% 6% 19% 37% 19% 3	380
	11th to 20th	Night	13% 4 18% 40% 21% 3	380
	21st to end	Night	13% 4 20% 33% 28% 2	380
July	1st to 10th	Night	8% 3 13% 47% 25% 4	370
	11th to 20th	Night	10% 2 19% 34% 30% 4	414
	21st to end	Night	9% 3 14% 41% 30% 3	624
August	1st to 10th	Night	11% 2 16% 36% 28% 6%	567
	11th to 20th	Night	12% 3 16% 29% 31% 9%	564
	21st to end	Night	14% 4 19% 31% 26% 5%	627
September	1st to 10th	Night	14% 5% 18% 32% 23% 9%	736
	11th to 20th	Night	13% 5% 20% 29% 26% 7%	760
	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% 9% 1	760
	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

Page generated at 2024-11-03 02:11:09. Version ac07

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