Angel Light Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Angel Light Observatory clear sky chart from 2005-11-06 to 2024-03-01.

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	17% 12% 5% 8% 7% 4 5% 4 5% 6% 27%	78378
	Night	16% 13% 6% 10% 7% 3 4 4 4 6% 27%	81022
Transparency	Day	57% 6% 11% 17% 8% 1	78083
	Night	58% 6% 11% 17% 8% 1	80851
Seeing	Day	25% 19% 29% 17% 7% 2	13712
	Night	24% 19% 28% 21% 7% 1	23082

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	12% 20% 12% 13% 6% 3 4 4 3 4 17%	2517
	11th to 20th	Day	13% 21% 10% 16% 8% 6% 3 3 3 4 13%	2520
	21st to end	Day	17% 20% 9% 13% 11% 4 4 3 4 4 11%	3065
February	1st to 10th	Day	14% 17% 9% 13% 11% 4 6% 5% 4 4 13%	1710
	11th to 20th	Day	13% 15% 8% 18% 11% 4 5% 4 3 4 15%	1712
	21st to end	Day	18% 16% 7% 12% 11% 3 4 4 3 5% 19%	1570
March	1st to 10th	Day	14% 13% 9% 16% 8% 3 4 3 4 5% 20%	1818
	11th to 20th	Day	17% 17% 7% 9% 6% 3 4 3 3 5% 27%	1980
	21st to end	Day	19% 14% 6% 8% 7% 3 3 3 4 6% 28%	2284
April	1st to 10th	Day	16% 16% 7% 8% 6% 3 3 3 4 5% 28%	2160
	11th to 20th	Day	17% 13% 4 5% 6% 2 4 4 5% 5% 34%	2134
	21st to end	Day	20% 16% 3 6% 5% 3 4 4 4 5% 31%	2160
May	1st to 10th	Day	20% 11% 4 7% 6% 3 4 4 4 6% 30%	2493
	11th to 20th	Day	18% 8% 4 6% 6% 3 5% 4 5% 8% 34%	2478
	21st to end	Day	14% 7% 3 6% 5% 3 5% 5% 6% 9% 36%	2725
June	1st to 10th	Day	20% 7% 4 6% 6% 3 4 5% 5% 8% 31%	2520
	11th to 20th	Day	18% 7% 3 4 6% 4 5% 5% 6% 9% 33%	2520
	21st to end	Day	16% 8% 2 6% 6% 4 6% 6% 6% 9% 32%	2520
July	1st to 10th	Day	14% 7% 3 5% 5% 3 6% 6% 6% 9% 36%	2520
	11th to 20th	Day	13% 6% 3 6% 6% 4 6% 6% 6% 9% 35%	2520
	21st to end	Day	14% 6% 3 7% 6% 5% 6% 7% 7% 8% 32%	2744
August	1st to 10th	Day	13% 6% 3 5% 7% 4 7% 6% 8% 9% 33%	2360
	11th to 20th	Day	15% 7% 3 7% 7% 4 6% 6% 8% 8% 30%	2227
	21st to end	Day	16% 7% 3 6% 6% 4 5% 6% 6% 7% 34%	2376
September	1st to 10th	Day	17% 6% 2 6% 5% 3 5% 6% 6% 8% 36%	2160
	11th to 20th	Day	17% 7% 3 6% 4 3 4 3 4 6% 41%	2160
	21st to end	Day	17% 8% 2 6% 5% 4 5% 6% 6% 5% 35%	1926
October	1st to 10th	Day	20% 10% 3 7% 6% 4 5% 5% 5% 6% 30%	1800
	11th to 20th	Day	20% 10% 3 6% 6% 4 5% 5% 6% 7% 29%	1800
	21st to end	Day	25% 19% 3 6% 6% 3 4 4 4 5% 22%	1890
November	1st to 10th	Day	14% 15% 3 6% 7% 3 5% 5% 4 6% 32%	1671
	11th to 20th	Day	17% 17% 5% 9% 6% 3 5% 4 4 5% 25%	1620
	21st to end	Day	17% 18% 6% 11% 6% 3 4 4 4 7% 19%	1520
December	1st to 10th	Day	17% 22% 8% 14% 6% 2 3 3 5% 5% 13%	1520
	11th to 20th	Day	20% 17% 9% 12% 6% 2 3 3 4 4 20%	1906
	21st to end	Day	20% 21% 10% 10% 8% 4 4 4 3 4 11%	2772
January	1st to 10th	Night	15% 16% 10% 18% 9% 4 4 3 4 4 12%	4429
	11th to 20th	Night	11% 19% 10% 15% 11% 4 4 3 3 4 15%	4440
	21st to end	Night	19% 17% 8% 15% 12% 5% 4 3 3 3 11%	4587
February	1st to 10th	Night	13% 17% 8% 15% 9% 4 5% 4 4 4 17%	2470
	11th to 20th	Night	11% 14% 8% 16% 12% 4 6% 5% 4 3 17%	2352
	21st to end	Night	17% 14% 6% 14% 9% 3 5% 3 5% 6% 18%	1884
March	1st to 10th	Night	14% 12% 8% 14% 9% 4 5% 3 4 5% 22%	2135
	11th to 20th	Night	14% 16% 8% 10% 8% 3 4 4 4 4 26%	1979
	21st to end	Night	17% 15% 5% 8% 5% 3 5% 4 4 5% 28%	2071
April	1st to 10th	Night	17% 15% 5% 8% 6% 4 5% 4 4 6% 27%	1800
	11th to 20th	Night	17% 13% 3 7% 5% 3 4 4 5% 6% 32%	1775
	21st to end	Night	20% 12% 4 6% 6% 2 4 4 4 5% 33%	1800
May	1st to 10th	Night	20% 12% 3 7% 5% 3 4 4 5% 6% 32%	1481
	11th to 20th	Night	13% 7% 2 5% 5% 3 5% 5% 5% 10% 38%	1415
	21st to end	Night	14% 6% 3 6% 6% 3 4 6% 6% 7% 40%	1550
June	1st to 10th	Night	17% 6% 3 5% 5% 3 4 5% 7% 7% 37%	1440
	11th to 20th	Night	18% 7% 3 4 4 4 5% 5% 5% 9% 37%	1440
	21st to end	Night	15% 4 2 5% 5% 3 5% 5% 6% 7% 43%	1440
July	1st to 10th	Night	8% 6% 2 5% 6% 3 6% 6% 6% 9% 43%	1440
	11th to 20th	Night	11% 4 2 5% 5% 3 5% 5% 6% 8% 45%	1440
	21st to end	Night	13% 5% 2 4 5% 3 4 5% 6% 8% 44%	1574
August	1st to 10th	Night	11% 5% 2 4 5% 3 6% 5% 5% 10% 44%	1575
	11th to 20th	Night	12% 6% 3 6% 7% 4 5% 6% 6% 8% 38%	1656
	21st to end	Night	14% 6% 2 6% 4 3 4 5% 6% 8% 43%	1980
September	1st to 10th	Night	15% 7% 3 5% 5% 3 4 5% 5% 7% 42%	1800
	11th to 20th	Night	16% 7% 2 5% 4 3 2 4 4 6% 47%	1800
	21st to end	Night	17% 8% 2 5% 5% 3 5% 5% 5% 6% 38%	2031
October	1st to 10th	Night	20% 9% 3 6% 6% 3 4 5% 5% 5% 34%	2160
	11th to 20th	Night	21% 11% 3 6% 6% 2 5% 4 4 6% 32%	2160
	21st to end	Night	26% 16% 4 7% 5% 3 3 4 3 5% 23%	2464
November	1st to 10th	Night	13% 16% 5% 8% 5% 2 4 4 4 5% 34%	2407
	11th to 20th	Night	19% 16% 6% 7% 6% 3 4 5% 4 5% 26%	2555
	21st to end	Night	15% 16% 9% 10% 7% 3 4 4 5% 6% 21%	2660
December	1st to 10th	Night	18% 20% 10% 12% 6% 3 4 4 3 4 16%	2660
	11th to 20th	Night	18% 16% 8% 13% 6% 3 4 4 4 5% 18%	3288
	21st to end	Night	24% 17% 10% 12% 6% 3 3 3 4 5% 13%	4884

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	70% 4 10% 12% 4	2518
	11th to 20th	Day	75% 5% 7% 9% 4 1	2520
	21st to end	Day	77% 4 8% 8% 3 1	3065
February	1st to 10th	Day	72% 5% 8% 12% 3	1710
	11th to 20th	Day	71% 5% 6% 14% 4	1721
	21st to end	Day	69% 4 8% 13% 6%	1570
March	1st to 10th	Day	66% 5% 11% 12% 5% 1	1821
	11th to 20th	Day	62% 3 9% 14% 10% 2	1980
	21st to end	Day	59% 4 8% 15% 13% 2	2284
April	1st to 10th	Day	58% 4 8% 20% 9% 3	2160
	11th to 20th	Day	52% 5% 10% 20% 12% 1	2134
	21st to end	Day	56% 4 8% 17% 11% 3	2160
May	1st to 10th	Day	56% 5% 8% 19% 11% 2	2493
	11th to 20th	Day	49% 6% 10% 23% 11% 2	2482
	21st to end	Day	44% 9% 16% 20% 10% 1	2725
June	1st to 10th	Day	49% 8% 10% 20% 12% 1	2520
	11th to 20th	Day	47% 7% 12% 22% 10% 1	2520
	21st to end	Day	47% 8% 13% 20% 10% 1	2520
July	1st to 10th	Day	43% 9% 12% 24% 12% 1	2450
	11th to 20th	Day	44% 7% 15% 26% 8%	2445
	21st to end	Day	46% 9% 13% 21% 10% 1	2591
August	1st to 10th	Day	44% 9% 15% 22% 10% 1	2346
	11th to 20th	Day	49% 8% 14% 22% 7%	2227
	21st to end	Day	46% 8% 11% 23% 10% 1	2376
September	1st to 10th	Day	46% 6% 12% 25% 10% 1	2160
	11th to 20th	Day	44% 5% 10% 24% 17% 1	2160
	21st to end	Day	49% 6% 11% 22% 12% 1	1926
October	1st to 10th	Day	53% 6% 10% 17% 12% 1	1800
	11th to 20th	Day	53% 6% 14% 21% 6% 1	1800
	21st to end	Day	64% 4 12% 12% 6% 1	1890
November	1st to 10th	Day	51% 7% 12% 18% 11% 1	1671
	11th to 20th	Day	61% 5% 10% 14% 7% 2	1620
	21st to end	Day	66% 5% 12% 12% 4	1520
December	1st to 10th	Day	74% 4 10% 9% 3	1520
	11th to 20th	Day	70% 4 13% 9% 3	1906
	21st to end	Day	77% 4 6% 9% 3	2772
January	1st to 10th	Night	73% 6% 10% 7% 3	4429
	11th to 20th	Night	72% 5% 7% 10% 6%	4440
	21st to end	Night	78% 4 7% 9% 3 1	4587
February	1st to 10th	Night	69% 4 9% 13% 5%	2470
	11th to 20th	Night	69% 6% 7% 13% 4 1	2354
	21st to end	Night	64% 5% 10% 13% 7% 1	1884
March	1st to 10th	Night	63% 5% 11% 15% 6%	2145
	11th to 20th	Night	60% 6% 10% 17% 8% 1	1980
	21st to end	Night	58% 4 8% 16% 11% 2	2071
April	1st to 10th	Night	58% 6% 10% 17% 8% 2	1800
	11th to 20th	Night	52% 5% 10% 18% 14% 1	1775
	21st to end	Night	53% 4 10% 18% 12% 3	1800
May	1st to 10th	Night	54% 5% 9% 17% 13% 2	1481
	11th to 20th	Night	43% 7% 12% 24% 13% 2	1416
	21st to end	Night	42% 7% 16% 22% 11% 2	1551
June	1st to 10th	Night	44% 6% 12% 23% 14% 1	1440
	11th to 20th	Night	43% 7% 15% 21% 13% 1	1440
	21st to end	Night	39% 7% 13% 28% 12% 1	1440
July	1st to 10th	Night	34% 9% 15% 25% 15% 2	1400
	11th to 20th	Night	36% 8% 15% 28% 12% 1	1392
	21st to end	Night	38% 7% 14% 24% 15% 2	1486
August	1st to 10th	Night	35% 9% 16% 28% 11% 1	1565
	11th to 20th	Night	42% 7% 18% 25% 8%	1656
	21st to end	Night	39% 7% 15% 25% 12% 1	1980
September	1st to 10th	Night	41% 6% 15% 26% 13% 1	1800
	11th to 20th	Night	38% 5% 11% 29% 17% 1	1800
	21st to end	Night	45% 5% 12% 24% 13% 1	2031
October	1st to 10th	Night	51% 6% 12% 19% 12% 1	2160
	11th to 20th	Night	52% 6% 14% 18% 9% 1	2160
	21st to end	Night	63% 4 11% 14% 7% 1	2464
November	1st to 10th	Night	52% 5% 11% 18% 12% 2	2407
	11th to 20th	Night	58% 5% 11% 16% 7% 3	2555
	21st to end	Night	62% 5% 15% 15% 4	2660
December	1st to 10th	Night	72% 5% 11% 10% 2	2660
	11th to 20th	Night	68% 6% 9% 12% 4	3288
	21st to end	Night	73% 5% 8% 10% 3	4884

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	30% 27% 30% 10% 3	180
	11th to 20th	Day	40% 28% 26% 5% 1	180
	21st to end	Day	27% 33% 30% 8% 2	282
February	1st to 10th	Day	36% 33% 27% 5%	270
	11th to 20th	Day	36% 33% 27% 3	319
	21st to end	Day	38% 25% 27% 8% 2	382
March	1st to 10th	Day	32% 26% 29% 11% 2	420
	11th to 20th	Day	35% 25% 26% 12% 2	420
	21st to end	Day	35% 23% 27% 10% 3 1	462
April	1st to 10th	Day	33% 22% 32% 10% 4	420
	11th to 20th	Day	32% 25% 23% 11% 7% 3	412
	21st to end	Day	34% 20% 26% 14% 4 1	420
May	1st to 10th	Day	28% 20% 30% 12% 7% 2	423
	11th to 20th	Day	22% 15% 37% 17% 8% 1	447
	21st to end	Day	18% 15% 32% 22% 10% 2	485
June	1st to 10th	Day	24% 14% 30% 23% 8% 1	450
	11th to 20th	Day	20% 14% 32% 23% 8% 3	450
	21st to end	Day	18% 17% 36% 20% 8% 1	450
July	1st to 10th	Day	16% 8% 29% 30% 14% 3	445
	11th to 20th	Day	13% 12% 30% 30% 13% 4	444
	21st to end	Day	13% 9% 41% 25% 9% 3	488
August	1st to 10th	Day	14% 8% 29% 28% 15% 6%	446
	11th to 20th	Day	16% 10% 28% 27% 12% 7%	439
	21st to end	Day	13% 16% 30% 23% 12% 6%	495
September	1st to 10th	Day	19% 12% 27% 22% 14% 6%	450
	11th to 20th	Day	16% 12% 27% 21% 16% 7%	450
	21st to end	Day	20% 17% 27% 20% 12% 4	450
October	1st to 10th	Day	25% 16% 27% 14% 15% 3	450
	11th to 20th	Day	27% 20% 28% 17% 6% 1	450
	21st to end	Day	37% 19% 27% 14% 3	450
November	1st to 10th	Day	27% 23% 24% 17% 8%	365
	11th to 20th	Day	31% 27% 24% 13% 4	280
	21st to end	Day	36% 25% 34% 6%	180
December	1st to 10th	Day	41% 21% 28% 7% 3 1	180
	11th to 20th	Day	37% 28% 27% 7% 1	180
	21st to end	Day	32% 29% 33% 5% 1	198
January	1st to 10th	Night	26% 30% 31% 12% 1	760
	11th to 20th	Night	27% 26% 29% 14% 3	760
	21st to end	Night	22% 28% 33% 15% 2	836
February	1st to 10th	Night	28% 25% 32% 14% 1	760
	11th to 20th	Night	27% 29% 31% 12% 1	743
	21st to end	Night	30% 27% 26% 14% 3	628
March	1st to 10th	Night	26% 26% 30% 16% 2	723
	11th to 20th	Night	28% 24% 30% 16% 2	720
	21st to end	Night	28% 22% 27% 17% 6% 1	792
April	1st to 10th	Night	28% 20% 32% 18% 3	720
	11th to 20th	Night	26% 22% 26% 17% 8% 1	711
	21st to end	Night	28% 15% 29% 22% 5%	720
May	1st to 10th	Night	25% 14% 25% 27% 8%	401
	11th to 20th	Night	17% 11% 34% 30% 7% 1	354
	21st to end	Night	15% 9% 34% 30% 11% 1	389
June	1st to 10th	Night	19% 8% 27% 34% 11% 1	360
	11th to 20th	Night	18% 12% 23% 34% 14%	360
	21st to end	Night	14% 6% 30% 41% 9%	360
July	1st to 10th	Night	9% 4 27% 43% 15% 2	350
	11th to 20th	Night	12% 4 24% 41% 19% 1	348
	21st to end	Night	15% 5% 24% 42% 13% 1	394
August	1st to 10th	Night	13% 5% 28% 33% 19% 1	501
	11th to 20th	Night	15% 5% 21% 34% 23% 3	592
	21st to end	Night	15% 7% 24% 33% 19% 1	792
September	1st to 10th	Night	17% 9% 26% 31% 14% 2	720
	11th to 20th	Night	18% 9% 23% 29% 18% 4	720
	21st to end	Night	19% 13% 25% 25% 17% 2	720
October	1st to 10th	Night	25% 15% 24% 20% 14% 2	720
	11th to 20th	Night	27% 16% 31% 21% 5% 1	720
	21st to end	Night	34% 18% 27% 17% 3	792
November	1st to 10th	Night	24% 18% 32% 18% 7%	740
	11th to 20th	Night	31% 21% 28% 14% 5% 1	760
	21st to end	Night	26% 25% 29% 16% 4	760
December	1st to 10th	Night	32% 28% 25% 13% 3	760
	11th to 20th	Night	30% 26% 29% 13% 2	760
	21st to end	Night	30% 31% 28% 10% 2	836

Page generated at 2024-02-27 06:30:16. Version ac07

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