Angwin
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Angwin clear sky chart from 2005-11-06 to 2025-08-05.

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	7% 4 1 3 2 2 3 3 4 6% 65%	129574
	Night	9% 5% 1 4 3 3 3 2 4 6% 61%	127377
Transparency	Day	23% 2 7% 19% 25% 23%	129289
	Night	27% 2 8% 19% 23% 20%	127177
Seeing	Day	11% 9% 23% 30% 21% 7%	16272
	Night	11% 5% 15% 34% 31% 5%	25515

By Month

Percentage Cloud Cover SW (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% 7% 2 5% 4 4 4 3 5% 6% 47%	3805
	11th to 20th	Day	15% 6% 3 6% 3 2 2 2 3 4 53%	4100
	21st to end	Day	18% 6% 2 6% 4 3 3 2 4 6% 48%	4510
February	1st to 10th	Day	17% 6% 2 6% 5% 4 4 3 5% 5% 45%	1800
	11th to 20th	Day	13% 7% 3 7% 4 5% 3 3 5% 8% 43%	1806
	21st to end	Day	15% 6% 2 6% 5% 3 4 3 3 6% 48%	1650
March	1st to 10th	Day	15% 10% 3 7% 5% 4 4 3 4 6% 37%	4897
	11th to 20th	Day	13% 6% 2 5% 3 3 4 4 5% 6% 49%	5000
	21st to end	Day	15% 8% 2 6% 5% 5% 5% 4 5% 7% 37%	5546
April	1st to 10th	Day	11% 8% 2 7% 4 5% 4 4 5% 7% 43%	2400
	11th to 20th	Day	8% 5% 2 4 3 4 4 4 4 6% 54%	2369
	21st to end	Day	4 4 2 4 3 3 4 3 6% 8% 60%	2400
May	1st to 10th	Day	3 2 1 2 2 1 2 2 3 5% 77%	5748
	11th to 20th	Day	2 1 2 1 2 2 2 3 5% 78%	6027
	21st to end	Day	2 1 1 2 1 2 1 2 3 5% 81%	6902
June	1st to 10th	Day	4 3 2 3 2 3 3 3 5% 7% 65%	2800
	11th to 20th	Day	1 1 1 2 1 2 2 2 4 6% 78%	2800
	21st to end	Day	1 1 2 1 1 2 1 3 4 83%	2800
July	1st to 10th	Day	1 1 1 1 1 2 3 90%	6319
	11th to 20th	Day	1 1 1 1 1 2 3 88%	5995
	21st to end	Day	1 1 1 2 2 3 5% 85%	6402
August	1st to 10th	Day	1 1 1 1 1 2 2 4 5% 6% 75%	4144
	11th to 20th	Day	1 1 1 1 2 2 4 87%	3887
	21st to end	Day	1 1 1 1 2 2 2 2 4 85%	4180
September	1st to 10th	Day	1 1 1 2 2 1 2 4 86%	2280
	11th to 20th	Day	3 1 1 3 2 2 2 3 4 6% 73%	2280
	21st to end	Day	2 1 1 2 2 2 2 2 3 5% 78%	2002
October	1st to 10th	Day	2 2 1 3 2 2 2 2 3 6% 76%	3180
	11th to 20th	Day	4 3 1 4 2 2 2 2 3 5% 71%	3161
	21st to end	Day	5% 3 1 4 3 3 4 3 5% 8% 62%	3448
November	1st to 10th	Day	9% 4 2 3 3 3 3 3 4 8% 57%	1909
	11th to 20th	Day	10% 5% 2 5% 3 5% 4 3 5% 6% 51%	1775
	21st to end	Day	11% 4 1 5% 4 4 4 3 5% 5% 53%	1600
December	1st to 10th	Day	9% 4 2 4 3 2 3 3 6% 9% 54%	2620
	11th to 20th	Day	25% 9% 2 6% 4 3 4 4 6% 7% 31%	3028
	21st to end	Day	26% 9% 2 5% 4 4 4 3 5% 7% 30%	4004
January	1st to 10th	Night	10% 6% 2 4 3 4 4 4 6% 9% 48%	6130
	11th to 20th	Night	15% 6% 2 5% 3 3 3 2 4 4 52%	5880
	21st to end	Night	17% 7% 2 5% 3 3 3 2 4 5% 48%	6468
February	1st to 10th	Night	17% 7% 2 4 3 4 3 3 4 5% 49%	2600
	11th to 20th	Night	11% 7% 2 6% 5% 4 4 4 5% 7% 46%	2479
	21st to end	Night	11% 6% 1 4 4 3 4 2 4 6% 54%	1980
March	1st to 10th	Night	15% 12% 2 5% 4 4 4 3 4 7% 41%	4994
	11th to 20th	Night	14% 9% 1 4 3 3 3 2 4 5% 51%	5000
	21st to end	Night	15% 11% 2 7% 4 3 3 3 4 7% 41%	5429
April	1st to 10th	Night	8% 6% 2 6% 5% 4 4 3 5% 6% 52%	2000
	11th to 20th	Night	7% 5% 1 4 3 3 3 3 4 7% 58%	1979
	21st to end	Night	4 4 1 3 3 3 3 3 6% 7% 64%	2000
May	1st to 10th	Night	3 2 1 2 2 2 2 1 3 5% 76%	4220
	11th to 20th	Night	2 1 1 3 2 2 2 2 4 5% 75%	3873
	21st to end	Night	2 1 1 1 1 2 2 2 4 83%	3972
June	1st to 10th	Night	4 3 2 3 3 3 2 3 5% 6% 67%	1600
	11th to 20th	Night	1 1 1 2 2 2 2 4 6% 81%	1600
	21st to end	Night	1 2 1 2 2 1 2 2 2 3 82%	1600
July	1st to 10th	Night	1 1 1 1 1 1 2 92%	3640
	11th to 20th	Night	1 1 1 1 1 1 1 1 2 4 87%	3961
	21st to end	Night	2 2 2 2 2 4 7% 79%	4472
August	1st to 10th	Night	2 1 1 2 2 2 2 3 4 6% 75%	2884
	11th to 20th	Night	1 1 1 1 1 1 2 4 87%	2954
	21st to end	Night	2 1 2 1 2 2 1 2 3 84%	3498
September	1st to 10th	Night	1 1 1 1 1 3 5% 85%	1900
	11th to 20th	Night	3 2 1 3 2 3 2 2 3 6% 73%	1900
	21st to end	Night	1 1 2 2 2 2 2 3 5% 79%	2171
October	1st to 10th	Night	2 2 1 2 2 2 2 2 3 4 77%	3800
	11th to 20th	Night	3 2 1 3 3 2 3 3 4 6% 70%	3777
	21st to end	Night	6% 2 1 3 2 3 4 3 5% 6% 64%	4121
November	1st to 10th	Night	8% 6% 2 5% 3 3 2 3 5% 7% 57%	2387
	11th to 20th	Night	9% 6% 2 5% 3 3 4 3 5% 8% 52%	2620
	21st to end	Night	8% 5% 1 5% 3 4 4 3 4 5% 57%	2800
December	1st to 10th	Night	10% 4 1 4 3 3 4 3 5% 7% 57%	4560
	11th to 20th	Night	23% 9% 3 6% 4 4 4 4 6% 7% 30%	5176
	21st to end	Night	28% 10% 2 6% 4 3 3 3 3 5% 33%	6952

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	38% 5% 14% 22% 12% 9%	3805
	11th to 20th	Day	38% 2 7% 15% 23% 15%	4100
	21st to end	Day	41% 2 7% 15% 12% 23%	4510
February	1st to 10th	Day	41% 4 7% 18% 20% 9%	1800
	11th to 20th	Day	40% 4 10% 18% 21% 7%	1815
	21st to end	Day	39% 2 9% 24% 19% 8%	1650
March	1st to 10th	Day	47% 3 7% 17% 21% 4	4897
	11th to 20th	Day	36% 3 13% 24% 16% 8%	5000
	21st to end	Day	45% 5% 14% 22% 12% 2	5546
April	1st to 10th	Day	40% 5% 11% 21% 18% 5%	2400
	11th to 20th	Day	31% 3 14% 22% 22% 8%	2372
	21st to end	Day	23% 3 15% 29% 23% 6%	2400
May	1st to 10th	Day	12% 2 8% 24% 31% 23%	5748
	11th to 20th	Day	11% 1 7% 29% 38% 14%	6030
	21st to end	Day	9% 2 10% 32% 39% 8%	6904
June	1st to 10th	Day	20% 3 11% 29% 27% 8%	2800
	11th to 20th	Day	9% 2 11% 21% 36% 21%	2800
	21st to end	Day	8% 1 4 17% 36% 34%	2800
July	1st to 10th	Day	5% 1 1 7% 24% 63%	6251
	11th to 20th	Day	5% 1 2 5% 16% 70%	5923
	21st to end	Day	6% 1 3 19% 28% 44%	6217
August	1st to 10th	Day	11% 1 2 11% 28% 47%	4129
	11th to 20th	Day	5% 1 2 11% 45% 37%	3926
	21st to end	Day	8% 1 4 14% 37% 37%	4179
September	1st to 10th	Day	5% 1 3 11% 31% 48%	2280
	11th to 20th	Day	14% 2 7% 25% 36% 15%	2280
	21st to end	Day	12% 2 5% 20% 32% 29%	2002
October	1st to 10th	Day	13% 1 4 11% 25% 45%	3180
	11th to 20th	Day	19% 1 5% 24% 29% 23%	3161
	21st to end	Day	23% 1 8% 23% 32% 14%	3448
November	1st to 10th	Day	27% 4 7% 28% 22% 12%	1909
	11th to 20th	Day	34% 5% 10% 23% 19% 8%	1775
	21st to end	Day	33% 3 9% 19% 22% 14%	1600
December	1st to 10th	Day	28% 2 6% 27% 28% 9%	2620
	11th to 20th	Day	54% 2 7% 18% 15% 3	3028
	21st to end	Day	54% 4 11% 17% 11% 3	4004
January	1st to 10th	Night	33% 5% 22% 20% 16% 4	6130
	11th to 20th	Night	37% 3 9% 16% 22% 13%	5880
	21st to end	Night	41% 2 8% 10% 20% 19%	6468
February	1st to 10th	Night	39% 3 7% 18% 20% 13%	2600
	11th to 20th	Night	38% 3 10% 21% 19% 9%	2481
	21st to end	Night	32% 4 9% 25% 21% 8%	1980
March	1st to 10th	Night	45% 3 10% 18% 20% 5%	4994
	11th to 20th	Night	37% 2 12% 22% 20% 8%	5000
	21st to end	Night	43% 5% 13% 24% 13% 2	5429
April	1st to 10th	Night	34% 5% 13% 22% 22% 6%	2000
	11th to 20th	Night	27% 4 13% 28% 17% 11%	1979
	21st to end	Night	20% 3 14% 33% 25% 6%	2000
May	1st to 10th	Night	14% 2 5% 26% 33% 21%	4217
	11th to 20th	Night	13% 1 9% 30% 30% 17%	3873
	21st to end	Night	8% 3 7% 29% 39% 14%	3972
June	1st to 10th	Night	18% 4 11% 25% 33% 9%	1600
	11th to 20th	Night	8% 3 6% 23% 35% 25%	1600
	21st to end	Night	10% 1 3 16% 34% 35%	1600
July	1st to 10th	Night	4 1 7% 23% 65%	3600
	11th to 20th	Night	6% 1 2 6% 15% 71%	3911
	21st to end	Night	9% 1 4 17% 26% 43%	4345
August	1st to 10th	Night	13% 1 2 10% 25% 49%	2875
	11th to 20th	Night	5% 1 3 8% 39% 44%	2981
	21st to end	Night	9% 3 13% 36% 38%	3498
September	1st to 10th	Night	5% 2 3 10% 31% 50%	1900
	11th to 20th	Night	16% 1 6% 20% 39% 17%	1900
	21st to end	Night	11% 1 4 21% 29% 34%	2171
October	1st to 10th	Night	14% 1 4 11% 20% 50%	3800
	11th to 20th	Night	17% 1 8% 27% 25% 22%	3777
	21st to end	Night	22% 1 7% 23% 26% 21%	4121
November	1st to 10th	Night	27% 4 10% 25% 20% 15%	2387
	11th to 20th	Night	30% 3 12% 25% 19% 10%	2620
	21st to end	Night	30% 3 8% 21% 23% 15%	2800
December	1st to 10th	Night	29% 3 6% 23% 26% 14%	4560
	11th to 20th	Night	54% 3 8% 16% 15% 4	5176
	21st to end	Night	56% 3 10% 17% 11% 3	6952

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	32% 9% 23% 22% 11% 3	275
	11th to 20th	Day	21% 12% 23% 28% 13% 4	400
	21st to end	Day	16% 13% 22% 27% 17% 5%	440
February	1st to 10th	Day	24% 12% 24% 22% 15% 4	400
	11th to 20th	Day	22% 16% 28% 21% 12% 1	421
	21st to end	Day	18% 12% 28% 29% 10% 3	414
March	1st to 10th	Day	23% 16% 27% 20% 12% 1	500
	11th to 20th	Day	26% 12% 25% 20% 14% 3	500
	21st to end	Day	21% 15% 29% 23% 10% 2	550
April	1st to 10th	Day	17% 16% 26% 26% 12% 4	500
	11th to 20th	Day	15% 16% 28% 20% 15% 5%	492
	21st to end	Day	8% 14% 27% 28% 17% 7%	500
May	1st to 10th	Day	11% 14% 32% 27% 14% 4	503
	11th to 20th	Day	9% 12% 26% 27% 19% 7%	528
	21st to end	Day	6% 9% 26% 32% 20% 7%	573
June	1st to 10th	Day	7% 10% 25% 34% 18% 7%	530
	11th to 20th	Day	3 10% 28% 37% 17% 4	530
	21st to end	Day	1 5% 24% 37% 24% 9%	530
July	1st to 10th	Day	1 3 18% 42% 29% 7%	526
	11th to 20th	Day	1 1 15% 40% 34% 9%	524
	21st to end	Day	1 1 14% 41% 33% 11%	575
August	1st to 10th	Day	3 2 19% 39% 29% 9%	504
	11th to 20th	Day	2 2 20% 34% 27% 13%	477
	21st to end	Day	1 2 14% 37% 34% 12%	539
September	1st to 10th	Day	3 16% 37% 33% 11%	490
	11th to 20th	Day	4 5% 19% 32% 29% 10%	490
	21st to end	Day	3 6% 22% 34% 26% 9%	490
October	1st to 10th	Day	4 10% 22% 28% 24% 12%	490
	11th to 20th	Day	7% 8% 22% 26% 27% 11%	490
	21st to end	Day	12% 7% 22% 30% 23% 6%	539
November	1st to 10th	Day	13% 9% 19% 22% 29% 7%	445
	11th to 20th	Day	11% 8% 21% 25% 25% 10%	287
	21st to end	Day	16% 9% 21% 32% 17% 6%	200
December	1st to 10th	Day	22% 12% 18% 22% 16% 11%	200
	11th to 20th	Day	25% 12% 19% 23% 16% 6%	200
	21st to end	Day	24% 10% 24% 23% 13% 5%	220
January	1st to 10th	Night	21% 4 16% 32% 22% 5%	800
	11th to 20th	Night	19% 7% 15% 36% 18% 5%	800
	21st to end	Night	18% 6% 15% 32% 25% 4	880
February	1st to 10th	Night	21% 7% 15% 36% 19% 3	800
	11th to 20th	Night	15% 9% 17% 33% 21% 4	784
	21st to end	Night	14% 8% 21% 35% 19% 2	660
March	1st to 10th	Night	19% 8% 20% 30% 20% 4	800
	11th to 20th	Night	18% 8% 18% 30% 24% 2	800
	21st to end	Night	17% 9% 24% 31% 18% 2	880
April	1st to 10th	Night	12% 8% 22% 36% 21% 3	800
	11th to 20th	Night	11% 10% 20% 31% 23% 5%	791
	21st to end	Night	6% 9% 21% 34% 25% 6%	800
May	1st to 10th	Night	7% 7% 24% 30% 28% 5%	655
	11th to 20th	Night	4 4 18% 33% 36% 5%	508
	21st to end	Night	5% 6% 17% 35% 34% 4	433
June	1st to 10th	Night	6% 6% 16% 38% 33% 3	400
	11th to 20th	Night	2 4 17% 42% 35% 1	400
	21st to end	Night	1 2 13% 42% 39% 4	400
July	1st to 10th	Night	1 8% 36% 51% 4	390
	11th to 20th	Night	1 4 30% 55% 9%	525
	21st to end	Night	1 4 31% 55% 9%	657
August	1st to 10th	Night	2 1 6% 32% 51% 9%	579
	11th to 20th	Night	1 7% 37% 45% 9%	641
	21st to end	Night	1 1 6% 32% 53% 7%	836
September	1st to 10th	Night	1 1 7% 34% 49% 7%	760
	11th to 20th	Night	4 2 8% 36% 43% 7%	760
	21st to end	Night	2 3 11% 37% 37% 11%	760
October	1st to 10th	Night	4 5% 13% 33% 36% 9%	760
	11th to 20th	Night	6% 5% 11% 35% 36% 7%	760
	21st to end	Night	9% 3 15% 31% 32% 10%	836
November	1st to 10th	Night	9% 4 17% 30% 31% 9%	780
	11th to 20th	Night	12% 4 15% 36% 29% 5%	800
	21st to end	Night	12% 7% 14% 37% 24% 6%	800
December	1st to 10th	Night	19% 5% 14% 35% 23% 6%	800
	11th to 20th	Night	25% 5% 14% 32% 21% 3	800
	21st to end	Night	23% 7% 18% 33% 18% 1	880

Page generated at 2025-08-01 22:32:24. Version ac07

Copyright © 2025 Attilla Danko.