Ash Observatory
Clear Sky Chart
History

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

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% 6% 2 6% 4 4 4 4 6% 7% 41%	101252
	Night	16% 7% 2 6% 4 4 4 3 5% 7% 42%	97698
Transparency	Day	41% 7% 12% 23% 15% 2	101009
	Night	41% 6% 12% 23% 16% 3	97561
Seeing	Day	17% 13% 26% 25% 14% 5%	15030
	Night	18% 9% 31% 31% 11% 1	24300

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	15% 9% 2 5% 4 3 3 3 4 5% 46%	2517
	11th to 20th	Day	15% 9% 2 10% 6% 4 4 3 4 5% 39%	2613
	21st to end	Day	22% 9% 2 6% 4 4 4 3 4 5% 38%	3124
February	1st to 10th	Day	15% 8% 2 7% 4 4 5% 4 6% 6% 40%	1710
	11th to 20th	Day	17% 7% 2 5% 5% 4 4 4 5% 7% 41%	1814
	21st to end	Day	17% 10% 3 6% 4 5% 4 3 5% 6% 38%	1570
March	1st to 10th	Day	25% 7% 2 6% 3 3 3 3 4 6% 40%	3116
	11th to 20th	Day	10% 4 2 6% 4 5% 4 4 6% 7% 49%	3479
	21st to end	Day	21% 5% 2 5% 4 4 3 3 4 5% 44%	4038
April	1st to 10th	Day	16% 7% 2 5% 4 4 4 4 6% 6% 42%	2280
	11th to 20th	Day	14% 6% 2 5% 5% 4 4 4 6% 7% 44%	2254
	21st to end	Day	16% 6% 2 5% 5% 4 5% 4 6% 7% 40%	2280
May	1st to 10th	Day	27% 8% 3 7% 4 4 3 4 5% 7% 28%	4006
	11th to 20th	Day	24% 8% 3 8% 5% 4 5% 3 5% 6% 29%	4398
	21st to end	Day	12% 4 2 6% 4 4 4 4 5% 9% 46%	4837
June	1st to 10th	Day	12% 5% 2 7% 5% 4 5% 5% 7% 9% 40%	2660
	11th to 20th	Day	12% 5% 2 6% 5% 5% 5% 5% 7% 8% 39%	2660
	21st to end	Day	10% 4 2 6% 5% 4 5% 4 7% 10% 43%	2660
July	1st to 10th	Day	7% 5% 2 6% 6% 6% 7% 5% 8% 12% 36%	4440
	11th to 20th	Day	10% 6% 2 6% 5% 4 6% 5% 7% 11% 39%	4440
	21st to end	Day	10% 8% 3 7% 6% 6% 6% 5% 7% 9% 32%	4706
August	1st to 10th	Day	17% 6% 2 7% 5% 5% 6% 5% 6% 8% 34%	4083
	11th to 20th	Day	12% 6% 2 6% 4 4 5% 4 6% 9% 42%	3730
	21st to end	Day	9% 5% 2 5% 4 4 4 4 7% 9% 47%	4180
September	1st to 10th	Day	13% 6% 2 6% 4 5% 4 4 5% 6% 45%	2280
	11th to 20th	Day	12% 4 2 4 4 4 4 4 4 7% 50%	2247
	21st to end	Day	18% 7% 2 7% 5% 4 4 3 4 6% 39%	2090
October	1st to 10th	Day	18% 5% 2 5% 4 3 5% 3 5% 5% 45%	1909
	11th to 20th	Day	15% 5% 2 4 3 3 4 3 5% 6% 50%	1900
	21st to end	Day	17% 5% 2 5% 4 3 4 3 3 6% 48%	2028
November	1st to 10th	Day	14% 5% 2 4 4 3 3 2 4 6% 53%	1689
	11th to 20th	Day	15% 6% 2 4 4 3 4 5% 5% 6% 47%	1710
	21st to end	Day	14% 5% 1 5% 4 3 4 3 5% 6% 48%	1606
December	1st to 10th	Day	19% 6% 3 5% 3 3 3 3 6% 8% 39%	1520
	11th to 20th	Day	19% 8% 2 5% 4 3 3 4 4 5% 43%	1906
	21st to end	Day	16% 6% 2 6% 4 5% 5% 4 7% 7% 37%	2772
January	1st to 10th	Night	17% 7% 2 5% 3 4 4 3 5% 5% 45%	4429
	11th to 20th	Night	19% 7% 2 6% 4 3 4 3 3 5% 44%	4344
	21st to end	Night	23% 7% 2 7% 4 4 4 3 4 5% 37%	4532
February	1st to 10th	Night	16% 9% 3 6% 3 4 4 4 5% 6% 41%	2470
	11th to 20th	Night	19% 9% 3 6% 4 3 3 3 4 6% 41%	2250
	21st to end	Night	18% 7% 2 5% 3 4 3 4 5% 6% 42%	1884
March	1st to 10th	Night	22% 10% 2 6% 3 4 4 2 4 6% 38%	3709
	11th to 20th	Night	11% 6% 2 5% 5% 4 4 3 5% 5% 51%	3479
	21st to end	Night	18% 9% 2 5% 4 3 4 3 3 4 45%	3579
April	1st to 10th	Night	16% 7% 2 7% 3 4 3 4 5% 7% 41%	1900
	11th to 20th	Night	14% 6% 2 7% 4 3 4 4 5% 7% 42%	1875
	21st to end	Night	18% 6% 2 6% 4 3 3 3 5% 6% 44%	1900
May	1st to 10th	Night	20% 10% 3 8% 5% 4 4 3 5% 8% 30%	2950
	11th to 20th	Night	23% 8% 3 7% 5% 4 4 4 5% 6% 30%	2495
	21st to end	Night	13% 5% 2 6% 4 4 5% 4 6% 9% 43%	2738
June	1st to 10th	Night	15% 5% 2 5% 4 4 3 4 6% 9% 43%	1520
	11th to 20th	Night	8% 7% 3 7% 5% 5% 5% 3 7% 9% 41%	1520
	21st to end	Night	9% 5% 2 5% 4 5% 5% 4 8% 10% 42%	1520
July	1st to 10th	Night	7% 6% 3 7% 5% 6% 6% 5% 7% 10% 37%	2520
	11th to 20th	Night	10% 6% 2 6% 5% 5% 4 5% 7% 10% 38%	2520
	21st to end	Night	14% 7% 3 7% 5% 5% 5% 4 7% 7% 36%	2830
August	1st to 10th	Night	20% 6% 3 6% 4 4 5% 4 6% 8% 33%	2825
	11th to 20th	Night	12% 7% 2 7% 4 5% 4 3 5% 7% 43%	3087
	21st to end	Night	11% 5% 2 5% 4 4 4 3 5% 8% 49%	3476
September	1st to 10th	Night	13% 6% 2 7% 4 4 4 3 5% 7% 46%	1900
	11th to 20th	Night	10% 5% 2 5% 4 2 3 3 5% 7% 55%	1933
	21st to end	Night	19% 7% 2 6% 4 3 3 3 5% 7% 40%	2090
October	1st to 10th	Night	18% 7% 2 5% 4 3 3 3 4 6% 44%	2269
	11th to 20th	Night	15% 4 1 5% 3 4 3 2 5% 6% 51%	2280
	21st to end	Night	17% 6% 2 5% 3 2 4 3 5% 6% 48%	2569
November	1st to 10th	Night	12% 5% 1 5% 4 3 3 3 5% 5% 53%	2434
	11th to 20th	Night	16% 6% 2 5% 4 3 3 3 3 5% 48%	2470
	21st to end	Night	12% 6% 1 5% 5% 4 4 4 7% 6% 45%	2569
December	1st to 10th	Night	20% 7% 3 6% 3 4 4 3 5% 6% 40%	2660
	11th to 20th	Night	17% 6% 2 6% 4 4 4 3 6% 6% 42%	3288
	21st to end	Night	16% 7% 2 6% 5% 4 5% 4 6% 7% 37%	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	39% 5% 12% 26% 14% 4	2518
	11th to 20th	Day	48% 4 10% 22% 14% 3	2613
	21st to end	Day	49% 5% 13% 21% 11% 2	3124
February	1st to 10th	Day	43% 6% 13% 24% 12% 2	1710
	11th to 20th	Day	42% 6% 9% 24% 15% 3	1823
	21st to end	Day	47% 5% 9% 20% 13% 5%	1570
March	1st to 10th	Day	46% 5% 8% 29% 10% 3	3119
	11th to 20th	Day	32% 6% 11% 30% 18% 4	3479
	21st to end	Day	43% 5% 9% 19% 21% 4	4038
April	1st to 10th	Day	40% 6% 11% 20% 16% 6%	2280
	11th to 20th	Day	39% 6% 12% 23% 16% 5%	2254
	21st to end	Day	41% 6% 11% 18% 20% 4	2280
May	1st to 10th	Day	54% 6% 10% 17% 10% 2	4018
	11th to 20th	Day	53% 8% 13% 16% 8% 1	4402
	21st to end	Day	34% 8% 12% 27% 17% 2	4837
June	1st to 10th	Day	38% 8% 14% 23% 15% 2	2660
	11th to 20th	Day	38% 10% 13% 24% 13% 3	2660
	21st to end	Day	34% 10% 15% 23% 17% 1	2660
July	1st to 10th	Day	37% 11% 18% 18% 14% 2	4370
	11th to 20th	Day	37% 8% 17% 23% 13% 1	4365
	21st to end	Day	45% 9% 12% 19% 14% 1	4556
August	1st to 10th	Day	45% 8% 15% 22% 9% 1	4070
	11th to 20th	Day	37% 7% 14% 28% 13% 1	3766
	21st to end	Day	32% 7% 13% 27% 20% 2	4180
September	1st to 10th	Day	39% 7% 11% 19% 21% 3	2280
	11th to 20th	Day	33% 5% 11% 22% 25% 4	2247
	21st to end	Day	46% 5% 8% 18% 20% 2	2090
October	1st to 10th	Day	40% 6% 11% 22% 19% 2	1909
	11th to 20th	Day	35% 5% 10% 23% 24% 3	1900
	21st to end	Day	39% 4 10% 25% 20% 3	2028
November	1st to 10th	Day	34% 4 12% 27% 20% 4	1689
	11th to 20th	Day	37% 6% 10% 23% 22% 3	1710
	21st to end	Day	37% 4 11% 31% 14% 3	1606
December	1st to 10th	Day	43% 5% 16% 23% 13% 1	1520
	11th to 20th	Day	43% 4 11% 29% 10% 3	1906
	21st to end	Day	43% 7% 14% 23% 13% 1	2772
January	1st to 10th	Night	40% 5% 10% 27% 14% 4	4429
	11th to 20th	Night	44% 4 10% 25% 14% 3	4344
	21st to end	Night	49% 5% 8% 25% 12% 2	4532
February	1st to 10th	Night	43% 5% 13% 24% 13% 2	2470
	11th to 20th	Night	44% 4 9% 20% 18% 5%	2252
	21st to end	Night	43% 5% 10% 22% 16% 6%	1884
March	1st to 10th	Night	48% 4 9% 23% 13% 2	3719
	11th to 20th	Night	34% 6% 12% 28% 17% 3	3480
	21st to end	Night	44% 4 8% 17% 23% 5%	3579
April	1st to 10th	Night	43% 5% 9% 20% 18% 4	1900
	11th to 20th	Night	40% 6% 11% 20% 19% 5%	1875
	21st to end	Night	41% 5% 9% 20% 20% 5%	1900
May	1st to 10th	Night	51% 8% 10% 18% 11% 1	2957
	11th to 20th	Night	52% 9% 13% 14% 10% 2	2496
	21st to end	Night	36% 9% 15% 21% 16% 3	2739
June	1st to 10th	Night	38% 7% 11% 22% 20% 1	1520
	11th to 20th	Night	38% 9% 13% 25% 14% 2	1520
	21st to end	Night	34% 9% 15% 25% 15% 3	1520
July	1st to 10th	Night	40% 10% 19% 17% 12% 2	2480
	11th to 20th	Night	38% 9% 19% 23% 9% 1	2472
	21st to end	Night	44% 8% 14% 19% 14% 1	2740
August	1st to 10th	Night	47% 8% 18% 17% 9%	2814
	11th to 20th	Night	39% 6% 13% 27% 14% 1	3117
	21st to end	Night	33% 6% 11% 29% 20% 2	3476
September	1st to 10th	Night	38% 7% 12% 19% 21% 4	1900
	11th to 20th	Night	30% 6% 10% 24% 26% 4	1933
	21st to end	Night	44% 6% 8% 21% 19% 2	2090
October	1st to 10th	Night	42% 6% 11% 21% 17% 3	2269
	11th to 20th	Night	34% 4 11% 28% 19% 4	2280
	21st to end	Night	37% 4 9% 26% 20% 3	2569
November	1st to 10th	Night	34% 5% 11% 25% 19% 6%	2434
	11th to 20th	Night	39% 5% 10% 22% 20% 5%	2470
	21st to end	Night	37% 5% 12% 26% 18% 2	2569
December	1st to 10th	Night	45% 5% 11% 23% 15% 1	2660
	11th to 20th	Night	41% 5% 12% 21% 18% 4	3288
	21st to end	Night	43% 7% 14% 24% 10% 1	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	18% 29% 26% 21% 5% 1	180
	11th to 20th	Day	18% 36% 27% 17% 2	206
	21st to end	Day	23% 23% 29% 19% 5% 2	297
February	1st to 10th	Day	22% 24% 31% 17% 6%	270
	11th to 20th	Day	23% 24% 36% 15% 2	422
	21st to end	Day	25% 19% 33% 20% 4 1	382
March	1st to 10th	Day	17% 24% 31% 21% 6%	460
	11th to 20th	Day	19% 23% 32% 19% 6% 1	460
	21st to end	Day	23% 17% 30% 23% 6% 1	506
April	1st to 10th	Day	20% 20% 33% 18% 8% 1	460
	11th to 20th	Day	17% 18% 28% 23% 11% 2	452
	21st to end	Day	21% 16% 31% 23% 7% 2	460
May	1st to 10th	Day	28% 17% 25% 17% 10% 2	463
	11th to 20th	Day	20% 12% 25% 26% 14% 3	487
	21st to end	Day	16% 7% 20% 29% 20% 8%	529
June	1st to 10th	Day	12% 9% 22% 31% 20% 6%	490
	11th to 20th	Day	14% 8% 24% 32% 18% 4	490
	21st to end	Day	11% 7% 25% 32% 18% 7%	490
July	1st to 10th	Day	12% 4 19% 31% 25% 10%	485
	11th to 20th	Day	7% 2 18% 36% 27% 10%	484
	21st to end	Day	12% 3 19% 33% 25% 8%	532
August	1st to 10th	Day	11% 5% 19% 31% 25% 10%	486
	11th to 20th	Day	13% 3 14% 32% 27% 10%	479
	21st to end	Day	11% 4 23% 29% 20% 13%	539
September	1st to 10th	Day	16% 8% 18% 24% 23% 11%	490
	11th to 20th	Day	13% 5% 22% 28% 20% 12%	490
	21st to end	Day	23% 9% 22% 27% 12% 6%	490
October	1st to 10th	Day	22% 6% 22% 26% 15% 9%	490
	11th to 20th	Day	16% 14% 24% 25% 17% 4	490
	21st to end	Day	18% 10% 32% 25% 12% 3	506
November	1st to 10th	Day	15% 10% 32% 25% 15% 2	371
	11th to 20th	Day	15% 16% 34% 25% 10%	370
	21st to end	Day	15% 15% 30% 29% 10% 1	266
December	1st to 10th	Day	22% 22% 32% 16% 8%	180
	11th to 20th	Day	27% 21% 29% 16% 7%	180
	21st to end	Day	25% 18% 26% 19% 10% 2	198
January	1st to 10th	Night	20% 13% 40% 24% 4	760
	11th to 20th	Night	22% 13% 36% 25% 4	760
	21st to end	Night	18% 13% 41% 26% 3	836
February	1st to 10th	Night	21% 11% 39% 26% 3	760
	11th to 20th	Night	24% 15% 41% 19% 1	743
	21st to end	Night	22% 14% 40% 21% 3	628
March	1st to 10th	Night	19% 13% 37% 25% 5%	760
	11th to 20th	Night	19% 14% 39% 22% 6% 1	760
	21st to end	Night	20% 8% 41% 25% 6%	836
April	1st to 10th	Night	20% 15% 39% 21% 5%	760
	11th to 20th	Night	16% 11% 37% 30% 5%	751
	21st to end	Night	21% 10% 32% 31% 7%	760
May	1st to 10th	Night	19% 10% 33% 30% 8%	648
	11th to 20th	Night	17% 5% 35% 36% 8%	374
	21st to end	Night	16% 4 20% 40% 18% 2	411
June	1st to 10th	Night	16% 3 24% 41% 16%	380
	11th to 20th	Night	11% 4 24% 47% 13% 1	380
	21st to end	Night	9% 3 21% 53% 13%	380
July	1st to 10th	Night	10% 2 20% 45% 22% 1	370
	11th to 20th	Night	8% 2 14% 52% 23% 1	368
	21st to end	Night	9% 3 18% 45% 23% 2	471
August	1st to 10th	Night	12% 1 14% 43% 28% 2	567
	11th to 20th	Night	14% 3 14% 40% 26% 2	740
	21st to end	Night	14% 3 16% 39% 26% 3	836
September	1st to 10th	Night	15% 3 21% 38% 21% 2	760
	11th to 20th	Night	13% 4 15% 37% 27% 4	760
	21st to end	Night	23% 4 21% 32% 19% 1	760
October	1st to 10th	Night	21% 4 26% 29% 16% 5%	760
	11th to 20th	Night	18% 7% 27% 34% 13% 1	760
	21st to end	Night	21% 8% 31% 30% 10% 1	836
November	1st to 10th	Night	14% 6% 36% 31% 13% 1	749
	11th to 20th	Night	19% 9% 34% 29% 9% 1	760
	21st to end	Night	14% 12% 36% 29% 9% 1	760
December	1st to 10th	Night	23% 12% 34% 23% 8% 1	760
	11th to 20th	Night	17% 11% 37% 30% 6%	760
	21st to end	Night	20% 12% 35% 31% 2	836

Page generated at 2024-10-31 03:39:21. Version ac07

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