Ayer Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ayer Observatory clear sky chart from 2005-11-06 to 2026-10-14.

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	16% 8% 2 6% 4 4 4 4 5% 7% 39%	148989
	Night	16% 8% 3 6% 4 4 4 3 5% 6% 40%	142758
Transparency	Day	43% 4 11% 23% 15% 2	152524
	Night	44% 4 11% 22% 16% 2	147312
Seeing	Day	22% 17% 34% 20% 7% 2	17595
	Night	21% 14% 34% 24% 7% 1	26397

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	10% 8% 3 8% 5% 4 4 3 5% 6% 42%	4661
	11th to 20th	Day	14% 9% 3 6% 3 3 4 3 4 5% 47%	4650
	21st to end	Day	14% 9% 2 5% 3 4 3 3 5% 6% 46%	5124
February	1st to 10th	Day	18% 7% 3 6% 4 5% 5% 3 5% 7% 37%	1973
	11th to 20th	Day	13% 7% 3 6% 4 4 4 4 5% 7% 42%	2048
	21st to end	Day	17% 10% 3 6% 4 4 4 3 4 6% 39%	1730
March	1st to 10th	Day	19% 12% 3 5% 4 4 4 3 5% 6% 35%	4576
	11th to 20th	Day	14% 9% 3 7% 4 4 4 4 6% 7% 39%	4901
	21st to end	Day	25% 9% 2 6% 5% 4 5% 4 6% 5% 30%	5857
April	1st to 10th	Day	20% 9% 3 6% 4 4 4 3 4 6% 37%	2520
	11th to 20th	Day	18% 7% 2 6% 4 4 4 4 6% 7% 39%	2520
	21st to end	Day	19% 7% 2 7% 4 4 4 3 6% 7% 38%	2730
May	1st to 10th	Day	24% 11% 3 7% 4 4 4 3 5% 6% 28%	7639
	11th to 20th	Day	18% 9% 3 7% 5% 4 4 3 5% 6% 35%	7676
	21st to end	Day	21% 8% 3 6% 4 5% 5% 4 6% 8% 31%	9060
June	1st to 10th	Day	18% 6% 2 6% 4 4 4 5% 7% 9% 35%	2940
	11th to 20th	Day	16% 6% 2 6% 5% 4 5% 5% 6% 8% 38%	2940
	21st to end	Day	15% 6% 2 7% 5% 5% 5% 4 6% 9% 37%	2940
July	1st to 10th	Day	17% 7% 2 5% 4 5% 5% 4 7% 9% 34%	8280
	11th to 20th	Day	7% 6% 2 6% 4 4 5% 5% 7% 10% 44%	8280
	21st to end	Day	15% 6% 2 6% 4 4 4 3 5% 7% 43%	8993
August	1st to 10th	Day	12% 7% 3 7% 5% 5% 6% 5% 8% 10% 32%	6023
	11th to 20th	Day	13% 6% 2 6% 4 4 4 4 6% 9% 41%	5911
	21st to end	Day	11% 4 2 5% 4 4 5% 5% 7% 10% 44%	6080
September	1st to 10th	Day	13% 6% 2 5% 4 4 4 3 5% 7% 46%	2501
	11th to 20th	Day	15% 6% 1 5% 2 3 3 2 5% 5% 52%	2310
	21st to end	Day	19% 6% 2 6% 3 3 4 3 6% 6% 41%	2310
October	1st to 10th	Day	22% 5% 3 5% 3 3 4 2 4 6% 45%	2310
	11th to 20th	Day	16% 6% 2 5% 3 3 4 3 4 6% 47%	1995
	21st to end	Day	19% 7% 2 5% 4 3 4 3 6% 6% 42%	1980
November	1st to 10th	Day	16% 6% 2 4 3 4 3 3 4 5% 49%	1851
	11th to 20th	Day	17% 7% 2 4 3 3 4 3 4 6% 47%	1890
	21st to end	Day	18% 7% 2 5% 5% 3 3 3 5% 6% 43%	1890
December	1st to 10th	Day	15% 8% 2 5% 3 3 3 3 4 5% 49%	2786
	11th to 20th	Day	17% 8% 2 4 3 3 3 3 4 5% 49%	3066
	21st to end	Day	19% 11% 3 7% 4 4 4 2 4 5% 38%	4048
January	1st to 10th	Night	14% 7% 2 6% 3 3 4 3 4 6% 48%	8234
	11th to 20th	Night	15% 10% 3 8% 5% 4 4 3 4 6% 38%	8235
	21st to end	Night	13% 10% 3 7% 5% 5% 4 3 4 5% 41%	9057
February	1st to 10th	Night	16% 9% 2 6% 4 5% 4 3 5% 7% 39%	2640
	11th to 20th	Night	15% 8% 3 6% 4 5% 4 4 5% 6% 39%	2458
	21st to end	Night	17% 9% 3 6% 4 4 4 4 4 5% 41%	2076
March	1st to 10th	Night	24% 9% 2 4 3 4 3 3 4 5% 37%	5484
	11th to 20th	Night	15% 11% 3 6% 4 4 4 3 5% 6% 38%	5277
	21st to end	Night	25% 10% 2 6% 4 4 4 4 5% 4 32%	5312
April	1st to 10th	Night	19% 8% 3 7% 4 5% 4 3 5% 6% 37%	2100
	11th to 20th	Night	18% 8% 2 7% 5% 4 3 3 5% 6% 38%	2048
	21st to end	Night	20% 7% 2 7% 4 3 4 3 5% 5% 40%	1890
May	1st to 10th	Night	26% 11% 3 6% 4 4 3 3 5% 6% 30%	5259
	11th to 20th	Night	18% 8% 3 7% 5% 4 5% 4 6% 7% 32%	5235
	21st to end	Night	20% 9% 3 8% 5% 5% 5% 4 5% 7% 30%	5118
June	1st to 10th	Night	16% 8% 2 7% 4 4 4 4 6% 7% 38%	1680
	11th to 20th	Night	16% 8% 3 6% 4 4 4 5% 6% 8% 36%	1680
	21st to end	Night	16% 5% 2 5% 4 4 5% 4 5% 9% 40%	1680
July	1st to 10th	Night	14% 8% 2 7% 4 4 5% 4 7% 9% 35%	4680
	11th to 20th	Night	8% 6% 2 6% 4 4 4 4 7% 9% 45%	4680
	21st to end	Night	14% 7% 2 6% 4 4 4 3 5% 7% 43%	5084
August	1st to 10th	Night	13% 7% 3 7% 5% 5% 5% 5% 8% 8% 34%	4065
	11th to 20th	Night	10% 7% 3 8% 5% 5% 6% 5% 7% 8% 38%	4130
	21st to end	Night	11% 4 2 5% 4 4 4 4 6% 8% 47%	5045
September	1st to 10th	Night	12% 6% 2 7% 4 5% 4 4 6% 6% 45%	2118
	11th to 20th	Night	13% 6% 1 5% 3 2 3 2 5% 7% 53%	2310
	21st to end	Night	19% 6% 3 6% 3 4 4 4 5% 5% 41%	2310
October	1st to 10th	Night	19% 6% 2 6% 4 4 3 2 4 5% 45%	2310
	11th to 20th	Night	16% 4 2 5% 4 4 4 3 5% 6% 47%	2466
	21st to end	Night	20% 8% 2 6% 3 4 3 3 5% 6% 40%	2860
November	1st to 10th	Night	14% 5% 2 6% 4 4 4 3 5% 6% 48%	2667
	11th to 20th	Night	16% 7% 2 5% 3 3 4 3 5% 5% 47%	2730
	21st to end	Night	16% 7% 2 5% 4 3 4 3 5% 6% 44%	2730
December	1st to 10th	Night	17% 9% 2 5% 4 3 3 3 4 6% 44%	4612
	11th to 20th	Night	19% 9% 3 6% 3 3 3 3 5% 4 41%	5348
	21st to end	Night	17% 10% 3 6% 4 4 3 3 4 6% 40%	7150

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	43% 3 9% 26% 18% 2	4662
	11th to 20th	Day	42% 3 9% 22% 20% 5%	4680
	21st to end	Day	40% 2 10% 25% 20% 2	5124
February	1st to 10th	Day	48% 4 11% 19% 16% 2	1973
	11th to 20th	Day	42% 4 11% 25% 16% 3	2057
	21st to end	Day	46% 4 10% 22% 15% 3	1730
March	1st to 10th	Day	51% 3 9% 15% 13% 9%	4579
	11th to 20th	Day	45% 4 11% 27% 11% 2	4901
	21st to end	Day	55% 3 6% 16% 13% 6%	5857
April	1st to 10th	Day	48% 4 8% 19% 18% 4	2520
	11th to 20th	Day	44% 4 10% 22% 16% 3	2520
	21st to end	Day	45% 3 8% 22% 18% 4	2730
May	1st to 10th	Day	57% 4 12% 17% 10% 1	7651
	11th to 20th	Day	49% 4 13% 20% 12% 1	7680
	21st to end	Day	50% 4 11% 23% 12% 1	9061
June	1st to 10th	Day	44% 6% 12% 24% 13% 1	2940
	11th to 20th	Day	41% 5% 14% 22% 15% 3	2940
	21st to end	Day	43% 5% 14% 23% 14% 1	2940
July	1st to 10th	Day	44% 7% 13% 22% 12% 1	8210
	11th to 20th	Day	33% 6% 15% 29% 16% 1	8205
	21st to end	Day	39% 5% 11% 25% 19% 2	8795
August	1st to 10th	Day	44% 7% 18% 20% 9% 1	6009
	11th to 20th	Day	39% 6% 14% 32% 9%	5950
	21st to end	Day	34% 5% 14% 29% 16% 2	6080
September	1st to 10th	Day	38% 5% 10% 26% 20% 2	2501
	11th to 20th	Day	34% 4 8% 29% 23% 3	2310
	21st to end	Day	44% 4 8% 22% 21% 2	2310
October	1st to 10th	Day	39% 5% 9% 26% 18% 4	3700
	11th to 20th	Day	34% 2 6% 30% 24% 4	3198
	21st to end	Day	33% 3 11% 30% 20% 3	3180
November	1st to 10th	Day	39% 3 11% 26% 18% 2	1851
	11th to 20th	Day	39% 3 8% 24% 23% 3	1890
	21st to end	Day	43% 4 12% 25% 13% 2	1890
December	1st to 10th	Day	40% 3 14% 28% 15% 1	2786
	11th to 20th	Day	40% 3 14% 18% 20% 5%	3066
	21st to end	Day	51% 2 7% 22% 14% 4	4048
January	1st to 10th	Night	38% 2 10% 31% 17% 2	8234
	11th to 20th	Night	49% 3 9% 20% 15% 4	8280
	21st to end	Night	48% 2 7% 19% 20% 3	9057
February	1st to 10th	Night	46% 3 12% 22% 15% 2	2640
	11th to 20th	Night	46% 4 8% 22% 16% 3	2460
	21st to end	Night	45% 3 7% 25% 15% 4	2076
March	1st to 10th	Night	51% 2 8% 16% 16% 7%	5494
	11th to 20th	Night	48% 3 14% 19% 13% 2	5278
	21st to end	Night	56% 2 4 13% 19% 6%	5312
April	1st to 10th	Night	49% 4 8% 20% 16% 4	2100
	11th to 20th	Night	47% 4 8% 20% 18% 3	2048
	21st to end	Night	48% 3 9% 20% 17% 3	1890
May	1st to 10th	Night	56% 3 13% 15% 11% 2	5265
	11th to 20th	Night	51% 5% 14% 19% 10% 2	5236
	21st to end	Night	55% 4 11% 21% 9% 1	5119
June	1st to 10th	Night	45% 5% 13% 24% 13%	1680
	11th to 20th	Night	45% 6% 13% 22% 14% 1	1680
	21st to end	Night	41% 6% 14% 25% 13% 1	1680
July	1st to 10th	Night	44% 5% 13% 21% 16%	4640
	11th to 20th	Night	34% 5% 17% 26% 16% 1	4632
	21st to end	Night	40% 5% 11% 27% 17% 1	4967
August	1st to 10th	Night	45% 7% 18% 18% 12% 1	4055
	11th to 20th	Night	40% 8% 17% 26% 10%	4157
	21st to end	Night	34% 6% 15% 28% 18% 1	5045
September	1st to 10th	Night	38% 5% 11% 26% 19% 2	2118
	11th to 20th	Night	33% 4 11% 24% 26% 3	2310
	21st to end	Night	43% 5% 8% 23% 19% 2	2310
October	1st to 10th	Night	39% 4 12% 26% 17% 3	3700
	11th to 20th	Night	28% 3 10% 28% 28% 3	4002
	21st to end	Night	34% 3 11% 30% 20% 2	4610
November	1st to 10th	Night	38% 4 11% 25% 19% 3	2667
	11th to 20th	Night	41% 3 9% 23% 21% 4	2730
	21st to end	Night	41% 4 13% 28% 13% 2	2730
December	1st to 10th	Night	43% 3 11% 28% 14% 1	4612
	11th to 20th	Night	47% 3 14% 16% 20% 2	5348
	21st to end	Night	46% 4 7% 24% 17% 2	7150

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	29% 24% 30% 14% 4	220
	11th to 20th	Day	23% 30% 37% 9% 1	220
	21st to end	Day	20% 29% 39% 10% 2	242
February	1st to 10th	Day	26% 23% 35% 14% 2	458
	11th to 20th	Day	26% 26% 35% 11% 1	536
	21st to end	Day	25% 22% 38% 12% 3	446
March	1st to 10th	Day	26% 23% 34% 13% 4	540
	11th to 20th	Day	26% 25% 34% 11% 3 1	540
	21st to end	Day	29% 23% 33% 9% 5% 1	594
April	1st to 10th	Day	31% 22% 29% 11% 5% 1	540
	11th to 20th	Day	28% 23% 32% 14% 3	532
	21st to end	Day	26% 17% 36% 16% 3 1	540
May	1st to 10th	Day	29% 25% 31% 11% 3 1	543
	11th to 20th	Day	21% 17% 40% 16% 5% 1	567
	21st to end	Day	20% 13% 35% 26% 5% 1	617
June	1st to 10th	Day	20% 12% 34% 25% 8% 1	570
	11th to 20th	Day	16% 14% 40% 21% 7% 1	570
	21st to end	Day	15% 11% 41% 24% 9% 1	570
July	1st to 10th	Day	18% 8% 32% 29% 12% 2	565
	11th to 20th	Day	13% 7% 32% 33% 12% 2	564
	21st to end	Day	12% 7% 36% 32% 11% 2	620
August	1st to 10th	Day	10% 9% 31% 35% 13% 2	566
	11th to 20th	Day	15% 7% 31% 30% 14% 3	559
	21st to end	Day	14% 9% 34% 25% 14% 4	627
September	1st to 10th	Day	17% 10% 30% 26% 14% 3	570
	11th to 20th	Day	19% 9% 28% 22% 14% 7%	570
	21st to end	Day	24% 11% 31% 20% 9% 4	570
October	1st to 10th	Day	23% 13% 28% 21% 10% 4	570
	11th to 20th	Day	21% 15% 32% 21% 9% 2	473
	21st to end	Day	24% 17% 33% 17% 8% 1	462
November	1st to 10th	Day	21% 16% 34% 17% 10% 1	425
	11th to 20th	Day	21% 24% 36% 14% 4	430
	21st to end	Day	25% 23% 30% 15% 5% 1	430
December	1st to 10th	Day	25% 23% 32% 15% 4	287
	11th to 20th	Day	31% 27% 33% 7% 2	220
	21st to end	Day	26% 20% 33% 16% 2 2	242
January	1st to 10th	Night	22% 21% 38% 16% 3	840
	11th to 20th	Night	26% 20% 36% 16% 2	840
	21st to end	Night	19% 22% 37% 19% 4	924
February	1st to 10th	Night	22% 20% 37% 20% 1	840
	11th to 20th	Night	25% 19% 40% 15% 1	823
	21st to end	Night	24% 21% 35% 17% 3	692
March	1st to 10th	Night	25% 19% 35% 17% 3	840
	11th to 20th	Night	23% 19% 37% 17% 3 1	840
	21st to end	Night	26% 18% 34% 17% 5%	924
April	1st to 10th	Night	25% 19% 33% 19% 3	840
	11th to 20th	Night	24% 17% 38% 17% 4	804
	21st to end	Night	24% 16% 35% 23% 2	630
May	1st to 10th	Night	28% 13% 34% 19% 5%	630
	11th to 20th	Night	21% 13% 40% 21% 5%	622
	21st to end	Night	19% 9% 38% 27% 7%	459
June	1st to 10th	Night	19% 7% 33% 34% 6%	420
	11th to 20th	Night	16% 8% 37% 31% 6%	420
	21st to end	Night	17% 5% 35% 36% 6%	420
July	1st to 10th	Night	14% 3 33% 40% 8% 1	410
	11th to 20th	Night	14% 3 31% 41% 9% 1	408
	21st to end	Night	13% 3 33% 41% 10%	460
August	1st to 10th	Night	11% 5% 32% 41% 11%	596
	11th to 20th	Night	12% 4 29% 42% 12% 1	634
	21st to end	Night	14% 5% 25% 37% 17% 2	924
September	1st to 10th	Night	14% 5% 30% 37% 14% 1	840
	11th to 20th	Night	15% 7% 27% 30% 18% 2	840
	21st to end	Night	24% 6% 25% 30% 12% 2	840
October	1st to 10th	Night	22% 11% 26% 26% 13% 2	840
	11th to 20th	Night	18% 13% 32% 25% 10% 2	813
	21st to end	Night	25% 12% 34% 22% 6%	880
November	1st to 10th	Night	18% 15% 34% 24% 7% 1	820
	11th to 20th	Night	21% 22% 32% 19% 6% 1	840
	21st to end	Night	20% 22% 33% 19% 5%	840
December	1st to 10th	Night	25% 18% 35% 17% 4	840
	11th to 20th	Night	24% 20% 33% 18% 3	840
	21st to end	Night	25% 18% 37% 18% 2	924

Page generated at 2026-10-11 05:06:27. Version ac07

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