Ajax
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Ajax clear sky chart from 2005-11-06 to 2026-09-14.

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	14% 8% 3 8% 5% 5% 5% 4 6% 7% 35%	149544
	Night	15% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	141499
Transparency	Day	46% 7% 13% 22% 11% 1	153135
	Night	47% 6% 12% 21% 12% 1	145995
Seeing	Day	17% 10% 27% 27% 14% 4	16793
	Night	19% 9% 29% 30% 12% 2	26093

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	16% 10% 4 9% 6% 5% 5% 4 5% 5% 30%	4661
	11th to 20th	Day	10% 18% 7% 16% 8% 6% 5% 4 4 5% 15%	4989
	21st to end	Day	14% 14% 6% 13% 6% 5% 5% 4 5% 6% 21%	5780
February	1st to 10th	Day	15% 14% 5% 11% 7% 5% 5% 5% 5% 5% 23%	1890
	11th to 20th	Day	13% 13% 5% 12% 7% 5% 5% 3 5% 6% 26%	1843
	21st to end	Day	16% 13% 5% 10% 6% 5% 4 4 5% 5% 27%	1609
March	1st to 10th	Day	20% 8% 4 8% 5% 4 4 4 5% 6% 32%	5042
	11th to 20th	Day	11% 11% 4 9% 6% 6% 6% 4 6% 8% 30%	5090
	21st to end	Day	18% 9% 3 7% 5% 4 4 3 5% 6% 36%	5586
April	1st to 10th	Day	18% 10% 4 8% 6% 4 5% 3 6% 7% 31%	2502
	11th to 20th	Day	19% 8% 3 8% 4 4 5% 3 5% 6% 35%	2545
	21st to end	Day	18% 8% 3 7% 6% 5% 5% 3 6% 6% 34%	2730
May	1st to 10th	Day	18% 8% 3 8% 5% 5% 5% 4 6% 7% 32%	7696
	11th to 20th	Day	14% 8% 3 8% 5% 4 5% 5% 6% 8% 35%	8221
	21st to end	Day	17% 5% 2 5% 4 4 4 3 6% 7% 43%	9060
June	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 5% 7% 8% 38%	2940
	11th to 20th	Day	10% 5% 2 7% 5% 5% 5% 4 6% 10% 41%	2940
	21st to end	Day	13% 6% 2 6% 4 5% 4 4 7% 9% 42%	2940
July	1st to 10th	Day	8% 5% 2 7% 5% 5% 5% 5% 7% 9% 42%	8280
	11th to 20th	Day	6% 3 2 5% 4 4 4 4 7% 10% 51%	8280
	21st to end	Day	8% 4 2 5% 4 5% 5% 5% 7% 8% 46%	8527
August	1st to 10th	Day	16% 7% 3 6% 5% 4 5% 3 5% 7% 39%	6014
	11th to 20th	Day	9% 5% 2 7% 5% 5% 5% 4 8% 10% 41%	5951
	21st to end	Day	10% 6% 3 7% 4 5% 5% 4 6% 9% 40%	6293
September	1st to 10th	Day	12% 5% 1 6% 3 4 5% 5% 5% 8% 44%	2520
	11th to 20th	Day	10% 5% 1 5% 4 4 4 4 5% 8% 50%	2351
	21st to end	Day	13% 6% 2 6% 5% 5% 4 4 6% 9% 40%	2139
October	1st to 10th	Day	15% 7% 2 7% 4 5% 4 4 6% 6% 40%	2000
	11th to 20th	Day	18% 9% 3 7% 5% 4 5% 4 6% 7% 33%	2000
	21st to end	Day	22% 9% 3 9% 5% 5% 5% 4 6% 7% 26%	2200
November	1st to 10th	Day	14% 9% 3 8% 6% 5% 5% 4 6% 8% 34%	1771
	11th to 20th	Day	14% 11% 3 9% 6% 7% 5% 4 7% 8% 25%	1680
	21st to end	Day	18% 11% 4 9% 6% 5% 5% 3 6% 8% 23%	1680
December	1st to 10th	Day	19% 14% 4 11% 7% 7% 7% 4 7% 6% 15%	2680
	11th to 20th	Day	20% 14% 5% 8% 5% 5% 4 4 5% 7% 23%	3066
	21st to end	Day	27% 15% 6% 11% 6% 5% 4 3 4 5% 14%	4048
January	1st to 10th	Night	15% 12% 4 9% 5% 5% 5% 4 6% 6% 27%	8234
	11th to 20th	Night	13% 15% 5% 14% 8% 6% 6% 4 6% 7% 16%	7879
	21st to end	Night	14% 13% 5% 10% 6% 6% 6% 5% 6% 7% 23%	8401
February	1st to 10th	Night	13% 15% 4 9% 6% 5% 6% 4 6% 6% 26%	2730
	11th to 20th	Night	14% 12% 4 10% 7% 6% 5% 4 5% 6% 27%	2663
	21st to end	Night	14% 12% 4 9% 5% 5% 5% 5% 6% 5% 29%	2193
March	1st to 10th	Night	17% 10% 4 8% 5% 5% 5% 3 5% 6% 32%	5018
	11th to 20th	Night	11% 8% 3 8% 6% 5% 5% 4 6% 8% 37%	5089
	21st to end	Night	21% 7% 3 6% 4 4 4 3 4 5% 39%	5588
April	1st to 10th	Night	18% 10% 3 7% 6% 4 4 3 5% 7% 33%	2117
	11th to 20th	Night	16% 7% 3 7% 5% 5% 5% 4 5% 6% 38%	2041
	21st to end	Night	16% 8% 2 7% 4 5% 5% 4 7% 7% 36%	1890
May	1st to 10th	Night	22% 7% 3 6% 4 5% 4 3 5% 6% 34%	5200
	11th to 20th	Night	15% 7% 3 8% 5% 5% 6% 4 6% 7% 34%	4639
	21st to end	Night	17% 5% 2 6% 4 4 4 4 5% 7% 44%	5114
June	1st to 10th	Night	12% 5% 2 6% 4 5% 4 3 6% 8% 43%	1680
	11th to 20th	Night	8% 5% 3 6% 5% 5% 4 4 6% 9% 44%	1680
	21st to end	Night	12% 6% 2 5% 5% 5% 5% 4 6% 9% 41%	1680
July	1st to 10th	Night	7% 6% 2 6% 4 4 4 5% 6% 9% 46%	4680
	11th to 20th	Night	8% 3 1 6% 4 4 4 3 6% 10% 51%	4680
	21st to end	Night	10% 5% 2 5% 4 3 4 3 6% 8% 50%	5534
August	1st to 10th	Night	13% 7% 3 7% 4 4 4 4 5% 6% 43%	4135
	11th to 20th	Night	10% 5% 2 6% 3 4 4 3 5% 8% 51%	4095
	21st to end	Night	11% 5% 2 6% 4 5% 4 4 6% 9% 43%	4785
September	1st to 10th	Night	9% 5% 2 6% 4 5% 4 5% 5% 8% 47%	2100
	11th to 20th	Night	10% 4 2 4 3 3 3 3 5% 6% 55%	2113
	21st to end	Night	16% 6% 3 6% 4 4 4 3 4 6% 42%	2260
October	1st to 10th	Night	15% 7% 3 6% 5% 4 4 3 5% 7% 42%	2400
	11th to 20th	Night	20% 8% 2 7% 5% 4 5% 4 5% 6% 34%	2400
	21st to end	Night	21% 8% 3 7% 4 4 5% 4 5% 6% 32%	2640
November	1st to 10th	Night	17% 7% 2 7% 5% 5% 4 4 6% 7% 37%	2743
	11th to 20th	Night	17% 9% 3 8% 6% 5% 5% 5% 6% 7% 30%	2940
	21st to end	Night	20% 10% 3 8% 6% 5% 5% 4 6% 8% 25%	2940
December	1st to 10th	Night	17% 13% 4 10% 6% 5% 6% 4 6% 7% 21%	4720
	11th to 20th	Night	18% 15% 4 9% 5% 5% 5% 4 6% 6% 24%	5348
	21st to end	Night	26% 13% 6% 11% 6% 5% 5% 4 5% 5% 15%	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	54% 7% 14% 14% 11%	4662
	11th to 20th	Day	70% 6% 10% 9% 5%	5022
	21st to end	Day	62% 6% 11% 11% 9%	5780
February	1st to 10th	Day	61% 8% 10% 16% 5% 1	1890
	11th to 20th	Day	58% 7% 12% 16% 7% 1	1852
	21st to end	Day	58% 6% 12% 18% 6%	1609
March	1st to 10th	Day	52% 6% 14% 14% 14% 1	5045
	11th to 20th	Day	49% 7% 18% 16% 7% 2	5090
	21st to end	Day	49% 5% 10% 20% 15% 1	5586
April	1st to 10th	Day	52% 7% 11% 18% 11% 1	2502
	11th to 20th	Day	48% 6% 11% 21% 13% 1	2545
	21st to end	Day	49% 7% 12% 18% 12% 3	2730
May	1st to 10th	Day	49% 7% 13% 17% 11% 3	7705
	11th to 20th	Day	44% 9% 12% 19% 14% 1	8225
	21st to end	Day	39% 5% 12% 28% 14% 2	9061
June	1st to 10th	Day	41% 8% 13% 21% 16% 1	2940
	11th to 20th	Day	37% 8% 13% 25% 15% 1	2940
	21st to end	Day	38% 8% 16% 24% 14% 1	2940
July	1st to 10th	Day	36% 7% 15% 27% 13% 1	8210
	11th to 20th	Day	28% 6% 13% 38% 15% 1	8205
	21st to end	Day	33% 7% 14% 31% 14%	8339
August	1st to 10th	Day	44% 7% 13% 18% 14% 3	6001
	11th to 20th	Day	35% 7% 20% 30% 8%	5990
	21st to end	Day	38% 7% 16% 31% 8%	6293
September	1st to 10th	Day	35% 9% 14% 30% 11% 1	2520
	11th to 20th	Day	31% 8% 15% 31% 15% 1	2351
	21st to end	Day	41% 9% 14% 26% 10%	2139
October	1st to 10th	Day	35% 6% 15% 27% 15% 1	3260
	11th to 20th	Day	37% 6% 17% 25% 14%	3240
	21st to end	Day	49% 7% 15% 22% 6%	3538
November	1st to 10th	Day	48% 8% 14% 20% 9% 1	1771
	11th to 20th	Day	54% 9% 14% 15% 8% 1	1680
	21st to end	Day	58% 7% 15% 16% 3	1680
December	1st to 10th	Day	67% 8% 14% 8% 3	2680
	11th to 20th	Day	60% 8% 13% 10% 9%	3066
	21st to end	Day	70% 8% 6% 10% 5%	4048
January	1st to 10th	Night	54% 7% 14% 19% 6%	8234
	11th to 20th	Night	64% 9% 10% 11% 7% 1	7921
	21st to end	Night	56% 7% 12% 13% 12% 1	8401
February	1st to 10th	Night	56% 8% 12% 18% 5% 1	2730
	11th to 20th	Night	57% 6% 12% 16% 8%	2665
	21st to end	Night	53% 7% 12% 19% 8% 1	2193
March	1st to 10th	Night	52% 5% 11% 17% 13% 3	5028
	11th to 20th	Night	43% 6% 18% 22% 10% 2	5090
	21st to end	Night	46% 4 11% 20% 17% 2	5588
April	1st to 10th	Night	51% 4 11% 19% 13% 1	2117
	11th to 20th	Night	47% 6% 9% 20% 16% 2	2041
	21st to end	Night	45% 8% 11% 20% 15% 2	1890
May	1st to 10th	Night	49% 6% 10% 16% 14% 4	5207
	11th to 20th	Night	46% 8% 11% 19% 14% 2	4640
	21st to end	Night	40% 5% 11% 26% 16% 2	5115
June	1st to 10th	Night	38% 7% 10% 25% 17% 2	1680
	11th to 20th	Night	37% 5% 13% 23% 21% 2	1680
	21st to end	Night	38% 8% 12% 24% 16% 1	1680
July	1st to 10th	Night	34% 6% 16% 26% 16% 2	4640
	11th to 20th	Night	29% 5% 12% 34% 20% 1	4632
	21st to end	Night	32% 6% 12% 28% 22% 1	5407
August	1st to 10th	Night	41% 5% 13% 22% 15% 4	4124
	11th to 20th	Night	32% 5% 16% 35% 11% 1	4122
	21st to end	Night	35% 6% 16% 30% 12% 1	4785
September	1st to 10th	Night	34% 7% 14% 30% 14% 1	2100
	11th to 20th	Night	28% 6% 13% 34% 18%	2113
	21st to end	Night	41% 7% 12% 26% 13% 1	2260
October	1st to 10th	Night	34% 4 9% 28% 24% 1	3920
	11th to 20th	Night	42% 5% 14% 28% 10%	3899
	21st to end	Night	40% 6% 15% 27% 11% 1	4252
November	1st to 10th	Night	45% 7% 13% 22% 13% 1	2743
	11th to 20th	Night	51% 7% 15% 17% 9% 1	2940
	21st to end	Night	56% 9% 16% 15% 4	2940
December	1st to 10th	Night	61% 7% 14% 14% 4	4720
	11th to 20th	Night	59% 8% 11% 12% 9% 1	5348
	21st to end	Night	69% 9% 9% 8% 5%	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	22% 21% 29% 19% 8%	220
	11th to 20th	Day	24% 21% 34% 15% 5% 1	283
	21st to end	Day	25% 19% 33% 19% 3	363
February	1st to 10th	Day	22% 22% 35% 16% 5% 1	330
	11th to 20th	Day	22% 21% 38% 16% 3	330
	21st to end	Day	25% 23% 33% 17% 2	315
March	1st to 10th	Day	21% 16% 36% 22% 5%	540
	11th to 20th	Day	21% 19% 32% 22% 6%	540
	21st to end	Day	26% 14% 32% 22% 6% 1	594
April	1st to 10th	Day	20% 17% 31% 25% 5%	540
	11th to 20th	Day	25% 12% 30% 20% 10% 2	532
	21st to end	Day	22% 11% 35% 21% 10% 2	540
May	1st to 10th	Day	20% 8% 28% 29% 10% 5%	543
	11th to 20th	Day	14% 6% 28% 32% 16% 4	567
	21st to end	Day	15% 4 24% 32% 19% 5%	617
June	1st to 10th	Day	14% 7% 27% 35% 16% 2	570
	11th to 20th	Day	12% 6% 26% 35% 20% 2	570
	21st to end	Day	14% 4 24% 38% 16% 4	570
July	1st to 10th	Day	8% 3 20% 36% 27% 6%	565
	11th to 20th	Day	12% 4 20% 35% 23% 8%	564
	21st to end	Day	7% 3 22% 41% 20% 6%	620
August	1st to 10th	Day	11% 3 18% 35% 23% 9%	566
	11th to 20th	Day	10% 4 20% 31% 26% 9%	559
	21st to end	Day	12% 4 19% 36% 21% 8%	627
September	1st to 10th	Day	11% 6% 21% 31% 21% 10%	570
	11th to 20th	Day	8% 4 15% 32% 25% 16%	542
	21st to end	Day	15% 5% 28% 29% 16% 7%	530
October	1st to 10th	Day	18% 8% 26% 25% 17% 6%	530
	11th to 20th	Day	19% 13% 35% 24% 7% 2	530
	21st to end	Day	24% 14% 31% 22% 8% 1	583
November	1st to 10th	Day	21% 16% 29% 24% 10% 1	321
	11th to 20th	Day	18% 20% 35% 21% 6%	220
	21st to end	Day	24% 18% 38% 16% 4	220
December	1st to 10th	Day	22% 18% 32% 21% 5% 2	220
	11th to 20th	Day	25% 24% 29% 17% 5%	220
	21st to end	Day	29% 18% 38% 12% 2	242
January	1st to 10th	Night	24% 15% 38% 17% 5%	840
	11th to 20th	Night	26% 16% 33% 22% 3	840
	21st to end	Night	19% 13% 36% 25% 6% 1	924
February	1st to 10th	Night	20% 15% 39% 23% 3	840
	11th to 20th	Night	22% 15% 36% 23% 5%	823
	21st to end	Night	21% 14% 40% 21% 4	692
March	1st to 10th	Night	19% 13% 36% 28% 4 1	840
	11th to 20th	Night	21% 13% 34% 26% 5% 1	840
	21st to end	Night	20% 10% 31% 28% 10%	924
April	1st to 10th	Night	24% 10% 32% 28% 6%	840
	11th to 20th	Night	17% 10% 32% 30% 10% 1	797
	21st to end	Night	20% 6% 30% 33% 10%	630
May	1st to 10th	Night	20% 4 27% 32% 17% 1	611
	11th to 20th	Night	15% 3 28% 38% 15%	414
	21st to end	Night	11% 4 20% 42% 21% 2	455
June	1st to 10th	Night	14% 3 23% 41% 18% 1	420
	11th to 20th	Night	10% 3 25% 39% 21% 2	420
	21st to end	Night	12% 3 20% 42% 21% 1	420
July	1st to 10th	Night	6% 2 14% 46% 30% 2	410
	11th to 20th	Night	11% 3 15% 43% 26% 2	408
	21st to end	Night	9% 2 15% 47% 24% 2	614
August	1st to 10th	Night	11% 2 13% 39% 32% 4	627
	11th to 20th	Night	8% 2 14% 39% 32% 4	624
	21st to end	Night	11% 2 20% 38% 25% 3	803
September	1st to 10th	Night	10% 5% 21% 35% 24% 5%	840
	11th to 20th	Night	12% 4 17% 29% 30% 8%	813
	21st to end	Night	18% 5% 19% 31% 22% 5%	800
October	1st to 10th	Night	17% 5% 28% 28% 21% 2	800
	11th to 20th	Night	22% 10% 29% 29% 9% 1	800
	21st to end	Night	24% 9% 30% 30% 6% 1	880
November	1st to 10th	Night	21% 10% 26% 32% 10% 1	820
	11th to 20th	Night	21% 14% 30% 26% 6% 2	840
	21st to end	Night	25% 11% 34% 24% 6%	840
December	1st to 10th	Night	28% 13% 33% 22% 4	840
	11th to 20th	Night	22% 15% 38% 22% 4	840
	21st to end	Night	30% 11% 36% 20% 2	924

Page generated at 2026-09-11 04:11:51. Version ac07

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