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 2025-05-11.

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% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	113994
	Night	15% 9% 3 8% 5% 5% 5% 4 6% 7% 33%	114928
Transparency	Day	46% 8% 14% 20% 11% 1	117629
	Night	48% 7% 13% 20% 12% 1	119456
Seeing	Day	17% 10% 27% 27% 14% 4	15017
	Night	19% 9% 29% 29% 13% 2	24518

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	13% 10% 4 9% 6% 5% 5% 4 5% 6% 33%	3597
	11th to 20th	Day	10% 16% 7% 17% 9% 7% 5% 4 4 5% 16%	3825
	21st to end	Day	17% 16% 7% 12% 6% 5% 5% 4 4 6% 19%	4466
February	1st to 10th	Day	15% 14% 5% 11% 7% 5% 6% 5% 5% 5% 22%	1800
	11th to 20th	Day	13% 14% 5% 12% 6% 5% 5% 3 5% 6% 27%	1753
	21st to end	Day	17% 13% 4 10% 6% 5% 4 4 4 6% 28%	1535
March	1st to 10th	Day	20% 8% 4 8% 5% 4 5% 4 5% 6% 32%	4932
	11th to 20th	Day	11% 11% 4 9% 6% 6% 6% 4 6% 8% 31%	4980
	21st to end	Day	18% 9% 3 7% 5% 4 4 3 5% 6% 36%	5465
April	1st to 10th	Day	17% 10% 4 8% 5% 4 5% 3 6% 7% 31%	2383
	11th to 20th	Day	18% 8% 3 8% 4 4 5% 3 5% 6% 36%	2423
	21st to end	Day	18% 8% 3 7% 6% 5% 5% 3 6% 6% 34%	2600
May	1st to 10th	Day	17% 8% 3 7% 5% 5% 5% 4 6% 8% 33%	4266
	11th to 20th	Day	14% 9% 4 9% 5% 4 5% 5% 6% 8% 31%	4398
	21st to end	Day	11% 5% 2 5% 4 4 4 4 6% 9% 47%	4837
June	1st to 10th	Day	13% 6% 2 7% 5% 4 5% 5% 7% 8% 37%	2660
	11th to 20th	Day	11% 5% 2 6% 4 5% 5% 4 6% 10% 42%	2660
	21st to end	Day	12% 5% 2 6% 4 5% 4 4 7% 9% 42%	2660
July	1st to 10th	Day	9% 4 2 5% 4 5% 5% 5% 7% 10% 44%	4440
	11th to 20th	Day	9% 4 2 5% 4 4 4 5% 7% 9% 47%	4440
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 7% 9% 44%	4571
August	1st to 10th	Day	15% 7% 3 5% 5% 4 5% 4 5% 8% 40%	4104
	11th to 20th	Day	10% 6% 2 7% 5% 5% 5% 4 7% 10% 40%	4044
	21st to end	Day	8% 5% 2 6% 4 5% 5% 4 6% 10% 45%	4322
September	1st to 10th	Day	12% 5% 2 6% 3 4 6% 5% 5% 8% 45%	2280
	11th to 20th	Day	10% 5% 2 5% 4 4 4 4 5% 8% 49%	2199
	21st to end	Day	12% 6% 2 6% 5% 5% 4 4 6% 9% 40%	2032
October	1st to 10th	Day	16% 7% 2 7% 5% 5% 5% 4 6% 6% 38%	1900
	11th to 20th	Day	18% 9% 3 7% 5% 4 4 3 6% 7% 33%	1900
	21st to end	Day	22% 10% 3 9% 5% 5% 5% 4 6% 7% 26%	2090
November	1st to 10th	Day	13% 8% 3 8% 6% 5% 5% 4 6% 8% 34%	1686
	11th to 20th	Day	15% 11% 3 9% 6% 7% 5% 4 7% 7% 26%	1600
	21st to end	Day	19% 12% 4 9% 6% 5% 5% 3 6% 8% 24%	1600
December	1st to 10th	Day	19% 13% 4 11% 7% 7% 7% 4 7% 6% 15%	2600
	11th to 20th	Day	20% 14% 5% 8% 5% 5% 4 4 5% 7% 24%	2986
	21st to end	Day	27% 15% 6% 11% 6% 5% 4 3 4 5% 14%	3960
January	1st to 10th	Night	15% 11% 4 8% 5% 5% 5% 4 7% 7% 29%	6349
	11th to 20th	Night	13% 13% 5% 14% 8% 6% 6% 4 6% 8% 17%	6043
	21st to end	Night	17% 13% 5% 10% 6% 5% 5% 4 6% 7% 23%	6490
February	1st to 10th	Night	13% 15% 4 10% 6% 5% 6% 4 6% 6% 25%	2600
	11th to 20th	Night	15% 13% 4 10% 7% 6% 5% 4 6% 6% 27%	2533
	21st to end	Night	14% 11% 4 9% 5% 5% 5% 5% 6% 5% 29%	2092
March	1st to 10th	Night	17% 10% 4 8% 5% 5% 5% 3 5% 7% 32%	4908
	11th to 20th	Night	11% 8% 3 8% 6% 5% 5% 4 6% 8% 37%	4979
	21st to end	Night	21% 7% 3 6% 4 4 4 3 4 5% 39%	5467
April	1st to 10th	Night	18% 10% 3 7% 6% 4 4 3 5% 7% 33%	2016
	11th to 20th	Night	16% 7% 3 6% 6% 4 6% 4 5% 6% 38%	1943
	21st to end	Night	16% 8% 2 7% 4 5% 5% 4 7% 7% 35%	1800
May	1st to 10th	Night	20% 7% 3 6% 4 5% 4 3 5% 6% 38%	2892
	11th to 20th	Night	15% 7% 3 8% 5% 5% 6% 4 7% 8% 33%	2501
	21st to end	Night	9% 4 2 5% 4 4 4 4 6% 8% 50%	2738
June	1st to 10th	Night	12% 5% 2 6% 4 6% 5% 4 7% 8% 42%	1520
	11th to 20th	Night	8% 5% 3 6% 6% 5% 4 4 6% 9% 43%	1520
	21st to end	Night	12% 6% 1 5% 5% 5% 5% 4 6% 9% 42%	1520
July	1st to 10th	Night	6% 5% 2 6% 4 4 4 4 6% 9% 49%	2520
	11th to 20th	Night	11% 4 2 6% 4 3 4 3 5% 9% 50%	2520
	21st to end	Night	8% 4 2 5% 3 4 4 4 7% 8% 51%	2979
August	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 8% 47%	2825
	11th to 20th	Night	12% 6% 2 6% 3 3 4 3 5% 8% 50%	2785
	21st to end	Night	9% 5% 2 6% 4 5% 4 4 6% 8% 47%	3302
September	1st to 10th	Night	10% 5% 2 6% 4 5% 5% 5% 5% 7% 47%	1900
	11th to 20th	Night	11% 4 2 4 3 3 3 3 5% 7% 55%	1978
	21st to end	Night	16% 6% 3 7% 4 4 4 3 4 6% 42%	2147
October	1st to 10th	Night	15% 7% 3 6% 5% 4 4 4 5% 7% 40%	2280
	11th to 20th	Night	20% 8% 3 8% 5% 4 5% 4 5% 6% 34%	2280
	21st to end	Night	21% 8% 3 7% 4 4 6% 4 5% 6% 31%	2508
November	1st to 10th	Night	17% 7% 2 7% 5% 5% 4 4 6% 7% 38%	2609
	11th to 20th	Night	17% 9% 3 8% 6% 4 5% 5% 6% 7% 31%	2800
	21st to end	Night	20% 10% 3 8% 5% 5% 5% 4 6% 8% 26%	2800
December	1st to 10th	Night	17% 13% 4 10% 6% 5% 5% 4 6% 7% 21%	4580
	11th to 20th	Night	18% 15% 4 9% 5% 5% 5% 4 6% 6% 24%	5208
	21st to end	Night	26% 13% 6% 11% 6% 5% 5% 4 5% 5% 15%	6996

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	50% 8% 15% 15% 11% 1	3598
	11th to 20th	Day	69% 7% 10% 9% 4 1	3858
	21st to end	Day	65% 7% 11% 9% 7%	4466
February	1st to 10th	Day	62% 8% 10% 15% 4 1	1800
	11th to 20th	Day	57% 7% 12% 16% 7% 1	1762
	21st to end	Day	57% 6% 12% 17% 6% 1	1535
March	1st to 10th	Day	51% 6% 14% 14% 14% 1	4935
	11th to 20th	Day	48% 7% 19% 17% 7% 2	4980
	21st to end	Day	48% 5% 10% 21% 15% 1	5465
April	1st to 10th	Day	52% 7% 12% 17% 11% 1	2383
	11th to 20th	Day	47% 6% 11% 21% 14% 1	2423
	21st to end	Day	49% 7% 12% 18% 12% 3	2600
May	1st to 10th	Day	46% 8% 14% 17% 13% 2	4278
	11th to 20th	Day	47% 12% 14% 18% 9% 1	4402
	21st to end	Day	33% 8% 15% 25% 16% 3	4837
June	1st to 10th	Day	42% 9% 13% 21% 14% 1	2660
	11th to 20th	Day	37% 8% 13% 25% 16% 2	2660
	21st to end	Day	37% 8% 17% 23% 14% 1	2660
July	1st to 10th	Day	33% 8% 15% 27% 15% 2	4370
	11th to 20th	Day	31% 8% 14% 36% 10% 1	4365
	21st to end	Day	34% 9% 16% 24% 17% 1	4425
August	1st to 10th	Day	41% 8% 14% 22% 14% 1	4091
	11th to 20th	Day	37% 7% 17% 28% 10% 1	4083
	21st to end	Day	32% 8% 14% 35% 10% 1	4322
September	1st to 10th	Day	36% 9% 14% 28% 11% 1	2280
	11th to 20th	Day	32% 8% 15% 29% 15% 1	2199
	21st to end	Day	40% 9% 14% 26% 10%	2032
October	1st to 10th	Day	36% 6% 16% 26% 15% 1	3160
	11th to 20th	Day	37% 6% 17% 25% 14%	3140
	21st to end	Day	49% 7% 15% 22% 6%	3428
November	1st to 10th	Day	48% 8% 14% 20% 9% 1	1686
	11th to 20th	Day	54% 9% 14% 15% 8% 1	1600
	21st to end	Day	58% 7% 15% 16% 3	1600
December	1st to 10th	Day	67% 8% 14% 8% 3	2600
	11th to 20th	Day	60% 8% 13% 10% 9%	2986
	21st to end	Day	71% 8% 6% 10% 5%	3960
January	1st to 10th	Night	52% 7% 14% 20% 6%	6349
	11th to 20th	Night	63% 11% 10% 11% 5%	6085
	21st to end	Night	58% 8% 12% 11% 10% 1	6490
February	1st to 10th	Night	58% 8% 12% 17% 4 1	2600
	11th to 20th	Night	57% 6% 12% 16% 9% 1	2535
	21st to end	Night	53% 7% 12% 19% 8% 1	2092
March	1st to 10th	Night	52% 5% 11% 17% 12% 3	4918
	11th to 20th	Night	43% 6% 19% 22% 10% 2	4980
	21st to end	Night	46% 4 11% 20% 17% 2	5467
April	1st to 10th	Night	52% 4 11% 19% 13% 1	2016
	11th to 20th	Night	46% 6% 9% 19% 16% 2	1943
	21st to end	Night	46% 8% 10% 19% 15% 2	1800
May	1st to 10th	Night	46% 7% 13% 16% 16% 3	2899
	11th to 20th	Night	44% 10% 16% 18% 11% 2	2502
	21st to end	Night	30% 8% 13% 25% 21% 2	2739
June	1st to 10th	Night	39% 7% 11% 24% 16% 2	1520
	11th to 20th	Night	37% 6% 13% 22% 21% 2	1520
	21st to end	Night	37% 8% 12% 24% 17% 1	1520
July	1st to 10th	Night	31% 5% 16% 25% 21% 3	2480
	11th to 20th	Night	32% 5% 14% 34% 15% 1	2472
	21st to end	Night	30% 8% 15% 30% 17% 1	2884
August	1st to 10th	Night	35% 5% 14% 28% 15% 2	2814
	11th to 20th	Night	33% 6% 16% 32% 12% 1	2812
	21st to end	Night	34% 7% 12% 32% 15% 1	3302
September	1st to 10th	Night	34% 8% 14% 27% 15% 1	1900
	11th to 20th	Night	29% 6% 13% 33% 18%	1978
	21st to end	Night	42% 7% 12% 26% 12%	2147
October	1st to 10th	Night	34% 5% 10% 27% 24% 1	3800
	11th to 20th	Night	42% 5% 15% 28% 10%	3779
	21st to end	Night	40% 6% 15% 27% 11%	4120
November	1st to 10th	Night	44% 7% 13% 22% 13% 1	2609
	11th to 20th	Night	50% 8% 15% 17% 10% 1	2800
	21st to end	Night	55% 9% 17% 15% 4	2800
December	1st to 10th	Night	61% 7% 14% 13% 4	4580
	11th to 20th	Night	59% 8% 11% 12% 9% 1	5208
	21st to end	Night	69% 9% 9% 8% 5%	6996

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% 20% 31% 20% 8% 1	200
	11th to 20th	Day	23% 20% 34% 16% 5% 1	257
	21st to end	Day	24% 20% 34% 19% 3	330
February	1st to 10th	Day	22% 23% 35% 15% 4	300
	11th to 20th	Day	22% 23% 37% 15% 3	300
	21st to end	Day	26% 22% 33% 18% 2	289
March	1st to 10th	Day	21% 15% 36% 22% 5%	500
	11th to 20th	Day	19% 18% 34% 23% 6%	500
	21st to end	Day	27% 12% 33% 22% 7% 1	550
April	1st to 10th	Day	20% 16% 32% 26% 5%	500
	11th to 20th	Day	24% 12% 30% 21% 10% 2	492
	21st to end	Day	21% 12% 35% 20% 10% 2	500
May	1st to 10th	Day	20% 8% 26% 30% 11% 5%	503
	11th to 20th	Day	16% 4 24% 34% 18% 4	487
	21st to end	Day	13% 3 21% 35% 21% 6%	529
June	1st to 10th	Day	14% 7% 27% 35% 16% 2	490
	11th to 20th	Day	12% 4 24% 36% 21% 2	490
	21st to end	Day	14% 4 23% 37% 17% 4	490
July	1st to 10th	Day	8% 3 18% 36% 29% 7%	485
	11th to 20th	Day	13% 2 18% 35% 23% 8%	484
	21st to end	Day	7% 3 22% 41% 21% 6%	532
August	1st to 10th	Day	12% 3 17% 34% 24% 10%	486
	11th to 20th	Day	10% 4 18% 30% 29% 9%	479
	21st to end	Day	11% 3 19% 36% 22% 9%	539
September	1st to 10th	Day	11% 6% 21% 31% 21% 10%	490
	11th to 20th	Day	9% 4 16% 32% 24% 16%	490
	21st to end	Day	14% 6% 29% 28% 16% 7%	490
October	1st to 10th	Day	19% 9% 26% 25% 16% 6%	490
	11th to 20th	Day	19% 13% 33% 24% 8% 2	490
	21st to end	Day	24% 14% 31% 22% 8% 1	539
November	1st to 10th	Day	19% 16% 28% 25% 11% 1	296
	11th to 20th	Day	18% 19% 35% 23% 6% 1	200
	21st to end	Day	25% 16% 39% 16% 5% 1	200
December	1st to 10th	Day	22% 17% 33% 21% 6% 2	200
	11th to 20th	Day	27% 23% 29% 17% 5%	200
	21st to end	Day	28% 17% 39% 13% 3	220
January	1st to 10th	Night	24% 14% 39% 17% 6%	800
	11th to 20th	Night	26% 14% 33% 23% 3 1	800
	21st to end	Night	19% 12% 37% 26% 6% 1	880
February	1st to 10th	Night	20% 16% 40% 22% 3	800
	11th to 20th	Night	22% 15% 36% 22% 5%	783
	21st to end	Night	21% 14% 40% 21% 4	660
March	1st to 10th	Night	19% 13% 36% 27% 4 1	800
	11th to 20th	Night	20% 13% 35% 27% 5% 1	800
	21st to end	Night	20% 10% 31% 28% 11%	880
April	1st to 10th	Night	24% 10% 31% 29% 6%	800
	11th to 20th	Night	17% 10% 32% 30% 10% 1	759
	21st to end	Night	20% 6% 30% 32% 11%	600
May	1st to 10th	Night	20% 4 26% 31% 17% 2	582
	11th to 20th	Night	16% 2 27% 37% 17% 1	376
	21st to end	Night	9% 3 19% 43% 24% 2	411
June	1st to 10th	Night	13% 3 24% 41% 18% 1	380
	11th to 20th	Night	9% 3 24% 41% 22% 2	380
	21st to end	Night	12% 3 20% 41% 22% 1	380
July	1st to 10th	Night	6% 2 15% 46% 29% 2	370
	11th to 20th	Night	12% 3 15% 42% 26% 2	368
	21st to end	Night	9% 2 15% 47% 25% 3	555
August	1st to 10th	Night	10% 1 13% 40% 32% 4	567
	11th to 20th	Night	8% 2 14% 38% 32% 5%	564
	21st to end	Night	11% 2 20% 37% 26% 3	727
September	1st to 10th	Night	10% 5% 21% 34% 24% 6%	760
	11th to 20th	Night	13% 4 18% 29% 29% 8%	760
	21st to end	Night	18% 5% 19% 31% 22% 5%	760
October	1st to 10th	Night	17% 5% 28% 28% 20% 2	760
	11th to 20th	Night	22% 11% 28% 29% 9% 1	760
	21st to end	Night	25% 9% 30% 30% 6% 1	836
November	1st to 10th	Night	21% 10% 26% 32% 10% 1	780
	11th to 20th	Night	21% 14% 30% 27% 7% 2	800
	21st to end	Night	24% 11% 34% 24% 6%	800
December	1st to 10th	Night	28% 13% 33% 21% 4	800
	11th to 20th	Night	22% 14% 38% 22% 4 1	800
	21st to end	Night	30% 11% 36% 21% 2	880

Page generated at 2025-05-08 09:33:55. Version ac07

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