Hamilton
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hamilton clear sky chart from 2005-11-06 to 2025-05-26.

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	4 6% 2 7% 5% 5% 5% 5% 7% 8% 46%	117300
	Night	4 6% 3 7% 5% 5% 6% 5% 7% 8% 45%	120748
Transparency	Day	36% 9% 14% 22% 14% 5%	117036
	Night	37% 11% 16% 23% 11% 3	120600
Seeing	Day	12% 19% 29% 25% 12% 3	15766
	Night	9% 13% 26% 34% 16% 2	25223

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	1 6% 5% 9% 8% 9% 9% 8% 11% 11% 26%	3476
	11th to 20th	Day	1 4 3 12% 7% 7% 9% 6% 7% 9% 37%	3640
	21st to end	Day	1 5% 2 6% 6% 5% 6% 5% 7% 10% 46%	4004
February	1st to 10th	Day	2 10% 4 11% 8% 8% 9% 7% 7% 9% 24%	1740
	11th to 20th	Day	1 10% 4 10% 8% 7% 7% 5% 7% 9% 31%	1826
	21st to end	Day	2 9% 5% 11% 8% 7% 7% 7% 7% 9% 28%	1650
March	1st to 10th	Day	2 7% 3 8% 5% 5% 6% 6% 9% 10% 40%	4518
	11th to 20th	Day	2 8% 3 9% 6% 5% 6% 5% 8% 8% 39%	4990
	21st to end	Day	3 10% 4 11% 10% 9% 8% 6% 7% 9% 23%	5486
April	1st to 10th	Day	5% 10% 4 10% 7% 7% 7% 5% 9% 10% 27%	2400
	11th to 20th	Day	6% 11% 3 10% 6% 6% 6% 4 6% 8% 34%	2552
	21st to end	Day	9% 12% 4 9% 6% 5% 5% 5% 6% 7% 33%	2600
May	1st to 10th	Day	10% 12% 4 10% 6% 5% 5% 4 6% 7% 32%	4240
	11th to 20th	Day	7% 5% 2 6% 5% 5% 5% 5% 8% 9% 43%	4322
	21st to end	Day	14% 8% 3 8% 5% 5% 5% 5% 7% 7% 33%	5198
June	1st to 10th	Day	14% 8% 2 7% 6% 4 6% 5% 6% 7% 34%	2850
	11th to 20th	Day	14% 8% 2 7% 4 5% 5% 4 6% 7% 39%	2850
	21st to end	Day	4 3 1 4 4 4 5% 4 6% 7% 59%	2850
July	1st to 10th	Day	1 1 1 2 2 3 3 3 5% 6% 73%	4740
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 78%	4424
	21st to end	Day	2 1 1 2 2 2 3 3 5% 6% 73%	4790
August	1st to 10th	Day	3 2 1 3 4 3 4 4 7% 8% 61%	4194
	11th to 20th	Day	3 2 1 3 2 4 5% 4 7% 8% 62%	4028
	21st to end	Day	3 3 1 3 3 3 3 3 4 6% 69%	4508
September	1st to 10th	Day	4 3 1 3 3 3 4 4 5% 7% 63%	2233
	11th to 20th	Day	6% 3 2 5% 4 4 5% 5% 7% 7% 51%	2090
	21st to end	Day	8% 6% 2 4 3 3 4 3 5% 7% 57%	2090
October	1st to 10th	Day	5% 5% 2 5% 3 4 4 3 4 5% 60%	3429
	11th to 20th	Day	5% 5% 2 5% 4 3 4 4 5% 7% 56%	3034
	21st to end	Day	3 7% 3 8% 6% 5% 5% 5% 7% 8% 44%	3101
November	1st to 10th	Day	5% 9% 3 9% 6% 7% 7% 6% 7% 8% 33%	1764
	11th to 20th	Day	3 6% 2 9% 7% 5% 7% 6% 9% 9% 38%	1691
	21st to end	Day	4 7% 3 6% 6% 5% 6% 4 9% 9% 42%	1559
December	1st to 10th	Day	1 4 1 6% 7% 7% 6% 4 8% 8% 49%	2290
	11th to 20th	Day	1 6% 3 9% 8% 7% 8% 7% 10% 11% 31%	2645
	21st to end	Day	2 7% 4 10% 6% 6% 7% 6% 6% 8% 39%	3498
January	1st to 10th	Night	1 7% 3 11% 7% 8% 9% 7% 8% 10% 29%	6513
	11th to 20th	Night	1 5% 5% 11% 8% 8% 7% 5% 7% 9% 34%	6320
	21st to end	Night	1 6% 3 6% 5% 5% 6% 5% 7% 9% 47%	6952
February	1st to 10th	Night	3 11% 5% 11% 8% 6% 7% 6% 7% 9% 27%	2660
	11th to 20th	Night	1 9% 3 11% 8% 6% 7% 6% 8% 8% 32%	2458
	21st to end	Night	2 7% 5% 9% 7% 7% 8% 6% 8% 10% 33%	1980
March	1st to 10th	Night	2 6% 3 7% 5% 5% 6% 5% 8% 8% 47%	5376
	11th to 20th	Night	2 8% 3 9% 6% 6% 6% 5% 7% 8% 40%	5010
	21st to end	Night	5% 12% 4 10% 6% 7% 8% 6% 8% 9% 26%	5474
April	1st to 10th	Night	6% 12% 4 7% 6% 5% 6% 5% 7% 9% 35%	2000
	11th to 20th	Night	6% 9% 3 9% 5% 5% 6% 5% 8% 8% 36%	1796
	21st to end	Night	8% 10% 2 8% 5% 5% 5% 5% 7% 9% 38%	1800
May	1st to 10th	Night	11% 13% 3 7% 4 4 4 4 5% 6% 39%	2950
	11th to 20th	Night	8% 7% 1 5% 4 3 4 4 6% 8% 50%	2794
	21st to end	Night	14% 11% 3 6% 5% 5% 5% 3 5% 8% 35%	2489
June	1st to 10th	Night	12% 7% 3 6% 5% 4 5% 5% 7% 8% 39%	1330
	11th to 20th	Night	13% 8% 3 5% 4 4 5% 4 5% 7% 43%	1330
	21st to end	Night	4 2 1 3 3 3 4 4 6% 8% 62%	1330
July	1st to 10th	Night	1 1 1 2 1 2 3 5% 8% 75%	2220
	11th to 20th	Night	1 1 2 2 2 3 2 4 7% 76%	2531
	21st to end	Night	1 1 2 1 2 3 2 4 6% 78%	2761
August	1st to 10th	Night	2 1 1 3 3 3 3 4 7% 9% 65%	2742
	11th to 20th	Night	2 1 1 2 2 3 4 3 6% 6% 69%	2811
	21st to end	Night	4 2 1 2 2 2 3 3 5% 6% 69%	3146
September	1st to 10th	Night	3 3 1 3 3 3 4 3 5% 6% 66%	1945
	11th to 20th	Night	6% 3 1 5% 3 4 4 4 6% 9% 54%	2090
	21st to end	Night	7% 4 2 4 3 3 4 3 5% 6% 59%	2090
October	1st to 10th	Night	5% 5% 2 4 3 3 4 3 5% 6% 61%	3548
	11th to 20th	Night	5% 5% 2 5% 3 4 4 4 5% 7% 56%	3890
	21st to end	Night	5% 7% 2 7% 5% 5% 4 4 6% 9% 47%	4457
November	1st to 10th	Night	6% 7% 2 8% 5% 6% 5% 5% 7% 10% 39%	2535
	11th to 20th	Night	5% 9% 2 7% 6% 5% 7% 6% 9% 10% 34%	2686
	21st to end	Night	2 7% 3 6% 4 5% 6% 5% 8% 10% 43%	2840
December	1st to 10th	Night	1 5% 2 5% 4 4 6% 4 7% 8% 53%	4890
	11th to 20th	Night	2 6% 2 8% 6% 6% 8% 7% 10% 11% 34%	5546
	21st to end	Night	2 7% 3 10% 7% 7% 8% 6% 8% 9% 34%	7458

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	49% 17% 14% 17% 3	3476
	11th to 20th	Day	45% 10% 19% 18% 7% 1	3640
	21st to end	Day	36% 11% 17% 25% 11%	4004
February	1st to 10th	Day	60% 12% 12% 12% 4 1	1740
	11th to 20th	Day	55% 12% 15% 11% 6% 1	1835
	21st to end	Day	57% 11% 12% 11% 8% 1	1650
March	1st to 10th	Day	39% 9% 14% 20% 17% 1	4518
	11th to 20th	Day	39% 10% 16% 29% 6%	4990
	21st to end	Day	54% 14% 16% 15% 2	5486
April	1st to 10th	Day	56% 12% 15% 15% 3	2400
	11th to 20th	Day	50% 9% 15% 18% 7% 1	2555
	21st to end	Day	52% 11% 13% 17% 6%	2600
May	1st to 10th	Day	51% 8% 14% 21% 6%	4240
	11th to 20th	Day	37% 11% 17% 26% 9% 1	4325
	21st to end	Day	50% 11% 16% 20% 4	5199
June	1st to 10th	Day	49% 10% 17% 18% 6%	2850
	11th to 20th	Day	46% 8% 13% 17% 13% 4	2850
	21st to end	Day	26% 9% 12% 29% 21% 3	2850
July	1st to 10th	Day	13% 6% 7% 16% 35% 23%	4666
	11th to 20th	Day	11% 4 5% 13% 25% 42%	4348
	21st to end	Day	12% 5% 6% 19% 38% 21%	4639
August	1st to 10th	Day	19% 7% 9% 27% 28% 10%	4178
	11th to 20th	Day	18% 7% 12% 32% 26% 5%	4067
	21st to end	Day	17% 5% 9% 32% 29% 8%	4507
September	1st to 10th	Day	22% 7% 10% 27% 25% 9%	2233
	11th to 20th	Day	32% 8% 12% 26% 18% 4	2090
	21st to end	Day	30% 6% 17% 28% 16% 2	2090
October	1st to 10th	Day	28% 6% 8% 36% 20% 1	3429
	11th to 20th	Day	31% 7% 20% 30% 12% 1	3034
	21st to end	Day	36% 10% 14% 26% 14%	3101
November	1st to 10th	Day	50% 11% 16% 17% 5% 1	1764
	11th to 20th	Day	47% 13% 19% 17% 4 1	1691
	21st to end	Day	44% 12% 16% 19% 9% 1	1559
December	1st to 10th	Day	31% 12% 26% 23% 7% 1	2290
	11th to 20th	Day	45% 15% 20% 15% 4	2644
	21st to end	Day	43% 10% 17% 20% 7% 3	3498
January	1st to 10th	Night	49% 14% 17% 17% 2	6513
	11th to 20th	Night	45% 11% 19% 17% 7% 1	6320
	21st to end	Night	36% 12% 18% 26% 8%	6952
February	1st to 10th	Night	59% 13% 13% 11% 4	2660
	11th to 20th	Night	52% 14% 15% 13% 5%	2459
	21st to end	Night	51% 13% 13% 14% 8% 1	1980
March	1st to 10th	Night	35% 10% 14% 24% 16% 1	5376
	11th to 20th	Night	40% 13% 15% 27% 5%	5010
	21st to end	Night	50% 16% 16% 14% 3	5474
April	1st to 10th	Night	49% 12% 17% 18% 4	2000
	11th to 20th	Night	47% 11% 16% 18% 7% 1	1796
	21st to end	Night	45% 12% 15% 20% 8%	1800
May	1st to 10th	Night	46% 9% 15% 23% 7%	2950
	11th to 20th	Night	30% 11% 18% 27% 13%	2794
	21st to end	Night	48% 10% 17% 22% 3	2489
June	1st to 10th	Night	44% 12% 19% 20% 5%	1330
	11th to 20th	Night	41% 9% 14% 21% 11% 3	1330
	21st to end	Night	20% 9% 17% 33% 18% 2	1330
July	1st to 10th	Night	9% 4 9% 24% 35% 20%	2185
	11th to 20th	Night	9% 4 6% 19% 22% 40%	2484
	21st to end	Night	10% 4 7% 28% 29% 23%	2674
August	1st to 10th	Night	15% 6% 12% 28% 31% 8%	2734
	11th to 20th	Night	15% 6% 13% 38% 23% 5%	2838
	21st to end	Night	17% 6% 10% 36% 26% 5%	3146
September	1st to 10th	Night	21% 7% 12% 26% 26% 9%	1945
	11th to 20th	Night	29% 9% 12% 27% 18% 6%	2090
	21st to end	Night	30% 7% 15% 27% 19% 3	2090
October	1st to 10th	Night	25% 7% 11% 34% 21% 1	3548
	11th to 20th	Night	29% 9% 20% 30% 12% 1	3890
	21st to end	Night	35% 10% 16% 28% 11% 1	4457
November	1st to 10th	Night	45% 11% 16% 20% 5% 2	2535
	11th to 20th	Night	49% 13% 16% 17% 3 2	2686
	21st to end	Night	41% 12% 17% 20% 10% 1	2840
December	1st to 10th	Night	30% 11% 29% 21% 7% 1	4890
	11th to 20th	Night	42% 16% 23% 17% 1	5547
	21st to end	Night	44% 12% 18% 17% 7% 2	7458

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	8% 24% 23% 31% 13% 3	200
	11th to 20th	Day	8% 18% 37% 21% 11% 6%	200
	21st to end	Day	11% 21% 30% 25% 9% 3	220
February	1st to 10th	Day	13% 34% 34% 15% 4	270
	11th to 20th	Day	9% 27% 32% 25% 7% 1	371
	21st to end	Day	14% 27% 35% 21% 5%	414
March	1st to 10th	Day	13% 24% 35% 20% 7% 1	500
	11th to 20th	Day	13% 22% 26% 24% 14%	500
	21st to end	Day	15% 28% 30% 21% 6% 1	550
April	1st to 10th	Day	16% 27% 29% 21% 7% 1	500
	11th to 20th	Day	19% 27% 28% 19% 6% 1	492
	21st to end	Day	21% 23% 27% 19% 8% 2	500
May	1st to 10th	Day	16% 17% 29% 26% 11% 1	503
	11th to 20th	Day	23% 16% 27% 20% 12% 2	528
	21st to end	Day	20% 16% 19% 25% 16% 3	732
June	1st to 10th	Day	16% 15% 25% 27% 13% 4	680
	11th to 20th	Day	21% 20% 26% 20% 11% 1	680
	21st to end	Day	7% 14% 28% 31% 18% 3	680
July	1st to 10th	Day	3 13% 29% 30% 21% 4	671
	11th to 20th	Day	6% 16% 35% 28% 12% 2	487
	21st to end	Day	4 11% 33% 33% 15% 4	531
August	1st to 10th	Day	5% 11% 30% 31% 18% 5%	487
	11th to 20th	Day	6% 11% 25% 35% 17% 6%	477
	21st to end	Day	5% 14% 31% 31% 14% 6%	539
September	1st to 10th	Day	5% 13% 31% 28% 16% 7%	490
	11th to 20th	Day	8% 15% 31% 27% 16% 4	490
	21st to end	Day	13% 18% 25% 24% 16% 4	490
October	1st to 10th	Day	15% 19% 24% 25% 14% 3	455
	11th to 20th	Day	13% 20% 25% 22% 13% 7%	300
	21st to end	Day	16% 22% 30% 23% 8% 2	330
November	1st to 10th	Day	18% 21% 28% 23% 10% 1	300
	11th to 20th	Day	11% 21% 35% 19% 11% 1	300
	21st to end	Day	10% 18% 33% 30% 8% 1	279
December	1st to 10th	Day	13% 18% 28% 24% 13% 5%	200
	11th to 20th	Day	12% 26% 31% 25% 6% 1	200
	21st to end	Day	10% 23% 35% 24% 7% 2	220
January	1st to 10th	Night	9% 19% 31% 31% 10% 1	890
	11th to 20th	Night	8% 20% 34% 27% 11% 2	800
	21st to end	Night	8% 15% 35% 31% 10% 1	880
February	1st to 10th	Night	11% 18% 36% 28% 7% 1	800
	11th to 20th	Night	8% 20% 33% 29% 8% 2	784
	21st to end	Night	8% 17% 35% 28% 10% 1	660
March	1st to 10th	Night	6% 15% 35% 34% 9% 1	800
	11th to 20th	Night	10% 16% 30% 30% 13% 1	800
	21st to end	Night	10% 14% 30% 35% 10% 1	880
April	1st to 10th	Night	14% 13% 26% 32% 14% 2	800
	11th to 20th	Night	12% 17% 30% 30% 11%	608
	21st to end	Night	13% 15% 24% 36% 11% 2	600
May	1st to 10th	Night	13% 11% 22% 35% 19% 2	600
	11th to 20th	Night	13% 9% 19% 34% 23% 2	514
	21st to end	Night	19% 8% 14% 28% 28% 3	422
June	1st to 10th	Night	17% 7% 18% 35% 22% 2	380
	11th to 20th	Night	19% 8% 21% 38% 14% 1	380
	21st to end	Night	4 4 15% 51% 24% 1	380
July	1st to 10th	Night	2 2 12% 51% 32% 1	370
	11th to 20th	Night	4 2 13% 57% 24%	368
	21st to end	Night	2 2 8% 53% 34% 2	416
August	1st to 10th	Night	3 3 12% 47% 33% 2	500
	11th to 20th	Night	4 2 11% 47% 33% 3	564
	21st to end	Night	5% 4 17% 44% 28% 1	627
September	1st to 10th	Night	4 4 15% 45% 26% 5%	758
	11th to 20th	Night	8% 4 15% 40% 28% 5%	760
	21st to end	Night	10% 6% 17% 39% 23% 5%	760
October	1st to 10th	Night	13% 9% 20% 37% 19% 2	760
	11th to 20th	Night	11% 10% 21% 37% 18% 3	760
	21st to end	Night	12% 15% 26% 32% 13% 3	836
November	1st to 10th	Night	12% 15% 29% 33% 11% 1	780
	11th to 20th	Night	14% 18% 32% 28% 8%	886
	21st to end	Night	8% 17% 33% 31% 10% 1	1000
December	1st to 10th	Night	10% 18% 30% 29% 11% 2	1000
	11th to 20th	Night	9% 17% 38% 29% 8%	1000
	21st to end	Night	8% 21% 37% 25% 8%	1100

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