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 2026-06-01.

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% 4 5% 4 5% 7% 36%	132169
	Night	14% 10% 3 8% 5% 4 4 3 5% 7% 36%	131031
Transparency	Day	45% 6% 12% 22% 13% 1	135794
	Night	46% 6% 11% 22% 14% 2	135476
Seeing	Day	18% 12% 27% 23% 15% 6%	17444
	Night	18% 9% 27% 30% 15% 2	27213

By Month

Percentage Cloud Cover SE (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% 16% 6% 11% 5% 4 4 3 4 5% 25%	4691
	11th to 20th	Day	12% 14% 5% 16% 6% 5% 4 3 3 4 27%	5026
	21st to end	Day	19% 12% 6% 11% 7% 6% 5% 4 5% 5% 21%	5780
February	1st to 10th	Day	18% 14% 4 10% 4 4 4 2 5% 5% 29%	2004
	11th to 20th	Day	16% 12% 4 11% 6% 4 4 3 4 5% 31%	2048
	21st to end	Day	17% 12% 4 11% 5% 4 3 2 4 5% 32%	1730
March	1st to 10th	Day	18% 14% 6% 10% 5% 4 4 3 3 4 30%	4598
	11th to 20th	Day	12% 8% 3 7% 4 4 4 4 6% 7% 40%	5090
	21st to end	Day	17% 9% 3 8% 5% 4 4 4 5% 7% 33%	5586
April	1st to 10th	Day	15% 10% 2 8% 5% 4 4 4 4 7% 35%	2489
	11th to 20th	Day	15% 8% 3 6% 5% 4 4 4 5% 7% 38%	2547
	21st to end	Day	17% 8% 3 7% 4 5% 4 3 6% 6% 36%	2730
May	1st to 10th	Day	19% 9% 4 8% 5% 5% 5% 4 6% 7% 29%	5992
	11th to 20th	Day	20% 9% 3 7% 4 3 4 3 5% 7% 35%	5991
	21st to end	Day	13% 7% 3 7% 5% 5% 6% 5% 8% 9% 32%	6576
June	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 5% 6% 8% 40%	2631
	11th to 20th	Day	9% 5% 2 7% 5% 4 5% 5% 6% 9% 43%	2800
	21st to end	Day	8% 6% 3 7% 5% 5% 5% 4 7% 11% 39%	2800
July	1st to 10th	Day	9% 6% 3 7% 5% 5% 5% 5% 7% 10% 40%	6360
	11th to 20th	Day	9% 5% 2 7% 5% 4 5% 5% 7% 12% 38%	6204
	21st to end	Day	10% 7% 3 8% 6% 6% 7% 5% 8% 10% 32%	6424
August	1st to 10th	Day	8% 5% 3 6% 4 4 6% 5% 6% 10% 43%	4234
	11th to 20th	Day	8% 5% 3 6% 4 5% 5% 4 7% 11% 42%	4174
	21st to end	Day	6% 5% 2 5% 4 3 4 3 6% 9% 53%	4327
September	1st to 10th	Day	11% 5% 2 4 4 4 5% 3 5% 7% 49%	2285
	11th to 20th	Day	8% 5% 2 6% 4 4 4 3 6% 8% 50%	2200
	21st to end	Day	15% 6% 3 5% 4 3 4 3 4 6% 47%	2200
October	1st to 10th	Day	10% 4 3 6% 4 4 4 3 4 7% 50%	2200
	11th to 20th	Day	12% 6% 2 6% 3 4 4 3 5% 6% 47%	2035
	21st to end	Day	20% 8% 3 6% 4 3 4 3 4 7% 38%	2035
November	1st to 10th	Day	9% 7% 2 5% 5% 4 4 4 4 6% 51%	1852
	11th to 20th	Day	15% 10% 4 9% 4 3 4 3 4 7% 37%	1890
	21st to end	Day	18% 11% 5% 7% 5% 4 4 3 4 5% 34%	1890
December	1st to 10th	Day	17% 10% 3 9% 4 4 3 3 4 5% 37%	3020
	11th to 20th	Day	18% 15% 5% 9% 6% 4 5% 3 5% 5% 26%	3463
	21st to end	Day	22% 19% 5% 11% 5% 4 4 3 4 6% 18%	4267
January	1st to 10th	Night	14% 15% 5% 13% 7% 5% 5% 4 5% 5% 21%	8232
	11th to 20th	Night	11% 17% 6% 15% 7% 5% 5% 3 4 5% 23%	7940
	21st to end	Night	18% 13% 4 8% 5% 5% 4 3 4 5% 31%	8401
February	1st to 10th	Night	20% 11% 4 9% 5% 4 5% 4 4 5% 29%	2614
	11th to 20th	Night	17% 10% 4 9% 6% 4 5% 4 5% 6% 30%	2458
	21st to end	Night	20% 13% 3 9% 4 4 3 3 4 6% 31%	2076
March	1st to 10th	Night	20% 12% 3 6% 4 4 3 3 4 5% 37%	5460
	11th to 20th	Night	11% 7% 2 7% 4 4 5% 4 6% 6% 43%	5090
	21st to end	Night	16% 9% 4 8% 4 4 5% 4 5% 6% 34%	5588
April	1st to 10th	Night	15% 9% 3 6% 4 4 5% 3 6% 7% 38%	2147
	11th to 20th	Night	14% 7% 2 6% 4 4 4 5% 6% 8% 40%	2020
	21st to end	Night	16% 6% 3 6% 5% 5% 4 4 6% 8% 37%	1890
May	1st to 10th	Night	20% 10% 3 8% 5% 4 5% 4 6% 7% 28%	4123
	11th to 20th	Night	16% 7% 3 7% 4 4 4 4 7% 9% 35%	4122
	21st to end	Night	14% 6% 3 7% 5% 5% 4 4 6% 9% 38%	4520
June	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 4 9% 49%	1762
	11th to 20th	Night	8% 6% 2 4 4 3 5% 4 8% 9% 46%	1600
	21st to end	Night	5% 5% 1 5% 4 5% 5% 5% 9% 10% 47%	1600
July	1st to 10th	Night	10% 5% 2 6% 4 4 4 4 6% 9% 46%	3618
	11th to 20th	Night	7% 6% 3 7% 4 5% 5% 4 7% 10% 42%	3749
	21st to end	Night	9% 6% 3 7% 4 5% 5% 4 7% 11% 39%	4427
August	1st to 10th	Night	8% 6% 2 7% 5% 5% 5% 4 6% 9% 43%	2915
	11th to 20th	Night	7% 5% 2 6% 5% 4 5% 4 7% 8% 47%	2875
	21st to end	Night	3 3 1 5% 3 4 5% 4 6% 9% 57%	3553
September	1st to 10th	Night	11% 5% 2 4 4 4 4 3 5% 7% 51%	2095
	11th to 20th	Night	8% 4 1 4 3 4 4 3 5% 9% 56%	2200
	21st to end	Night	14% 7% 2 6% 4 3 4 3 4 5% 49%	2200
October	1st to 10th	Night	9% 4 2 6% 3 4 4 3 4 6% 56%	2200
	11th to 20th	Night	14% 6% 2 6% 4 4 4 3 5% 7% 45%	2364
	21st to end	Night	17% 8% 3 7% 4 4 5% 4 4 6% 40%	2801
November	1st to 10th	Night	12% 8% 2 5% 3 4 4 3 4 7% 47%	2666
	11th to 20th	Night	16% 8% 3 7% 4 3 4 3 5% 6% 40%	2730
	21st to end	Night	20% 12% 4 8% 5% 4 4 3 5% 7% 30%	2730
December	1st to 10th	Night	17% 10% 4 8% 5% 4 3 3 4 5% 37%	4380
	11th to 20th	Night	19% 13% 5% 10% 6% 4 4 3 5% 5% 26%	4952
	21st to end	Night	24% 18% 6% 11% 5% 4 4 3 4 5% 17%	6933

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	61% 4 13% 18% 5%	4692
	11th to 20th	Day	60% 4 9% 13% 12% 1	5026
	21st to end	Day	65% 4 10% 13% 6% 2	5780
February	1st to 10th	Day	58% 5% 10% 18% 9% 1	2004
	11th to 20th	Day	56% 4 10% 20% 9% 1	2058
	21st to end	Day	54% 5% 11% 20% 8% 2	1730
March	1st to 10th	Day	57% 5% 6% 19% 11% 2	4598
	11th to 20th	Day	41% 6% 10% 28% 15% 1	5090
	21st to end	Day	49% 6% 12% 20% 10% 2	5586
April	1st to 10th	Day	47% 7% 12% 21% 12% 1	2489
	11th to 20th	Day	45% 6% 11% 22% 13% 3	2547
	21st to end	Day	47% 6% 11% 24% 11% 2	2730
May	1st to 10th	Day	52% 7% 12% 17% 10%	6027
	11th to 20th	Day	49% 6% 12% 23% 10%	5995
	21st to end	Day	43% 10% 13% 22% 12% 1	6590
June	1st to 10th	Day	39% 8% 12% 22% 17% 2	2631
	11th to 20th	Day	37% 8% 14% 22% 17% 2	2800
	21st to end	Day	38% 9% 15% 19% 16% 4	2800
July	1st to 10th	Day	38% 6% 12% 26% 17% 1	6291
	11th to 20th	Day	37% 8% 15% 25% 14% 1	6129
	21st to end	Day	45% 7% 14% 18% 13% 2	6240
August	1st to 10th	Day	33% 8% 16% 30% 12% 1	4221
	11th to 20th	Day	34% 6% 12% 32% 14% 1	4213
	21st to end	Day	27% 6% 15% 30% 19% 3	4326
September	1st to 10th	Day	33% 7% 11% 24% 19% 5%	2285
	11th to 20th	Day	31% 6% 11% 26% 23% 3	2200
	21st to end	Day	38% 6% 11% 22% 22% 2	2200
October	1st to 10th	Day	24% 4 9% 31% 31% 2	3590
	11th to 20th	Day	29% 5% 9% 30% 25% 2	3288
	21st to end	Day	37% 3 10% 24% 23% 2	3259
November	1st to 10th	Day	35% 4 10% 29% 19% 2	1852
	11th to 20th	Day	48% 4 10% 21% 14% 3	1890
	21st to end	Day	52% 5% 9% 22% 11% 1	1890
December	1st to 10th	Day	51% 4 10% 25% 10%	3020
	11th to 20th	Day	60% 5% 9% 18% 7%	3460
	21st to end	Day	69% 5% 10% 12% 5%	4267
January	1st to 10th	Night	63% 5% 11% 14% 7%	8233
	11th to 20th	Night	64% 4 8% 12% 9% 2	7940
	21st to end	Night	55% 4 11% 21% 8% 1	8401
February	1st to 10th	Night	58% 5% 10% 19% 8% 1	2614
	11th to 20th	Night	54% 5% 12% 19% 10% 1	2459
	21st to end	Night	54% 5% 12% 17% 9% 2	2076
March	1st to 10th	Night	50% 5% 6% 26% 11% 2	5460
	11th to 20th	Night	38% 7% 12% 27% 16% 1	5090
	21st to end	Night	49% 6% 9% 24% 10% 2	5588
April	1st to 10th	Night	44% 6% 12% 24% 12% 2	2147
	11th to 20th	Night	40% 7% 13% 19% 17% 5%	2020
	21st to end	Night	43% 7% 12% 22% 13% 3	1890
May	1st to 10th	Night	54% 8% 10% 18% 8% 2	4150
	11th to 20th	Night	44% 7% 14% 24% 11% 1	4123
	21st to end	Night	40% 8% 17% 22% 10% 3	4528
June	1st to 10th	Night	33% 7% 10% 26% 21% 2	1762
	11th to 20th	Night	32% 9% 13% 23% 20% 4	1600
	21st to end	Night	30% 9% 17% 21% 18% 5%	1600
July	1st to 10th	Night	34% 7% 12% 24% 19% 4	3580
	11th to 20th	Night	34% 9% 15% 26% 15% 1	3704
	21st to end	Night	37% 7% 14% 25% 14% 3	4297
August	1st to 10th	Night	35% 9% 13% 29% 12% 2	2904
	11th to 20th	Night	33% 7% 13% 30% 17% 1	2902
	21st to end	Night	22% 7% 15% 32% 21% 3	3553
September	1st to 10th	Night	31% 6% 13% 24% 19% 7%	2095
	11th to 20th	Night	26% 7% 10% 26% 27% 3	2200
	21st to end	Night	38% 4 10% 20% 25% 2	2200
October	1st to 10th	Night	21% 4 7% 29% 36% 3	3590
	11th to 20th	Night	28% 4 10% 26% 31% 2	3850
	21st to end	Night	34% 4 8% 22% 27% 4	4527
November	1st to 10th	Night	37% 4 12% 22% 22% 3	2666
	11th to 20th	Night	44% 5% 9% 23% 17% 3	2730
	21st to end	Night	54% 5% 11% 20% 10% 1	2730
December	1st to 10th	Night	50% 4 12% 24% 10%	4380
	11th to 20th	Night	59% 5% 12% 17% 7% 1	4954
	21st to end	Night	70% 5% 10% 12% 3	6933

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	27% 20% 33% 14% 6%	330
	11th to 20th	Day	29% 26% 31% 11% 2	330
	21st to end	Day	33% 21% 29% 15% 3	363
February	1st to 10th	Day	25% 22% 34% 15% 3	330
	11th to 20th	Day	28% 25% 33% 12% 2	330
	21st to end	Day	27% 22% 41% 8% 2	273
March	1st to 10th	Day	25% 26% 35% 12% 2	335
	11th to 20th	Day	27% 16% 34% 16% 6% 1	540
	21st to end	Day	29% 17% 32% 15% 6%	594
April	1st to 10th	Day	22% 19% 31% 19% 8% 1	540
	11th to 20th	Day	20% 22% 26% 19% 10% 3	532
	21st to end	Day	21% 17% 32% 19% 9% 3	540
May	1st to 10th	Day	22% 17% 31% 18% 8% 3	543
	11th to 20th	Day	23% 11% 27% 22% 12% 5%	567
	21st to end	Day	15% 9% 25% 27% 15% 10%	617
June	1st to 10th	Day	14% 6% 24% 30% 18% 8%	561
	11th to 20th	Day	10% 6% 24% 32% 22% 6%	730
	21st to end	Day	9% 7% 24% 33% 19% 8%	730
July	1st to 10th	Day	13% 4 17% 26% 24% 17%	721
	11th to 20th	Day	8% 5% 22% 28% 25% 12%	653
	21st to end	Day	11% 3 17% 28% 26% 13%	576
August	1st to 10th	Day	8% 4 18% 27% 26% 17%	526
	11th to 20th	Day	11% 5% 19% 29% 23% 13%	519
	21st to end	Day	9% 3 15% 31% 28% 14%	583
September	1st to 10th	Day	11% 5% 16% 27% 31% 11%	530
	11th to 20th	Day	8% 3 15% 26% 33% 15%	530
	21st to end	Day	16% 11% 19% 24% 22% 9%	530
October	1st to 10th	Day	10% 6% 23% 32% 21% 8%	530
	11th to 20th	Day	15% 13% 30% 25% 15% 3	530
	21st to end	Day	23% 16% 33% 22% 6% 1	418
November	1st to 10th	Day	12% 15% 38% 22% 10% 2	330
	11th to 20th	Day	25% 17% 31% 19% 7% 2	330
	21st to end	Day	27% 15% 37% 17% 4	330
December	1st to 10th	Day	20% 21% 39% 15% 4 1	330
	11th to 20th	Day	22% 21% 38% 13% 6%	330
	21st to end	Day	31% 18% 33% 15% 4	363
January	1st to 10th	Night	22% 16% 32% 24% 6%	1050
	11th to 20th	Night	27% 15% 34% 20% 3	931
	21st to end	Night	23% 15% 35% 25% 3	924
February	1st to 10th	Night	28% 10% 32% 27% 2	840
	11th to 20th	Night	22% 16% 36% 23% 3	823
	21st to end	Night	26% 14% 32% 25% 2	692
March	1st to 10th	Night	22% 13% 35% 25% 5%	840
	11th to 20th	Night	21% 15% 33% 25% 5%	840
	21st to end	Night	22% 14% 30% 28% 6%	924
April	1st to 10th	Night	19% 13% 35% 25% 8%	677
	11th to 20th	Night	18% 10% 33% 28% 9% 1	623
	21st to end	Night	17% 8% 38% 27% 9%	630
May	1st to 10th	Night	22% 9% 31% 27% 11% 1	630
	11th to 20th	Night	17% 6% 25% 33% 17% 2	621
	21st to end	Night	11% 3 24% 35% 24% 2	683
June	1st to 10th	Night	10% 3 21% 42% 23% 1	600
	11th to 20th	Night	10% 3 18% 43% 23% 3	600
	21st to end	Night	6% 1 21% 43% 27% 3	600
July	1st to 10th	Night	11% 2 12% 35% 33% 6%	585
	11th to 20th	Night	7% 2 15% 38% 33% 4	582
	21st to end	Night	10% 3 12% 38% 33% 3	657
August	1st to 10th	Night	8% 3 13% 37% 37% 3	597
	11th to 20th	Night	7% 1 15% 41% 30% 6%	594
	21st to end	Night	6% 1 13% 35% 39% 6%	660
September	1st to 10th	Night	13% 3 14% 32% 34% 4	695
	11th to 20th	Night	9% 4 12% 35% 36% 5%	800
	21st to end	Night	16% 6% 19% 32% 25% 2	800
October	1st to 10th	Night	12% 5% 21% 35% 24% 4	800
	11th to 20th	Night	18% 8% 26% 32% 15% 1	800
	21st to end	Night	22% 10% 31% 26% 10%	880
November	1st to 10th	Night	17% 8% 30% 29% 15% 1	820
	11th to 20th	Night	21% 14% 29% 27% 9%	840
	21st to end	Night	25% 12% 25% 31% 7% 1	840
December	1st to 10th	Night	25% 13% 32% 25% 5%	840
	11th to 20th	Night	25% 9% 34% 26% 5% 1	840
	21st to end	Night	31% 16% 28% 22% 3	1055

Page generated at 2026-05-29 01:53:25. Version ac07

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