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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 2026-07-01.

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	13% 9% 3 8% 6% 5% 5% 4 6% 7% 33%	138076
	Night	14% 10% 3 8% 5% 5% 5% 4 6% 7% 34%	134251
Transparency	Day	31% 38% 8% 15% 8% 1	141672
	Night	31% 37% 8% 14% 9% 1	138742
Seeing	Day	16% 9% 24% 28% 17% 6%	16519
	Night	18% 9% 28% 30% 14% 2	25997

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% 12% 5% 13% 6% 6% 5% 5% 6% 6% 24%	4677
	11th to 20th	Day	8% 17% 8% 19% 9% 8% 6% 3 4 5% 13%	5246
	21st to end	Day	14% 14% 6% 13% 7% 8% 6% 5% 5% 5% 19%	5780
February	1st to 10th	Day	14% 15% 3 12% 7% 6% 6% 4 5% 5% 22%	1890
	11th to 20th	Day	12% 13% 6% 13% 8% 6% 5% 4 4 7% 23%	1843
	21st to end	Day	16% 13% 5% 12% 6% 5% 4 4 5% 6% 24%	1638
March	1st to 10th	Day	17% 10% 4 11% 6% 5% 4 3 5% 6% 30%	5042
	11th to 20th	Day	10% 11% 5% 10% 6% 5% 5% 4 6% 8% 31%	5090
	21st to end	Day	15% 11% 3 8% 6% 5% 5% 4 5% 5% 32%	5586
April	1st to 10th	Day	17% 10% 3 8% 6% 6% 5% 4 5% 6% 30%	2457
	11th to 20th	Day	16% 8% 3 8% 5% 5% 4 3 6% 6% 35%	2586
	21st to end	Day	18% 9% 3 7% 6% 4 4 3 5% 6% 34%	2730
May	1st to 10th	Day	17% 8% 3 8% 5% 5% 5% 4 5% 7% 34%	7642
	11th to 20th	Day	13% 9% 3 8% 5% 5% 5% 4 6% 7% 34%	8019
	21st to end	Day	15% 5% 2 5% 4 5% 4 4 5% 7% 43%	9060
June	1st to 10th	Day	12% 5% 2 6% 5% 5% 5% 4 6% 8% 41%	2940
	11th to 20th	Day	9% 5% 2 7% 5% 6% 6% 5% 8% 9% 39%	2940
	21st to end	Day	11% 6% 2 6% 4 5% 5% 4 7% 9% 41%	2940
July	1st to 10th	Day	8% 4 2 6% 4 5% 6% 5% 7% 10% 42%	6360
	11th to 20th	Day	7% 5% 2 5% 5% 5% 5% 5% 7% 10% 43%	6271
	21st to end	Day	7% 4 2 5% 5% 4 5% 4 7% 9% 48%	6424
August	1st to 10th	Day	16% 6% 2 6% 4 5% 5% 4 6% 8% 38%	4234
	11th to 20th	Day	10% 6% 3 7% 5% 5% 6% 4 8% 10% 36%	4174
	21st to end	Day	8% 6% 2 6% 4 5% 6% 4 7% 9% 43%	4512
September	1st to 10th	Day	12% 6% 3 6% 4 5% 5% 5% 6% 7% 42%	2400
	11th to 20th	Day	8% 6% 2 5% 5% 5% 4 3 6% 8% 48%	2250
	21st to end	Day	13% 7% 2 7% 6% 5% 6% 4 6% 8% 35%	2176
October	1st to 10th	Day	17% 8% 3 7% 4 4 4 4 6% 7% 37%	2000
	11th to 20th	Day	16% 9% 4 8% 5% 5% 5% 3 6% 6% 33%	2000
	21st to end	Day	22% 10% 3 9% 6% 5% 5% 4 5% 7% 25%	2200
November	1st to 10th	Day	11% 10% 3 8% 5% 5% 5% 5% 7% 8% 33%	1815
	11th to 20th	Day	11% 12% 3 10% 6% 5% 6% 4 6% 7% 29%	1680
	21st to end	Day	17% 14% 4 7% 7% 5% 6% 4 7% 7% 22%	1680
December	1st to 10th	Day	16% 15% 6% 11% 7% 6% 6% 5% 6% 6% 16%	2680
	11th to 20th	Day	17% 15% 4 10% 6% 5% 5% 4 5% 5% 24%	3066
	21st to end	Day	24% 15% 7% 11% 7% 6% 5% 4 5% 4 13%	4048
January	1st to 10th	Night	14% 13% 4 11% 6% 5% 5% 4 6% 7% 25%	8216
	11th to 20th	Night	8% 13% 5% 15% 8% 7% 6% 5% 6% 6% 19%	7639
	21st to end	Night	14% 12% 5% 12% 7% 6% 5% 4 6% 7% 23%	8401
February	1st to 10th	Night	14% 13% 5% 10% 6% 5% 5% 4 6% 6% 26%	2730
	11th to 20th	Night	13% 13% 5% 11% 6% 5% 5% 4 6% 7% 27%	2663
	21st to end	Night	14% 12% 5% 10% 7% 4 5% 4 6% 5% 27%	2162
March	1st to 10th	Night	15% 8% 4 8% 5% 5% 6% 4 6% 6% 32%	5018
	11th to 20th	Night	11% 9% 3 7% 5% 5% 4 4 6% 8% 38%	5089
	21st to end	Night	17% 10% 3 7% 5% 4 4 3 4 5% 38%	5588
April	1st to 10th	Night	17% 10% 3 8% 6% 5% 4 4 6% 5% 32%	2162
	11th to 20th	Night	17% 9% 2 6% 6% 4 4 4 5% 7% 36%	1999
	21st to end	Night	15% 8% 3 7% 6% 4 4 4 4 7% 38%	1890
May	1st to 10th	Night	19% 8% 3 8% 5% 4 5% 4 5% 6% 34%	5257
	11th to 20th	Night	13% 8% 3 9% 6% 6% 5% 4 6% 6% 33%	4825
	21st to end	Night	18% 5% 2 5% 4 4 3 3 5% 7% 44%	5114
June	1st to 10th	Night	11% 5% 2 5% 5% 5% 5% 5% 6% 8% 43%	1680
	11th to 20th	Night	9% 5% 2 5% 5% 5% 5% 5% 8% 8% 43%	1680
	21st to end	Night	11% 5% 2 6% 4 5% 5% 4 5% 8% 45%	1680
July	1st to 10th	Night	6% 5% 2 6% 5% 5% 5% 4 6% 9% 48%	3600
	11th to 20th	Night	9% 4 2 5% 4 4 4 4 6% 9% 48%	3685
	21st to end	Night	7% 5% 1 6% 4 4 4 4 6% 9% 51%	4428
August	1st to 10th	Night	10% 5% 2 6% 4 4 5% 4 5% 9% 45%	2915
	11th to 20th	Night	11% 5% 2 5% 5% 4 4 4 6% 8% 46%	2875
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 8% 49%	3351
September	1st to 10th	Night	11% 5% 3 6% 5% 4 4 4 6% 7% 45%	2000
	11th to 20th	Night	9% 4 2 4 4 4 4 3 4 7% 56%	2145
	21st to end	Night	14% 7% 3 7% 5% 5% 5% 4 6% 6% 38%	2223
October	1st to 10th	Night	16% 6% 2 7% 4 3 4 3 5% 7% 43%	2400
	11th to 20th	Night	17% 9% 2 7% 5% 5% 5% 4 5% 6% 35%	2400
	21st to end	Night	20% 9% 3 7% 4 5% 5% 3 6% 5% 32%	2640
November	1st to 10th	Night	14% 8% 2 7% 4 5% 5% 3 5% 7% 40%	2698
	11th to 20th	Night	13% 11% 4 8% 5% 5% 5% 4 5% 8% 30%	2940
	21st to end	Night	17% 11% 3 8% 7% 5% 5% 4 6% 6% 27%	2940
December	1st to 10th	Night	12% 14% 5% 10% 7% 7% 5% 5% 7% 9% 21%	4720
	11th to 20th	Night	14% 15% 4 10% 6% 5% 5% 4 5% 6% 26%	5348
	21st to end	Night	24% 16% 5% 12% 6% 4 4 4 4 5% 16%	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	44% 27% 10% 10% 8% 1	4678
	11th to 20th	Day	57% 24% 7% 8% 4 1	5280
	21st to end	Day	49% 29% 8% 9% 5%	5780
February	1st to 10th	Day	25% 60% 5% 7% 3	1890
	11th to 20th	Day	24% 59% 4 10% 2	1852
	21st to end	Day	21% 60% 6% 8% 4 1	1638
March	1st to 10th	Day	38% 32% 11% 11% 9%	5045
	11th to 20th	Day	34% 32% 15% 13% 4 1	5090
	21st to end	Day	34% 32% 8% 17% 9% 1	5586
April	1st to 10th	Day	23% 61% 3 8% 5%	2457
	11th to 20th	Day	21% 60% 4 10% 4 1	2586
	21st to end	Day	20% 60% 5% 8% 5% 1	2730
May	1st to 10th	Day	38% 26% 11% 14% 8% 4	7651
	11th to 20th	Day	37% 27% 9% 17% 9% 1	8023
	21st to end	Day	32% 25% 7% 21% 13% 1	9061
June	1st to 10th	Day	16% 61% 4 10% 8% 1	2940
	11th to 20th	Day	17% 60% 5% 10% 7% 1	2940
	21st to end	Day	15% 59% 5% 11% 9%	2940
July	1st to 10th	Day	26% 29% 9% 22% 12% 1	6290
	11th to 20th	Day	22% 28% 10% 26% 13% 1	6196
	21st to end	Day	21% 29% 10% 23% 16% 1	6240
August	1st to 10th	Day	28% 38% 9% 14% 10% 1	4221
	11th to 20th	Day	27% 39% 10% 18% 5%	4213
	21st to end	Day	20% 39% 8% 24% 8%	4512
September	1st to 10th	Day	15% 59% 7% 13% 6%	2400
	11th to 20th	Day	11% 60% 6% 14% 9%	2250
	21st to end	Day	17% 58% 8% 11% 5%	2176
October	1st to 10th	Day	21% 38% 10% 21% 10% 1	3260
	11th to 20th	Day	21% 38% 9% 18% 14%	3240
	21st to end	Day	29% 38% 10% 17% 7%	3538
November	1st to 10th	Day	25% 55% 7% 9% 4	1815
	11th to 20th	Day	23% 57% 6% 9% 5% 1	1680
	21st to end	Day	27% 57% 7% 6% 4	1680
December	1st to 10th	Day	43% 38% 12% 5% 2	2680
	11th to 20th	Day	43% 33% 8% 8% 8% 1	3066
	21st to end	Day	54% 32% 6% 5% 3	4048
January	1st to 10th	Night	43% 27% 9% 16% 5%	8216
	11th to 20th	Night	48% 26% 8% 10% 7% 1	7680
	21st to end	Night	47% 26% 7% 11% 9%	8401
February	1st to 10th	Night	24% 60% 5% 9% 2	2730
	11th to 20th	Night	21% 59% 5% 11% 4	2665
	21st to end	Night	22% 61% 6% 6% 4	2162
March	1st to 10th	Night	35% 32% 10% 12% 9% 2	5028
	11th to 20th	Night	26% 31% 19% 15% 8% 1	5090
	21st to end	Night	33% 31% 7% 15% 13% 1	5588
April	1st to 10th	Night	25% 60% 4 7% 4 1	2162
	11th to 20th	Night	20% 60% 4 10% 5% 1	1999
	21st to end	Night	17% 60% 5% 11% 6% 1	1890
May	1st to 10th	Night	40% 26% 8% 11% 10% 4	5264
	11th to 20th	Night	39% 29% 9% 14% 9%	4826
	21st to end	Night	33% 25% 8% 20% 14% 1	5115
June	1st to 10th	Night	16% 60% 3 10% 10% 1	1680
	11th to 20th	Night	16% 59% 5% 10% 9% 1	1680
	21st to end	Night	17% 58% 4 10% 9% 1	1680
July	1st to 10th	Night	26% 28% 10% 18% 16% 2	3560
	11th to 20th	Night	23% 28% 9% 23% 14% 1	3637
	21st to end	Night	23% 28% 10% 20% 19% 1	4297
August	1st to 10th	Night	24% 37% 9% 17% 11% 2	2904
	11th to 20th	Night	27% 38% 10% 19% 6%	2902
	21st to end	Night	21% 39% 8% 22% 9%	3351
September	1st to 10th	Night	13% 57% 7% 13% 9%	2000
	11th to 20th	Night	11% 57% 6% 13% 11% 1	2145
	21st to end	Night	19% 57% 5% 13% 6%	2223
October	1st to 10th	Night	19% 37% 6% 19% 19%	3920
	11th to 20th	Night	27% 37% 9% 18% 9%	3899
	21st to end	Night	20% 37% 12% 22% 9%	4252
November	1st to 10th	Night	24% 54% 6% 10% 6%	2698
	11th to 20th	Night	22% 56% 6% 12% 5%	2940
	21st to end	Night	26% 56% 8% 7% 3	2940
December	1st to 10th	Night	38% 39% 12% 8% 3	4720
	11th to 20th	Night	40% 34% 6% 10% 9% 1	5348
	21st to end	Night	51% 32% 7% 7% 3	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	17% 14% 37% 22% 9% 2	223
	11th to 20th	Day	21% 18% 37% 15% 7% 2	330
	21st to end	Day	19% 14% 35% 26% 5% 1	363
February	1st to 10th	Day	20% 12% 41% 18% 9%	330
	11th to 20th	Day	19% 19% 34% 23% 4	330
	21st to end	Day	22% 21% 32% 20% 5%	281
March	1st to 10th	Day	18% 11% 35% 27% 9%	540
	11th to 20th	Day	18% 15% 34% 25% 8% 1	540
	21st to end	Day	23% 10% 30% 27% 9% 1	594
April	1st to 10th	Day	18% 11% 31% 30% 9% 1	540
	11th to 20th	Day	20% 13% 26% 25% 12% 4	532
	21st to end	Day	21% 9% 27% 29% 11% 3	540
May	1st to 10th	Day	18% 9% 22% 30% 16% 5%	543
	11th to 20th	Day	15% 7% 20% 32% 21% 6%	567
	21st to end	Day	13% 3 19% 36% 21% 8%	617
June	1st to 10th	Day	11% 5% 24% 36% 20% 4	570
	11th to 20th	Day	11% 5% 19% 34% 25% 6%	570
	21st to end	Day	14% 4 17% 34% 24% 7%	570
July	1st to 10th	Day	7% 2 11% 34% 31% 14%	525
	11th to 20th	Day	12% 4 15% 31% 25% 13%	524
	21st to end	Day	9% 4 13% 36% 28% 10%	576
August	1st to 10th	Day	10% 3 13% 31% 26% 16%	526
	11th to 20th	Day	8% 4 15% 28% 29% 15%	519
	21st to end	Day	12% 3 16% 30% 26% 12%	583
September	1st to 10th	Day	12% 6% 18% 27% 24% 14%	530
	11th to 20th	Day	9% 3 16% 23% 29% 21%	530
	21st to end	Day	16% 7% 21% 27% 20% 9%	530
October	1st to 10th	Day	19% 9% 20% 24% 17% 11%	530
	11th to 20th	Day	17% 10% 31% 27% 12% 3	530
	21st to end	Day	26% 9% 30% 23% 11% 1	583
November	1st to 10th	Day	14% 17% 31% 27% 9% 2	331
	11th to 20th	Day	22% 16% 25% 26% 9% 1	220
	21st to end	Day	23% 13% 34% 22% 8%	220
December	1st to 10th	Day	19% 20% 32% 21% 7% 1	220
	11th to 20th	Day	25% 22% 29% 19% 5%	220
	21st to end	Day	23% 14% 41% 16% 5% 1	242
January	1st to 10th	Night	20% 15% 37% 22% 6% 1	840
	11th to 20th	Night	24% 17% 34% 21% 4 1	840
	21st to end	Night	16% 12% 41% 24% 7%	924
February	1st to 10th	Night	22% 13% 38% 22% 4	840
	11th to 20th	Night	19% 14% 37% 26% 4	823
	21st to end	Night	22% 12% 36% 26% 4	692
March	1st to 10th	Night	18% 10% 35% 29% 8%	840
	11th to 20th	Night	20% 12% 35% 25% 7%	840
	21st to end	Night	20% 8% 30% 31% 9% 1	924
April	1st to 10th	Night	20% 9% 31% 31% 7% 1	840
	11th to 20th	Night	21% 8% 29% 29% 11% 1	755
	21st to end	Night	18% 5% 25% 38% 14%	630
May	1st to 10th	Night	18% 4 23% 34% 18% 3	630
	11th to 20th	Night	18% 4 21% 38% 18% 1	485
	21st to end	Night	14% 3 20% 36% 25% 2	455
June	1st to 10th	Night	12% 3 22% 43% 18% 2	420
	11th to 20th	Night	12% 3 23% 40% 21% 1	420
	21st to end	Night	12% 3 18% 42% 24% 1	420
July	1st to 10th	Night	6% 1 16% 46% 29% 2	390
	11th to 20th	Night	10% 2 17% 40% 26% 4	421
	21st to end	Night	10% 1 11% 47% 28% 3	657
August	1st to 10th	Night	9% 2 11% 41% 32% 5%	597
	11th to 20th	Night	8% 3 12% 36% 37% 5%	594
	21st to end	Night	11% 2 17% 35% 32% 3	736
September	1st to 10th	Night	12% 4 18% 32% 26% 8%	800
	11th to 20th	Night	11% 3 16% 29% 32% 10%	800
	21st to end	Night	17% 6% 18% 29% 25% 6%	800
October	1st to 10th	Night	19% 6% 23% 26% 22% 4	800
	11th to 20th	Night	21% 9% 29% 29% 10% 2	800
	21st to end	Night	24% 10% 29% 29% 8%	880
November	1st to 10th	Night	18% 11% 28% 30% 11% 2	820
	11th to 20th	Night	19% 14% 33% 23% 9% 2	840
	21st to end	Night	23% 12% 34% 24% 7%	840
December	1st to 10th	Night	24% 14% 34% 20% 7% 1	840
	11th to 20th	Night	22% 17% 35% 22% 5%	840
	21st to end	Night	28% 13% 33% 23% 3	924

Page generated at 2026-06-28 02:10:43. Version ac07

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