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 2025-02-06.

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 4 4 5% 7% 36%	108851
	Night	15% 10% 3 8% 5% 4 4 3 5% 7% 36%	109436
Transparency	Day	45% 6% 11% 22% 14% 2	112520
	Night	45% 6% 11% 21% 14% 2	113906
Seeing	Day	17% 12% 27% 23% 15% 6%	15656
	Night	18% 9% 26% 30% 15% 2	25489

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	15% 18% 7% 12% 4 4 3 3 4 5% 26%	3617
	11th to 20th	Day	14% 15% 6% 13% 6% 5% 5% 3 3 4 27%	3858
	21st to end	Day	22% 12% 7% 9% 5% 5% 4 3 4 4 24%	4466
February	1st to 10th	Day	19% 14% 4 10% 5% 4 4 3 5% 5% 29%	1858
	11th to 20th	Day	15% 12% 4 10% 6% 4 4 3 4 6% 31%	1848
	21st to end	Day	19% 12% 4 11% 5% 4 3 2 4 5% 31%	1570
March	1st to 10th	Day	22% 16% 5% 9% 5% 4 4 2 4 5% 25%	3138
	11th to 20th	Day	14% 9% 3 6% 4 4 4 4 5% 7% 41%	3480
	21st to end	Day	18% 8% 3 8% 5% 4 4 4 5% 7% 34%	3815
April	1st to 10th	Day	14% 10% 2 8% 5% 4 4 5% 4 7% 35%	2251
	11th to 20th	Day	16% 9% 3 6% 5% 4 4 3 5% 7% 39%	2301
	21st to end	Day	18% 8% 3 7% 4 5% 4 3 6% 6% 36%	2470
May	1st to 10th	Day	21% 9% 3 7% 5% 4 5% 4 5% 7% 29%	4082
	11th to 20th	Day	20% 8% 3 6% 4 3 3 3 5% 7% 38%	4081
	21st to end	Day	10% 6% 3 8% 5% 5% 6% 6% 8% 9% 34%	4476
June	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 5% 7% 8% 40%	2499
	11th to 20th	Day	9% 5% 2 7% 5% 4 5% 5% 6% 9% 44%	2660
	21st to end	Day	8% 6% 3 6% 5% 5% 5% 4 7% 11% 39%	2660
July	1st to 10th	Day	11% 7% 3 7% 5% 5% 5% 4 7% 9% 36%	4440
	11th to 20th	Day	7% 4 2 6% 5% 4 4 5% 7% 11% 46%	4326
	21st to end	Day	10% 7% 2 7% 6% 6% 6% 5% 8% 9% 32%	4466
August	1st to 10th	Day	8% 5% 3 6% 4 4 6% 5% 6% 10% 43%	4104
	11th to 20th	Day	8% 5% 3 6% 4 5% 5% 4 6% 11% 43%	4044
	21st to end	Day	6% 5% 2 5% 4 3 4 3 6% 9% 53%	4194
September	1st to 10th	Day	12% 6% 2 4 4 4 5% 3 6% 7% 48%	2171
	11th to 20th	Day	8% 5% 2 6% 4 4 4 3 6% 9% 49%	2090
	21st to end	Day	14% 6% 3 5% 4 3 4 3 5% 6% 47%	2090
October	1st to 10th	Day	10% 5% 3 6% 4 4 4 3 4 7% 49%	2090
	11th to 20th	Day	13% 6% 2 7% 3 4 4 3 5% 6% 47%	1933
	21st to end	Day	20% 8% 3 6% 4 3 4 3 5% 7% 38%	1933
November	1st to 10th	Day	9% 6% 2 5% 5% 4 4 3 4 6% 52%	1762
	11th to 20th	Day	15% 10% 4 9% 4 4 4 3 5% 7% 37%	1800
	21st to end	Day	18% 12% 4 7% 5% 4 4 3 4 6% 35%	1800
December	1st to 10th	Day	17% 10% 3 8% 4 4 3 3 4 5% 38%	2930
	11th to 20th	Day	18% 15% 4 9% 6% 5% 5% 3 5% 5% 26%	3373
	21st to end	Day	22% 19% 5% 11% 5% 4 4 3 4 6% 18%	4175
January	1st to 10th	Night	16% 16% 6% 14% 7% 5% 4 4 4 5% 19%	6348
	11th to 20th	Night	14% 18% 6% 13% 8% 5% 5% 3 3 5% 20%	6104
	21st to end	Night	21% 13% 5% 8% 5% 4 4 3 4 5% 30%	6490
February	1st to 10th	Night	21% 12% 4 9% 5% 4 4 4 4 5% 29%	2431
	11th to 20th	Night	16% 10% 4 9% 6% 5% 5% 4 5% 6% 31%	2218
	21st to end	Night	21% 13% 3 8% 4 4 3 4 4 6% 30%	1884
March	1st to 10th	Night	22% 12% 3 5% 4 4 4 3 4 5% 33%	3714
	11th to 20th	Night	12% 8% 2 6% 4 4 4 4 6% 6% 44%	3480
	21st to end	Night	16% 8% 3 7% 4 4 5% 3 4 6% 39%	3817
April	1st to 10th	Night	14% 8% 3 6% 4 5% 5% 4 6% 7% 38%	1943
	11th to 20th	Night	14% 7% 2 6% 4 4 4 4 6% 8% 41%	1826
	21st to end	Night	16% 6% 3 6% 5% 5% 4 5% 6% 7% 37%	1710
May	1st to 10th	Night	21% 11% 3 9% 5% 4 5% 3 5% 6% 27%	2813
	11th to 20th	Night	18% 7% 2 6% 4 4 4 3 6% 8% 38%	2812
	21st to end	Night	9% 5% 2 5% 5% 5% 4 4 7% 10% 45%	3079
June	1st to 10th	Night	9% 5% 2 6% 4 4 4 3 4 9% 49%	1674
	11th to 20th	Night	8% 5% 2 4 4 3 5% 4 8% 10% 47%	1520
	21st to end	Night	5% 5% 1 5% 4 5% 5% 4 9% 10% 46%	1520
July	1st to 10th	Night	12% 5% 2 5% 4 3 4 3 5% 8% 48%	2530
	11th to 20th	Night	5% 3 2 5% 4 5% 4 4 7% 10% 52%	2623
	21st to end	Night	11% 7% 3 7% 4 5% 5% 4 7% 11% 37%	3087
August	1st to 10th	Night	8% 6% 3 6% 5% 5% 5% 4 6% 9% 43%	2825
	11th to 20th	Night	7% 5% 2 7% 4 4 5% 4 7% 8% 46%	2785
	21st to end	Night	3 3 1 5% 4 4 5% 4 6% 9% 57%	3444
September	1st to 10th	Night	12% 5% 2 5% 4 3 4 3 5% 7% 50%	1990
	11th to 20th	Night	8% 4 1 4 3 4 4 3 5% 9% 55%	2090
	21st to end	Night	13% 7% 2 6% 4 3 4 3 4 5% 49%	2090
October	1st to 10th	Night	9% 4 2 6% 3 4 4 3 4 6% 55%	2090
	11th to 20th	Night	14% 7% 2 6% 4 4 4 3 5% 7% 44%	2246
	21st to end	Night	17% 8% 3 7% 4 4 5% 4 4 6% 41%	2661
November	1st to 10th	Night	13% 8% 2 5% 3 4 4 3 4 7% 47%	2536
	11th to 20th	Night	16% 8% 3 7% 4 3 5% 3 5% 6% 40%	2600
	21st to end	Night	19% 12% 4 7% 5% 4 4 3 5% 7% 30%	2600
December	1st to 10th	Night	17% 10% 4 8% 5% 4 3 3 4 5% 37%	4250
	11th to 20th	Night	19% 13% 5% 10% 6% 4 4 3 5% 5% 26%	4822
	21st to end	Night	24% 18% 6% 10% 5% 4 4 3 4 5% 17%	6784

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 9% 19% 7%	3618
	11th to 20th	Day	61% 5% 10% 12% 11% 1	3858
	21st to end	Day	64% 4 10% 14% 7% 2	4466
February	1st to 10th	Day	58% 5% 11% 16% 9% 1	1858
	11th to 20th	Day	55% 5% 10% 21% 9% 1	1858
	21st to end	Day	56% 5% 10% 20% 8% 2	1570
March	1st to 10th	Day	61% 6% 7% 17% 8% 1	3138
	11th to 20th	Day	41% 7% 10% 25% 16% 1	3480
	21st to end	Day	47% 8% 12% 22% 10% 1	3815
April	1st to 10th	Day	47% 7% 11% 22% 12% 1	2251
	11th to 20th	Day	45% 6% 10% 22% 14% 3	2301
	21st to end	Day	48% 6% 11% 22% 11% 2	2470
May	1st to 10th	Day	52% 8% 12% 19% 8% 1	4120
	11th to 20th	Day	46% 7% 13% 23% 11%	4085
	21st to end	Day	38% 12% 14% 19% 15% 1	4489
June	1st to 10th	Day	39% 9% 12% 22% 16% 2	2499
	11th to 20th	Day	36% 8% 14% 22% 17% 2	2660
	21st to end	Day	38% 9% 15% 19% 15% 4	2660
July	1st to 10th	Day	42% 6% 12% 21% 17% 2	4371
	11th to 20th	Day	30% 8% 12% 29% 20% 1	4251
	21st to end	Day	44% 8% 15% 17% 13% 3	4324
August	1st to 10th	Day	33% 8% 16% 29% 13% 1	4091
	11th to 20th	Day	34% 6% 12% 32% 15% 1	4083
	21st to end	Day	27% 6% 15% 30% 19% 3	4193
September	1st to 10th	Day	34% 7% 12% 23% 19% 6%	2171
	11th to 20th	Day	32% 6% 12% 26% 22% 3	2090
	21st to end	Day	38% 6% 11% 23% 21% 2	2090
October	1st to 10th	Day	24% 4 9% 31% 31% 2	3480
	11th to 20th	Day	29% 5% 9% 30% 25% 2	3186
	21st to end	Day	37% 3 10% 24% 23% 2	3157
November	1st to 10th	Day	35% 4 10% 29% 19% 3	1762
	11th to 20th	Day	48% 4 10% 21% 14% 3	1800
	21st to end	Day	51% 5% 10% 22% 12% 1	1800
December	1st to 10th	Day	50% 4 10% 26% 11%	2930
	11th to 20th	Day	60% 5% 9% 18% 7%	3370
	21st to end	Day	68% 5% 10% 12% 5%	4175
January	1st to 10th	Night	65% 6% 8% 13% 7%	6348
	11th to 20th	Night	67% 4 8% 11% 7% 2	6104
	21st to end	Night	57% 4 9% 20% 9% 1	6490
February	1st to 10th	Night	58% 5% 10% 19% 7% 1	2431
	11th to 20th	Night	53% 5% 11% 19% 11% 1	2219
	21st to end	Night	55% 6% 11% 16% 9% 2	1884
March	1st to 10th	Night	52% 6% 8% 23% 11% 1	3714
	11th to 20th	Night	37% 8% 13% 25% 16% 2	3480
	21st to end	Night	45% 7% 10% 27% 10% 1	3817
April	1st to 10th	Night	43% 7% 12% 24% 12% 2	1943
	11th to 20th	Night	40% 7% 11% 18% 18% 5%	1826
	21st to end	Night	44% 7% 12% 20% 13% 3	1710
May	1st to 10th	Night	56% 8% 9% 18% 7% 2	2840
	11th to 20th	Night	43% 7% 15% 25% 9% 1	2813
	21st to end	Night	30% 10% 17% 25% 13% 5%	3087
June	1st to 10th	Night	33% 7% 10% 27% 20% 2	1674
	11th to 20th	Night	30% 9% 14% 23% 20% 4	1520
	21st to end	Night	30% 9% 17% 21% 18% 5%	1520
July	1st to 10th	Night	34% 7% 11% 24% 18% 6%	2490
	11th to 20th	Night	25% 8% 14% 31% 20% 2	2575
	21st to end	Night	39% 6% 14% 23% 14% 4	2988
August	1st to 10th	Night	35% 9% 14% 29% 12% 2	2814
	11th to 20th	Night	33% 7% 13% 29% 17% 1	2812
	21st to end	Night	22% 7% 15% 31% 21% 3	3444
September	1st to 10th	Night	32% 7% 13% 23% 19% 7%	1990
	11th to 20th	Night	26% 7% 11% 26% 26% 3	2090
	21st to end	Night	38% 4 10% 21% 25% 2	2090
October	1st to 10th	Night	21% 4 7% 29% 36% 3	3480
	11th to 20th	Night	28% 4 10% 26% 31% 2	3732
	21st to end	Night	34% 4 9% 23% 27% 4	4387
November	1st to 10th	Night	38% 4 12% 21% 22% 3	2536
	11th to 20th	Night	44% 5% 9% 22% 17% 3	2600
	21st to end	Night	54% 5% 11% 20% 10% 1	2600
December	1st to 10th	Night	49% 5% 12% 24% 10%	4250
	11th to 20th	Night	59% 5% 12% 17% 7% 1	4824
	21st to end	Night	70% 6% 10% 12% 3	6784

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	26% 20% 33% 15% 6%	300
	11th to 20th	Day	28% 25% 33% 12% 3	300
	21st to end	Day	31% 21% 30% 15% 3	330
February	1st to 10th	Day	26% 24% 32% 15% 2	285
	11th to 20th	Day	27% 24% 34% 13% 2	270
	21st to end	Day	30% 19% 41% 8% 2	225
March	1st to 10th	Day	23% 27% 34% 13% 3	275
	11th to 20th	Day	26% 14% 35% 18% 6% 1	460
	21st to end	Day	29% 16% 32% 15% 7%	506
April	1st to 10th	Day	22% 18% 31% 20% 9% 2	460
	11th to 20th	Day	19% 21% 25% 21% 11% 4	452
	21st to end	Day	22% 16% 33% 17% 8% 3	460
May	1st to 10th	Day	22% 15% 31% 20% 9% 4	463
	11th to 20th	Day	23% 9% 27% 22% 13% 6%	487
	21st to end	Day	12% 7% 26% 28% 17% 11%	529
June	1st to 10th	Day	14% 6% 23% 29% 19% 9%	519
	11th to 20th	Day	10% 6% 25% 31% 22% 6%	680
	21st to end	Day	10% 7% 25% 33% 18% 8%	680
July	1st to 10th	Day	13% 4 17% 26% 23% 17%	671
	11th to 20th	Day	8% 5% 23% 27% 24% 13%	606
	21st to end	Day	12% 3 18% 28% 26% 13%	532
August	1st to 10th	Day	8% 4 18% 26% 26% 17%	486
	11th to 20th	Day	12% 5% 19% 30% 22% 13%	479
	21st to end	Day	9% 3 14% 30% 29% 15%	539
September	1st to 10th	Day	11% 4 16% 28% 31% 11%	490
	11th to 20th	Day	9% 3 15% 27% 32% 14%	490
	21st to end	Day	16% 11% 20% 24% 21% 8%	490
October	1st to 10th	Day	10% 7% 24% 32% 20% 7%	490
	11th to 20th	Day	15% 13% 30% 23% 15% 3	490
	21st to end	Day	21% 15% 34% 23% 6% 1	382
November	1st to 10th	Day	11% 14% 39% 23% 11% 3	300
	11th to 20th	Day	22% 18% 31% 20% 8% 2	300
	21st to end	Day	25% 15% 38% 17% 5%	300
December	1st to 10th	Day	20% 20% 38% 16% 5% 1	300
	11th to 20th	Day	21% 22% 38% 14% 6%	300
	21st to end	Day	31% 18% 32% 15% 4	330
January	1st to 10th	Night	22% 17% 32% 24% 6%	1000
	11th to 20th	Night	28% 14% 34% 21% 3	887
	21st to end	Night	23% 14% 34% 26% 3	880
February	1st to 10th	Night	29% 10% 31% 27% 2	781
	11th to 20th	Night	21% 16% 37% 24% 3	743
	21st to end	Night	28% 14% 32% 25% 2	628
March	1st to 10th	Night	21% 13% 35% 25% 5%	760
	11th to 20th	Night	20% 15% 33% 25% 6% 1	760
	21st to end	Night	22% 12% 31% 28% 7%	836
April	1st to 10th	Night	17% 12% 36% 26% 8%	613
	11th to 20th	Night	18% 10% 32% 29% 10% 1	563
	21st to end	Night	17% 8% 38% 27% 9% 1	570
May	1st to 10th	Night	22% 9% 30% 27% 12% 1	570
	11th to 20th	Night	18% 4 24% 34% 18% 2	561
	21st to end	Night	9% 2 23% 36% 27% 2	617
June	1st to 10th	Night	10% 3 20% 42% 24% 1	570
	11th to 20th	Night	9% 3 18% 44% 23% 3	570
	21st to end	Night	6% 1 22% 43% 26% 2	570
July	1st to 10th	Night	12% 2 13% 35% 32% 7%	555
	11th to 20th	Night	7% 2 15% 38% 33% 4	552
	21st to end	Night	11% 3 12% 39% 32% 3	624
August	1st to 10th	Night	8% 3 13% 37% 37% 3	567
	11th to 20th	Night	8% 1 16% 42% 29% 5%	564
	21st to end	Night	6% 1 12% 34% 40% 6%	627
September	1st to 10th	Night	14% 3 14% 31% 33% 4	660
	11th to 20th	Night	9% 4 13% 35% 35% 5%	760
	21st to end	Night	16% 6% 20% 31% 25% 2	760
October	1st to 10th	Night	12% 4 22% 34% 23% 4	760
	11th to 20th	Night	18% 8% 27% 31% 15% 1	760
	21st to end	Night	22% 10% 31% 26% 11%	836
November	1st to 10th	Night	17% 8% 29% 29% 16% 1	780
	11th to 20th	Night	21% 13% 29% 27% 9%	800
	21st to end	Night	24% 12% 25% 31% 7% 1	800
December	1st to 10th	Night	25% 12% 32% 25% 6%	800
	11th to 20th	Night	25% 9% 34% 26% 6% 1	800
	21st to end	Night	30% 15% 29% 23% 3	1005

Page generated at 2025-02-03 03:05:50. Version ac07

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