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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 2025-02-21.

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% 5% 5% 5% 4 6% 7% 33%	108773
	Night	14% 10% 3 8% 5% 5% 5% 4 6% 7% 33%	109794
Transparency	Day	27% 46% 8% 12% 7% 1	112413
	Night	28% 44% 8% 12% 7%	114317
Seeing	Day	16% 9% 25% 27% 16% 6%	14729
	Night	18% 9% 28% 29% 14% 2	24340

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	11% 12% 5% 13% 6% 6% 4 4 5% 6% 27%	3613
	11th to 20th	Day	9% 17% 8% 18% 8% 6% 6% 4 4 5% 14%	4026
	21st to end	Day	17% 16% 6% 14% 6% 6% 5% 4 4 4 18%	4466
February	1st to 10th	Day	15% 15% 4 13% 7% 6% 6% 4 5% 6% 21%	1800
	11th to 20th	Day	12% 13% 6% 13% 8% 6% 5% 4 5% 7% 23%	1753
	21st to end	Day	17% 13% 4 12% 5% 5% 5% 4 5% 5% 24%	1486
March	1st to 10th	Day	18% 11% 3 10% 6% 6% 5% 3 5% 6% 27%	3432
	11th to 20th	Day	10% 13% 4 10% 6% 5% 5% 3 6% 7% 30%	3480
	21st to end	Day	15% 10% 3 7% 6% 5% 5% 4 6% 7% 34%	3815
April	1st to 10th	Day	16% 10% 3 8% 6% 6% 4 4 5% 6% 30%	2223
	11th to 20th	Day	16% 8% 3 8% 5% 4 4 3 6% 6% 36%	2337
	21st to end	Day	19% 9% 3 7% 6% 4 4 3 5% 6% 34%	2470
May	1st to 10th	Day	16% 8% 2 7% 5% 5% 5% 3 5% 8% 36%	4108
	11th to 20th	Day	15% 11% 4 8% 6% 5% 5% 4 5% 7% 30%	4292
	21st to end	Day	9% 6% 3 6% 4 5% 4 3 5% 8% 47%	4837
June	1st to 10th	Day	13% 6% 2 6% 6% 5% 5% 4 6% 8% 39%	2660
	11th to 20th	Day	10% 6% 2 7% 5% 5% 6% 5% 7% 9% 40%	2660
	21st to end	Day	11% 6% 2 6% 4 5% 5% 4 6% 10% 40%	2660
July	1st to 10th	Day	9% 4 2 5% 4 5% 5% 5% 8% 11% 43%	4440
	11th to 20th	Day	8% 5% 2 5% 4 5% 5% 6% 7% 10% 43%	4379
	21st to end	Day	7% 5% 2 6% 5% 5% 6% 5% 8% 10% 43%	4466
August	1st to 10th	Day	17% 6% 2 6% 4 5% 5% 4 6% 8% 37%	4104
	11th to 20th	Day	9% 6% 3 7% 5% 5% 6% 4 8% 10% 36%	4044
	21st to end	Day	8% 6% 2 6% 4 5% 6% 4 7% 9% 43%	4374
September	1st to 10th	Day	12% 6% 3 6% 5% 5% 5% 5% 6% 7% 41%	2280
	11th to 20th	Day	8% 6% 2 5% 5% 5% 4 3 6% 8% 48%	2138
	21st to end	Day	13% 7% 2 7% 6% 5% 5% 4 6% 8% 35%	2067
October	1st to 10th	Day	17% 8% 3 7% 5% 4 4 4 6% 7% 35%	1900
	11th to 20th	Day	17% 9% 4 8% 5% 5% 5% 3 6% 6% 32%	1900
	21st to end	Day	22% 10% 3 9% 6% 5% 5% 4 5% 7% 25%	2090
November	1st to 10th	Day	10% 9% 3 8% 6% 5% 5% 5% 7% 8% 33%	1727
	11th to 20th	Day	11% 12% 4 10% 6% 6% 6% 4 6% 7% 28%	1600
	21st to end	Day	18% 14% 4 7% 7% 5% 5% 4 7% 7% 23%	1600
December	1st to 10th	Day	16% 15% 6% 11% 7% 6% 6% 5% 7% 6% 16%	2600
	11th to 20th	Day	18% 15% 4 10% 6% 5% 5% 4 5% 5% 24%	2986
	21st to end	Day	24% 16% 7% 11% 7% 6% 5% 4 5% 4 13%	3960
January	1st to 10th	Night	13% 11% 4 11% 6% 5% 5% 4 6% 7% 28%	6331
	11th to 20th	Night	10% 13% 5% 14% 9% 7% 6% 5% 6% 6% 19%	5859
	21st to end	Night	17% 14% 5% 12% 5% 5% 5% 4 5% 7% 23%	6490
February	1st to 10th	Night	15% 14% 5% 10% 6% 5% 5% 4 6% 6% 24%	2600
	11th to 20th	Night	13% 13% 4 11% 6% 5% 5% 4 6% 7% 27%	2533
	21st to end	Night	15% 12% 5% 10% 7% 4 5% 4 6% 5% 28%	1964
March	1st to 10th	Night	19% 9% 3 8% 5% 5% 5% 3 5% 6% 31%	3408
	11th to 20th	Night	13% 10% 3 7% 4 5% 4 3 5% 8% 37%	3479
	21st to end	Night	15% 10% 3 7% 4 3 4 3 4 5% 41%	3817
April	1st to 10th	Night	17% 10% 3 8% 5% 5% 4 4 5% 5% 33%	1956
	11th to 20th	Night	17% 8% 2 6% 6% 4 4 3 5% 7% 37%	1807
	21st to end	Night	15% 8% 3 6% 6% 4 4 4 4 7% 37%	1710
May	1st to 10th	Night	18% 9% 3 8% 4 3 3 3 4 6% 37%	2833
	11th to 20th	Night	14% 9% 3 9% 6% 5% 5% 4 6% 7% 32%	2599
	21st to end	Night	9% 4 2 5% 4 4 4 3 6% 8% 50%	2738
June	1st to 10th	Night	11% 5% 2 5% 5% 6% 5% 5% 6% 8% 42%	1520
	11th to 20th	Night	9% 5% 2 5% 5% 5% 5% 5% 8% 7% 43%	1520
	21st to end	Night	12% 5% 2 6% 4 5% 5% 4 6% 8% 45%	1520
July	1st to 10th	Night	6% 4 2 5% 4 5% 4 3 6% 8% 51%	2520
	11th to 20th	Night	10% 4 2 5% 4 4 3 4 6% 10% 50%	2577
	21st to end	Night	7% 4 1 6% 4 4 4 4 7% 9% 49%	3087
August	1st to 10th	Night	11% 5% 2 6% 4 5% 5% 4 5% 8% 44%	2825
	11th to 20th	Night	11% 5% 2 6% 5% 4 4 4 6% 8% 46%	2785
	21st to end	Night	7% 5% 2 5% 4 4 4 4 6% 8% 49%	3248
September	1st to 10th	Night	11% 5% 3 6% 5% 4 4 4 6% 7% 45%	1900
	11th to 20th	Night	9% 4 2 4 4 4 4 3 4 7% 55%	2038
	21st to end	Night	14% 7% 3 7% 5% 5% 5% 4 6% 6% 38%	2112
October	1st to 10th	Night	17% 6% 2 7% 4 3 4 3 5% 7% 42%	2280
	11th to 20th	Night	17% 9% 2 7% 6% 5% 5% 3 5% 6% 35%	2280
	21st to end	Night	21% 9% 3 7% 4 5% 5% 3 6% 5% 31%	2508
November	1st to 10th	Night	14% 8% 2 6% 4 5% 5% 3 5% 7% 41%	2566
	11th to 20th	Night	13% 11% 4 8% 5% 5% 5% 4 5% 8% 31%	2800
	21st to end	Night	17% 11% 3 7% 6% 5% 5% 4 6% 6% 27%	2800
December	1st to 10th	Night	12% 14% 5% 10% 7% 7% 5% 5% 7% 9% 21%	4580
	11th to 20th	Night	14% 16% 4 10% 6% 5% 5% 4 5% 6% 25%	5208
	21st to end	Night	24% 16% 5% 12% 6% 5% 4 4 4 5% 16%	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	40% 32% 8% 9% 9% 1	3614
	11th to 20th	Day	52% 29% 8% 7% 3 1	4060
	21st to end	Day	48% 32% 7% 7% 5%	4466
February	1st to 10th	Day	24% 63% 5% 6% 3	1800
	11th to 20th	Day	22% 62% 4 9% 2	1762
	21st to end	Day	17% 66% 6% 7% 4 1	1486
March	1st to 10th	Day	34% 45% 7% 11% 4	3435
	11th to 20th	Day	29% 44% 11% 12% 4 1	3480
	21st to end	Day	25% 44% 8% 14% 7% 1	3815
April	1st to 10th	Day	21% 67% 3 6% 4	2223
	11th to 20th	Day	17% 66% 4 8% 4 1	2337
	21st to end	Day	18% 66% 4 6% 5% 1	2470
May	1st to 10th	Day	27% 43% 10% 11% 7% 2	4120
	11th to 20th	Day	32% 46% 10% 11% 2	4296
	21st to end	Day	20% 42% 8% 15% 13% 2	4837
June	1st to 10th	Day	15% 66% 4 9% 5% 1	2660
	11th to 20th	Day	14% 66% 5% 8% 6% 1	2660
	21st to end	Day	14% 65% 3 10% 8%	2660
July	1st to 10th	Day	22% 38% 9% 19% 10% 1	4370
	11th to 20th	Day	19% 37% 9% 26% 9%	4304
	21st to end	Day	21% 40% 10% 15% 15% 1	4324
August	1st to 10th	Day	29% 39% 8% 14% 9% 1	4091
	11th to 20th	Day	27% 41% 10% 17% 5%	4083
	21st to end	Day	20% 40% 8% 24% 8%	4374
September	1st to 10th	Day	14% 62% 6% 12% 5%	2280
	11th to 20th	Day	11% 62% 5% 12% 9%	2138
	21st to end	Day	16% 61% 7% 11% 5%	2067
October	1st to 10th	Day	21% 39% 10% 19% 10%	3160
	11th to 20th	Day	21% 39% 9% 17% 13%	3140
	21st to end	Day	28% 39% 10% 17% 6%	3428
November	1st to 10th	Day	23% 57% 7% 9% 3	1727
	11th to 20th	Day	22% 59% 6% 9% 5% 1	1600
	21st to end	Day	26% 59% 7% 5% 3	1600
December	1st to 10th	Day	42% 39% 12% 4 2	2600
	11th to 20th	Day	43% 34% 8% 8% 8% 1	2986
	21st to end	Day	53% 33% 6% 5% 3	3960
January	1st to 10th	Night	37% 32% 8% 16% 6%	6331
	11th to 20th	Night	45% 31% 10% 10% 3	5900
	21st to end	Night	44% 31% 7% 9% 8%	6490
February	1st to 10th	Night	23% 63% 5% 7% 2	2600
	11th to 20th	Night	20% 61% 5% 9% 4	2535
	21st to end	Night	18% 66% 6% 6% 4	1964
March	1st to 10th	Night	31% 43% 9% 13% 4	3418
	11th to 20th	Night	23% 43% 15% 11% 7% 1	3480
	21st to end	Night	22% 43% 6% 16% 12% 1	3817
April	1st to 10th	Night	21% 65% 3 7% 4	1956
	11th to 20th	Night	17% 66% 3 9% 4 1	1807
	21st to end	Night	15% 66% 4 8% 5% 1	1710
May	1st to 10th	Night	29% 43% 8% 10% 9% 2	2840
	11th to 20th	Night	31% 47% 11% 9% 2	2600
	21st to end	Night	18% 44% 8% 14% 16% 1	2739
June	1st to 10th	Night	14% 66% 3 9% 8%	1520
	11th to 20th	Night	14% 65% 4 9% 7% 1	1520
	21st to end	Night	15% 64% 3 10% 8% 1	1520
July	1st to 10th	Night	20% 37% 9% 17% 14% 3	2480
	11th to 20th	Night	18% 38% 9% 24% 11%	2529
	21st to end	Night	19% 38% 13% 17% 12% 1	2988
August	1st to 10th	Night	24% 38% 8% 17% 11% 2	2814
	11th to 20th	Night	27% 39% 10% 19% 6%	2812
	21st to end	Night	21% 40% 8% 22% 10%	3248
September	1st to 10th	Night	13% 60% 7% 11% 9%	1900
	11th to 20th	Night	11% 59% 6% 12% 11% 1	2038
	21st to end	Night	18% 60% 5% 13% 5%	2112
October	1st to 10th	Night	19% 38% 6% 17% 19%	3800
	11th to 20th	Night	26% 38% 9% 18% 9%	3779
	21st to end	Night	20% 38% 12% 21% 8%	4120
November	1st to 10th	Night	22% 57% 6% 9% 6%	2566
	11th to 20th	Night	20% 58% 5% 11% 4	2800
	21st to end	Night	24% 58% 8% 7% 3	2800
December	1st to 10th	Night	37% 40% 12% 8% 3	4580
	11th to 20th	Night	40% 35% 6% 10% 9% 1	5208
	21st to end	Night	51% 32% 7% 7% 3	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	17% 14% 36% 22% 9% 2	203
	11th to 20th	Day	21% 17% 37% 16% 8% 2	300
	21st to end	Day	20% 14% 35% 26% 5%	330
February	1st to 10th	Day	21% 13% 44% 17% 6%	300
	11th to 20th	Day	19% 21% 36% 21% 3 1	300
	21st to end	Day	25% 24% 33% 16% 2	232
March	1st to 10th	Day	18% 12% 35% 25% 9%	460
	11th to 20th	Day	17% 16% 33% 25% 8% 1	460
	21st to end	Day	23% 11% 31% 24% 9% 1	506
April	1st to 10th	Day	17% 11% 32% 30% 10% 1	460
	11th to 20th	Day	21% 14% 27% 23% 11% 4	452
	21st to end	Day	21% 10% 30% 27% 10% 3	460
May	1st to 10th	Day	18% 9% 25% 27% 15% 6%	463
	11th to 20th	Day	16% 6% 21% 29% 21% 7%	487
	21st to end	Day	12% 3 21% 33% 21% 9%	529
June	1st to 10th	Day	11% 6% 27% 34% 18% 4	490
	11th to 20th	Day	12% 5% 20% 31% 25% 7%	490
	21st to end	Day	13% 4 20% 35% 20% 7%	490
July	1st to 10th	Day	7% 2 12% 36% 28% 14%	485
	11th to 20th	Day	13% 4 15% 30% 23% 14%	484
	21st to end	Day	9% 4 14% 36% 26% 11%	532
August	1st to 10th	Day	10% 3 15% 32% 25% 15%	486
	11th to 20th	Day	7% 5% 16% 29% 28% 15%	479
	21st to end	Day	12% 4 17% 30% 25% 12%	539
September	1st to 10th	Day	12% 6% 19% 28% 23% 13%	490
	11th to 20th	Day	9% 3 17% 24% 27% 19%	490
	21st to end	Day	16% 8% 23% 28% 18% 9%	490
October	1st to 10th	Day	20% 9% 21% 24% 17% 9%	490
	11th to 20th	Day	18% 10% 31% 24% 12% 4	490
	21st to end	Day	26% 9% 31% 23% 10% 1	539
November	1st to 10th	Day	14% 17% 30% 27% 10% 3	303
	11th to 20th	Day	23% 17% 24% 27% 8% 2	200
	21st to end	Day	24% 13% 34% 22% 8%	200
December	1st to 10th	Day	19% 19% 34% 20% 8% 1	200
	11th to 20th	Day	27% 21% 30% 19% 4	200
	21st to end	Day	23% 13% 41% 16% 5% 1	220
January	1st to 10th	Night	20% 15% 37% 22% 6% 1	800
	11th to 20th	Night	24% 16% 34% 22% 4 1	800
	21st to end	Night	16% 12% 40% 25% 7%	880
February	1st to 10th	Night	23% 14% 38% 21% 4	800
	11th to 20th	Night	19% 15% 37% 25% 4	783
	21st to end	Night	22% 12% 37% 25% 4	629
March	1st to 10th	Night	18% 10% 34% 28% 9%	760
	11th to 20th	Night	20% 13% 36% 24% 7%	760
	21st to end	Night	20% 8% 30% 32% 10% 1	836
April	1st to 10th	Night	20% 9% 32% 31% 7% 1	760
	11th to 20th	Night	20% 9% 30% 28% 11% 1	683
	21st to end	Night	19% 5% 26% 38% 12%	570
May	1st to 10th	Night	17% 5% 24% 33% 18% 3	570
	11th to 20th	Night	18% 4 21% 38% 18% 1	439
	21st to end	Night	12% 4 21% 35% 27% 2	411
June	1st to 10th	Night	12% 3 24% 43% 16% 2	380
	11th to 20th	Night	13% 2 24% 39% 22% 1	380
	21st to end	Night	12% 4 19% 43% 22% 1	380
July	1st to 10th	Night	6% 1 17% 45% 28% 2	370
	11th to 20th	Night	10% 2 18% 40% 26% 4	399
	21st to end	Night	10% 1 12% 47% 27% 3	624
August	1st to 10th	Night	9% 2 11% 43% 31% 4	567
	11th to 20th	Night	8% 3 12% 36% 37% 5%	564
	21st to end	Night	11% 2 17% 34% 32% 3	699
September	1st to 10th	Night	13% 4 18% 33% 26% 7%	760
	11th to 20th	Night	11% 3 17% 30% 30% 9%	760
	21st to end	Night	17% 6% 18% 29% 24% 6%	760
October	1st to 10th	Night	20% 6% 23% 26% 21% 3	760
	11th to 20th	Night	21% 10% 28% 29% 10% 2	760
	21st to end	Night	24% 10% 28% 29% 8%	836
November	1st to 10th	Night	19% 10% 28% 30% 11% 2	780
	11th to 20th	Night	19% 14% 33% 24% 9% 2	800
	21st to end	Night	23% 11% 34% 24% 7%	800
December	1st to 10th	Night	24% 15% 34% 19% 7% 1	800
	11th to 20th	Night	23% 17% 34% 22% 5%	800
	21st to end	Night	29% 12% 32% 23% 4	880

Page generated at 2025-02-18 03:26:28. Version ac07

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