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 2024-01-31.

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% 4 7% 6% 4 5% 5% 6% 8% 33%	74798
	Night	15% 9% 4 7% 6% 3 5% 5% 5% 7% 34%	75320
Transparency	Day	32% 39% 10% 12% 6% 1	74515
	Night	33% 39% 9% 12% 6% 1	75137
Seeing	Day	16% 9% 26% 27% 16% 6%	13384
	Night	18% 9% 28% 29% 14% 2	22993

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	14% 12% 5% 10% 8% 5% 5% 6% 5% 6% 24%	1519
	11th to 20th	Day	14% 13% 7% 11% 8% 4 7% 6% 5% 6% 19%	1710
	21st to end	Day	12% 14% 7% 11% 9% 6% 6% 7% 5% 5% 17%	1872
February	1st to 10th	Day	16% 15% 4 11% 8% 5% 7% 5% 5% 6% 19%	1620
	11th to 20th	Day	12% 12% 7% 12% 8% 4 5% 5% 5% 6% 23%	1573
	21st to end	Day	17% 12% 5% 11% 6% 4 6% 4 5% 6% 24%	1400
March	1st to 10th	Day	13% 10% 5% 8% 8% 4 6% 5% 5% 7% 28%	1977
	11th to 20th	Day	12% 12% 5% 8% 7% 3 5% 4 4 6% 33%	1980
	21st to end	Day	17% 10% 4 7% 7% 3 5% 5% 5% 6% 32%	2178
April	1st to 10th	Day	16% 9% 4 7% 8% 5% 5% 5% 5% 7% 30%	2106
	11th to 20th	Day	16% 7% 3 7% 6% 3 4 4 5% 7% 36%	2212
	21st to end	Day	19% 8% 3 6% 7% 3 4 4 5% 6% 34%	2340
May	1st to 10th	Day	13% 9% 3 7% 6% 4 5% 5% 5% 7% 35%	2340
	11th to 20th	Day	14% 7% 3 7% 6% 4 5% 4 5% 7% 37%	2414
	21st to end	Day	10% 5% 3 5% 5% 4 5% 5% 6% 9% 44%	2725
June	1st to 10th	Day	12% 5% 2 5% 6% 3 5% 5% 6% 9% 41%	2520
	11th to 20th	Day	10% 5% 2 6% 5% 4 6% 6% 7% 8% 41%	2520
	21st to end	Day	11% 5% 2 5% 5% 4 5% 6% 6% 10% 42%	2520
July	1st to 10th	Day	6% 4 2 4 5% 4 5% 6% 7% 10% 47%	2520
	11th to 20th	Day	11% 5% 3 4 5% 4 6% 7% 7% 11% 38%	2487
	21st to end	Day	9% 5% 2 5% 6% 4 6% 6% 9% 10% 39%	2547
August	1st to 10th	Day	7% 5% 2 5% 6% 4 6% 7% 7% 11% 40%	2327
	11th to 20th	Day	8% 4 3 6% 7% 4 6% 6% 7% 10% 39%	2303
	21st to end	Day	10% 6% 3 5% 5% 4 5% 5% 6% 9% 42%	2486
September	1st to 10th	Day	12% 5% 4 6% 5% 3 5% 6% 6% 7% 40%	2160
	11th to 20th	Day	9% 5% 3 5% 5% 4 4 5% 6% 8% 47%	2026
	21st to end	Day	13% 6% 3 6% 6% 4 5% 5% 6% 8% 37%	1959
October	1st to 10th	Day	18% 9% 3 6% 6% 3 5% 4 5% 7% 35%	1800
	11th to 20th	Day	17% 9% 4 7% 7% 4 5% 4 6% 6% 31%	1800
	21st to end	Day	23% 10% 3 8% 6% 4 5% 5% 4 7% 25%	1980
November	1st to 10th	Day	11% 9% 4 8% 6% 4 4 6% 7% 9% 34%	1639
	11th to 20th	Day	11% 11% 4 9% 7% 4 6% 6% 6% 8% 28%	1520
	21st to end	Day	18% 13% 4 6% 7% 3 5% 6% 7% 8% 24%	1520
December	1st to 10th	Day	14% 13% 5% 10% 8% 4 7% 6% 5% 6% 21%	1520
	11th to 20th	Day	17% 13% 4 10% 8% 4 6% 6% 5% 6% 21%	1906
	21st to end	Day	27% 17% 9% 10% 7% 4 4 4 4 4 10%	2772
January	1st to 10th	Night	14% 13% 5% 9% 8% 4 5% 5% 5% 6% 25%	2645
	11th to 20th	Night	17% 13% 5% 10% 8% 5% 6% 6% 5% 6% 18%	2470
	21st to end	Night	13% 12% 7% 9% 8% 4 7% 4 5% 8% 24%	2711
February	1st to 10th	Night	16% 13% 5% 10% 7% 4 6% 5% 6% 6% 23%	2340
	11th to 20th	Night	13% 13% 5% 10% 7% 4 6% 4 6% 6% 27%	2273
	21st to end	Night	16% 11% 5% 9% 8% 3 5% 5% 5% 5% 28%	1851
March	1st to 10th	Night	13% 9% 3 8% 6% 4 6% 6% 6% 6% 33%	1970
	11th to 20th	Night	16% 10% 4 8% 5% 4 5% 4 4 6% 34%	1979
	21st to end	Night	16% 9% 4 8% 5% 3 5% 4 4 5% 37%	2178
April	1st to 10th	Night	17% 9% 4 7% 6% 4 4 5% 6% 6% 34%	1854
	11th to 20th	Night	16% 8% 3 6% 6% 3 4 5% 5% 6% 38%	1711
	21st to end	Night	16% 7% 4 6% 7% 3 5% 4 5% 7% 37%	1620
May	1st to 10th	Night	15% 7% 4 6% 6% 3 4 4 5% 8% 38%	1620
	11th to 20th	Night	15% 4 3 6% 6% 2 5% 5% 6% 7% 42%	1477
	21st to end	Night	10% 4 1 5% 5% 3 4 4 6% 8% 49%	1550
June	1st to 10th	Night	11% 5% 2 4 6% 4 6% 5% 6% 8% 43%	1440
	11th to 20th	Night	10% 5% 3 5% 6% 3 5% 5% 8% 7% 44%	1440
	21st to end	Night	11% 4 1 5% 4 3 5% 5% 5% 8% 46%	1440
July	1st to 10th	Night	6% 4 3 3 4 3 5% 4 5% 8% 57%	1440
	11th to 20th	Night	9% 4 2 3 5% 2 4 5% 5% 10% 49%	1469
	21st to end	Night	8% 4 2 5% 4 3 4 5% 7% 9% 49%	1771
August	1st to 10th	Night	8% 4 1 5% 5% 4 4 7% 6% 10% 47%	1611
	11th to 20th	Night	8% 3 1 4 5% 3 4 5% 7% 10% 50%	1598
	21st to end	Night	8% 5% 2 5% 5% 4 4 5% 7% 7% 48%	1850
September	1st to 10th	Night	12% 5% 3 5% 6% 3 4 5% 6% 8% 43%	1800
	11th to 20th	Night	10% 4 2 4 4 3 4 4 4 7% 55%	1930
	21st to end	Night	13% 7% 3 6% 6% 4 5% 6% 6% 7% 39%	2000
October	1st to 10th	Night	18% 6% 2 6% 5% 2 4 4 4 7% 41%	2160
	11th to 20th	Night	17% 8% 3 6% 6% 4 5% 5% 5% 7% 34%	2160
	21st to end	Night	22% 9% 4 7% 5% 4 5% 4 6% 5% 30%	2376
November	1st to 10th	Night	15% 7% 2 5% 4 3 6% 4 5% 8% 41%	2434
	11th to 20th	Night	14% 10% 4 7% 6% 3 6% 5% 5% 8% 31%	2660
	21st to end	Night	17% 10% 4 7% 7% 4 6% 5% 7% 7% 28%	2660
December	1st to 10th	Night	17% 13% 6% 8% 7% 4 5% 6% 5% 7% 22%	2660
	11th to 20th	Night	15% 14% 5% 9% 7% 4 5% 5% 6% 6% 24%	3288
	21st to end	Night	28% 17% 6% 11% 7% 3 4 4 4 4 11%	4884

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% 43% 9% 7% 2	1520
	11th to 20th	Day	43% 41% 8% 5% 2	1710
	21st to end	Day	46% 39% 7% 7% 2	1872
February	1st to 10th	Day	43% 38% 9% 9% 2	1620
	11th to 20th	Day	41% 39% 9% 8% 3	1582
	21st to end	Day	44% 38% 7% 7% 3	1400
March	1st to 10th	Day	35% 39% 10% 12% 3 1	1980
	11th to 20th	Day	36% 38% 8% 11% 6% 2	1980
	21st to end	Day	34% 38% 9% 13% 6% 1	2178
April	1st to 10th	Day	32% 37% 9% 13% 9% 1	2106
	11th to 20th	Day	31% 37% 9% 13% 10%	2212
	21st to end	Day	33% 37% 8% 12% 8% 1	2340
May	1st to 10th	Day	31% 37% 8% 13% 9% 1	2340
	11th to 20th	Day	29% 38% 7% 13% 10% 2	2418
	21st to end	Day	22% 40% 11% 15% 10% 2	2725
June	1st to 10th	Day	26% 39% 8% 15% 10% 1	2520
	11th to 20th	Day	24% 39% 10% 15% 10% 1	2520
	21st to end	Day	25% 39% 12% 16% 8%	2520
July	1st to 10th	Day	20% 40% 11% 16% 11% 1	2450
	11th to 20th	Day	23% 41% 14% 15% 7% 1	2412
	21st to end	Day	24% 42% 11% 14% 8% 1	2405
August	1st to 10th	Day	24% 40% 10% 17% 8%	2314
	11th to 20th	Day	22% 40% 12% 16% 9% 1	2303
	21st to end	Day	24% 39% 10% 17% 9% 1	2486
September	1st to 10th	Day	26% 39% 11% 16% 7% 1	2160
	11th to 20th	Day	23% 38% 12% 19% 8%	2026
	21st to end	Day	31% 40% 10% 14% 5%	1959
October	1st to 10th	Day	32% 38% 10% 12% 7% 1	1800
	11th to 20th	Day	36% 39% 10% 12% 3 1	1800
	21st to end	Day	40% 39% 9% 9% 2 1	1980
November	1st to 10th	Day	25% 43% 11% 14% 5% 1	1639
	11th to 20th	Day	33% 43% 11% 8% 5%	1520
	21st to end	Day	35% 43% 10% 10% 2	1520
December	1st to 10th	Day	38% 44% 10% 7% 1	1520
	11th to 20th	Day	41% 37% 13% 6% 4	1906
	21st to end	Day	60% 30% 5% 3	2772
January	1st to 10th	Night	41% 43% 7% 7% 2	2645
	11th to 20th	Night	42% 42% 9% 4 2	2470
	21st to end	Night	41% 39% 9% 11% 1	2711
February	1st to 10th	Night	40% 38% 9% 10% 3 1	2340
	11th to 20th	Night	39% 39% 10% 9% 4	2275
	21st to end	Night	38% 39% 10% 10% 4	1851
March	1st to 10th	Night	32% 40% 11% 13% 4 1	1980
	11th to 20th	Night	36% 38% 6% 13% 6% 1	1980
	21st to end	Night	33% 38% 6% 15% 7% 2	2178
April	1st to 10th	Night	31% 38% 8% 14% 9% 1	1854
	11th to 20th	Night	32% 37% 6% 13% 11% 1	1711
	21st to end	Night	33% 38% 7% 11% 10% 2	1620
May	1st to 10th	Night	29% 38% 6% 15% 10% 2	1620
	11th to 20th	Night	24% 38% 8% 16% 10% 4	1478
	21st to end	Night	20% 39% 9% 16% 13% 3	1551
June	1st to 10th	Night	24% 40% 9% 16% 10% 1	1440
	11th to 20th	Night	22% 39% 10% 16% 12% 2	1440
	21st to end	Night	24% 39% 11% 16% 10%	1440
July	1st to 10th	Night	19% 39% 8% 15% 17% 2	1400
	11th to 20th	Night	21% 40% 10% 17% 10% 2	1421
	21st to end	Night	19% 41% 11% 17% 12% 1	1672
August	1st to 10th	Night	24% 41% 9% 18% 7% 1	1600
	11th to 20th	Night	17% 39% 13% 20% 10% 1	1598
	21st to end	Night	21% 40% 9% 18% 10% 2	1850
September	1st to 10th	Night	28% 38% 7% 17% 9% 2	1800
	11th to 20th	Night	22% 38% 9% 23% 9%	1930
	21st to end	Night	30% 39% 9% 15% 7%	2000
October	1st to 10th	Night	28% 38% 8% 17% 8% 1	2160
	11th to 20th	Night	33% 38% 11% 13% 5%	2160
	21st to end	Night	38% 39% 7% 10% 6%	2376
November	1st to 10th	Night	23% 43% 9% 17% 7% 1	2434
	11th to 20th	Night	31% 43% 9% 10% 6% 1	2660
	21st to end	Night	34% 43% 12% 10% 2	2660
December	1st to 10th	Night	39% 44% 9% 7% 1	2660
	11th to 20th	Night	41% 38% 11% 9% 2	3288
	21st to end	Night	59% 30% 7% 2 1	4884

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% 21% 9% 3	182
	11th to 20th	Day	22% 16% 38% 15% 7% 2	270
	21st to end	Day	21% 15% 33% 26% 5%	294
February	1st to 10th	Day	22% 13% 47% 15% 4	240
	11th to 20th	Day	18% 20% 40% 19% 2	240
	21st to end	Day	26% 23% 33% 16% 2	204
March	1st to 10th	Day	18% 11% 37% 25% 8%	420
	11th to 20th	Day	18% 15% 33% 25% 8% 1	420
	21st to end	Day	24% 11% 30% 24% 10% 2	462
April	1st to 10th	Day	15% 12% 32% 30% 11% 1	420
	11th to 20th	Day	21% 13% 26% 24% 12% 5%	412
	21st to end	Day	21% 9% 30% 28% 10% 3	420
May	1st to 10th	Day	19% 9% 26% 26% 15% 6%	423
	11th to 20th	Day	16% 6% 22% 29% 20% 7%	447
	21st to end	Day	12% 3 20% 33% 21% 9%	485
June	1st to 10th	Day	10% 5% 28% 35% 18% 4	450
	11th to 20th	Day	13% 5% 22% 31% 24% 6%	450
	21st to end	Day	14% 4 21% 32% 21% 8%	450
July	1st to 10th	Day	7% 2 12% 34% 28% 16%	445
	11th to 20th	Day	13% 4 16% 30% 23% 14%	444
	21st to end	Day	9% 4 16% 38% 25% 9%	488
August	1st to 10th	Day	8% 4 15% 32% 26% 15%	446
	11th to 20th	Day	7% 4 17% 29% 28% 14%	439
	21st to end	Day	12% 4 19% 32% 22% 13%	495
September	1st to 10th	Day	12% 6% 18% 28% 24% 12%	450
	11th to 20th	Day	10% 3 18% 26% 26% 16%	450
	21st to end	Day	14% 8% 24% 28% 17% 9%	450
October	1st to 10th	Day	22% 10% 21% 22% 16% 9%	450
	11th to 20th	Day	19% 11% 33% 24% 10% 2	450
	21st to end	Day	28% 9% 30% 22% 9% 1	495
November	1st to 10th	Day	14% 18% 31% 25% 9% 3	275
	11th to 20th	Day	23% 18% 25% 25% 8% 2	180
	21st to end	Day	24% 14% 33% 22% 8%	180
December	1st to 10th	Day	18% 21% 34% 19% 7% 1	180
	11th to 20th	Day	26% 22% 31% 18% 4	180
	21st to end	Day	24% 14% 42% 16% 3 1	198
January	1st to 10th	Night	20% 15% 37% 21% 6% 1	760
	11th to 20th	Night	25% 16% 33% 22% 4 1	760
	21st to end	Night	17% 11% 39% 26% 7%	835
February	1st to 10th	Night	24% 14% 39% 19% 3	720
	11th to 20th	Night	19% 15% 36% 25% 4	703
	21st to end	Night	23% 11% 37% 25% 4	592
March	1st to 10th	Night	18% 11% 35% 28% 9%	720
	11th to 20th	Night	21% 12% 36% 24% 7%	720
	21st to end	Night	20% 8% 29% 32% 10% 1	792
April	1st to 10th	Night	19% 9% 32% 31% 7% 1	720
	11th to 20th	Night	19% 9% 30% 28% 12% 2	647
	21st to end	Night	19% 5% 26% 37% 13%	540
May	1st to 10th	Night	18% 4 25% 32% 18% 3	540
	11th to 20th	Night	18% 4 22% 38% 17% 1	416
	21st to end	Night	12% 3 22% 34% 27% 2	389
June	1st to 10th	Night	11% 3 24% 44% 16% 2	360
	11th to 20th	Night	13% 2 25% 39% 21% 1	360
	21st to end	Night	11% 4 19% 43% 23% 1	360
July	1st to 10th	Night	7% 1 18% 43% 29% 3	350
	11th to 20th	Night	10% 2 18% 41% 24% 4	377
	21st to end	Night	10% 1 13% 49% 25% 3	591
August	1st to 10th	Night	9% 2 11% 43% 31% 5%	537
	11th to 20th	Night	7% 2 12% 37% 37% 4	534
	21st to end	Night	11% 2 18% 34% 31% 4	662
September	1st to 10th	Night	13% 4 17% 33% 26% 7%	720
	11th to 20th	Night	12% 3 18% 31% 29% 7%	720
	21st to end	Night	15% 6% 19% 30% 24% 6%	720
October	1st to 10th	Night	21% 6% 23% 26% 21% 3	720
	11th to 20th	Night	21% 10% 29% 29% 10% 2	720
	21st to end	Night	26% 11% 28% 29% 7%	792
November	1st to 10th	Night	19% 10% 27% 30% 12% 2	740
	11th to 20th	Night	19% 14% 33% 23% 9% 2	760
	21st to end	Night	23% 11% 34% 24% 7%	760
December	1st to 10th	Night	25% 15% 33% 19% 6% 1	760
	11th to 20th	Night	23% 17% 34% 22% 5%	760
	21st to end	Night	29% 12% 33% 22% 3	836

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