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 2024-03-06.

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	5% 6% 3 6% 6% 4 6% 6% 7% 8% 42%	78185
	Night	4 7% 3 6% 6% 4 6% 6% 7% 9% 42%	81343
Transparency	Day	43% 10% 14% 19% 12% 3	77881
	Night	43% 11% 15% 19% 10% 2	81167
Seeing	Day	13% 20% 29% 24% 12% 3	14127
	Night	10% 13% 26% 34% 16% 2	23665

By Month

Percentage Cloud Cover SW (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	1 7% 7% 9% 9% 6% 7% 9% 9% 11% 26%	2418
	11th to 20th	Day	1 5% 3 8% 8% 7% 10% 9% 8% 9% 32%	2540
	21st to end	Day	2 7% 3 7% 9% 5% 7% 8% 8% 10% 35%	2794
February	1st to 10th	Day	2 9% 4 9% 9% 6% 9% 9% 6% 11% 26%	1653
	11th to 20th	Day	2 9% 5% 9% 10% 5% 8% 7% 6% 9% 31%	1732
	21st to end	Day	2 8% 6% 9% 11% 6% 8% 8% 7% 9% 27%	1570
March	1st to 10th	Day	3 8% 4 8% 8% 4 8% 8% 9% 9% 31%	1851
	11th to 20th	Day	4 8% 3 9% 9% 4 7% 8% 7% 8% 31%	1970
	21st to end	Day	3 11% 6% 9% 11% 5% 8% 7% 7% 9% 24%	2178
April	1st to 10th	Day	5% 9% 5% 8% 8% 5% 8% 7% 9% 10% 26%	2160
	11th to 20th	Day	7% 10% 4 9% 7% 4 6% 5% 6% 8% 33%	2293
	21st to end	Day	9% 12% 4 8% 7% 4 6% 6% 6% 7% 32%	2340
May	1st to 10th	Day	6% 9% 4 8% 6% 4 6% 6% 7% 8% 36%	2340
	11th to 20th	Day	11% 7% 2 6% 7% 5% 6% 7% 7% 8% 34%	2366
	21st to end	Day	16% 8% 4 7% 7% 4 6% 7% 6% 7% 27%	2891
June	1st to 10th	Day	14% 8% 2 6% 7% 3 6% 6% 6% 8% 34%	2700
	11th to 20th	Day	14% 8% 3 6% 5% 3 6% 5% 6% 7% 37%	2700
	21st to end	Day	4 3 2 4 4 2 5% 5% 6% 7% 59%	2700
July	1st to 10th	Day	2 1 1 3 3 3 4 5% 7% 8% 63%	2700
	11th to 20th	Day	3 3 1 2 3 2 3 4 4 5% 70%	2524
	21st to end	Day	3 1 1 2 3 2 3 4 6% 6% 70%	2741
August	1st to 10th	Day	3 2 1 3 4 3 4 5% 6% 8% 61%	2392
	11th to 20th	Day	4 2 1 2 3 3 4 5% 6% 7% 65%	2299
	21st to end	Day	5% 4 1 3 3 2 3 4 5% 7% 63%	2574
September	1st to 10th	Day	5% 3 1 3 4 2 4 4 5% 7% 63%	2116
	11th to 20th	Day	6% 3 2 4 5% 3 5% 7% 7% 7% 52%	1980
	21st to end	Day	8% 6% 2 4 3 2 4 4 5% 7% 55%	1980
October	1st to 10th	Day	10% 8% 3 6% 6% 4 6% 5% 6% 6% 41%	1947
	11th to 20th	Day	7% 7% 3 5% 6% 3 5% 6% 5% 8% 46%	1734
	21st to end	Day	4 8% 3 8% 8% 5% 5% 6% 7% 7% 39%	1782
November	1st to 10th	Day	6% 8% 4 8% 7% 5% 7% 7% 7% 8% 33%	1674
	11th to 20th	Day	3 6% 2 8% 7% 4 6% 7% 8% 10% 38%	1607
	21st to end	Day	4 6% 3 5% 6% 4 5% 5% 9% 10% 42%	1482
December	1st to 10th	Day	1 6% 3 8% 8% 7% 7% 7% 9% 10% 35%	1330
	11th to 20th	Day	1 7% 3 7% 8% 5% 8% 9% 10% 12% 29%	1685
	21st to end	Day	2 6% 2 7% 6% 4 6% 7% 7% 9% 43%	2442
January	1st to 10th	Night	2 9% 4 9% 8% 5% 7% 8% 8% 11% 30%	4557
	11th to 20th	Night	1 6% 6% 10% 9% 6% 8% 7% 8% 10% 30%	4420
	21st to end	Night	2 8% 4 7% 7% 4 7% 7% 8% 10% 35%	4862
February	1st to 10th	Night	3 11% 5% 9% 8% 4 8% 7% 7% 10% 28%	2527
	11th to 20th	Night	1 9% 3 10% 9% 5% 7% 7% 7% 9% 33%	2332
	21st to end	Night	2 7% 5% 8% 7% 5% 8% 7% 8% 11% 32%	1884
March	1st to 10th	Night	3 7% 3 6% 6% 4 7% 8% 9% 11% 34%	2225
	11th to 20th	Night	3 8% 4 5% 7% 4 7% 6% 9% 11% 36%	1990
	21st to end	Night	5% 12% 4 8% 8% 4 6% 6% 7% 10% 30%	2175
April	1st to 10th	Night	7% 10% 4 6% 6% 4 6% 6% 7% 9% 34%	1800
	11th to 20th	Night	6% 9% 3 9% 5% 4 6% 7% 7% 8% 35%	1615
	21st to end	Night	8% 9% 2 6% 6% 4 5% 6% 7% 10% 38%	1620
May	1st to 10th	Night	9% 7% 3 6% 6% 4 5% 6% 6% 8% 43%	1620
	11th to 20th	Night	10% 8% 2 5% 5% 4 5% 6% 7% 9% 40%	1524
	21st to end	Night	14% 10% 3 6% 6% 4 6% 5% 6% 8% 33%	1380
June	1st to 10th	Night	12% 6% 3 5% 6% 3 5% 6% 7% 9% 38%	1260
	11th to 20th	Night	14% 7% 3 5% 4 3 5% 5% 5% 7% 42%	1260
	21st to end	Night	4 2 1 3 2 2 4 5% 6% 8% 63%	1260
July	1st to 10th	Night	2 1 1 2 2 2 3 5% 5% 9% 69%	1260
	11th to 20th	Night	2 2 2 3 2 3 3 4 8% 71%	1431
	21st to end	Night	2 1 1 2 1 2 2 3 4 6% 76%	1576
August	1st to 10th	Night	3 2 1 2 3 2 3 4 6% 8% 67%	1547
	11th to 20th	Night	3 2 1 2 3 2 4 5% 5% 5% 67%	1602
	21st to end	Night	5% 3 1 3 3 2 4 4 6% 7% 62%	1782
September	1st to 10th	Night	3 3 1 3 3 2 4 3 5% 7% 66%	1842
	11th to 20th	Night	6% 2 1 4 4 3 4 5% 6% 9% 55%	1980
	21st to end	Night	7% 4 2 4 4 3 5% 4 5% 5% 58%	1980
October	1st to 10th	Night	8% 8% 3 6% 4 3 4 4 5% 8% 46%	2012
	11th to 20th	Night	5% 6% 2 5% 5% 3 5% 5% 6% 7% 49%	2222
	21st to end	Night	6% 6% 3 7% 7% 4 5% 5% 6% 9% 41%	2574
November	1st to 10th	Night	6% 7% 2 7% 6% 5% 6% 6% 7% 10% 39%	2405
	11th to 20th	Night	5% 8% 3 6% 6% 3 7% 7% 8% 10% 36%	2551
	21st to end	Night	3 7% 3 5% 5% 3 6% 6% 8% 10% 44%	2698
December	1st to 10th	Night	2 8% 4 7% 6% 4 6% 7% 8% 10% 38%	2850
	11th to 20th	Night	2 7% 3 7% 8% 5% 7% 8% 8% 10% 34%	3506
	21st to end	Night	3 6% 3 8% 7% 4 6% 7% 7% 9% 39%	5214

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	57% 14% 14% 12% 3 1	2418
	11th to 20th	Day	52% 11% 15% 14% 7% 2	2540
	21st to end	Day	50% 13% 18% 14% 5% 1	2794
February	1st to 10th	Day	60% 12% 12% 12% 3	1653
	11th to 20th	Day	56% 12% 15% 11% 5% 1	1741
	21st to end	Day	59% 11% 11% 11% 7% 1	1570
March	1st to 10th	Day	52% 13% 18% 12% 5%	1851
	11th to 20th	Day	54% 11% 13% 18% 3	1970
	21st to end	Day	63% 10% 14% 11% 2	2178
April	1st to 10th	Day	58% 12% 14% 14% 3	2160
	11th to 20th	Day	52% 9% 15% 16% 7% 1	2296
	21st to end	Day	54% 11% 14% 16% 5%	2340
May	1st to 10th	Day	46% 10% 16% 20% 7%	2340
	11th to 20th	Day	48% 12% 16% 17% 6%	2369
	21st to end	Day	57% 11% 14% 13% 4	2892
June	1st to 10th	Day	50% 10% 17% 18% 5%	2700
	11th to 20th	Day	47% 8% 13% 16% 12% 4	2700
	21st to end	Day	27% 9% 13% 29% 20% 3	2700
July	1st to 10th	Day	20% 8% 12% 24% 27% 10%	2626
	11th to 20th	Day	18% 7% 8% 19% 29% 20%	2448
	21st to end	Day	14% 6% 8% 19% 35% 18%	2589
August	1st to 10th	Day	22% 7% 10% 27% 25% 10%	2375
	11th to 20th	Day	19% 8% 10% 27% 28% 9%	2299
	21st to end	Day	23% 6% 11% 29% 25% 6%	2574
September	1st to 10th	Day	23% 7% 10% 27% 24% 9%	2116
	11th to 20th	Day	31% 8% 12% 26% 19% 4	1980
	21st to end	Day	31% 7% 17% 27% 15% 2	1980
October	1st to 10th	Day	46% 8% 13% 23% 9% 1	1947
	11th to 20th	Day	42% 10% 13% 25% 10% 2	1734
	21st to end	Day	48% 10% 13% 20% 8% 1	1782
November	1st to 10th	Day	52% 11% 14% 17% 5% 1	1674
	11th to 20th	Day	48% 12% 19% 17% 3 1	1607
	21st to end	Day	44% 12% 15% 19% 9% 1	1482
December	1st to 10th	Day	50% 12% 18% 12% 6% 2	1330
	11th to 20th	Day	53% 14% 17% 13% 2	1684
	21st to end	Day	42% 9% 16% 19% 9% 4	2442
January	1st to 10th	Night	54% 13% 14% 15% 3	4557
	11th to 20th	Night	53% 10% 12% 16% 7% 2	4420
	21st to end	Night	48% 15% 17% 14% 4	4862
February	1st to 10th	Night	59% 13% 13% 11% 4	2527
	11th to 20th	Night	54% 13% 15% 13% 5%	2333
	21st to end	Night	52% 13% 12% 14% 7% 1	1884
March	1st to 10th	Night	47% 14% 17% 15% 5% 1	2225
	11th to 20th	Night	47% 15% 16% 15% 7%	1990
	21st to end	Night	54% 14% 17% 12% 3	2175
April	1st to 10th	Night	49% 13% 17% 17% 4	1800
	11th to 20th	Night	49% 11% 16% 17% 7% 1	1615
	21st to end	Night	46% 12% 15% 19% 7%	1620
May	1st to 10th	Night	41% 11% 18% 23% 7%	1620
	11th to 20th	Night	40% 13% 21% 18% 9%	1524
	21st to end	Night	52% 10% 17% 18% 3	1380
June	1st to 10th	Night	44% 12% 20% 20% 4	1260
	11th to 20th	Night	42% 10% 15% 21% 10% 2	1260
	21st to end	Night	21% 9% 17% 33% 18% 2	1260
July	1st to 10th	Night	14% 6% 13% 31% 26% 9%	1225
	11th to 20th	Night	14% 5% 9% 28% 27% 16%	1384
	21st to end	Night	11% 4 7% 28% 33% 17%	1488
August	1st to 10th	Night	16% 7% 13% 28% 27% 10%	1539
	11th to 20th	Night	19% 6% 10% 28% 28% 9%	1602
	21st to end	Night	23% 8% 11% 27% 27% 5%	1782
September	1st to 10th	Night	22% 7% 11% 24% 26% 10%	1842
	11th to 20th	Night	28% 8% 11% 28% 19% 6%	1980
	21st to end	Night	31% 7% 15% 26% 18% 3	1980
October	1st to 10th	Night	39% 9% 15% 25% 11% 1	2012
	11th to 20th	Night	37% 10% 15% 25% 12% 1	2222
	21st to end	Night	44% 12% 14% 19% 10% 1	2574
November	1st to 10th	Night	46% 11% 15% 20% 5% 2	2405
	11th to 20th	Night	50% 13% 15% 17% 3 2	2551
	21st to end	Night	41% 11% 16% 19% 10% 1	2698
December	1st to 10th	Night	48% 14% 15% 14% 8% 2	2850
	11th to 20th	Night	50% 16% 15% 17% 2 1	3507
	21st to end	Night	46% 11% 15% 17% 8% 3	5214

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	8% 26% 22% 28% 13% 3	180
	11th to 20th	Day	8% 19% 37% 18% 11% 7%	180
	21st to end	Day	13% 23% 29% 25% 7% 3	198
February	1st to 10th	Day	14% 36% 33% 13% 4	243
	11th to 20th	Day	9% 29% 32% 22% 7%	337
	21st to end	Day	15% 28% 32% 20% 5%	382
March	1st to 10th	Day	15% 25% 37% 16% 7% 1	441
	11th to 20th	Day	14% 23% 26% 25% 13%	420
	21st to end	Day	16% 28% 30% 20% 5% 1	462
April	1st to 10th	Day	15% 28% 29% 21% 5%	420
	11th to 20th	Day	20% 28% 29% 17% 5% 1	412
	21st to end	Day	23% 25% 26% 17% 8% 2	420
May	1st to 10th	Day	15% 18% 30% 26% 10% 1	423
	11th to 20th	Day	25% 14% 27% 21% 12% 2	448
	21st to end	Day	21% 16% 20% 24% 16% 4	653
June	1st to 10th	Day	15% 16% 24% 27% 14% 4	630
	11th to 20th	Day	22% 21% 25% 20% 11% 1	630
	21st to end	Day	7% 14% 26% 31% 19% 3	630
July	1st to 10th	Day	4 14% 29% 29% 21% 3	621
	11th to 20th	Day	7% 18% 35% 27% 12% 2	447
	21st to end	Day	4 11% 33% 33% 15% 4	487
August	1st to 10th	Day	6% 12% 29% 31% 17% 5%	447
	11th to 20th	Day	6% 12% 24% 35% 17% 6%	437
	21st to end	Day	6% 14% 31% 31% 14% 5%	495
September	1st to 10th	Day	5% 14% 34% 28% 15% 4	450
	11th to 20th	Day	8% 16% 32% 25% 15% 4	450
	21st to end	Day	15% 18% 26% 23% 16% 3	450
October	1st to 10th	Day	16% 20% 24% 25% 13% 3	417
	11th to 20th	Day	13% 23% 26% 20% 12% 6%	270
	21st to end	Day	17% 24% 29% 21% 7% 2	297
November	1st to 10th	Day	20% 22% 29% 22% 8%	270
	11th to 20th	Day	12% 21% 35% 19% 12% 1	270
	21st to end	Day	12% 18% 33% 29% 8% 1	252
December	1st to 10th	Day	14% 19% 29% 20% 12% 5%	180
	11th to 20th	Day	13% 28% 31% 24% 4 1	180
	21st to end	Day	11% 24% 33% 23% 8% 2	198
January	1st to 10th	Night	9% 19% 30% 30% 11% 1	846
	11th to 20th	Night	8% 21% 33% 26% 11% 2	760
	21st to end	Night	8% 16% 34% 31% 9% 1	836
February	1st to 10th	Night	12% 18% 36% 26% 7% 1	760
	11th to 20th	Night	8% 21% 34% 28% 8% 2	744
	21st to end	Night	8% 18% 34% 28% 10% 1	628
March	1st to 10th	Night	7% 16% 35% 32% 9% 1	742
	11th to 20th	Night	10% 17% 29% 29% 14% 1	720
	21st to end	Night	10% 14% 30% 34% 10% 1	792
April	1st to 10th	Night	13% 13% 26% 32% 14% 2	720
	11th to 20th	Night	12% 17% 30% 29% 12% 1	547
	21st to end	Night	14% 16% 24% 34% 12% 1	540
May	1st to 10th	Night	12% 11% 22% 34% 19% 2	540
	11th to 20th	Night	13% 9% 18% 33% 24% 2	462
	21st to end	Night	20% 7% 15% 27% 29% 3	389
June	1st to 10th	Night	16% 7% 19% 34% 23% 2	360
	11th to 20th	Night	19% 8% 20% 38% 14% 1	360
	21st to end	Night	4 4 14% 51% 26% 1	360
July	1st to 10th	Night	2 2 11% 50% 34% 1	350
	11th to 20th	Night	4 2 14% 55% 24%	348
	21st to end	Night	2 2 8% 52% 34% 2	394
August	1st to 10th	Night	3 3 12% 46% 34% 2	473
	11th to 20th	Night	4 2 11% 46% 34% 3	534
	21st to end	Night	5% 4 16% 44% 29% 1	594
September	1st to 10th	Night	5% 4 16% 46% 25% 4	718
	11th to 20th	Night	8% 4 15% 39% 28% 6%	720
	21st to end	Night	10% 7% 17% 38% 23% 5%	720
October	1st to 10th	Night	13% 10% 19% 36% 20% 2	720
	11th to 20th	Night	11% 11% 21% 36% 17% 3	720
	21st to end	Night	12% 15% 26% 31% 13% 3	792
November	1st to 10th	Night	13% 15% 29% 32% 11% 1	740
	11th to 20th	Night	15% 18% 31% 27% 8%	841
	21st to end	Night	8% 18% 32% 30% 11% 1	950
December	1st to 10th	Night	10% 19% 30% 29% 11% 2	950
	11th to 20th	Night	9% 17% 37% 28% 8%	950
	21st to end	Night	8% 22% 37% 25% 8%	1045

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