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 2024-11-02.

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% 2 7% 5% 5% 5% 5% 6% 8% 46%	101384
	Night	4 7% 3 7% 5% 5% 5% 4 7% 8% 45%	97714
Transparency	Day	38% 9% 13% 20% 14% 6%	101120
	Night	40% 11% 14% 21% 11% 4	97566
Seeing	Day	13% 19% 29% 25% 12% 3	15116
	Night	10% 13% 26% 34% 16% 2	24405

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% 6% 11% 7% 7% 7% 7% 10% 11% 26%	2418
	11th to 20th	Day	1 5% 3 9% 7% 9% 10% 7% 8% 9% 32%	2540
	21st to end	Day	2 8% 3 8% 8% 7% 7% 6% 7% 10% 34%	2794
February	1st to 10th	Day	2 10% 3 10% 8% 8% 9% 7% 7% 10% 25%	1653
	11th to 20th	Day	1 10% 5% 10% 8% 7% 7% 5% 6% 9% 31%	1732
	21st to end	Day	2 9% 5% 11% 9% 8% 7% 7% 7% 8% 27%	1570
March	1st to 10th	Day	2 6% 3 8% 6% 6% 8% 7% 9% 9% 36%	3138
	11th to 20th	Day	2 8% 3 9% 6% 5% 6% 5% 5% 6% 47%	3480
	21st to end	Day	4 13% 6% 12% 9% 7% 8% 6% 7% 9% 19%	3825
April	1st to 10th	Day	5% 10% 4 10% 7% 7% 7% 5% 9% 10% 27%	2280
	11th to 20th	Day	7% 11% 3 10% 6% 6% 6% 4 6% 8% 33%	2423
	21st to end	Day	9% 12% 4 9% 6% 5% 5% 5% 6% 7% 32%	2470
May	1st to 10th	Day	10% 12% 4 10% 6% 5% 5% 4 6% 7% 31%	4110
	11th to 20th	Day	7% 5% 2 5% 5% 5% 5% 5% 7% 9% 45%	4188
	21st to end	Day	14% 9% 3 8% 5% 5% 6% 5% 7% 7% 33%	5121
June	1st to 10th	Day	14% 8% 2 7% 6% 4 6% 5% 6% 7% 34%	2850
	11th to 20th	Day	14% 8% 2 7% 4 5% 5% 4 6% 7% 39%	2850
	21st to end	Day	4 3 1 4 4 4 5% 4 6% 7% 59%	2850
July	1st to 10th	Day	1 1 1 2 2 3 3 3 5% 6% 73%	4740
	11th to 20th	Day	2 2 1 2 2 2 2 2 3 4 78%	4424
	21st to end	Day	2 1 1 2 2 2 3 3 5% 6% 73%	4790
August	1st to 10th	Day	3 2 1 3 4 3 4 4 7% 8% 61%	4194
	11th to 20th	Day	3 2 1 3 2 4 5% 4 7% 8% 62%	4028
	21st to end	Day	3 3 1 3 3 3 3 3 4 6% 69%	4508
September	1st to 10th	Day	4 3 1 3 3 3 4 4 5% 7% 63%	2233
	11th to 20th	Day	6% 3 2 5% 4 4 5% 5% 7% 7% 51%	2090
	21st to end	Day	8% 6% 2 4 3 3 4 3 5% 7% 57%	2090
October	1st to 10th	Day	9% 8% 3 7% 5% 5% 5% 4 5% 6% 43%	2055
	11th to 20th	Day	6% 7% 3 6% 5% 4 5% 4 5% 7% 47%	1830
	21st to end	Day	4 9% 3 10% 6% 6% 4 5% 8% 7% 40%	1881
November	1st to 10th	Day	5% 9% 3 9% 6% 7% 7% 6% 7% 8% 33%	1683
	11th to 20th	Day	3 6% 2 9% 6% 5% 7% 6% 9% 9% 38%	1607
	21st to end	Day	4 7% 3 6% 6% 5% 6% 4 9% 10% 42%	1482
December	1st to 10th	Day	1 6% 3 9% 8% 8% 7% 5% 10% 10% 34%	1330
	11th to 20th	Day	1 7% 3 9% 7% 6% 9% 7% 11% 12% 29%	1685
	21st to end	Day	2 6% 2 8% 5% 5% 6% 6% 7% 9% 43%	2442
January	1st to 10th	Night	2 9% 4 10% 7% 7% 7% 6% 9% 11% 30%	4557
	11th to 20th	Night	1 6% 5% 11% 7% 8% 7% 5% 8% 10% 30%	4420
	21st to end	Night	2 9% 4 8% 6% 6% 7% 6% 8% 10% 35%	4862
February	1st to 10th	Night	3 11% 5% 10% 7% 6% 7% 6% 7% 9% 28%	2527
	11th to 20th	Night	1 9% 3 11% 8% 6% 7% 5% 8% 8% 33%	2332
	21st to end	Night	2 7% 5% 9% 7% 6% 8% 6% 9% 10% 32%	1884
March	1st to 10th	Night	2 6% 3 7% 5% 5% 6% 5% 9% 10% 43%	3736
	11th to 20th	Night	2 9% 3 7% 5% 4 4 4 6% 7% 48%	3500
	21st to end	Night	6% 15% 4 11% 6% 6% 6% 5% 8% 9% 23%	3813
April	1st to 10th	Night	6% 12% 4 7% 5% 5% 6% 5% 7% 9% 34%	1900
	11th to 20th	Night	6% 9% 3 10% 5% 5% 6% 5% 8% 8% 35%	1705
	21st to end	Night	8% 10% 2 8% 5% 5% 5% 5% 7% 9% 38%	1710
May	1st to 10th	Night	12% 14% 3 7% 4 4 4 4 5% 6% 38%	2860
	11th to 20th	Night	7% 6% 1 5% 4 3 4 4 6% 8% 50%	2708
	21st to end	Night	14% 11% 3 6% 5% 5% 5% 3 5% 8% 35%	2450
June	1st to 10th	Night	12% 7% 3 6% 5% 4 5% 5% 7% 8% 39%	1330
	11th to 20th	Night	13% 8% 3 5% 4 4 5% 4 5% 7% 43%	1330
	21st to end	Night	4 2 1 3 3 3 4 4 6% 8% 62%	1330
July	1st to 10th	Night	1 1 1 2 1 2 3 5% 8% 75%	2220
	11th to 20th	Night	1 1 2 2 2 3 2 4 7% 76%	2531
	21st to end	Night	1 1 2 1 2 3 2 4 6% 78%	2761
August	1st to 10th	Night	2 1 1 3 3 3 3 4 7% 9% 65%	2742
	11th to 20th	Night	2 1 1 2 2 3 4 3 6% 6% 69%	2811
	21st to end	Night	4 2 1 2 2 2 3 3 5% 6% 69%	3146
September	1st to 10th	Night	3 3 1 3 3 3 4 3 5% 6% 66%	1945
	11th to 20th	Night	6% 3 1 5% 3 4 4 4 6% 9% 54%	2090
	21st to end	Night	7% 4 2 4 3 3 4 3 5% 6% 59%	2090
October	1st to 10th	Night	8% 8% 2 6% 3 4 4 3 5% 8% 48%	2124
	11th to 20th	Night	5% 6% 2 6% 4 4 5% 4 6% 7% 50%	2346
	21st to end	Night	6% 7% 3 8% 6% 6% 5% 4 6% 9% 42%	2717
November	1st to 10th	Night	6% 7% 2 8% 5% 6% 5% 5% 7% 10% 39%	2418
	11th to 20th	Night	5% 9% 2 7% 5% 5% 7% 5% 9% 9% 35%	2551
	21st to end	Night	2 7% 3 6% 4 4 6% 5% 8% 10% 44%	2698
December	1st to 10th	Night	2 9% 3 7% 6% 5% 7% 5% 9% 9% 37%	2850
	11th to 20th	Night	2 8% 3 9% 7% 6% 8% 6% 9% 10% 33%	3506
	21st to end	Night	2 7% 3 9% 6% 6% 6% 5% 8% 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	44% 10% 16% 16% 13%	3138
	11th to 20th	Day	41% 8% 17% 30% 3	3480
	21st to end	Day	64% 12% 15% 9% 1	3825
April	1st to 10th	Day	58% 12% 15% 14% 3	2280
	11th to 20th	Day	51% 9% 15% 17% 7% 1	2426
	21st to end	Day	54% 11% 13% 16% 6%	2470
May	1st to 10th	Day	52% 9% 14% 20% 6%	4110
	11th to 20th	Day	36% 11% 17% 26% 9% 1	4191
	21st to end	Day	51% 11% 16% 19% 4	5122
June	1st to 10th	Day	49% 10% 17% 18% 6%	2850
	11th to 20th	Day	46% 8% 13% 17% 13% 4	2850
	21st to end	Day	26% 9% 12% 29% 21% 3	2850
July	1st to 10th	Day	13% 6% 7% 16% 35% 23%	4666
	11th to 20th	Day	11% 4 5% 13% 25% 42%	4348
	21st to end	Day	12% 5% 6% 19% 38% 21%	4639
August	1st to 10th	Day	19% 7% 9% 27% 28% 10%	4178
	11th to 20th	Day	18% 7% 12% 32% 26% 5%	4067
	21st to end	Day	17% 5% 9% 32% 29% 8%	4507
September	1st to 10th	Day	22% 7% 10% 27% 25% 9%	2233
	11th to 20th	Day	32% 8% 12% 26% 18% 4	2090
	21st to end	Day	30% 6% 17% 28% 16% 2	2090
October	1st to 10th	Day	44% 7% 12% 25% 11% 1	2055
	11th to 20th	Day	40% 9% 13% 25% 10% 2	1830
	21st to end	Day	46% 10% 13% 21% 10%	1881
November	1st to 10th	Day	51% 11% 15% 17% 5% 1	1683
	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	39% 11% 14% 20% 15%	3736
	11th to 20th	Night	39% 11% 17% 29% 4	3500
	21st to end	Night	58% 16% 15% 10% 2	3813
April	1st to 10th	Night	50% 12% 17% 17% 4	1900
	11th to 20th	Night	48% 11% 15% 17% 7% 1	1705
	21st to end	Night	46% 12% 15% 19% 8%	1710
May	1st to 10th	Night	47% 9% 15% 22% 7%	2860
	11th to 20th	Night	30% 12% 17% 28% 13%	2708
	21st to end	Night	48% 11% 17% 22% 2	2450
June	1st to 10th	Night	44% 12% 19% 20% 5%	1330
	11th to 20th	Night	41% 9% 14% 21% 11% 3	1330
	21st to end	Night	20% 9% 17% 33% 18% 2	1330
July	1st to 10th	Night	9% 4 9% 24% 35% 20%	2185
	11th to 20th	Night	9% 4 6% 19% 22% 40%	2484
	21st to end	Night	10% 4 7% 28% 29% 23%	2674
August	1st to 10th	Night	15% 6% 12% 28% 31% 8%	2734
	11th to 20th	Night	15% 6% 13% 38% 23% 5%	2838
	21st to end	Night	17% 6% 10% 36% 26% 5%	3146
September	1st to 10th	Night	21% 7% 12% 26% 26% 9%	1945
	11th to 20th	Night	29% 9% 12% 27% 18% 6%	2090
	21st to end	Night	30% 7% 15% 27% 19% 3	2090
October	1st to 10th	Night	38% 8% 15% 26% 12% 1	2124
	11th to 20th	Night	36% 10% 16% 25% 12% 1	2346
	21st to end	Night	44% 12% 14% 20% 10% 1	2717
November	1st to 10th	Night	46% 11% 16% 20% 6% 2	2418
	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	14% 25% 36% 18% 6% 1	460
	11th to 20th	Day	13% 23% 26% 24% 14%	460
	21st to end	Day	16% 27% 30% 21% 5% 1	506
April	1st to 10th	Day	17% 28% 29% 20% 6%	460
	11th to 20th	Day	19% 27% 29% 18% 5% 1	452
	21st to end	Day	23% 24% 27% 17% 8% 2	460
May	1st to 10th	Day	16% 18% 29% 26% 10% 1	463
	11th to 20th	Day	24% 16% 27% 20% 12% 1	488
	21st to end	Day	21% 16% 19% 24% 16% 4	707
June	1st to 10th	Day	16% 15% 25% 27% 13% 4	680
	11th to 20th	Day	21% 20% 26% 20% 11% 1	680
	21st to end	Day	7% 14% 28% 31% 18% 3	680
July	1st to 10th	Day	3 13% 29% 30% 21% 4	671
	11th to 20th	Day	6% 16% 35% 28% 12% 2	487
	21st to end	Day	4 11% 33% 33% 15% 4	531
August	1st to 10th	Day	5% 11% 30% 31% 18% 5%	487
	11th to 20th	Day	6% 11% 25% 35% 17% 6%	477
	21st to end	Day	5% 14% 31% 31% 14% 6%	539
September	1st to 10th	Day	5% 13% 31% 28% 16% 7%	490
	11th to 20th	Day	8% 15% 31% 27% 16% 4	490
	21st to end	Day	13% 18% 25% 24% 16% 4	490
October	1st to 10th	Day	15% 19% 24% 25% 14% 3	455
	11th to 20th	Day	13% 20% 25% 22% 13% 7%	300
	21st to end	Day	16% 22% 29% 23% 8% 2	330
November	1st to 10th	Day	19% 22% 29% 22% 8%	273
	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% 33% 9% 1	760
	11th to 20th	Night	10% 16% 29% 30% 14% 1	760
	21st to end	Night	11% 14% 29% 35% 10% 1	836
April	1st to 10th	Night	14% 13% 26% 32% 13% 2	760
	11th to 20th	Night	12% 18% 29% 29% 12% 1	577
	21st to end	Night	13% 16% 24% 34% 11% 2	570
May	1st to 10th	Night	13% 11% 22% 34% 18% 2	570
	11th to 20th	Night	13% 9% 18% 34% 24% 2	488
	21st to end	Night	20% 8% 15% 27% 27% 3	411
June	1st to 10th	Night	17% 7% 18% 35% 22% 2	380
	11th to 20th	Night	19% 8% 21% 38% 14% 1	380
	21st to end	Night	4 4 15% 51% 24% 1	380
July	1st to 10th	Night	2 2 12% 51% 32% 1	370
	11th to 20th	Night	4 2 13% 57% 24%	368
	21st to end	Night	2 2 8% 53% 34% 2	416
August	1st to 10th	Night	3 3 12% 47% 33% 2	500
	11th to 20th	Night	4 2 11% 47% 33% 3	564
	21st to end	Night	5% 4 17% 44% 28% 1	627
September	1st to 10th	Night	4 4 15% 45% 26% 5%	758
	11th to 20th	Night	8% 4 15% 40% 28% 5%	760
	21st to end	Night	10% 6% 17% 39% 23% 5%	760
October	1st to 10th	Night	13% 9% 20% 37% 19% 2	760
	11th to 20th	Night	11% 10% 21% 37% 18% 3	760
	21st to end	Night	12% 15% 26% 31% 13% 3	836
November	1st to 10th	Night	13% 15% 29% 32% 10% 1	744
	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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