Hammond
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hammond clear sky chart from 2005-11-06 to 2025-04-11.

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	17% 10% 3 8% 5% 5% 4 4 5% 7% 32%	113482
	Night	17% 12% 4 9% 5% 4 4 3 5% 6% 30%	114793
Transparency	Day	50% 7% 13% 20% 9% 1	116982
	Night	53% 7% 12% 18% 9% 1	119457
Seeing	Day	21% 12% 28% 23% 12% 4	14947
	Night	22% 9% 29% 30% 10% 1	24164

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	15% 18% 7% 15% 6% 5% 4 3 4 4 19%	3597
	11th to 20th	Day	13% 16% 6% 14% 7% 5% 4 3 5% 5% 22%	3570
	21st to end	Day	12% 18% 7% 12% 6% 6% 5% 4 5% 5% 21%	4067
February	1st to 10th	Day	14% 15% 5% 12% 7% 5% 4 4 5% 5% 25%	1800
	11th to 20th	Day	14% 15% 5% 10% 6% 4 5% 3 4 6% 26%	1784
	21st to end	Day	20% 13% 5% 9% 6% 5% 5% 3 5% 6% 24%	1650
March	1st to 10th	Day	18% 14% 5% 11% 6% 4 5% 3 4 5% 25%	4476
	11th to 20th	Day	13% 13% 4 9% 5% 5% 5% 4 5% 7% 30%	4904
	21st to end	Day	19% 10% 3 8% 5% 4 4 3 4 5% 35%	5826
April	1st to 10th	Day	22% 12% 3 6% 5% 4 3 3 4 5% 33%	2400
	11th to 20th	Day	23% 9% 3 7% 5% 3 3 3 5% 5% 35%	2254
	21st to end	Day	23% 10% 3 9% 5% 4 5% 2 4 5% 31%	2410
May	1st to 10th	Day	21% 8% 3 7% 5% 5% 5% 4 5% 7% 30%	4426
	11th to 20th	Day	19% 9% 3 8% 5% 4 4 3 5% 7% 34%	4398
	21st to end	Day	16% 7% 2 7% 5% 4 5% 4 6% 7% 37%	4837
June	1st to 10th	Day	23% 8% 3 7% 5% 5% 5% 4 5% 6% 30%	2660
	11th to 20th	Day	14% 6% 2 6% 5% 5% 5% 4 7% 8% 37%	2660
	21st to end	Day	20% 7% 3 6% 5% 4 4 6% 6% 7% 31%	2660
July	1st to 10th	Day	12% 7% 3 7% 6% 6% 5% 5% 6% 9% 35%	4440
	11th to 20th	Day	13% 6% 2 6% 5% 5% 5% 5% 7% 8% 38%	4440
	21st to end	Day	11% 6% 2 5% 4 4 4 4 7% 8% 44%	4814
August	1st to 10th	Day	15% 7% 3 7% 5% 5% 5% 4 6% 9% 35%	4423
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 7% 8% 38%	3953
	21st to end	Day	16% 5% 2 6% 4 4 5% 4 6% 10% 39%	4180
September	1st to 10th	Day	13% 6% 3 5% 5% 4 4 4 6% 8% 43%	2280
	11th to 20th	Day	13% 5% 1 5% 4 4 4 3 6% 7% 48%	2259
	21st to end	Day	18% 7% 2 5% 4 4 4 3 5% 7% 41%	2062
October	1st to 10th	Day	21% 8% 4 6% 4 3 4 4 4 7% 35%	1900
	11th to 20th	Day	22% 10% 3 8% 5% 5% 4 4 5% 6% 30%	1900
	21st to end	Day	26% 10% 2 8% 5% 5% 4 3 5% 6% 28%	1910
November	1st to 10th	Day	17% 9% 3 6% 6% 5% 5% 4 6% 7% 33%	1761
	11th to 20th	Day	18% 12% 4 9% 5% 5% 4 3 4 6% 30%	1635
	21st to end	Day	22% 14% 4 9% 6% 4 5% 3 5% 5% 22%	1600
December	1st to 10th	Day	21% 20% 6% 10% 5% 5% 5% 5% 5% 5% 15%	2600
	11th to 20th	Day	18% 17% 4 10% 5% 4 4 3 4 4 25%	2986
	21st to end	Day	23% 17% 4 10% 5% 4 4 3 3 4 22%	3960
January	1st to 10th	Night	15% 20% 8% 14% 5% 4 3 2 3 4 20%	6349
	11th to 20th	Night	11% 15% 6% 15% 9% 6% 5% 4 5% 5% 20%	6315
	21st to end	Night	14% 18% 7% 12% 6% 6% 4 3 5% 6% 20%	6886
February	1st to 10th	Night	14% 14% 5% 11% 7% 6% 5% 4 5% 5% 24%	2600
	11th to 20th	Night	13% 14% 5% 11% 7% 5% 5% 3 4 5% 28%	2501
	21st to end	Night	16% 14% 4 9% 6% 5% 4 3 5% 5% 29%	1980
March	1st to 10th	Night	22% 14% 4 8% 6% 4 4 3 4 4 28%	5364
	11th to 20th	Night	13% 12% 3 9% 5% 5% 5% 4 6% 8% 30%	5053
	21st to end	Night	18% 11% 3 9% 5% 4 4 3 5% 5% 33%	5105
April	1st to 10th	Night	22% 11% 2 6% 6% 4 4 3 5% 5% 31%	2000
	11th to 20th	Night	20% 10% 3 7% 5% 3 4 3 5% 5% 35%	1882
	21st to end	Night	22% 9% 3 7% 4 4 4 3 4 6% 35%	1786
May	1st to 10th	Night	21% 8% 3 8% 5% 4 4 4 6% 7% 29%	2514
	11th to 20th	Night	21% 8% 3 7% 3 5% 4 3 5% 6% 36%	2495
	21st to end	Night	16% 6% 3 6% 5% 4 5% 5% 6% 7% 38%	2738
June	1st to 10th	Night	16% 7% 2 6% 6% 4 5% 4 8% 9% 32%	1520
	11th to 20th	Night	15% 6% 3 5% 4 4 4 3 6% 7% 40%	1520
	21st to end	Night	19% 8% 2 6% 4 4 5% 4 6% 7% 35%	1520
July	1st to 10th	Night	13% 8% 3 8% 5% 6% 6% 4 5% 7% 35%	2520
	11th to 20th	Night	13% 6% 2 6% 5% 4 4 4 6% 8% 42%	2520
	21st to end	Night	11% 6% 3 6% 4 5% 5% 4 6% 7% 43%	2739
August	1st to 10th	Night	12% 8% 3 7% 5% 4 4 4 6% 8% 38%	2506
	11th to 20th	Night	15% 7% 2 6% 5% 3 5% 3 5% 7% 41%	2839
	21st to end	Night	12% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	3476
September	1st to 10th	Night	11% 5% 2 7% 5% 5% 5% 3 5% 7% 45%	1900
	11th to 20th	Night	14% 5% 1 5% 4 3 3 3 5% 7% 49%	1920
	21st to end	Night	17% 6% 2 5% 4 4 4 3 4 6% 44%	2116
October	1st to 10th	Night	19% 7% 2 6% 4 4 5% 3 5% 6% 38%	2280
	11th to 20th	Night	20% 9% 2 7% 6% 5% 4 3 5% 5% 33%	2280
	21st to end	Night	23% 9% 3 9% 6% 4 4 3 5% 6% 29%	2688
November	1st to 10th	Night	20% 10% 2 8% 5% 4 4 3 5% 6% 33%	2537
	11th to 20th	Night	19% 10% 3 8% 5% 5% 5% 4 4 5% 31%	2760
	21st to end	Night	22% 14% 4 9% 5% 4 4 3 5% 6% 24%	2800
December	1st to 10th	Night	19% 17% 6% 12% 6% 6% 5% 4 4 5% 17%	4580
	11th to 20th	Night	24% 15% 5% 9% 5% 4 3 3 4 4 26%	5208
	21st to end	Night	27% 17% 3 9% 4 4 4 3 5% 5% 20%	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	65% 8% 8% 14% 4 1	3598
	11th to 20th	Day	62% 6% 11% 13% 7% 1	3600
	21st to end	Day	63% 8% 11% 11% 8% 1	4067
February	1st to 10th	Day	60% 5% 10% 18% 5% 2	1800
	11th to 20th	Day	59% 5% 12% 15% 8% 1	1793
	21st to end	Day	60% 6% 10% 14% 10% 1	1650
March	1st to 10th	Day	60% 6% 9% 12% 11% 3	4479
	11th to 20th	Day	50% 7% 17% 15% 9% 1	4904
	21st to end	Day	50% 5% 7% 18% 17% 2	5826
April	1st to 10th	Day	53% 5% 8% 19% 12% 3	2400
	11th to 20th	Day	50% 5% 8% 21% 14% 2	2254
	21st to end	Day	55% 6% 9% 14% 13% 4	2410
May	1st to 10th	Day	49% 9% 11% 20% 9% 2	4440
	11th to 20th	Day	47% 10% 14% 18% 9% 2	4402
	21st to end	Day	42% 9% 13% 21% 13% 1	4837
June	1st to 10th	Day	52% 7% 12% 17% 10% 1	2660
	11th to 20th	Day	42% 9% 13% 22% 12% 2	2660
	21st to end	Day	48% 10% 14% 18% 9% 1	2660
July	1st to 10th	Day	44% 7% 15% 23% 11%	4370
	11th to 20th	Day	40% 8% 16% 29% 8%	4365
	21st to end	Day	34% 9% 15% 31% 10%	4661
August	1st to 10th	Day	45% 7% 16% 22% 9%	4409
	11th to 20th	Day	41% 8% 17% 28% 7%	3992
	21st to end	Day	40% 6% 18% 23% 11% 1	4180
September	1st to 10th	Day	37% 8% 16% 26% 12%	2280
	11th to 20th	Day	34% 8% 14% 31% 13% 1	2259
	21st to end	Day	42% 8% 15% 26% 10%	2062
October	1st to 10th	Day	47% 5% 12% 28% 8%	3160
	11th to 20th	Day	47% 6% 15% 26% 7%	3140
	21st to end	Day	49% 4 19% 21% 6%	3122
November	1st to 10th	Day	48% 8% 15% 21% 6% 1	1761
	11th to 20th	Day	56% 6% 13% 16% 9% 1	1635
	21st to end	Day	62% 9% 12% 11% 5%	1600
December	1st to 10th	Day	69% 6% 13% 10% 1	2600
	11th to 20th	Day	62% 6% 10% 15% 7%	2986
	21st to end	Day	66% 5% 9% 16% 5%	3960
January	1st to 10th	Night	68% 6% 11% 11% 4	6349
	11th to 20th	Night	64% 8% 11% 11% 5%	6360
	21st to end	Night	64% 6% 10% 9% 10% 1	6886
February	1st to 10th	Night	60% 7% 11% 15% 6% 1	2600
	11th to 20th	Night	57% 6% 10% 19% 7%	2503
	21st to end	Night	56% 7% 9% 18% 10%	1980
March	1st to 10th	Night	58% 6% 8% 13% 12% 3	5374
	11th to 20th	Night	50% 7% 16% 16% 8% 3	5054
	21st to end	Night	51% 5% 8% 22% 12% 2	5105
April	1st to 10th	Night	53% 6% 9% 17% 13% 2	2000
	11th to 20th	Night	48% 7% 8% 21% 14% 1	1882
	21st to end	Night	50% 5% 10% 17% 15% 3	1786
May	1st to 10th	Night	49% 11% 10% 17% 9% 3	2520
	11th to 20th	Night	44% 12% 11% 18% 12% 2	2496
	21st to end	Night	41% 11% 12% 18% 17% 2	2739
June	1st to 10th	Night	46% 9% 15% 20% 10% 1	1520
	11th to 20th	Night	40% 8% 11% 22% 17% 2	1520
	21st to end	Night	45% 9% 14% 18% 13% 1	1520
July	1st to 10th	Night	46% 7% 16% 18% 12% 1	2480
	11th to 20th	Night	39% 6% 16% 31% 8%	2472
	21st to end	Night	38% 8% 16% 23% 14%	2651
August	1st to 10th	Night	43% 7% 16% 25% 8%	2496
	11th to 20th	Night	42% 6% 16% 29% 6%	2866
	21st to end	Night	40% 8% 16% 23% 12% 2	3476
September	1st to 10th	Night	38% 8% 13% 26% 15% 1	1900
	11th to 20th	Night	34% 7% 13% 33% 11% 1	1920
	21st to end	Night	39% 8% 13% 29% 11%	2116
October	1st to 10th	Night	42% 6% 15% 26% 10%	3800
	11th to 20th	Night	46% 6% 15% 25% 9%	3779
	21st to end	Night	47% 6% 17% 20% 10%	4426
November	1st to 10th	Night	50% 6% 15% 20% 8% 1	2537
	11th to 20th	Night	54% 7% 12% 17% 10% 1	2760
	21st to end	Night	60% 8% 12% 15% 4	2800
December	1st to 10th	Night	69% 6% 12% 11% 2	4580
	11th to 20th	Night	63% 5% 8% 14% 10% 1	5208
	21st to end	Night	66% 5% 6% 13% 9%	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	27% 16% 28% 25% 4 1	200
	11th to 20th	Day	23% 16% 34% 21% 5% 2	200
	21st to end	Day	18% 12% 38% 24% 7%	242
February	1st to 10th	Day	29% 16% 37% 14% 4	300
	11th to 20th	Day	25% 17% 38% 18% 2	331
	21st to end	Day	26% 12% 35% 24% 2	414
March	1st to 10th	Day	25% 12% 32% 24% 7%	500
	11th to 20th	Day	18% 20% 33% 23% 5% 1	500
	21st to end	Day	28% 14% 31% 17% 8% 2	550
April	1st to 10th	Day	25% 19% 29% 18% 8% 1	500
	11th to 20th	Day	27% 19% 23% 17% 11% 3	452
	21st to end	Day	25% 16% 30% 17% 9% 2	460
May	1st to 10th	Day	22% 11% 31% 25% 8% 3	463
	11th to 20th	Day	17% 14% 31% 22% 13% 3	487
	21st to end	Day	17% 12% 32% 24% 13% 3	529
June	1st to 10th	Day	23% 9% 28% 27% 10% 3	490
	11th to 20th	Day	14% 12% 29% 26% 17% 3	490
	21st to end	Day	21% 13% 24% 25% 14% 3	490
July	1st to 10th	Day	13% 6% 25% 33% 17% 6%	485
	11th to 20th	Day	11% 7% 30% 29% 19% 4	484
	21st to end	Day	14% 5% 26% 32% 18% 5%	532
August	1st to 10th	Day	15% 6% 22% 32% 21% 5%	486
	11th to 20th	Day	14% 5% 19% 30% 23% 9%	479
	21st to end	Day	13% 7% 26% 27% 18% 9%	539
September	1st to 10th	Day	16% 8% 26% 22% 17% 11%	490
	11th to 20th	Day	12% 6% 21% 22% 24% 15%	490
	21st to end	Day	18% 10% 26% 23% 17% 7%	490
October	1st to 10th	Day	23% 11% 26% 19% 15% 5%	490
	11th to 20th	Day	24% 14% 31% 21% 9% 1	490
	21st to end	Day	30% 11% 25% 25% 8%	444
November	1st to 10th	Day	25% 15% 25% 24% 10% 1	395
	11th to 20th	Day	26% 13% 36% 19% 5% 1	235
	21st to end	Day	27% 20% 28% 18% 7% 2	200
December	1st to 10th	Day	28% 11% 32% 23% 7% 1	200
	11th to 20th	Day	28% 18% 34% 18% 3 1	200
	21st to end	Day	30% 15% 35% 13% 6%	220
January	1st to 10th	Night	27% 10% 33% 26% 5%	800
	11th to 20th	Night	27% 11% 30% 26% 5% 1	800
	21st to end	Night	19% 8% 33% 32% 7% 1	880
February	1st to 10th	Night	20% 9% 38% 28% 5%	800
	11th to 20th	Night	20% 12% 39% 25% 5%	783
	21st to end	Night	22% 11% 32% 31% 4	660
March	1st to 10th	Night	24% 10% 33% 27% 6%	800
	11th to 20th	Night	22% 13% 33% 26% 5%	800
	21st to end	Night	23% 11% 29% 28% 8%	880
April	1st to 10th	Night	26% 11% 31% 28% 5%	800
	11th to 20th	Night	22% 14% 30% 26% 8% 1	754
	21st to end	Night	25% 11% 27% 29% 8% 1	646
May	1st to 10th	Night	23% 7% 29% 29% 9% 2	380
	11th to 20th	Night	22% 8% 27% 34% 9% 1	374
	21st to end	Night	14% 6% 34% 32% 13%	411
June	1st to 10th	Night	16% 4 28% 42% 11%	380
	11th to 20th	Night	14% 6% 28% 38% 12% 1	380
	21st to end	Night	20% 4 27% 32% 16% 1	380
July	1st to 10th	Night	8% 4 26% 44% 18%	370
	11th to 20th	Night	10% 5% 23% 43% 18% 1	368
	21st to end	Night	13% 3 20% 45% 19% 1	416
August	1st to 10th	Night	13% 4 18% 45% 19% 1	378
	11th to 20th	Night	12% 4 18% 35% 28% 2	592
	21st to end	Night	13% 5% 23% 37% 19% 2	836
September	1st to 10th	Night	13% 4 25% 38% 19% 1	760
	11th to 20th	Night	16% 5% 17% 33% 25% 3	760
	21st to end	Night	19% 6% 24% 28% 21% 2	760
October	1st to 10th	Night	22% 6% 28% 28% 16% 1	760
	11th to 20th	Night	24% 7% 31% 27% 10% 1	760
	21st to end	Night	26% 10% 28% 30% 6%	836
November	1st to 10th	Night	25% 11% 26% 29% 9% 1	780
	11th to 20th	Night	23% 12% 34% 21% 10%	800
	21st to end	Night	27% 12% 29% 24% 7%	800
December	1st to 10th	Night	29% 12% 31% 23% 5% 1	800
	11th to 20th	Night	28% 12% 31% 23% 5% 1	800
	21st to end	Night	33% 10% 29% 23% 4	880

Page generated at 2025-04-08 09:17:29. Version ac07

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