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 2026-08-15.

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% 5% 4 5% 7% 32%	144325
	Night	17% 12% 4 9% 5% 5% 4 3 5% 6% 30%	137557
Transparency	Day	50% 6% 13% 21% 10% 1	147779
	Night	53% 6% 12% 19% 10% 1	142192
Seeing	Day	21% 12% 29% 23% 11% 3	16713
	Night	21% 9% 29% 30% 10% 1	25693

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% 17% 6% 14% 6% 5% 4 3 5% 5% 20%	4661
	11th to 20th	Day	12% 19% 7% 15% 6% 5% 4 3 4 5% 20%	4650
	21st to end	Day	9% 16% 7% 13% 9% 8% 5% 4 5% 5% 20%	5259
February	1st to 10th	Day	14% 15% 5% 12% 7% 5% 4 4 5% 5% 25%	1890
	11th to 20th	Day	14% 15% 5% 10% 6% 4 5% 3 5% 6% 26%	1876
	21st to end	Day	19% 14% 5% 10% 6% 5% 5% 3 5% 6% 23%	1730
March	1st to 10th	Day	18% 13% 5% 11% 6% 4 5% 3 4 5% 25%	4576
	11th to 20th	Day	13% 13% 4 9% 5% 5% 5% 4 5% 7% 30%	5012
	21st to end	Day	19% 10% 3 8% 6% 4 4 3 4 5% 35%	5955
April	1st to 10th	Day	22% 13% 3 6% 5% 4 3 3 4 5% 33%	2520
	11th to 20th	Day	23% 10% 3 7% 4 3 3 3 4 5% 35%	2494
	21st to end	Day	22% 9% 3 8% 5% 4 4 2 4 5% 31%	2664
May	1st to 10th	Day	22% 9% 3 8% 5% 5% 5% 4 5% 7% 26%	8237
	11th to 20th	Day	18% 9% 3 8% 5% 4 5% 4 5% 7% 32%	8221
	21st to end	Day	21% 9% 2 6% 4 4 4 3 5% 6% 34%	9060
June	1st to 10th	Day	22% 8% 3 7% 5% 5% 5% 4 5% 7% 31%	2940
	11th to 20th	Day	14% 7% 2 6% 5% 5% 5% 4 6% 8% 37%	2940
	21st to end	Day	19% 7% 3 6% 5% 4 4 5% 7% 7% 32%	2940
July	1st to 10th	Day	12% 6% 2 7% 6% 6% 5% 5% 7% 9% 36%	8280
	11th to 20th	Day	10% 5% 2 6% 4 5% 4 4 7% 9% 44%	8280
	21st to end	Day	11% 5% 2 6% 4 5% 5% 4 7% 10% 40%	8993
August	1st to 10th	Day	14% 7% 3 7% 5% 5% 5% 4 6% 8% 36%	4703
	11th to 20th	Day	16% 7% 2 6% 4 4 4 4 7% 8% 39%	4132
	21st to end	Day	15% 5% 2 6% 4 4 5% 4 6% 10% 39%	4312
September	1st to 10th	Day	13% 6% 2 5% 5% 4 4 4 6% 8% 43%	2400
	11th to 20th	Day	13% 4 1 5% 4 4 4 3 5% 7% 49%	2378
	21st to end	Day	18% 7% 3 5% 4 4 4 3 5% 7% 41%	2170
October	1st to 10th	Day	20% 8% 3 6% 4 4 4 4 4 7% 37%	2000
	11th to 20th	Day	21% 10% 3 7% 5% 5% 4 5% 5% 6% 31%	2000
	21st to end	Day	26% 10% 2 8% 5% 5% 4 3 4 6% 28%	2010
November	1st to 10th	Day	17% 10% 3 7% 6% 5% 4 4 6% 7% 32%	1851
	11th to 20th	Day	17% 13% 4 9% 5% 6% 4 3 4 6% 29%	1717
	21st to end	Day	22% 15% 4 9% 6% 4 5% 4 5% 5% 21%	1680
December	1st to 10th	Day	20% 20% 6% 10% 4 5% 4 4 5% 5% 15%	2680
	11th to 20th	Day	18% 17% 4 10% 5% 4 5% 3 4 4 25%	3066
	21st to end	Day	23% 17% 4 10% 5% 4 4 3 3 4 22%	4048
January	1st to 10th	Night	14% 19% 8% 15% 5% 4 4 3 4 4 21%	8234
	11th to 20th	Night	12% 18% 7% 16% 8% 6% 4 3 4 4 17%	8235
	21st to end	Night	12% 16% 6% 11% 6% 7% 5% 4 5% 6% 22%	8919
February	1st to 10th	Night	13% 14% 5% 11% 7% 6% 5% 4 5% 5% 25%	2730
	11th to 20th	Night	13% 14% 5% 11% 8% 5% 5% 3 4 5% 28%	2629
	21st to end	Night	16% 14% 4 9% 6% 5% 4 3 5% 5% 28%	2076
March	1st to 10th	Night	22% 14% 4 8% 6% 4 4 3 4 4 28%	5484
	11th to 20th	Night	14% 12% 3 9% 5% 5% 4 4 6% 8% 30%	5165
	21st to end	Night	18% 11% 3 9% 5% 4 4 3 5% 5% 33%	5218
April	1st to 10th	Night	22% 11% 2 6% 6% 4 4 3 5% 5% 31%	2100
	11th to 20th	Night	20% 10% 3 7% 5% 3 3 3 5% 5% 35%	2075
	21st to end	Night	22% 9% 3 7% 4 4 4 3 5% 6% 36%	1974
May	1st to 10th	Night	20% 9% 3 9% 5% 5% 5% 4 6% 8% 23%	4660
	11th to 20th	Night	17% 8% 3 8% 4 4 4 4 5% 7% 35%	4639
	21st to end	Night	19% 9% 3 7% 5% 4 5% 4 5% 7% 32%	5114
June	1st to 10th	Night	15% 7% 2 7% 5% 5% 5% 4 7% 9% 33%	1680
	11th to 20th	Night	15% 6% 3 6% 4 4 4 3 6% 7% 41%	1680
	21st to end	Night	18% 8% 2 6% 4 4 5% 4 6% 7% 35%	1680
July	1st to 10th	Night	12% 6% 3 7% 5% 6% 6% 5% 7% 9% 35%	4680
	11th to 20th	Night	11% 6% 2 6% 4 4 4 4 6% 7% 46%	4680
	21st to end	Night	13% 6% 2 7% 5% 5% 5% 4 6% 7% 41%	5084
August	1st to 10th	Night	12% 8% 3 7% 5% 4 4 4 6% 8% 38%	2666
	11th to 20th	Night	15% 7% 2 6% 4 4 4 3 5% 7% 42%	2967
	21st to end	Night	12% 6% 2 7% 5% 5% 5% 4 6% 8% 40%	3586
September	1st to 10th	Night	11% 5% 2 7% 5% 5% 5% 3 5% 7% 45%	2000
	11th to 20th	Night	14% 5% 1 5% 4 3 3 3 5% 7% 50%	2021
	21st to end	Night	17% 7% 2 5% 4 4 4 3 4 6% 44%	2228
October	1st to 10th	Night	18% 7% 2 6% 4 4 5% 3 5% 6% 40%	2400
	11th to 20th	Night	20% 9% 2 7% 6% 5% 4 4 5% 5% 34%	2400
	21st to end	Night	23% 9% 3 9% 6% 4 4 3 5% 6% 30%	2830
November	1st to 10th	Night	20% 10% 3 8% 5% 4 4 3 5% 6% 33%	2667
	11th to 20th	Night	18% 11% 3 9% 5% 5% 5% 4 4 5% 31%	2898
	21st to end	Night	21% 14% 4 10% 5% 4 4 3 5% 6% 24%	2940
December	1st to 10th	Night	19% 17% 6% 12% 6% 5% 5% 4 4 5% 17%	4720
	11th to 20th	Night	24% 15% 5% 9% 5% 4 4 3 4 4 25%	5348
	21st to end	Night	27% 17% 4 9% 4 4 4 3 5% 5% 20%	7150

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	63% 7% 10% 15% 4 1	4662
	11th to 20th	Day	66% 5% 9% 11% 8% 1	4680
	21st to end	Day	65% 6% 10% 10% 8% 1	5259
February	1st to 10th	Day	60% 5% 10% 19% 5% 1	1890
	11th to 20th	Day	58% 5% 12% 16% 7% 1	1885
	21st to end	Day	61% 6% 9% 14% 9% 1	1730
March	1st to 10th	Day	60% 6% 10% 11% 11% 3	4579
	11th to 20th	Day	51% 7% 17% 15% 9% 1	5012
	21st to end	Day	51% 5% 7% 18% 17% 2	5955
April	1st to 10th	Day	54% 5% 8% 18% 12% 3	2520
	11th to 20th	Day	51% 5% 8% 21% 14% 2	2494
	21st to end	Day	54% 5% 9% 15% 13% 3	2664
May	1st to 10th	Day	54% 7% 9% 19% 10% 1	8249
	11th to 20th	Day	50% 7% 13% 20% 9% 1	8225
	21st to end	Day	48% 6% 11% 22% 11% 1	9061
June	1st to 10th	Day	51% 7% 11% 18% 12% 1	2940
	11th to 20th	Day	42% 8% 13% 23% 12% 2	2940
	21st to end	Day	47% 9% 14% 19% 9% 1	2940
July	1st to 10th	Day	42% 6% 15% 25% 11%	8210
	11th to 20th	Day	34% 6% 15% 33% 12%	8205
	21st to end	Day	37% 7% 16% 31% 8%	8795
August	1st to 10th	Day	44% 7% 17% 22% 9%	4689
	11th to 20th	Day	40% 8% 16% 28% 7% 1	4171
	21st to end	Day	40% 7% 18% 23% 11% 1	4312
September	1st to 10th	Day	36% 8% 16% 28% 12%	2400
	11th to 20th	Day	33% 8% 14% 32% 13% 1	2378
	21st to end	Day	42% 8% 14% 25% 11%	2170
October	1st to 10th	Day	46% 5% 12% 28% 8%	3260
	11th to 20th	Day	46% 6% 15% 25% 7% 1	3240
	21st to end	Day	49% 4 19% 21% 6%	3222
November	1st to 10th	Day	49% 8% 14% 21% 7% 1	1851
	11th to 20th	Day	57% 6% 13% 15% 9% 1	1717
	21st to end	Day	63% 9% 12% 11% 5%	1680
December	1st to 10th	Day	69% 6% 13% 10% 2	2680
	11th to 20th	Day	63% 6% 10% 15% 7%	3066
	21st to end	Day	66% 5% 9% 16% 5%	4048
January	1st to 10th	Night	67% 5% 11% 12% 5% 1	8234
	11th to 20th	Night	68% 7% 9% 10% 6%	8280
	21st to end	Night	62% 5% 9% 13% 11% 1	8919
February	1st to 10th	Night	59% 6% 11% 16% 7% 1	2730
	11th to 20th	Night	57% 5% 10% 20% 7%	2631
	21st to end	Night	55% 6% 9% 18% 10%	2076
March	1st to 10th	Night	58% 6% 8% 13% 12% 3	5494
	11th to 20th	Night	51% 7% 15% 15% 8% 3	5166
	21st to end	Night	51% 5% 8% 22% 12% 2	5218
April	1st to 10th	Night	53% 6% 9% 17% 13% 2	2100
	11th to 20th	Night	49% 7% 8% 21% 14% 1	2075
	21st to end	Night	49% 5% 10% 17% 16% 3	1974
May	1st to 10th	Night	54% 8% 11% 17% 8% 2	4666
	11th to 20th	Night	46% 9% 12% 20% 12% 1	4640
	21st to end	Night	49% 7% 11% 18% 15% 1	5115
June	1st to 10th	Night	45% 8% 13% 20% 12% 1	1680
	11th to 20th	Night	41% 8% 11% 22% 16% 2	1680
	21st to end	Night	45% 9% 13% 19% 13% 1	1680
July	1st to 10th	Night	43% 6% 18% 20% 12% 1	4640
	11th to 20th	Night	36% 5% 14% 32% 12%	4632
	21st to end	Night	42% 6% 13% 26% 13%	4967
August	1st to 10th	Night	43% 7% 16% 25% 9% 1	2656
	11th to 20th	Night	42% 5% 16% 29% 7%	2994
	21st to end	Night	40% 8% 16% 23% 11% 2	3586
September	1st to 10th	Night	37% 7% 13% 27% 14% 1	2000
	11th to 20th	Night	33% 7% 13% 34% 11% 1	2021
	21st to end	Night	40% 8% 13% 28% 12%	2228
October	1st to 10th	Night	41% 6% 15% 26% 11% 1	3920
	11th to 20th	Night	45% 6% 15% 25% 9%	3899
	21st to end	Night	47% 6% 17% 20% 10%	4568
November	1st to 10th	Night	51% 6% 15% 20% 8% 1	2667
	11th to 20th	Night	54% 6% 12% 17% 9% 1	2898
	21st to end	Night	61% 8% 12% 15% 4	2940
December	1st to 10th	Night	68% 6% 12% 11% 3	4720
	11th to 20th	Night	63% 5% 8% 14% 10% 1	5348
	21st to end	Night	66% 5% 6% 13% 9%	7150

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	25% 16% 29% 25% 5% 1	220
	11th to 20th	Day	25% 16% 34% 19% 5% 1	220
	21st to end	Day	17% 14% 37% 24% 8%	266
February	1st to 10th	Day	28% 15% 38% 15% 4	330
	11th to 20th	Day	24% 17% 38% 19% 2	363
	21st to end	Day	26% 12% 37% 23% 2	446
March	1st to 10th	Day	25% 13% 32% 23% 7%	540
	11th to 20th	Day	22% 19% 32% 22% 5% 1	540
	21st to end	Day	27% 15% 31% 16% 8% 2	594
April	1st to 10th	Day	28% 19% 29% 17% 7% 1	540
	11th to 20th	Day	29% 18% 23% 17% 10% 3	532
	21st to end	Day	26% 16% 31% 16% 8% 2	540
May	1st to 10th	Day	24% 12% 30% 23% 7% 3	543
	11th to 20th	Day	18% 14% 32% 20% 12% 3	567
	21st to end	Day	20% 12% 30% 23% 12% 3	617
June	1st to 10th	Day	22% 9% 29% 26% 10% 4	570
	11th to 20th	Day	15% 12% 31% 24% 15% 3	570
	21st to end	Day	19% 13% 28% 25% 13% 3	570
July	1st to 10th	Day	13% 7% 27% 32% 16% 6%	565
	11th to 20th	Day	11% 9% 31% 28% 17% 4	564
	21st to end	Day	15% 6% 28% 30% 17% 5%	620
August	1st to 10th	Day	14% 6% 24% 32% 20% 4	566
	11th to 20th	Day	14% 6% 21% 30% 22% 8%	535
	21st to end	Day	14% 7% 26% 27% 17% 9%	583
September	1st to 10th	Day	17% 8% 25% 22% 18% 11%	530
	11th to 20th	Day	11% 6% 21% 23% 25% 15%	530
	21st to end	Day	19% 10% 26% 23% 16% 6%	530
October	1st to 10th	Day	22% 11% 26% 20% 16% 5%	530
	11th to 20th	Day	23% 14% 32% 21% 9% 1	530
	21st to end	Day	30% 11% 26% 25% 8%	478
November	1st to 10th	Day	26% 15% 25% 23% 10% 1	425
	11th to 20th	Day	27% 12% 35% 18% 5% 2	257
	21st to end	Day	29% 19% 28% 17% 6% 1	220
December	1st to 10th	Day	27% 11% 33% 21% 7%	220
	11th to 20th	Day	27% 17% 36% 17% 3	220
	21st to end	Day	31% 15% 35% 12% 6%	242
January	1st to 10th	Night	26% 10% 33% 26% 5%	840
	11th to 20th	Night	28% 12% 29% 25% 5% 1	840
	21st to end	Night	19% 9% 33% 31% 7% 1	924
February	1st to 10th	Night	19% 9% 38% 29% 5%	840
	11th to 20th	Night	19% 12% 39% 25% 5%	823
	21st to end	Night	22% 11% 32% 31% 4	692
March	1st to 10th	Night	24% 11% 33% 26% 6%	840
	11th to 20th	Night	23% 13% 33% 25% 5%	840
	21st to end	Night	23% 12% 30% 28% 8%	924
April	1st to 10th	Night	27% 11% 31% 27% 5%	840
	11th to 20th	Night	23% 13% 31% 26% 7%	831
	21st to end	Night	25% 11% 28% 28% 7% 1	714
May	1st to 10th	Night	23% 8% 29% 29% 9% 2	420
	11th to 20th	Night	22% 8% 29% 32% 9%	414
	21st to end	Night	16% 6% 34% 32% 12%	455
June	1st to 10th	Night	16% 3 28% 42% 10%	420
	11th to 20th	Night	15% 6% 29% 37% 11% 1	420
	21st to end	Night	19% 4 28% 32% 15% 1	420
July	1st to 10th	Night	8% 5% 27% 45% 16%	410
	11th to 20th	Night	10% 6% 24% 43% 17%	408
	21st to end	Night	13% 3 21% 45% 18% 1	460
August	1st to 10th	Night	12% 4 18% 45% 19% 1	418
	11th to 20th	Night	12% 5% 18% 35% 28% 2	636
	21st to end	Night	13% 5% 23% 37% 19% 2	880
September	1st to 10th	Night	12% 4 25% 38% 20% 1	800
	11th to 20th	Night	16% 5% 17% 34% 25% 3	800
	21st to end	Night	19% 6% 25% 28% 21% 2	800
October	1st to 10th	Night	21% 5% 28% 29% 16% 1	800
	11th to 20th	Night	24% 7% 32% 28% 10% 1	800
	21st to end	Night	26% 9% 28% 30% 7%	880
November	1st to 10th	Night	26% 11% 25% 29% 9% 1	820
	11th to 20th	Night	23% 12% 34% 21% 10%	840
	21st to end	Night	27% 12% 29% 24% 7%	840
December	1st to 10th	Night	29% 12% 30% 23% 5%	840
	11th to 20th	Night	28% 13% 31% 22% 5% 1	840
	21st to end	Night	33% 11% 29% 23% 4	924

Page generated at 2026-08-12 03:37:17. Version ac07

Copyright © 2026 Attilla Danko.