Chatham
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Chatham clear sky chart from 2005-11-06 to 2026-08-30.

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% 3 8% 5% 5% 5% 4 6% 8% 34%	143606
	Night	14% 10% 4 9% 5% 5% 4 4 5% 7% 34%	138572
Transparency	Day	46% 6% 13% 21% 12% 1	147204
	Night	48% 6% 12% 20% 13% 1	143046
Seeing	Day	17% 11% 29% 25% 14% 5%	17570
	Night	19% 10% 28% 28% 13% 2	26922

By Month

Percentage Cloud Cover SE (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	13% 14% 7% 16% 7% 5% 5% 4 5% 5% 19%	4691
	11th to 20th	Day	12% 19% 8% 16% 7% 7% 5% 4 4 6% 13%	5147
	21st to end	Day	17% 15% 6% 12% 6% 5% 5% 4 5% 6% 18%	5780
February	1st to 10th	Day	14% 17% 4 12% 7% 5% 5% 3 4 6% 23%	1890
	11th to 20th	Day	13% 12% 5% 12% 7% 6% 6% 4 5% 6% 24%	1917
	21st to end	Day	15% 13% 4 11% 7% 5% 5% 4 5% 5% 25%	1730
March	1st to 10th	Day	16% 12% 6% 10% 6% 4 4 3 5% 5% 29%	4832
	11th to 20th	Day	12% 12% 4 10% 5% 5% 4 3 5% 7% 34%	5090
	21st to end	Day	17% 13% 5% 10% 5% 4 4 3 5% 6% 28%	5586
April	1st to 10th	Day	15% 11% 3 8% 5% 4 5% 4 7% 7% 31%	2370
	11th to 20th	Day	17% 10% 3 7% 5% 4 5% 5% 5% 7% 34%	2702
	21st to end	Day	18% 8% 3 8% 5% 4 4 4 5% 7% 34%	2730
May	1st to 10th	Day	17% 9% 3 7% 5% 4 4 3 6% 8% 34%	7616
	11th to 20th	Day	13% 8% 3 7% 5% 5% 5% 4 6% 9% 35%	7626
	21st to end	Day	14% 6% 3 7% 4 4 4 4 6% 8% 41%	8588
June	1st to 10th	Day	10% 7% 2 6% 4 5% 6% 5% 6% 9% 40%	2977
	11th to 20th	Day	9% 6% 2 7% 5% 4 5% 5% 7% 9% 41%	3150
	21st to end	Day	9% 6% 2 6% 4 5% 5% 5% 8% 10% 41%	3150
July	1st to 10th	Day	10% 6% 2 6% 4 4 5% 4 7% 9% 43%	8368
	11th to 20th	Day	5% 4 2 5% 4 4 5% 4 7% 11% 48%	8004
	21st to end	Day	7% 5% 2 6% 5% 5% 5% 4 7% 9% 44%	8340
August	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 7% 9% 42%	4364
	11th to 20th	Day	8% 6% 3 6% 4 5% 6% 4 8% 12% 37%	4304
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 4 7% 9% 42%	4792
September	1st to 10th	Day	10% 6% 2 6% 4 5% 5% 5% 5% 8% 44%	2266
	11th to 20th	Day	9% 5% 1 5% 4 4 5% 3 6% 10% 47%	2200
	21st to end	Day	17% 6% 3 7% 4 5% 5% 4 5% 7% 38%	2200
October	1st to 10th	Day	12% 6% 2 6% 5% 4 5% 4 6% 7% 43%	2089
	11th to 20th	Day	16% 8% 3 7% 4 5% 5% 5% 6% 7% 35%	2000
	21st to end	Day	19% 9% 3 7% 4 5% 5% 4 6% 7% 31%	2120
November	1st to 10th	Day	13% 8% 4 8% 5% 5% 5% 4 5% 7% 36%	1852
	11th to 20th	Day	14% 12% 3 9% 5% 5% 5% 4 5% 7% 31%	1848
	21st to end	Day	17% 13% 5% 11% 7% 5% 5% 4 5% 6% 22%	1680
December	1st to 10th	Day	16% 13% 5% 13% 6% 5% 6% 5% 5% 6% 20%	2680
	11th to 20th	Day	21% 17% 5% 10% 7% 5% 5% 5% 6% 5% 15%	3070
	21st to end	Day	25% 19% 6% 12% 5% 4 4 3 4 5% 12%	3857
January	1st to 10th	Night	14% 15% 6% 13% 6% 5% 5% 4 5% 5% 22%	8232
	11th to 20th	Night	9% 17% 7% 14% 8% 7% 5% 4 5% 6% 17%	7786
	21st to end	Night	16% 15% 7% 10% 5% 5% 4 3 4 5% 26%	8401
February	1st to 10th	Night	16% 16% 5% 11% 6% 5% 4 3 4 5% 25%	2730
	11th to 20th	Night	15% 14% 5% 10% 6% 5% 5% 3 6% 6% 25%	2587
	21st to end	Night	18% 12% 4 9% 6% 5% 4 4 5% 5% 27%	2076
March	1st to 10th	Night	16% 12% 4 8% 5% 4 4 3 4 6% 34%	5240
	11th to 20th	Night	9% 7% 3 8% 6% 5% 5% 4 6% 8% 39%	5090
	21st to end	Night	16% 10% 4 8% 5% 4 4 3 5% 5% 37%	5588
April	1st to 10th	Night	16% 12% 4 7% 5% 4 4 4 5% 6% 33%	2265
	11th to 20th	Night	16% 7% 3 8% 5% 4 4 4 5% 8% 35%	1867
	21st to end	Night	16% 9% 2 7% 4 4 5% 4 5% 7% 38%	1890
May	1st to 10th	Night	19% 8% 3 8% 5% 5% 5% 4 5% 7% 31%	5237
	11th to 20th	Night	13% 9% 3 8% 5% 5% 4 3 6% 6% 38%	5234
	21st to end	Night	16% 6% 2 6% 4 4 4 4 6% 7% 42%	5546
June	1st to 10th	Night	11% 5% 2 6% 4 5% 4 3 6% 9% 46%	1637
	11th to 20th	Night	9% 5% 2 5% 5% 5% 6% 4 6% 10% 44%	1470
	21st to end	Night	9% 5% 2 6% 5% 4 4 4 6% 8% 47%	1470
July	1st to 10th	Night	7% 5% 2 6% 4 4 5% 4 7% 11% 46%	4587
	11th to 20th	Night	6% 4 2 6% 4 3 4 4 7% 11% 50%	4952
	21st to end	Night	8% 5% 2 6% 4 4 5% 4 6% 7% 50%	5736
August	1st to 10th	Night	11% 6% 2 6% 3 4 4 4 5% 8% 47%	3005
	11th to 20th	Night	8% 7% 2 7% 4 4 4 4 5% 7% 47%	2965
	21st to end	Night	7% 5% 2 7% 5% 5% 5% 4 6% 8% 47%	3304
September	1st to 10th	Night	9% 6% 2 6% 5% 4 4 4 5% 8% 47%	2109
	11th to 20th	Night	8% 5% 2 4 3 4 3 3 5% 8% 57%	2200
	21st to end	Night	13% 8% 3 7% 5% 4 4 3 5% 6% 41%	2200
October	1st to 10th	Night	12% 6% 2 7% 3 4 3 3 4 7% 48%	2310
	11th to 20th	Night	17% 8% 3 8% 5% 4 4 3 5% 6% 37%	2400
	21st to end	Night	20% 8% 3 8% 5% 5% 4 3 5% 6% 33%	2716
November	1st to 10th	Night	13% 10% 2 7% 5% 5% 4 4 5% 6% 41%	2666
	11th to 20th	Night	16% 10% 3 9% 6% 5% 5% 4 6% 7% 29%	2769
	21st to end	Night	21% 14% 4 9% 4 4 3 3 5% 5% 26%	2940
December	1st to 10th	Night	15% 14% 5% 11% 7% 7% 6% 4 5% 6% 20%	4720
	11th to 20th	Night	17% 17% 5% 10% 6% 5% 4 4 5% 5% 20%	5345
	21st to end	Night	24% 18% 6% 11% 5% 5% 4 3 4 5% 14%	7302

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% 7% 10% 12% 6% 1	4692
	11th to 20th	Day	72% 5% 7% 7% 8%	5147
	21st to end	Day	66% 4 11% 12% 7%	5780
February	1st to 10th	Day	62% 5% 10% 16% 7%	1890
	11th to 20th	Day	59% 6% 11% 16% 7% 1	1926
	21st to end	Day	58% 6% 12% 16% 7% 2	1730
March	1st to 10th	Day	55% 5% 13% 18% 7% 1	4832
	11th to 20th	Day	48% 5% 17% 20% 8% 1	5090
	21st to end	Day	55% 5% 13% 16% 10% 1	5586
April	1st to 10th	Day	49% 7% 13% 18% 12% 1	2370
	11th to 20th	Day	48% 6% 12% 20% 13% 2	2702
	21st to end	Day	48% 6% 13% 20% 11% 2	2730
May	1st to 10th	Day	47% 6% 12% 21% 10% 3	7651
	11th to 20th	Day	43% 8% 14% 22% 11% 1	7630
	21st to end	Day	39% 6% 11% 19% 20% 5%	8603
June	1st to 10th	Day	37% 8% 12% 27% 14% 2	2977
	11th to 20th	Day	37% 9% 14% 23% 16% 2	3150
	21st to end	Day	35% 8% 16% 24% 16% 2	3150
July	1st to 10th	Day	36% 6% 12% 29% 16% 1	8299
	11th to 20th	Day	28% 5% 13% 34% 19% 1	7929
	21st to end	Day	33% 6% 15% 22% 22% 2	8156
August	1st to 10th	Day	36% 7% 16% 26% 15% 1	4351
	11th to 20th	Day	36% 8% 14% 34% 7% 1	4343
	21st to end	Day	35% 7% 15% 31% 11% 1	4791
September	1st to 10th	Day	36% 7% 17% 25% 14% 1	2266
	11th to 20th	Day	31% 7% 15% 31% 15% 1	2200
	21st to end	Day	43% 7% 13% 23% 13% 1	2200
October	1st to 10th	Day	30% 6% 13% 29% 20% 1	3401
	11th to 20th	Day	38% 7% 12% 26% 17%	3240
	21st to end	Day	42% 6% 15% 23% 13% 1	3409
November	1st to 10th	Day	47% 7% 11% 23% 10% 2	1852
	11th to 20th	Day	51% 6% 15% 17% 10% 1	1848
	21st to end	Day	60% 8% 12% 16% 5%	1680
December	1st to 10th	Day	63% 7% 16% 11% 4	2680
	11th to 20th	Day	67% 7% 13% 9% 4	3066
	21st to end	Day	73% 6% 8% 8% 5%	3857
January	1st to 10th	Night	62% 6% 11% 14% 7% 1	8233
	11th to 20th	Night	65% 6% 10% 10% 7% 1	7786
	21st to end	Night	61% 4 9% 17% 9%	8401
February	1st to 10th	Night	61% 6% 10% 16% 5% 1	2730
	11th to 20th	Night	58% 5% 12% 16% 9% 1	2589
	21st to end	Night	57% 6% 11% 17% 8% 1	2076
March	1st to 10th	Night	51% 5% 14% 19% 11% 1	5240
	11th to 20th	Night	40% 7% 15% 24% 13% 1	5090
	21st to end	Night	49% 5% 11% 18% 16% 1	5588
April	1st to 10th	Night	51% 6% 10% 18% 12% 2	2265
	11th to 20th	Night	46% 6% 11% 20% 15% 2	1867
	21st to end	Night	43% 7% 11% 21% 15% 3	1890
May	1st to 10th	Night	52% 5% 10% 14% 14% 5%	5264
	11th to 20th	Night	45% 7% 10% 23% 12% 3	5235
	21st to end	Night	41% 5% 10% 21% 19% 4	5555
June	1st to 10th	Night	36% 6% 11% 24% 19% 4	1637
	11th to 20th	Night	34% 7% 14% 26% 16% 2	1470
	21st to end	Night	34% 5% 14% 25% 21% 1	1470
July	1st to 10th	Night	32% 6% 14% 28% 17% 3	4549
	11th to 20th	Night	28% 5% 12% 29% 24% 2	4907
	21st to end	Night	31% 6% 12% 21% 27% 2	5606
August	1st to 10th	Night	35% 6% 13% 26% 17% 2	2994
	11th to 20th	Night	35% 7% 12% 30% 15% 1	2992
	21st to end	Night	34% 7% 12% 30% 16% 1	3304
September	1st to 10th	Night	34% 7% 12% 27% 16% 3	2109
	11th to 20th	Night	27% 6% 13% 33% 20% 2	2200
	21st to end	Night	42% 7% 12% 24% 14% 2	2200
October	1st to 10th	Night	26% 5% 9% 30% 28% 1	3778
	11th to 20th	Night	45% 4 10% 21% 19%	3899
	21st to end	Night	40% 6% 13% 27% 14% 1	4377
November	1st to 10th	Night	43% 5% 12% 24% 14% 1	2666
	11th to 20th	Night	51% 7% 14% 18% 10% 1	2769
	21st to end	Night	59% 6% 13% 17% 6%	2940
December	1st to 10th	Night	61% 8% 15% 13% 3	4720
	11th to 20th	Night	63% 6% 13% 12% 5%	5348
	21st to end	Night	72% 6% 10% 9% 3	7302

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	21% 19% 40% 13% 5% 1	330
	11th to 20th	Day	25% 20% 40% 10% 4 1	330
	21st to end	Day	26% 17% 31% 22% 4 1	363
February	1st to 10th	Day	21% 18% 36% 20% 5% 1	330
	11th to 20th	Day	21% 19% 35% 21% 3	330
	21st to end	Day	23% 21% 41% 14% 1	273
March	1st to 10th	Day	21% 19% 37% 18% 4	439
	11th to 20th	Day	22% 15% 33% 24% 5% 1	540
	21st to end	Day	25% 14% 35% 21% 4 1	594
April	1st to 10th	Day	19% 14% 38% 22% 7% 1	540
	11th to 20th	Day	24% 17% 30% 15% 11% 4	532
	21st to end	Day	23% 10% 34% 20% 11% 2	540
May	1st to 10th	Day	20% 12% 34% 22% 9% 3	543
	11th to 20th	Day	15% 10% 32% 25% 15% 4	567
	21st to end	Day	14% 6% 28% 28% 17% 7%	617
June	1st to 10th	Day	11% 5% 31% 33% 15% 4	607
	11th to 20th	Day	12% 9% 29% 33% 15% 2	780
	21st to end	Day	12% 5% 31% 33% 15% 4	780
July	1st to 10th	Day	10% 4 23% 33% 22% 9%	771
	11th to 20th	Day	11% 5% 26% 28% 22% 8%	674
	21st to end	Day	9% 2 23% 30% 27% 9%	620
August	1st to 10th	Day	10% 4 19% 31% 24% 12%	566
	11th to 20th	Day	10% 4 19% 33% 21% 11%	559
	21st to end	Day	9% 4 19% 33% 26% 10%	619
September	1st to 10th	Day	13% 5% 23% 28% 20% 12%	530
	11th to 20th	Day	10% 3 17% 27% 30% 13%	530
	21st to end	Day	18% 8% 22% 26% 16% 9%	530
October	1st to 10th	Day	15% 8% 24% 26% 21% 6%	530
	11th to 20th	Day	17% 14% 33% 25% 8% 3	530
	21st to end	Day	23% 16% 35% 18% 7% 1	503
November	1st to 10th	Day	17% 16% 33% 24% 8% 2	330
	11th to 20th	Day	22% 22% 28% 20% 6% 1	308
	21st to end	Day	22% 20% 33% 20% 4	220
December	1st to 10th	Day	24% 20% 31% 17% 8%	220
	11th to 20th	Day	27% 23% 30% 15% 4	220
	21st to end	Day	30% 19% 32% 15% 4	275
January	1st to 10th	Night	24% 15% 36% 19% 5% 1	1050
	11th to 20th	Night	26% 17% 33% 19% 4	881
	21st to end	Night	22% 14% 34% 25% 6%	924
February	1st to 10th	Night	25% 13% 33% 25% 4	840
	11th to 20th	Night	23% 14% 33% 24% 6%	823
	21st to end	Night	23% 15% 37% 22% 2	692
March	1st to 10th	Night	20% 13% 36% 25% 6%	840
	11th to 20th	Night	18% 13% 40% 23% 6% 1	840
	21st to end	Night	23% 11% 34% 24% 8%	924
April	1st to 10th	Night	22% 13% 32% 27% 5% 1	819
	11th to 20th	Night	18% 10% 35% 27% 8% 1	623
	21st to end	Night	19% 9% 31% 32% 9%	630
May	1st to 10th	Night	20% 8% 28% 32% 12% 1	630
	11th to 20th	Night	17% 5% 28% 34% 15% 2	621
	21st to end	Night	12% 5% 24% 37% 21% 2	610
June	1st to 10th	Night	13% 3 26% 41% 16%	420
	11th to 20th	Night	12% 3 26% 45% 14% 1	420
	21st to end	Night	11% 2 25% 45% 16%	420
July	1st to 10th	Night	8% 2 15% 41% 31% 4	572
	11th to 20th	Night	10% 3 18% 38% 27% 4	612
	21st to end	Night	7% 2 16% 44% 27% 3	690
August	1st to 10th	Night	10% 1 11% 41% 31% 6%	627
	11th to 20th	Night	7% 2 14% 38% 33% 5%	624
	21st to end	Night	9% 2 16% 36% 32% 4	687
September	1st to 10th	Night	12% 5% 17% 31% 30% 5%	777
	11th to 20th	Night	9% 5% 16% 31% 34% 6%	800
	21st to end	Night	17% 5% 21% 31% 19% 6%	800
October	1st to 10th	Night	14% 7% 24% 31% 20% 4	800
	11th to 20th	Night	20% 15% 27% 26% 12% 2	800
	21st to end	Night	24% 13% 30% 27% 6%	880
November	1st to 10th	Night	18% 12% 31% 26% 12% 1	820
	11th to 20th	Night	22% 18% 31% 21% 7% 1	840
	21st to end	Night	28% 15% 31% 20% 6%	840
December	1st to 10th	Night	26% 17% 33% 18% 5% 1	840
	11th to 20th	Night	24% 17% 35% 20% 3	840
	21st to end	Night	31% 13% 31% 23% 2	1066

Page generated at 2026-08-27 04:07:13. Version ac07

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