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 2025-12-03.

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	12% 8% 3 7% 5% 4 4 4 6% 8% 39%	127103
	Night	13% 9% 3 7% 4 4 4 3 5% 7% 41%	123821
Transparency	Day	42% 6% 12% 23% 15% 2	130652
	Night	42% 5% 11% 23% 16% 2	128349
Seeing	Day	15% 11% 27% 26% 16% 5%	17153
	Night	17% 10% 30% 30% 13% 1	26963

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	14% 10% 5% 16% 7% 5% 4 3 4 6% 26%	3617
	11th to 20th	Day	10% 9% 6% 11% 7% 8% 8% 5% 6% 5% 25%	3684
	21st to end	Day	17% 16% 7% 9% 5% 3 3 2 3 5% 29%	4488
February	1st to 10th	Day	15% 12% 5% 10% 5% 4 4 3 5% 6% 30%	1801
	11th to 20th	Day	14% 11% 3 8% 7% 5% 4 4 5% 7% 32%	1948
	21st to end	Day	15% 12% 4 8% 5% 4 4 3 4 6% 35%	1650
March	1st to 10th	Day	17% 12% 3 5% 3 3 3 2 3 4 43%	4480
	11th to 20th	Day	8% 9% 3 5% 3 3 4 3 6% 8% 48%	4888
	21st to end	Day	16% 11% 3 9% 5% 5% 5% 4 5% 7% 31%	5861
April	1st to 10th	Day	14% 8% 4 8% 5% 3 5% 3 6% 8% 37%	2400
	11th to 20th	Day	15% 10% 3 7% 4 4 4 3 5% 6% 39%	2374
	21st to end	Day	19% 6% 2 7% 4 5% 5% 4 5% 7% 36%	2400
May	1st to 10th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 10% 31%	5757
	11th to 20th	Day	15% 8% 2 7% 4 4 4 3 5% 8% 39%	6222
	21st to end	Day	11% 7% 3 6% 5% 5% 5% 4 6% 8% 39%	6935
June	1st to 10th	Day	8% 5% 3 5% 4 5% 5% 4 6% 9% 47%	2800
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 5% 9% 49%	2800
	21st to end	Day	6% 6% 2 7% 5% 6% 6% 5% 7% 8% 43%	2800
July	1st to 10th	Day	10% 5% 2 6% 3 4 4 4 6% 10% 44%	6360
	11th to 20th	Day	7% 7% 3 8% 5% 5% 5% 5% 7% 10% 37%	6360
	21st to end	Day	9% 6% 2 7% 5% 5% 6% 5% 8% 11% 36%	6866
August	1st to 10th	Day	6% 4 2 5% 4 4 4 4 7% 10% 50%	4183
	11th to 20th	Day	10% 5% 3 7% 4 4 4 4 6% 9% 43%	3849
	21st to end	Day	5% 4 2 6% 5% 5% 4 4 7% 10% 49%	4312
September	1st to 10th	Day	11% 5% 2 5% 4 4 4 3 6% 6% 51%	2400
	11th to 20th	Day	9% 5% 3 6% 4 4 4 2 5% 7% 52%	2340
	21st to end	Day	9% 5% 3 5% 4 3 4 3 5% 7% 51%	2200
October	1st to 10th	Day	11% 4 2 5% 3 4 3 3 5% 7% 53%	2049
	11th to 20th	Day	15% 7% 2 5% 4 3 4 3 5% 5% 47%	2000
	21st to end	Day	17% 8% 3 6% 5% 3 4 3 4 6% 42%	2105
November	1st to 10th	Day	10% 6% 2 7% 4 3 4 3 4 5% 51%	1852
	11th to 20th	Day	14% 9% 3 8% 3 3 2 4 5% 7% 42%	1890
	21st to end	Day	17% 11% 4 7% 5% 4 3 3 4 5% 38%	1816
December	1st to 10th	Day	8% 9% 3 7% 5% 3 4 3 4 6% 48%	2626
	11th to 20th	Day	14% 11% 6% 10% 5% 5% 4 4 5% 7% 29%	3008
	21st to end	Day	26% 18% 6% 12% 4 3 3 3 4 5% 17%	3982
January	1st to 10th	Night	14% 12% 6% 11% 5% 4 4 3 4 5% 32%	6348
	11th to 20th	Night	12% 14% 5% 12% 6% 5% 7% 4 4 5% 28%	6291
	21st to end	Night	18% 13% 6% 10% 4 4 3 3 3 6% 30%	6490
February	1st to 10th	Night	17% 11% 5% 8% 5% 4 4 4 4 6% 32%	2596
	11th to 20th	Night	14% 11% 4 8% 5% 4 4 3 4 5% 38%	2338
	21st to end	Night	17% 9% 3 7% 5% 3 4 3 5% 5% 40%	1980
March	1st to 10th	Night	19% 12% 3 6% 3 3 3 2 4 5% 40%	5373
	11th to 20th	Night	11% 7% 2 5% 3 3 4 3 4 6% 51%	5100
	21st to end	Night	18% 9% 3 8% 5% 5% 5% 4 5% 6% 32%	5066
April	1st to 10th	Night	13% 8% 3 6% 5% 4 4 3 5% 7% 44%	2000
	11th to 20th	Night	15% 7% 2 6% 3 4 3 4 4 8% 45%	1975
	21st to end	Night	19% 7% 3 7% 4 3 4 3 5% 6% 38%	2000
May	1st to 10th	Night	18% 9% 3 7% 4 4 4 4 6% 7% 35%	4134
	11th to 20th	Night	14% 6% 2 5% 4 4 4 4 6% 9% 41%	3684
	21st to end	Night	13% 7% 3 7% 4 4 4 4 6% 9% 41%	3942
June	1st to 10th	Night	8% 6% 2 5% 3 3 3 4 5% 10% 51%	1600
	11th to 20th	Night	8% 4 2 6% 4 5% 4 3 5% 10% 49%	1600
	21st to end	Night	6% 5% 2 5% 4 4 5% 4 8% 9% 48%	1600
July	1st to 10th	Night	10% 5% 2 6% 5% 4 4 4 6% 9% 46%	3618
	11th to 20th	Night	5% 5% 2 6% 4 5% 5% 5% 7% 11% 45%	3617
	21st to end	Night	8% 6% 2 7% 4 4 5% 5% 7% 10% 41%	3989
August	1st to 10th	Night	5% 3 1 4 3 4 4 4 7% 9% 55%	2972
	11th to 20th	Night	7% 4 2 5% 4 4 5% 4 6% 8% 50%	3200
	21st to end	Night	5% 3 1 4 3 4 4 4 7% 10% 54%	3586
September	1st to 10th	Night	10% 4 2 5% 3 3 3 4 5% 7% 55%	2000
	11th to 20th	Night	6% 6% 2 5% 3 4 4 3 6% 8% 54%	2040
	21st to end	Night	9% 5% 2 5% 4 3 4 4 4 7% 53%	2200
October	1st to 10th	Night	9% 4 2 4 4 3 3 3 4 6% 57%	2348
	11th to 20th	Night	15% 6% 1 5% 3 3 2 3 4 7% 52%	2400
	21st to end	Night	18% 8% 2 6% 3 3 4 2 5% 5% 43%	2735
November	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 4 6% 53%	2666
	11th to 20th	Night	12% 8% 3 6% 3 3 3 3 5% 6% 48%	2730
	21st to end	Night	17% 10% 3 6% 4 4 4 3 4 5% 40%	2799
December	1st to 10th	Night	12% 9% 3 8% 4 3 3 3 4 5% 46%	4609
	11th to 20th	Night	15% 8% 4 8% 4 4 4 3 5% 6% 36%	5199
	21st to end	Night	26% 18% 5% 10% 5% 4 4 3 4 4 17%	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	59% 5% 9% 17% 8% 1	3618
	11th to 20th	Day	59% 5% 11% 13% 8% 3	3684
	21st to end	Day	60% 4 8% 19% 9% 1	4488
February	1st to 10th	Day	55% 5% 13% 18% 8% 1	1801
	11th to 20th	Day	52% 6% 11% 19% 11% 1	1958
	21st to end	Day	51% 4 10% 20% 12% 2	1650
March	1st to 10th	Day	46% 4 7% 22% 19% 2	4480
	11th to 20th	Day	33% 5% 13% 29% 19% 1	4888
	21st to end	Day	50% 8% 14% 21% 6% 1	5863
April	1st to 10th	Day	44% 7% 13% 22% 13% 1	2400
	11th to 20th	Day	44% 6% 11% 23% 13% 2	2374
	21st to end	Day	46% 8% 13% 20% 12% 2	2400
May	1st to 10th	Day	46% 8% 14% 21% 11% 1	5793
	11th to 20th	Day	43% 7% 11% 24% 13% 2	6226
	21st to end	Day	39% 8% 12% 23% 17% 1	6949
June	1st to 10th	Day	32% 8% 15% 26% 16% 3	2800
	11th to 20th	Day	32% 8% 12% 24% 22% 2	2800
	21st to end	Day	34% 10% 14% 22% 17% 3	2800
July	1st to 10th	Day	34% 6% 12% 29% 17% 2	6291
	11th to 20th	Day	40% 8% 14% 23% 14% 1	6285
	21st to end	Day	38% 8% 18% 26% 9% 1	6671
August	1st to 10th	Day	27% 7% 17% 35% 12% 1	4169
	11th to 20th	Day	36% 7% 14% 28% 14% 1	3885
	21st to end	Day	28% 7% 16% 30% 16% 4	4311
September	1st to 10th	Day	33% 6% 10% 26% 20% 5%	2400
	11th to 20th	Day	32% 5% 10% 26% 24% 4	2340
	21st to end	Day	32% 6% 10% 27% 23% 3	2200
October	1st to 10th	Day	22% 3 8% 19% 44% 4	3335
	11th to 20th	Day	26% 3 9% 23% 35% 5%	3240
	21st to end	Day	38% 4 8% 25% 23% 2	3382
November	1st to 10th	Day	36% 4 12% 25% 19% 4	1852
	11th to 20th	Day	42% 3 11% 25% 15% 4	1890
	21st to end	Day	50% 6% 10% 24% 11%	1816
December	1st to 10th	Day	38% 5% 16% 30% 11%	2626
	11th to 20th	Day	53% 5% 11% 22% 7% 1	3005
	21st to end	Day	70% 5% 11% 10% 4	3982
January	1st to 10th	Night	54% 5% 12% 22% 7%	6348
	11th to 20th	Night	59% 3 9% 14% 11% 3	6291
	21st to end	Night	57% 3 8% 19% 11% 2	6490
February	1st to 10th	Night	54% 4 11% 18% 11% 2	2596
	11th to 20th	Night	48% 4 11% 21% 14% 2	2339
	21st to end	Night	45% 5% 11% 22% 14% 2	1980
March	1st to 10th	Night	48% 4 7% 22% 17% 2	5373
	11th to 20th	Night	34% 5% 8% 29% 19% 4	5100
	21st to end	Night	50% 7% 10% 24% 8% 1	5066
April	1st to 10th	Night	40% 5% 11% 25% 15% 3	2000
	11th to 20th	Night	38% 5% 11% 25% 17% 3	1975
	21st to end	Night	47% 5% 10% 20% 15% 3	2000
May	1st to 10th	Night	47% 8% 13% 21% 10% 2	4162
	11th to 20th	Night	38% 7% 12% 27% 13% 3	3685
	21st to end	Night	39% 7% 13% 21% 17% 2	3950
June	1st to 10th	Night	28% 7% 15% 28% 18% 4	1600
	11th to 20th	Night	31% 7% 11% 25% 23% 4	1600
	21st to end	Night	29% 8% 15% 24% 19% 5%	1600
July	1st to 10th	Night	35% 5% 11% 30% 18% 2	3580
	11th to 20th	Night	31% 8% 15% 29% 15% 1	3572
	21st to end	Night	36% 8% 18% 24% 13% 1	3874
August	1st to 10th	Night	23% 8% 17% 34% 17% 1	2962
	11th to 20th	Night	30% 7% 13% 31% 18% 1	3230
	21st to end	Night	24% 7% 17% 32% 16% 4	3586
September	1st to 10th	Night	29% 5% 11% 26% 22% 7%	2000
	11th to 20th	Night	26% 6% 10% 25% 28% 5%	2040
	21st to end	Night	30% 6% 10% 23% 26% 5%	2200
October	1st to 10th	Night	21% 4 6% 20% 45% 4	3842
	11th to 20th	Night	24% 3 9% 23% 37% 4	3899
	21st to end	Night	34% 4 9% 23% 25% 5%	4408
November	1st to 10th	Night	33% 5% 11% 25% 21% 5%	2666
	11th to 20th	Night	37% 3 10% 28% 19% 2	2730
	21st to end	Night	47% 4 10% 23% 15% 1	2799
December	1st to 10th	Night	42% 3 12% 29% 14% 1	4609
	11th to 20th	Night	48% 5% 13% 23% 11%	5201
	21st to end	Night	70% 5% 9% 11% 4	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	23% 19% 35% 17% 6%	300
	11th to 20th	Day	25% 18% 39% 16% 1	300
	21st to end	Day	19% 19% 38% 18% 5%	330
February	1st to 10th	Day	25% 17% 38% 16% 4	300
	11th to 20th	Day	25% 21% 33% 17% 4	300
	21st to end	Day	27% 17% 42% 14% 1	249
March	1st to 10th	Day	21% 18% 42% 17% 2	300
	11th to 20th	Day	27% 20% 32% 16% 4	300
	21st to end	Day	31% 16% 30% 18% 4	496
April	1st to 10th	Day	21% 16% 34% 23% 6%	500
	11th to 20th	Day	22% 18% 29% 21% 9% 1	492
	21st to end	Day	23% 14% 36% 18% 8% 1	500
May	1st to 10th	Day	19% 13% 32% 25% 8% 2	503
	11th to 20th	Day	17% 12% 28% 27% 14% 3	680
	21st to end	Day	12% 9% 31% 29% 13% 7%	789
June	1st to 10th	Day	8% 6% 25% 30% 23% 7%	730
	11th to 20th	Day	9% 5% 24% 33% 23% 6%	730
	21st to end	Day	7% 7% 28% 33% 21% 5%	730
July	1st to 10th	Day	9% 3 15% 30% 27% 15%	721
	11th to 20th	Day	8% 3 20% 35% 25% 10%	718
	21st to end	Day	10% 3 16% 32% 30% 9%	794
August	1st to 10th	Day	8% 2 18% 36% 28% 8%	701
	11th to 20th	Day	8% 5% 18% 27% 29% 14%	519
	21st to end	Day	7% 2 16% 32% 34% 10%	583
September	1st to 10th	Day	11% 5% 18% 29% 27% 10%	530
	11th to 20th	Day	9% 3 11% 30% 32% 14%	530
	21st to end	Day	11% 9% 22% 22% 25% 10%	530
October	1st to 10th	Day	10% 9% 24% 31% 22% 4	379
	11th to 20th	Day	17% 19% 27% 25% 10% 2	330
	21st to end	Day	28% 16% 33% 18% 6%	363
November	1st to 10th	Day	11% 18% 35% 22% 11% 3	330
	11th to 20th	Day	20% 19% 34% 18% 8% 2	330
	21st to end	Day	25% 14% 34% 21% 6%	330
December	1st to 10th	Day	14% 24% 38% 17% 5% 3	306
	11th to 20th	Day	20% 19% 37% 21% 3	300
	21st to end	Day	29% 19% 34% 14% 3	330
January	1st to 10th	Night	18% 16% 39% 23% 5%	1000
	11th to 20th	Night	21% 15% 38% 22% 5%	1000
	21st to end	Night	19% 16% 36% 26% 3	1100
February	1st to 10th	Night	23% 14% 36% 24% 3	996
	11th to 20th	Night	21% 15% 36% 25% 2	783
	21st to end	Night	23% 14% 36% 25% 2	660
March	1st to 10th	Night	22% 14% 36% 25% 3	800
	11th to 20th	Night	21% 12% 34% 27% 6%	655
	21st to end	Night	27% 11% 32% 24% 7%	660
April	1st to 10th	Night	19% 16% 34% 27% 5%	600
	11th to 20th	Night	18% 12% 36% 25% 9% 1	593
	21st to end	Night	24% 8% 34% 27% 6% 1	600
May	1st to 10th	Night	18% 6% 36% 30% 9% 1	600
	11th to 20th	Night	16% 7% 26% 36% 13% 1	591
	21st to end	Night	12% 4 28% 35% 19% 2	650
June	1st to 10th	Night	10% 3 21% 45% 20%	600
	11th to 20th	Night	9% 3 24% 44% 20% 1	600
	21st to end	Night	7% 3 24% 42% 22% 2	600
July	1st to 10th	Night	11% 3 9% 35% 37% 5%	585
	11th to 20th	Night	5% 2 17% 42% 31% 3	582
	21st to end	Night	6% 2 14% 45% 30% 2	657
August	1st to 10th	Night	8% 2 13% 41% 33% 3	597
	11th to 20th	Night	7% 2 12% 41% 33% 5%	594
	21st to end	Night	6% 1 14% 38% 37% 3	660
September	1st to 10th	Night	10% 4 17% 38% 29% 4	600
	11th to 20th	Night	8% 3 16% 36% 33% 5%	640
	21st to end	Night	11% 7% 21% 31% 27% 4	800
October	1st to 10th	Night	11% 4 26% 33% 23% 3	800
	11th to 20th	Night	17% 11% 30% 30% 12% 1	800
	21st to end	Night	22% 11% 35% 25% 6%	880
November	1st to 10th	Night	13% 10% 40% 28% 9%	820
	11th to 20th	Night	17% 15% 37% 24% 7% 1	840
	21st to end	Night	23% 14% 34% 24% 5%	909
December	1st to 10th	Night	22% 12% 37% 23% 6% 1	1011
	11th to 20th	Night	23% 10% 33% 27% 6%	1000
	21st to end	Night	28% 16% 32% 22% 3	1100

Page generated at 2025-11-30 00:00:57. Version ac07

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