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 2025-04-26.

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	14% 10% 4 8% 5% 5% 5% 4 6% 7% 33%	113629
	Night	14% 11% 4 9% 5% 5% 4 4 5% 7% 33%	114948
Transparency	Day	47% 7% 13% 20% 11% 1	117271
	Night	49% 6% 12% 20% 12% 1	119445
Seeing	Day	17% 10% 29% 25% 14% 5%	15700
	Night	19% 10% 28% 28% 13% 2	25283

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	12% 14% 6% 17% 7% 5% 5% 4 5% 5% 20%	3617
	11th to 20th	Day	13% 18% 7% 16% 7% 7% 5% 4 5% 6% 12%	3953
	21st to end	Day	21% 16% 7% 12% 6% 4 5% 4 4 5% 16%	4466
February	1st to 10th	Day	15% 18% 4 12% 7% 5% 5% 3 4 6% 23%	1800
	11th to 20th	Day	13% 13% 5% 12% 7% 5% 6% 4 6% 6% 24%	1823
	21st to end	Day	16% 13% 4 10% 7% 5% 5% 4 5% 5% 26%	1650
March	1st to 10th	Day	16% 12% 6% 10% 6% 4 4 3 5% 5% 29%	4727
	11th to 20th	Day	11% 11% 4 10% 5% 5% 4 3 5% 7% 34%	4980
	21st to end	Day	17% 13% 4 10% 5% 4 4 3 5% 6% 28%	5465
April	1st to 10th	Day	15% 11% 3 8% 5% 4 5% 4 7% 7% 31%	2257
	11th to 20th	Day	17% 10% 3 6% 5% 4 5% 5% 5% 7% 34%	2572
	21st to end	Day	18% 9% 3 8% 5% 4 4 4 5% 7% 34%	2535
May	1st to 10th	Day	17% 8% 3 7% 5% 4 4 3 6% 7% 36%	4082
	11th to 20th	Day	17% 7% 3 6% 5% 5% 4 4 6% 9% 33%	4081
	21st to end	Day	9% 5% 2 7% 5% 4 4 4 6% 8% 46%	4589
June	1st to 10th	Day	10% 7% 3 7% 4 5% 6% 5% 7% 9% 38%	2693
	11th to 20th	Day	9% 6% 2 6% 5% 4 5% 5% 7% 9% 42%	2850
	21st to end	Day	10% 6% 2 6% 4 5% 5% 5% 8% 9% 40%	2850
July	1st to 10th	Day	9% 6% 2 7% 4 5% 5% 5% 7% 9% 42%	4486
	11th to 20th	Day	7% 4 1 5% 4 4 5% 4 8% 10% 47%	4290
	21st to end	Day	6% 4 2 6% 6% 6% 6% 5% 8% 9% 42%	4466
August	1st to 10th	Day	12% 6% 2 6% 4 4 5% 4 7% 10% 41%	4104
	11th to 20th	Day	8% 6% 3 6% 4 5% 6% 4 8% 12% 38%	4044
	21st to end	Day	8% 5% 2 6% 5% 5% 6% 4 7% 9% 42%	4532
September	1st to 10th	Day	10% 6% 2 6% 5% 5% 5% 5% 5% 8% 43%	2153
	11th to 20th	Day	9% 5% 1 5% 4 4 5% 4 6% 10% 46%	2090
	21st to end	Day	17% 6% 3 7% 4 5% 5% 4 5% 7% 37%	2090
October	1st to 10th	Day	12% 6% 3 7% 5% 4 5% 4 6% 7% 41%	1985
	11th to 20th	Day	16% 8% 3 7% 5% 5% 5% 4 6% 7% 34%	1900
	21st to end	Day	19% 10% 3 7% 4 5% 5% 4 6% 7% 30%	2014
November	1st to 10th	Day	12% 8% 4 8% 5% 5% 5% 5% 5% 7% 36%	1762
	11th to 20th	Day	15% 12% 3 8% 5% 4 5% 4 5% 7% 31%	1760
	21st to end	Day	17% 13% 5% 11% 7% 5% 5% 4 5% 6% 22%	1600
December	1st to 10th	Day	16% 13% 5% 13% 6% 6% 6% 5% 5% 6% 20%	2600
	11th to 20th	Day	22% 17% 5% 9% 7% 5% 5% 5% 6% 5% 15%	2990
	21st to end	Day	25% 19% 6% 12% 5% 4 5% 3 4 5% 12%	3773
January	1st to 10th	Night	13% 14% 6% 12% 6% 5% 5% 4 5% 5% 24%	6348
	11th to 20th	Night	11% 17% 7% 14% 7% 6% 5% 4 6% 7% 15%	5978
	21st to end	Night	20% 16% 7% 11% 5% 4 3 3 4 4 24%	6490
February	1st to 10th	Night	16% 16% 5% 11% 6% 5% 4 4 4 5% 24%	2600
	11th to 20th	Night	16% 14% 5% 10% 5% 5% 5% 3 6% 6% 26%	2461
	21st to end	Night	19% 12% 4 9% 6% 5% 4 4 5% 5% 27%	1980
March	1st to 10th	Night	16% 12% 4 8% 5% 4 4 3 4 6% 34%	5125
	11th to 20th	Night	8% 7% 3 8% 5% 5% 5% 4 6% 8% 40%	4980
	21st to end	Night	16% 10% 4 8% 5% 4 4 3 5% 5% 37%	5467
April	1st to 10th	Night	16% 12% 4 7% 5% 4 4 4 5% 7% 34%	2157
	11th to 20th	Night	17% 7% 3 8% 5% 4 4 3 5% 8% 36%	1777
	21st to end	Night	16% 9% 2 7% 4 4 5% 4 5% 7% 36%	1761
May	1st to 10th	Night	18% 7% 2 8% 6% 5% 4 4 6% 7% 34%	2813
	11th to 20th	Night	12% 8% 3 8% 5% 5% 5% 4 6% 8% 34%	2812
	21st to end	Night	10% 5% 2 6% 4 4 4 4 6% 7% 49%	2962
June	1st to 10th	Night	11% 5% 2 6% 4 5% 4 3 6% 9% 45%	1481
	11th to 20th	Night	9% 5% 2 5% 5% 5% 6% 4 6% 9% 44%	1330
	21st to end	Night	10% 5% 2 6% 5% 4 3 4 7% 8% 47%	1330
July	1st to 10th	Night	8% 4 2 5% 4 3 4 3 6% 12% 50%	2465
	11th to 20th	Night	8% 4 1 5% 4 3 4 3 7% 10% 51%	2671
	21st to end	Night	5% 4 2 6% 4 5% 6% 5% 8% 8% 48%	3087
August	1st to 10th	Night	12% 6% 2 6% 3 4 4 4 5% 8% 47%	2825
	11th to 20th	Night	8% 7% 2 6% 4 4 5% 4 5% 7% 47%	2785
	21st to end	Night	7% 5% 2 7% 5% 5% 5% 4 6% 8% 47%	3124
September	1st to 10th	Night	9% 6% 2 5% 5% 4 4 4 6% 8% 47%	2003
	11th to 20th	Night	8% 5% 2 4 3 4 3 3 5% 7% 55%	2090
	21st to end	Night	13% 8% 3 7% 5% 4 4 3 5% 6% 41%	2090
October	1st to 10th	Night	12% 6% 2 7% 3 4 4 3 4 7% 47%	2194
	11th to 20th	Night	17% 8% 3 7% 5% 4 4 3 5% 6% 36%	2280
	21st to end	Night	21% 8% 3 8% 5% 5% 4 4 5% 6% 31%	2580
November	1st to 10th	Night	13% 9% 2 7% 5% 5% 4 4 5% 6% 41%	2536
	11th to 20th	Night	17% 11% 3 9% 6% 5% 5% 4 5% 7% 29%	2637
	21st to end	Night	21% 14% 5% 9% 4 4 3 4 5% 5% 27%	2800
December	1st to 10th	Night	15% 14% 5% 11% 7% 7% 6% 4 5% 6% 19%	4580
	11th to 20th	Night	18% 17% 5% 10% 6% 5% 4 4 5% 5% 20%	5205
	21st to end	Night	24% 19% 6% 11% 5% 5% 4 3 4 5% 14%	7144

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	64% 7% 9% 12% 7% 2	3618
	11th to 20th	Day	71% 6% 9% 6% 8%	3953
	21st to end	Day	70% 5% 11% 9% 6%	4466
February	1st to 10th	Day	63% 5% 10% 15% 6%	1800
	11th to 20th	Day	60% 6% 11% 15% 7% 1	1832
	21st to end	Day	57% 6% 11% 16% 7% 2	1650
March	1st to 10th	Day	55% 5% 14% 18% 7% 1	4727
	11th to 20th	Day	47% 6% 17% 20% 8% 1	4980
	21st to end	Day	54% 5% 14% 16% 10% 1	5465
April	1st to 10th	Day	48% 7% 14% 18% 12% 1	2257
	11th to 20th	Day	47% 6% 12% 20% 13% 2	2572
	21st to end	Day	48% 6% 13% 20% 11% 2	2535
May	1st to 10th	Day	44% 8% 13% 23% 11% 2	4120
	11th to 20th	Day	42% 11% 17% 21% 8% 1	4085
	21st to end	Day	34% 8% 13% 19% 19% 7%	4603
June	1st to 10th	Day	38% 9% 13% 26% 13% 2	2693
	11th to 20th	Day	36% 9% 14% 23% 16% 2	2850
	21st to end	Day	35% 8% 16% 23% 16% 2	2850
July	1st to 10th	Day	37% 6% 13% 25% 17% 3	4417
	11th to 20th	Day	29% 6% 14% 33% 16% 1	4215
	21st to end	Day	34% 8% 18% 17% 21% 2	4324
August	1st to 10th	Day	36% 7% 16% 26% 15% 1	4091
	11th to 20th	Day	35% 8% 14% 34% 8% 1	4083
	21st to end	Day	35% 7% 15% 30% 12% 1	4531
September	1st to 10th	Day	36% 8% 17% 24% 14% 1	2153
	11th to 20th	Day	32% 7% 15% 30% 16% 1	2090
	21st to end	Day	43% 7% 14% 23% 12% 1	2090
October	1st to 10th	Day	31% 7% 14% 28% 20% 1	3297
	11th to 20th	Day	39% 7% 12% 25% 17%	3140
	21st to end	Day	42% 6% 16% 24% 12% 1	3303
November	1st to 10th	Day	47% 7% 12% 23% 10% 2	1762
	11th to 20th	Day	51% 6% 15% 17% 10% 1	1760
	21st to end	Day	60% 8% 12% 16% 5%	1600
December	1st to 10th	Day	63% 7% 16% 10% 4	2600
	11th to 20th	Day	68% 7% 13% 9% 4	2986
	21st to end	Day	73% 6% 8% 8% 5%	3773
January	1st to 10th	Night	59% 6% 11% 15% 7% 2	6348
	11th to 20th	Night	66% 8% 11% 10% 5% 1	5978
	21st to end	Night	64% 5% 9% 12% 10%	6490
February	1st to 10th	Night	62% 6% 11% 15% 5% 1	2600
	11th to 20th	Night	57% 5% 12% 17% 9% 1	2463
	21st to end	Night	57% 6% 11% 17% 8% 1	1980
March	1st to 10th	Night	51% 5% 14% 19% 11% 1	5125
	11th to 20th	Night	39% 7% 15% 24% 14% 1	4980
	21st to end	Night	48% 5% 11% 18% 17% 1	5467
April	1st to 10th	Night	50% 6% 11% 19% 12% 3	2157
	11th to 20th	Night	45% 6% 11% 20% 16% 2	1777
	21st to end	Night	44% 7% 11% 19% 15% 4	1761
May	1st to 10th	Night	47% 7% 9% 18% 15% 4	2840
	11th to 20th	Night	43% 10% 12% 22% 11% 2	2813
	21st to end	Night	32% 8% 12% 22% 20% 6%	2969
June	1st to 10th	Night	36% 7% 11% 26% 17% 3	1481
	11th to 20th	Night	34% 7% 13% 25% 17% 3	1330
	21st to end	Night	34% 5% 14% 24% 21% 1	1330
July	1st to 10th	Night	29% 6% 12% 29% 19% 6%	2425
	11th to 20th	Night	28% 5% 14% 32% 19% 2	2623
	21st to end	Night	29% 9% 16% 21% 23% 2	2988
August	1st to 10th	Night	35% 7% 13% 27% 16% 2	2814
	11th to 20th	Night	34% 7% 11% 31% 15% 1	2812
	21st to end	Night	34% 7% 12% 29% 17% 1	3124
September	1st to 10th	Night	34% 8% 13% 26% 17% 3	2003
	11th to 20th	Night	29% 6% 13% 31% 21% 2	2090
	21st to end	Night	42% 7% 12% 25% 14% 1	2090
October	1st to 10th	Night	27% 5% 10% 29% 28% 1	3662
	11th to 20th	Night	46% 4 11% 21% 18%	3779
	21st to end	Night	41% 6% 13% 27% 13% 1	4241
November	1st to 10th	Night	43% 6% 12% 24% 14% 1	2536
	11th to 20th	Night	52% 7% 13% 17% 10% 1	2637
	21st to end	Night	58% 6% 13% 17% 6%	2800
December	1st to 10th	Night	61% 8% 15% 12% 3	4580
	11th to 20th	Night	64% 7% 13% 12% 5%	5208
	21st to end	Night	72% 6% 10% 9% 3	7144

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% 39% 14% 6% 1	300
	11th to 20th	Day	24% 17% 43% 11% 4 1	300
	21st to end	Day	27% 16% 32% 21% 4 1	330
February	1st to 10th	Day	22% 20% 36% 17% 4	300
	11th to 20th	Day	21% 19% 37% 20% 3	300
	21st to end	Day	25% 20% 40% 14% 1	249
March	1st to 10th	Day	21% 18% 37% 19% 5%	404
	11th to 20th	Day	20% 15% 33% 25% 5% 1	500
	21st to end	Day	25% 13% 34% 22% 4 1	550
April	1st to 10th	Day	17% 13% 38% 23% 7% 1	500
	11th to 20th	Day	24% 16% 30% 16% 11% 4	492
	21st to end	Day	23% 10% 35% 19% 11% 2	480
May	1st to 10th	Day	19% 11% 33% 23% 11% 3	463
	11th to 20th	Day	16% 9% 30% 25% 16% 4	487
	21st to end	Day	12% 5% 26% 30% 19% 8%	529
June	1st to 10th	Day	11% 4 32% 33% 16% 4	523
	11th to 20th	Day	11% 8% 28% 34% 17% 3	680
	21st to end	Day	13% 6% 32% 31% 15% 4	680
July	1st to 10th	Day	9% 4 23% 32% 23% 10%	671
	11th to 20th	Day	11% 5% 26% 28% 21% 9%	583
	21st to end	Day	9% 2 23% 30% 28% 9%	532
August	1st to 10th	Day	10% 4 19% 30% 24% 13%	486
	11th to 20th	Day	10% 4 19% 33% 22% 12%	479
	21st to end	Day	9% 3 18% 34% 26% 10%	539
September	1st to 10th	Day	13% 4 22% 29% 20% 11%	490
	11th to 20th	Day	10% 3 18% 28% 30% 11%	490
	21st to end	Day	18% 9% 24% 26% 15% 9%	490
October	1st to 10th	Day	15% 9% 24% 25% 21% 6%	490
	11th to 20th	Day	18% 15% 31% 25% 8% 3	490
	21st to end	Day	23% 16% 35% 18% 7% 1	463
November	1st to 10th	Day	15% 17% 33% 25% 9% 2	300
	11th to 20th	Day	23% 21% 29% 20% 6% 1	280
	21st to end	Day	21% 21% 34% 21% 4 1	200
December	1st to 10th	Day	23% 19% 32% 17% 9% 1	200
	11th to 20th	Day	29% 21% 32% 16% 4	200
	21st to end	Day	30% 19% 32% 15% 4	250
January	1st to 10th	Night	24% 15% 36% 19% 6% 1	1000
	11th to 20th	Night	27% 16% 33% 20% 4	839
	21st to end	Night	23% 13% 33% 25% 6%	880
February	1st to 10th	Night	26% 13% 34% 24% 4	800
	11th to 20th	Night	23% 14% 32% 24% 6%	783
	21st to end	Night	23% 15% 37% 22% 2	660
March	1st to 10th	Night	21% 13% 35% 25% 6%	800
	11th to 20th	Night	17% 13% 40% 23% 6% 1	800
	21st to end	Night	23% 10% 34% 24% 9%	880
April	1st to 10th	Night	22% 13% 32% 28% 5% 1	780
	11th to 20th	Night	19% 10% 35% 27% 8% 1	593
	21st to end	Night	19% 8% 32% 32% 8%	587
May	1st to 10th	Night	19% 7% 27% 33% 13% 1	570
	11th to 20th	Night	17% 5% 26% 35% 16% 2	561
	21st to end	Night	11% 4 23% 37% 23% 2	551
June	1st to 10th	Night	14% 3 27% 40% 16% 1	380
	11th to 20th	Night	11% 3 24% 46% 15% 1	380
	21st to end	Night	11% 2 26% 45% 16% 1	380
July	1st to 10th	Night	8% 2 16% 40% 31% 4	516
	11th to 20th	Night	11% 3 18% 37% 27% 4	552
	21st to end	Night	7% 2 16% 44% 28% 3	624
August	1st to 10th	Night	9% 1 12% 42% 30% 6%	567
	11th to 20th	Night	7% 2 14% 37% 33% 6%	564
	21st to end	Night	10% 2 17% 35% 32% 5%	627
September	1st to 10th	Night	12% 5% 17% 32% 30% 5%	738
	11th to 20th	Night	10% 5% 16% 32% 31% 6%	760
	21st to end	Night	17% 5% 22% 31% 19% 6%	760
October	1st to 10th	Night	14% 7% 24% 31% 19% 4	760
	11th to 20th	Night	20% 15% 26% 26% 12% 2	760
	21st to end	Night	25% 13% 29% 26% 6%	836
November	1st to 10th	Night	18% 11% 30% 26% 13% 1	780
	11th to 20th	Night	23% 17% 32% 20% 8% 1	800
	21st to end	Night	28% 15% 31% 20% 7%	800
December	1st to 10th	Night	26% 17% 33% 18% 6% 1	800
	11th to 20th	Night	25% 17% 35% 21% 3	800
	21st to end	Night	31% 13% 30% 23% 3	1015

Page generated at 2025-04-23 09:25:03. Version ac07

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