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 2024-11-10.

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%	101312
	Night	13% 9% 3 7% 4 4 4 3 5% 7% 41%	97878
Transparency	Day	40% 7% 12% 23% 15% 2	104904
	Night	41% 6% 11% 23% 17% 2	102428
Seeing	Day	15% 10% 27% 26% 16% 5%	15706
	Night	17% 9% 30% 30% 13% 1	25521

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% 12% 6% 15% 6% 6% 4 4 4 6% 26%	2527
	11th to 20th	Day	13% 10% 5% 12% 7% 8% 8% 6% 6% 5% 20%	2566
	21st to end	Day	19% 19% 8% 9% 4 3 3 2 4 5% 23%	3135
February	1st to 10th	Day	15% 12% 5% 10% 5% 4 4 3 4 6% 30%	1710
	11th to 20th	Day	14% 11% 3 7% 7% 5% 4 4 5% 7% 33%	1848
	21st to end	Day	15% 12% 5% 9% 5% 4 4 3 5% 6% 33%	1570
March	1st to 10th	Day	18% 14% 4 6% 4 4 3 3 3 5% 36%	3120
	11th to 20th	Day	10% 9% 2 6% 4 3 4 3 6% 8% 46%	3416
	21st to end	Day	16% 11% 4 9% 5% 5% 5% 3 5% 7% 30%	4087
April	1st to 10th	Day	13% 8% 4 7% 5% 3 5% 4 6% 8% 37%	2280
	11th to 20th	Day	15% 10% 3 7% 4 4 4 3 5% 6% 40%	2254
	21st to end	Day	20% 6% 2 6% 4 5% 5% 4 5% 6% 36%	2280
May	1st to 10th	Day	14% 8% 3 7% 5% 6% 5% 5% 7% 11% 31%	4005
	11th to 20th	Day	17% 8% 2 7% 4 4 4 3 5% 7% 37%	4330
	21st to end	Day	9% 6% 2 5% 5% 5% 4 4 6% 8% 46%	4824
June	1st to 10th	Day	7% 5% 3 5% 4 5% 5% 5% 6% 9% 47%	2660
	11th to 20th	Day	8% 5% 2 6% 4 4 5% 4 5% 8% 50%	2660
	21st to end	Day	6% 6% 2 7% 5% 6% 6% 5% 7% 8% 43%	2660
July	1st to 10th	Day	12% 6% 2 6% 4 5% 4 4 7% 9% 40%	4440
	11th to 20th	Day	5% 5% 2 6% 4 5% 4 5% 7% 11% 45%	4440
	21st to end	Day	8% 5% 2 7% 5% 5% 6% 5% 8% 10% 38%	4769
August	1st to 10th	Day	6% 4 2 5% 4 4 4 4 7% 10% 50%	4054
	11th to 20th	Day	10% 5% 3 7% 4 4 4 4 6% 9% 43%	3729
	21st to end	Day	5% 4 2 6% 5% 5% 4 4 7% 11% 49%	4180
September	1st to 10th	Day	12% 5% 2 5% 4 3 4 3 6% 6% 50%	2280
	11th to 20th	Day	9% 5% 3 6% 4 4 4 2 5% 7% 51%	2223
	21st to end	Day	9% 5% 3 5% 4 3 4 4 5% 7% 50%	2090
October	1st to 10th	Day	11% 4 2 4 3 3 3 3 5% 7% 53%	1947
	11th to 20th	Day	15% 7% 2 5% 4 3 4 3 5% 5% 47%	1900
	21st to end	Day	18% 8% 3 6% 5% 3 4 3 4 6% 41%	2000
November	1st to 10th	Day	11% 6% 2 6% 4 3 4 3 4 5% 50%	1753
	11th to 20th	Day	15% 9% 3 8% 3 3 2 4 5% 7% 43%	1710
	21st to end	Day	17% 11% 4 7% 5% 5% 3 3 4 4 39%	1644
December	1st to 10th	Day	13% 12% 4 8% 6% 4 4 4 5% 6% 33%	1520
	11th to 20th	Day	14% 11% 3 9% 5% 4 5% 5% 5% 8% 31%	1918
	21st to end	Day	21% 16% 6% 14% 4 3 3 3 5% 5% 19%	2783
January	1st to 10th	Night	14% 13% 6% 10% 5% 4 4 3 4 5% 32%	4428
	11th to 20th	Night	14% 16% 5% 10% 5% 5% 7% 4 4 4 25%	4399
	21st to end	Night	23% 16% 7% 10% 4 3 3 2 3 6% 22%	4532
February	1st to 10th	Night	17% 11% 5% 8% 5% 4 4 4 4 6% 32%	2467
	11th to 20th	Night	13% 10% 4 8% 5% 4 4 3 5% 6% 39%	2218
	21st to end	Night	17% 10% 3 7% 5% 3 4 3 4 5% 38%	1884
March	1st to 10th	Night	20% 13% 2 5% 4 3 3 2 4 5% 39%	3733
	11th to 20th	Night	13% 6% 2 5% 3 3 3 3 4 6% 51%	3558
	21st to end	Night	19% 10% 4 8% 5% 4 4 3 5% 5% 33%	3542
April	1st to 10th	Night	12% 8% 3 6% 5% 4 4 3 5% 7% 44%	1900
	11th to 20th	Night	15% 7% 2 5% 4 4 3 4 4 8% 45%	1875
	21st to end	Night	20% 7% 3 7% 4 3 4 3 4 6% 39%	1900
May	1st to 10th	Night	17% 8% 3 7% 4 4 4 4 6% 7% 36%	2886
	11th to 20th	Night	16% 7% 3 6% 4 4 4 4 6% 7% 41%	2568
	21st to end	Night	9% 6% 2 6% 4 4 4 4 6% 10% 44%	2743
June	1st to 10th	Night	7% 6% 2 5% 3 3 3 4 6% 10% 51%	1520
	11th to 20th	Night	7% 4 2 6% 4 5% 4 3 5% 10% 50%	1520
	21st to end	Night	6% 5% 2 5% 4 3 5% 4 8% 9% 48%	1520
July	1st to 10th	Night	12% 5% 2 5% 4 4 4 4 7% 8% 43%	2530
	11th to 20th	Night	3 3 1 4 3 4 4 5% 6% 10% 56%	2530
	21st to end	Night	7% 6% 2 7% 4 4 5% 5% 8% 11% 42%	2778
August	1st to 10th	Night	5% 4 1 5% 3 4 4 4 7% 9% 55%	2881
	11th to 20th	Night	7% 4 2 5% 4 4 5% 4 6% 8% 50%	3100
	21st to end	Night	5% 3 1 4 3 4 4 4 7% 10% 53%	3476
September	1st to 10th	Night	10% 5% 2 5% 3 3 3 4 5% 7% 54%	1900
	11th to 20th	Night	7% 6% 2 6% 3 4 4 3 6% 8% 53%	1938
	21st to end	Night	9% 5% 2 5% 4 3 4 4 5% 7% 52%	2090
October	1st to 10th	Night	10% 4 2 4 4 3 3 3 4 6% 57%	2230
	11th to 20th	Night	15% 6% 1 5% 3 3 2 3 4 6% 52%	2280
	21st to end	Night	17% 8% 2 6% 3 3 4 3 5% 5% 44%	2598
November	1st to 10th	Night	10% 6% 2 5% 3 3 3 3 4 6% 53%	2530
	11th to 20th	Night	13% 8% 3 6% 3 3 3 3 5% 6% 49%	2470
	21st to end	Night	17% 9% 2 6% 4 4 4 3 5% 5% 42%	2531
December	1st to 10th	Night	16% 11% 3 8% 5% 4 4 4 5% 7% 33%	2660
	11th to 20th	Night	16% 8% 4 9% 4 3 3 2 4 5% 40%	3279
	21st to end	Night	22% 18% 5% 11% 5% 5% 4 3 4 4 19%	4884

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	58% 6% 10% 18% 8%	2528
	11th to 20th	Day	63% 7% 12% 13% 5%	2566
	21st to end	Day	64% 5% 10% 15% 6%	3135
February	1st to 10th	Day	55% 5% 12% 18% 8% 1	1710
	11th to 20th	Day	50% 6% 12% 20% 12% 1	1858
	21st to end	Day	52% 5% 10% 20% 11% 2	1570
March	1st to 10th	Day	50% 5% 9% 19% 14% 2	3120
	11th to 20th	Day	35% 7% 16% 23% 18% 1	3416
	21st to end	Day	51% 9% 16% 19% 4	4087
April	1st to 10th	Day	43% 8% 13% 22% 14% 1	2280
	11th to 20th	Day	44% 6% 11% 23% 14% 2	2254
	21st to end	Day	46% 8% 12% 20% 12% 2	2280
May	1st to 10th	Day	42% 10% 16% 23% 8%	4042
	11th to 20th	Day	45% 8% 12% 26% 9% 1	4334
	21st to end	Day	32% 10% 12% 23% 22% 2	4837
June	1st to 10th	Day	31% 9% 15% 25% 17% 3	2660
	11th to 20th	Day	31% 8% 12% 24% 23% 2	2660
	21st to end	Day	35% 10% 14% 22% 17% 3	2660
July	1st to 10th	Day	39% 7% 11% 27% 13% 2	4371
	11th to 20th	Day	30% 8% 14% 27% 19% 1	4365
	21st to end	Day	37% 9% 18% 23% 12% 1	4619
August	1st to 10th	Day	27% 7% 17% 35% 13% 1	4040
	11th to 20th	Day	36% 7% 13% 28% 15% 1	3765
	21st to end	Day	28% 7% 16% 30% 15% 4	4179
September	1st to 10th	Day	34% 6% 10% 26% 19% 5%	2280
	11th to 20th	Day	33% 5% 9% 26% 23% 4	2223
	21st to end	Day	32% 6% 10% 28% 22% 3	2090
October	1st to 10th	Day	22% 3 8% 19% 45% 4	3233
	11th to 20th	Day	26% 3 8% 23% 35% 5%	3140
	21st to end	Day	38% 4 8% 26% 23% 2	3277
November	1st to 10th	Day	36% 5% 12% 25% 18% 4	1753
	11th to 20th	Day	42% 3 11% 25% 15% 3	1710
	21st to end	Day	48% 6% 10% 24% 11%	1644
December	1st to 10th	Day	51% 7% 13% 18% 10%	1520
	11th to 20th	Day	49% 6% 12% 25% 6% 2	1915
	21st to end	Day	65% 7% 10% 12% 6%	2783
January	1st to 10th	Night	53% 6% 12% 20% 8%	4428
	11th to 20th	Night	63% 4 9% 16% 7%	4399
	21st to end	Night	65% 4 8% 16% 6% 1	4532
February	1st to 10th	Night	54% 5% 10% 19% 11% 2	2467
	11th to 20th	Night	46% 4 11% 22% 15% 2	2219
	21st to end	Night	47% 5% 11% 21% 13% 2	1884
March	1st to 10th	Night	48% 6% 9% 24% 12% 1	3733
	11th to 20th	Night	34% 6% 10% 26% 20% 4	3558
	21st to end	Night	50% 8% 11% 24% 7% 1	3542
April	1st to 10th	Night	40% 6% 11% 25% 15% 3	1900
	11th to 20th	Night	37% 5% 12% 25% 17% 3	1875
	21st to end	Night	47% 5% 10% 20% 15% 3	1900
May	1st to 10th	Night	44% 9% 15% 21% 10% 1	2914
	11th to 20th	Night	39% 9% 13% 29% 10% 1	2569
	21st to end	Night	33% 9% 14% 21% 20% 3	2751
June	1st to 10th	Night	28% 7% 15% 27% 19% 4	1520
	11th to 20th	Night	29% 7% 11% 25% 23% 5%	1520
	21st to end	Night	30% 8% 15% 25% 18% 5%	1520
July	1st to 10th	Night	37% 6% 11% 28% 15% 2	2490
	11th to 20th	Night	22% 6% 13% 35% 21% 2	2482
	21st to end	Night	33% 8% 18% 22% 16% 2	2690
August	1st to 10th	Night	23% 8% 17% 34% 17% 1	2871
	11th to 20th	Night	31% 7% 13% 31% 18% 1	3130
	21st to end	Night	24% 7% 17% 32% 16% 4	3476
September	1st to 10th	Night	30% 5% 11% 25% 21% 8%	1900
	11th to 20th	Night	27% 7% 9% 25% 27% 5%	1938
	21st to end	Night	30% 7% 10% 24% 25% 5%	2090
October	1st to 10th	Night	21% 4 6% 21% 45% 4	3724
	11th to 20th	Night	24% 2 9% 23% 37% 5%	3779
	21st to end	Night	33% 4 9% 24% 24% 5%	4271
November	1st to 10th	Night	32% 6% 11% 25% 20% 5%	2530
	11th to 20th	Night	37% 4 10% 28% 19% 2	2470
	21st to end	Night	44% 5% 10% 24% 16% 1	2531
December	1st to 10th	Night	50% 5% 14% 20% 10% 1	2660
	11th to 20th	Night	46% 5% 12% 25% 12% 1	3281
	21st to end	Night	68% 6% 8% 11% 6% 1	4884

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% 16% 6%	270
	11th to 20th	Day	27% 18% 37% 16% 1	270
	21st to end	Day	20% 20% 37% 19% 5%	297
February	1st to 10th	Day	26% 19% 34% 17% 4	270
	11th to 20th	Day	25% 20% 32% 19% 4	270
	21st to end	Day	28% 16% 42% 13% 1	225
March	1st to 10th	Day	22% 17% 42% 16% 2	270
	11th to 20th	Day	29% 18% 34% 16% 4	270
	21st to end	Day	31% 16% 30% 19% 5%	454
April	1st to 10th	Day	20% 16% 34% 24% 6%	460
	11th to 20th	Day	20% 18% 29% 22% 9% 2	452
	21st to end	Day	24% 13% 36% 18% 8% 1	460
May	1st to 10th	Day	19% 13% 33% 25% 8% 2	463
	11th to 20th	Day	17% 11% 28% 28% 14% 3	632
	21st to end	Day	12% 8% 30% 29% 14% 7%	734
June	1st to 10th	Day	8% 6% 25% 31% 24% 7%	680
	11th to 20th	Day	9% 4 25% 34% 22% 6%	680
	21st to end	Day	8% 7% 29% 32% 20% 4	680
July	1st to 10th	Day	10% 3 16% 29% 27% 14%	671
	11th to 20th	Day	7% 2 21% 35% 25% 10%	668
	21st to end	Day	10% 3 17% 32% 29% 9%	739
August	1st to 10th	Day	9% 2 18% 35% 28% 9%	652
	11th to 20th	Day	9% 5% 19% 28% 26% 14%	479
	21st to end	Day	7% 2 16% 30% 35% 10%	539
September	1st to 10th	Day	12% 5% 18% 30% 26% 10%	490
	11th to 20th	Day	10% 3 12% 31% 31% 13%	490
	21st to end	Day	11% 10% 24% 23% 24% 8%	490
October	1st to 10th	Day	11% 10% 25% 29% 21% 4	347
	11th to 20th	Day	18% 19% 29% 24% 9% 2	300
	21st to end	Day	29% 15% 32% 18% 6%	330
November	1st to 10th	Day	12% 15% 34% 22% 13% 4	297
	11th to 20th	Day	21% 19% 32% 19% 8% 2	270
	21st to end	Day	23% 13% 37% 21% 6%	270
December	1st to 10th	Day	14% 23% 37% 17% 6% 3	270
	11th to 20th	Day	19% 17% 39% 22% 3	270
	21st to end	Day	27% 21% 36% 13% 3	297
January	1st to 10th	Night	18% 16% 38% 22% 5%	950
	11th to 20th	Night	22% 15% 37% 22% 5%	950
	21st to end	Night	19% 17% 35% 26% 3	1045
February	1st to 10th	Night	23% 13% 35% 25% 3	947
	11th to 20th	Night	20% 15% 37% 26% 2	743
	21st to end	Night	24% 15% 35% 24% 2	628
March	1st to 10th	Night	22% 13% 37% 24% 4	760
	11th to 20th	Night	22% 11% 34% 27% 6%	622
	21st to end	Night	27% 10% 32% 24% 7%	627
April	1st to 10th	Night	18% 16% 34% 26% 6%	570
	11th to 20th	Night	17% 12% 36% 26% 10% 1	563
	21st to end	Night	24% 9% 35% 25% 6% 1	570
May	1st to 10th	Night	18% 7% 36% 29% 9% 1	570
	11th to 20th	Night	17% 6% 27% 36% 13% 1	561
	21st to end	Night	11% 3 29% 35% 20% 2	617
June	1st to 10th	Night	9% 3 21% 46% 21%	570
	11th to 20th	Night	9% 3 24% 44% 20% 1	570
	21st to end	Night	7% 4 25% 41% 21% 1	570
July	1st to 10th	Night	12% 3 9% 35% 36% 6%	555
	11th to 20th	Night	5% 2 18% 41% 31% 3	552
	21st to end	Night	6% 1 15% 46% 30% 2	624
August	1st to 10th	Night	9% 2 13% 41% 33% 3	567
	11th to 20th	Night	7% 2 12% 41% 32% 5%	564
	21st to end	Night	6% 1 15% 37% 37% 4	627
September	1st to 10th	Night	10% 4 16% 38% 28% 4	570
	11th to 20th	Night	8% 3 17% 36% 32% 4	608
	21st to end	Night	10% 7% 22% 31% 26% 4	760
October	1st to 10th	Night	11% 4 27% 32% 23% 3	760
	11th to 20th	Night	17% 11% 30% 28% 12% 1	760
	21st to end	Night	22% 11% 34% 25% 7%	836
November	1st to 10th	Night	13% 10% 39% 29% 9%	779
	11th to 20th	Night	17% 15% 36% 25% 7% 1	760
	21st to end	Night	22% 13% 34% 25% 6%	821
December	1st to 10th	Night	22% 12% 37% 22% 6% 1	950
	11th to 20th	Night	23% 9% 33% 27% 7%	950
	21st to end	Night	27% 16% 32% 22% 3	1045

Page generated at 2024-11-07 05:55:49. Version ac07

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