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 2024-10-25.

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% 9% 3 8% 5% 5% 5% 4 6% 8% 34%	101248
	Night	15% 10% 4 8% 5% 4 4 4 5% 7% 34%	97482
Transparency	Day	46% 7% 14% 21% 11% 1	101049
	Night	48% 7% 12% 20% 12% 1	97351
Seeing	Day	16% 10% 29% 25% 14% 5%	15140
	Night	19% 10% 28% 28% 13% 2	24551

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	16% 16% 6% 14% 6% 5% 5% 4 5% 5% 16%	2527
	11th to 20th	Day	14% 20% 7% 14% 7% 7% 6% 4 4 6% 10%	2759
	21st to end	Day	26% 19% 8% 11% 5% 3 4 3 4 4 13%	3124
February	1st to 10th	Day	15% 17% 4 11% 7% 5% 5% 3 4 6% 23%	1710
	11th to 20th	Day	13% 12% 5% 11% 7% 6% 6% 4 6% 6% 25%	1729
	21st to end	Day	16% 13% 4 10% 7% 5% 5% 4 5% 5% 26%	1570
March	1st to 10th	Day	19% 13% 5% 9% 6% 4 4 3 5% 5% 27%	3297
	11th to 20th	Day	12% 12% 5% 10% 6% 5% 4 4 5% 6% 32%	3480
	21st to end	Day	18% 12% 4 9% 5% 4 4 3 5% 6% 30%	3815
April	1st to 10th	Day	14% 11% 3 7% 5% 4 5% 4 7% 8% 32%	2144
	11th to 20th	Day	18% 10% 3 6% 4 4 5% 5% 5% 7% 34%	2442
	21st to end	Day	19% 9% 3 8% 5% 4 4 4 5% 7% 34%	2470
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	20% 10% 3 7% 4 5% 5% 4 6% 7% 30%	1955
November	1st to 10th	Day	12% 9% 4 8% 5% 5% 5% 5% 5% 8% 35%	1672
	11th to 20th	Day	15% 12% 3 8% 5% 4 5% 4 5% 7% 32%	1672
	21st to end	Day	18% 13% 5% 10% 7% 5% 5% 4 5% 6% 23%	1520
December	1st to 10th	Day	16% 15% 6% 10% 7% 5% 6% 5% 5% 6% 19%	1520
	11th to 20th	Day	18% 14% 5% 10% 6% 5% 5% 4 6% 6% 20%	1910
	21st to end	Day	26% 20% 7% 11% 5% 4 5% 3 5% 5% 10%	2647
January	1st to 10th	Night	16% 16% 6% 10% 6% 6% 5% 3 5% 5% 23%	4428
	11th to 20th	Night	14% 19% 7% 14% 6% 5% 4 4 5% 7% 14%	4184
	21st to end	Night	25% 18% 7% 11% 5% 4 3 3 3 3 18%	4532
February	1st to 10th	Night	16% 16% 4 11% 6% 5% 4 3 4 5% 24%	2470
	11th to 20th	Night	15% 14% 5% 10% 6% 5% 5% 3 6% 6% 26%	2335
	21st to end	Night	19% 12% 4 9% 6% 5% 4 4 5% 5% 27%	1884
March	1st to 10th	Night	18% 13% 4 8% 5% 4 4 3 4 5% 32%	3555
	11th to 20th	Night	10% 8% 3 8% 6% 4 6% 4 5% 8% 39%	3480
	21st to end	Night	16% 10% 4 7% 5% 3 4 3 5% 5% 38%	3817
April	1st to 10th	Night	15% 11% 4 7% 5% 4 4 4 5% 7% 35%	2049
	11th to 20th	Night	17% 7% 3 8% 5% 4 4 4 5% 8% 36%	1687
	21st to end	Night	16% 9% 2 7% 4 4 5% 4 5% 7% 37%	1710
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% 9% 3 8% 5% 5% 4 3 5% 6% 31%	2503
November	1st to 10th	Night	13% 10% 2 7% 5% 5% 4 4 5% 6% 41%	2406
	11th to 20th	Night	17% 11% 3 9% 6% 5% 5% 4 5% 8% 29%	2505
	21st to end	Night	21% 14% 4 8% 4 3 3 4 5% 6% 28%	2660
December	1st to 10th	Night	19% 15% 5% 10% 7% 6% 4 3 5% 6% 19%	2660
	11th to 20th	Night	15% 16% 5% 9% 6% 5% 5% 4 5% 6% 25%	3285
	21st to end	Night	26% 20% 6% 11% 6% 5% 4 3 4 4 13%	4990

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% 9% 11% 9% 5%	2528
	11th to 20th	Day	74% 8% 9% 6% 2	2759
	21st to end	Day	74% 6% 9% 7% 4	3124
February	1st to 10th	Day	63% 5% 11% 15% 7%	1710
	11th to 20th	Day	59% 6% 11% 15% 7% 1	1738
	21st to end	Day	57% 7% 11% 17% 7% 2	1570
March	1st to 10th	Day	57% 7% 10% 21% 5% 1	3297
	11th to 20th	Day	49% 7% 17% 18% 8% 1	3480
	21st to end	Day	53% 6% 14% 17% 10% 1	3815
April	1st to 10th	Day	47% 7% 14% 19% 12% 1	2144
	11th to 20th	Day	48% 7% 12% 19% 13% 2	2442
	21st to end	Day	48% 6% 13% 19% 11% 2	2470
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	40% 7% 13% 24% 15% 1	1985
	11th to 20th	Day	46% 8% 14% 20% 11% 1	1900
	21st to end	Day	51% 8% 14% 18% 8% 1	1955
November	1st to 10th	Day	47% 7% 12% 22% 10% 2	1672
	11th to 20th	Day	50% 7% 16% 16% 10% 1	1672
	21st to end	Day	59% 8% 12% 16% 5%	1520
December	1st to 10th	Day	64% 8% 13% 11% 4	1520
	11th to 20th	Day	61% 8% 14% 12% 4	1906
	21st to end	Day	75% 7% 9% 6% 2	2647
January	1st to 10th	Night	59% 8% 13% 14% 6%	4428
	11th to 20th	Night	68% 10% 11% 8% 2	4184
	21st to end	Night	71% 6% 9% 12% 3	4532
February	1st to 10th	Night	62% 6% 11% 15% 5% 1	2470
	11th to 20th	Night	57% 5% 11% 17% 9%	2337
	21st to end	Night	57% 6% 11% 17% 8% 1	1884
March	1st to 10th	Night	53% 5% 13% 20% 7%	3555
	11th to 20th	Night	40% 8% 16% 19% 16% 2	3480
	21st to end	Night	46% 6% 11% 21% 14% 1	3817
April	1st to 10th	Night	49% 6% 11% 19% 13% 2	2049
	11th to 20th	Night	45% 6% 11% 20% 15% 2	1687
	21st to end	Night	44% 7% 10% 19% 16% 4	1710
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	36% 7% 12% 23% 20% 1	2194
	11th to 20th	Night	48% 6% 13% 18% 15%	2280
	21st to end	Night	52% 7% 11% 19% 10% 1	2503
November	1st to 10th	Night	43% 6% 12% 24% 14% 1	2406
	11th to 20th	Night	52% 7% 14% 18% 9% 1	2505
	21st to end	Night	57% 6% 14% 17% 6%	2660
December	1st to 10th	Night	64% 7% 11% 13% 4	2660
	11th to 20th	Night	58% 8% 14% 14% 6%	3288
	21st to end	Night	75% 6% 9% 7% 3	4990

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% 20% 34% 15% 7% 1	270
	11th to 20th	Day	25% 15% 42% 11% 5% 1	270
	21st to end	Day	28% 14% 31% 23% 4 1	297
February	1st to 10th	Day	21% 19% 37% 17% 5%	270
	11th to 20th	Day	21% 18% 37% 20% 3	270
	21st to end	Day	23% 21% 40% 14% 1	225
March	1st to 10th	Day	22% 17% 35% 20% 5%	369
	11th to 20th	Day	20% 14% 34% 25% 6% 2	460
	21st to end	Day	24% 13% 35% 23% 5% 1	506
April	1st to 10th	Day	15% 13% 39% 24% 8% 1	460
	11th to 20th	Day	25% 15% 29% 17% 10% 4	452
	21st to end	Day	23% 10% 34% 20% 11% 2	460
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	24% 15% 34% 18% 7% 1	442
November	1st to 10th	Day	13% 18% 32% 26% 9% 2	270
	11th to 20th	Day	23% 21% 28% 20% 7% 1	252
	21st to end	Day	23% 17% 36% 20% 3 1	180
December	1st to 10th	Day	23% 19% 33% 16% 9% 1	180
	11th to 20th	Day	27% 22% 31% 16% 4	180
	21st to end	Day	28% 20% 32% 16% 4	225
January	1st to 10th	Night	25% 15% 35% 18% 6% 1	950
	11th to 20th	Night	28% 16% 32% 20% 4	798
	21st to end	Night	23% 12% 32% 26% 7%	836
February	1st to 10th	Night	26% 12% 34% 24% 4	760
	11th to 20th	Night	23% 14% 31% 25% 6%	743
	21st to end	Night	24% 14% 37% 22% 2	628
March	1st to 10th	Night	21% 12% 35% 26% 7%	760
	11th to 20th	Night	17% 12% 40% 24% 7% 1	760
	21st to end	Night	23% 10% 33% 25% 9%	836
April	1st to 10th	Night	20% 12% 33% 29% 5% 1	741
	11th to 20th	Night	19% 10% 35% 27% 9% 1	563
	21st to end	Night	20% 8% 32% 32% 9%	570
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% 14% 28% 26% 6%	812
November	1st to 10th	Night	18% 11% 30% 26% 14% 1	740
	11th to 20th	Night	23% 16% 32% 20% 8% 1	760
	21st to end	Night	28% 15% 30% 20% 7%	760
December	1st to 10th	Night	27% 17% 33% 17% 6% 1	760
	11th to 20th	Night	24% 17% 35% 20% 4	760
	21st to end	Night	31% 13% 31% 23% 2	964

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