Hartford
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Hartford clear sky chart from 2005-11-06 to 2025-09-04.

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	17% 7% 2 6% 4 4 4 4 5% 7% 38%	125924
	Night	18% 8% 3 6% 4 4 4 3 5% 6% 40%	122796
Transparency	Day	44% 6% 11% 21% 15% 2	129456
	Night	44% 6% 12% 21% 15% 2	127351
Seeing	Day	23% 16% 30% 20% 8% 2	16064
	Night	22% 13% 32% 25% 7% 1	25022

By Month

Percentage Cloud Cover NE (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% 8% 4 9% 5% 5% 4 3 4 5% 42%	3597
	11th to 20th	Day	14% 8% 3 7% 5% 4 4 4 5% 5% 42%	3570
	21st to end	Day	18% 8% 2 7% 3 4 4 3 4 6% 40%	4026
February	1st to 10th	Day	18% 7% 2 6% 4 4 6% 5% 6% 6% 36%	1872
	11th to 20th	Day	14% 7% 2 7% 5% 5% 5% 3 5% 7% 39%	1948
	21st to end	Day	20% 8% 2 5% 4 3 4 4 5% 6% 38%	1650
March	1st to 10th	Day	22% 9% 3 7% 4 3 3 3 4 5% 38%	4476
	11th to 20th	Day	15% 7% 2 6% 5% 5% 5% 3 5% 7% 40%	4648
	21st to end	Day	22% 7% 2 6% 4 4 4 4 5% 6% 36%	5970
April	1st to 10th	Day	19% 8% 3 6% 4 4 4 4 5% 6% 36%	2400
	11th to 20th	Day	19% 6% 2 5% 4 4 4 3 6% 6% 40%	2374
	21st to end	Day	20% 6% 2 6% 4 4 4 3 5% 6% 40%	2528
May	1st to 10th	Day	31% 8% 2 6% 4 4 4 3 5% 8% 25%	5886
	11th to 20th	Day	20% 8% 2 7% 5% 4 4 4 6% 7% 33%	6129
	21st to end	Day	19% 7% 3 6% 4 4 5% 4 6% 9% 34%	6949
June	1st to 10th	Day	16% 7% 3 5% 5% 4 5% 5% 6% 9% 35%	2800
	11th to 20th	Day	16% 6% 2 7% 5% 5% 4 3 6% 10% 36%	2800
	21st to end	Day	13% 7% 3 7% 5% 5% 4 5% 6% 9% 38%	2800
July	1st to 10th	Day	13% 7% 3 7% 5% 5% 5% 5% 7% 10% 33%	6360
	11th to 20th	Day	10% 6% 2 7% 6% 5% 6% 5% 7% 12% 35%	6360
	21st to end	Day	15% 7% 3 6% 4 4 4 3 5% 7% 42%	6926
August	1st to 10th	Day	16% 8% 3 8% 5% 6% 4 4 7% 8% 30%	4383
	11th to 20th	Day	10% 6% 3 7% 5% 5% 4 4 6% 8% 41%	3914
	21st to end	Day	8% 5% 2 6% 5% 4 5% 5% 6% 9% 44%	4312
September	1st to 10th	Day	13% 6% 2 7% 4 4 5% 3 5% 6% 45%	2316
	11th to 20th	Day	14% 5% 2 5% 2 3 3 3 5% 6% 50%	2141
	21st to end	Day	18% 6% 2 6% 4 4 5% 3 5% 7% 41%	2090
October	1st to 10th	Day	22% 7% 3 5% 4 4 4 3 5% 5% 40%	2071
	11th to 20th	Day	14% 7% 2 5% 3 4 3 3 5% 7% 46%	1870
	21st to end	Day	21% 6% 2 5% 3 4 4 4 4 7% 40%	1881
November	1st to 10th	Day	13% 7% 2 6% 4 4 4 3 4 6% 49%	1761
	11th to 20th	Day	19% 6% 2 4 3 3 3 3 4 7% 47%	1800
	21st to end	Day	18% 6% 3 6% 4 4 4 3 5% 6% 42%	1770
December	1st to 10th	Day	16% 7% 2 5% 4 3 4 3 5% 7% 44%	2600
	11th to 20th	Day	21% 7% 2 4 2 3 4 3 4 5% 45%	2986
	21st to end	Day	20% 10% 3 7% 4 4 3 3 4 5% 37%	3960
January	1st to 10th	Night	15% 6% 3 7% 4 4 3 4 5% 6% 43%	6349
	11th to 20th	Night	14% 8% 3 8% 4 4 3 3 5% 5% 42%	6315
	21st to end	Night	18% 10% 3 7% 4 4 4 3 5% 6% 37%	6927
February	1st to 10th	Night	17% 9% 3 6% 5% 4 4 4 5% 5% 39%	2526
	11th to 20th	Night	15% 8% 2 6% 5% 5% 4 4 5% 7% 39%	2338
	21st to end	Night	18% 9% 2 7% 5% 4 4 3 4 5% 40%	1980
March	1st to 10th	Night	23% 9% 3 5% 3 3 3 3 5% 5% 38%	5364
	11th to 20th	Night	17% 10% 3 7% 4 4 4 3 5% 6% 39%	5294
	21st to end	Night	23% 8% 3 6% 4 4 4 3 5% 5% 37%	4962
April	1st to 10th	Night	20% 7% 2 6% 5% 4 4 4 5% 6% 36%	2000
	11th to 20th	Night	19% 7% 2 6% 5% 3 4 4 5% 5% 39%	1975
	21st to end	Night	22% 7% 2 6% 4 4 2 3 5% 5% 41%	1870
May	1st to 10th	Night	29% 10% 3 7% 4 4 3 3 5% 6% 26%	4054
	11th to 20th	Night	20% 10% 3 8% 5% 4 4 3 6% 7% 30%	3806
	21st to end	Night	22% 7% 3 7% 4 4 4 4 6% 8% 31%	3926
June	1st to 10th	Night	17% 7% 2 6% 4 4 5% 3 6% 8% 39%	1600
	11th to 20th	Night	17% 6% 3 6% 4 4 4 4 6% 8% 38%	1600
	21st to end	Night	14% 6% 2 5% 4 4 4 4 7% 9% 42%	1600
July	1st to 10th	Night	14% 7% 3 7% 4 5% 4 4 6% 10% 37%	3600
	11th to 20th	Night	14% 8% 3 6% 5% 4 5% 4 6% 10% 36%	3600
	21st to end	Night	12% 7% 3 6% 4 4 4 4 6% 7% 44%	3926
August	1st to 10th	Night	21% 6% 3 7% 4 5% 5% 4 6% 8% 30%	2732
	11th to 20th	Night	13% 7% 2 8% 5% 5% 5% 4 6% 7% 39%	3129
	21st to end	Night	9% 4 1 5% 4 4 4 3 6% 8% 51%	3586
September	1st to 10th	Night	13% 5% 2 7% 4 4 4 4 5% 6% 45%	1931
	11th to 20th	Night	13% 6% 2 5% 4 3 3 3 4 6% 52%	2038
	21st to end	Night	17% 6% 2 6% 3 3 4 4 5% 6% 44%	2090
October	1st to 10th	Night	21% 7% 2 5% 4 4 4 3 4 6% 41%	2106
	11th to 20th	Night	15% 5% 2 6% 4 3 4 3 4 6% 47%	2308
	21st to end	Night	23% 6% 2 5% 4 4 3 4 4 5% 40%	2717
November	1st to 10th	Night	14% 5% 2 6% 3 4 4 3 5% 6% 48%	2537
	11th to 20th	Night	19% 7% 2 6% 3 3 3 3 5% 5% 45%	2600
	21st to end	Night	16% 7% 2 5% 4 3 4 3 4 6% 44%	2626
December	1st to 10th	Night	19% 7% 2 5% 3 4 3 3 5% 5% 44%	4580
	11th to 20th	Night	25% 7% 2 4 3 3 3 3 4 4 41%	5208
	21st to end	Night	19% 10% 3 7% 4 3 3 3 4 6% 39%	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	45% 4 8% 27% 13% 2	3598
	11th to 20th	Day	42% 5% 11% 22% 14% 7%	3600
	21st to end	Day	46% 5% 9% 23% 16% 1	4026
February	1st to 10th	Day	46% 7% 13% 19% 14% 2	1872
	11th to 20th	Day	45% 5% 10% 22% 15% 3	1957
	21st to end	Day	46% 6% 11% 20% 14% 3	1650
March	1st to 10th	Day	49% 4 11% 16% 12% 9%	4479
	11th to 20th	Day	41% 7% 13% 27% 10% 2	4648
	21st to end	Day	48% 5% 7% 15% 19% 6%	5970
April	1st to 10th	Day	47% 6% 10% 19% 15% 3	2400
	11th to 20th	Day	43% 5% 10% 20% 19% 3	2374
	21st to end	Day	46% 5% 9% 20% 16% 4	2528
May	1st to 10th	Day	57% 6% 14% 13% 9% 1	5897
	11th to 20th	Day	50% 6% 12% 18% 12% 1	6133
	21st to end	Day	46% 8% 11% 21% 13% 1	6949
June	1st to 10th	Day	45% 7% 12% 23% 12% 1	2800
	11th to 20th	Day	44% 7% 13% 21% 13% 2	2800
	21st to end	Day	42% 8% 13% 20% 15% 1	2800
July	1st to 10th	Day	46% 8% 11% 19% 15% 1	6290
	11th to 20th	Day	40% 8% 17% 23% 11% 1	6285
	21st to end	Day	41% 6% 10% 23% 18% 1	6728
August	1st to 10th	Day	50% 7% 15% 19% 8% 1	4369
	11th to 20th	Day	38% 8% 13% 29% 10% 1	3952
	21st to end	Day	35% 7% 12% 28% 16% 2	4312
September	1st to 10th	Day	40% 6% 10% 22% 20% 2	2316
	11th to 20th	Day	34% 6% 10% 24% 23% 3	2141
	21st to end	Day	43% 5% 8% 21% 19% 3	2090
October	1st to 10th	Day	39% 6% 9% 19% 23% 4	3448
	11th to 20th	Day	29% 4 8% 26% 30% 3	3086
	21st to end	Day	32% 5% 13% 26% 21% 3	3081
November	1st to 10th	Day	39% 4 12% 23% 19% 3	1761
	11th to 20th	Day	40% 4 10% 23% 21% 4	1800
	21st to end	Day	44% 4 12% 26% 12% 3	1770
December	1st to 10th	Day	41% 4 16% 28% 11% 1	2600
	11th to 20th	Day	41% 4 15% 19% 18% 3	2986
	21st to end	Day	50% 4 9% 19% 14% 4	3960
January	1st to 10th	Night	42% 4 9% 27% 15% 2	6349
	11th to 20th	Night	44% 5% 13% 20% 14% 5%	6360
	21st to end	Night	49% 4 11% 17% 16% 3	6927
February	1st to 10th	Night	46% 5% 13% 23% 11% 2	2526
	11th to 20th	Night	45% 5% 10% 19% 16% 4	2340
	21st to end	Night	46% 5% 9% 22% 15% 4	1980
March	1st to 10th	Night	47% 5% 10% 17% 15% 5%	5374
	11th to 20th	Night	45% 5% 16% 20% 11% 3	5295
	21st to end	Night	50% 4 6% 12% 23% 5%	4962
April	1st to 10th	Night	48% 6% 9% 19% 14% 3	2000
	11th to 20th	Night	46% 6% 10% 18% 20% 2	1975
	21st to end	Night	45% 5% 9% 21% 15% 5%	1870
May	1st to 10th	Night	58% 7% 14% 11% 9% 1	4060
	11th to 20th	Night	53% 8% 12% 15% 10% 3	3807
	21st to end	Night	49% 9% 13% 20% 9% 1	3927
June	1st to 10th	Night	44% 8% 12% 21% 15% 1	1600
	11th to 20th	Night	44% 8% 13% 25% 11% 1	1600
	21st to end	Night	37% 10% 14% 24% 14% 1	1600
July	1st to 10th	Night	44% 8% 14% 21% 13% 1	3560
	11th to 20th	Night	43% 8% 17% 21% 10% 1	3552
	21st to end	Night	39% 7% 12% 24% 17% 1	3809
August	1st to 10th	Night	50% 10% 12% 18% 9% 1	2722
	11th to 20th	Night	42% 8% 13% 28% 8% 1	3157
	21st to end	Night	28% 7% 16% 32% 16% 1	3586
September	1st to 10th	Night	39% 7% 9% 25% 18% 1	1931
	11th to 20th	Night	34% 5% 11% 22% 26% 2	2038
	21st to end	Night	41% 6% 9% 21% 20% 2	2090
October	1st to 10th	Night	39% 6% 13% 21% 18% 2	3509
	11th to 20th	Night	27% 4 8% 28% 30% 2	3831
	21st to end	Night	33% 3 13% 29% 19% 2	4467
November	1st to 10th	Night	37% 6% 12% 22% 20% 4	2537
	11th to 20th	Night	41% 5% 11% 20% 20% 4	2600
	21st to end	Night	41% 5% 13% 27% 12% 2	2626
December	1st to 10th	Night	43% 5% 12% 29% 11% 1	4580
	11th to 20th	Night	48% 3 14% 16% 18% 2	5208
	21st to end	Night	48% 3 9% 20% 17% 3	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	28% 24% 30% 17% 3	200
	11th to 20th	Day	28% 28% 30% 12% 3	200
	21st to end	Day	24% 25% 35% 11% 5%	235
February	1st to 10th	Day	31% 26% 27% 14% 2	372
	11th to 20th	Day	29% 27% 31% 11% 2	496
	21st to end	Day	29% 18% 36% 12% 4	414
March	1st to 10th	Day	28% 23% 33% 14% 3	500
	11th to 20th	Day	28% 24% 31% 14% 2	500
	21st to end	Day	31% 21% 32% 10% 5% 2	550
April	1st to 10th	Day	32% 25% 25% 13% 4 1	500
	11th to 20th	Day	33% 18% 29% 15% 4 1	492
	21st to end	Day	30% 19% 32% 13% 4 2	500
May	1st to 10th	Day	35% 17% 29% 12% 7% 1	503
	11th to 20th	Day	24% 19% 35% 15% 7%	527
	21st to end	Day	22% 14% 33% 21% 9% 2	573
June	1st to 10th	Day	18% 18% 29% 24% 10% 2	530
	11th to 20th	Day	17% 17% 35% 23% 8% 2	530
	21st to end	Day	16% 16% 31% 26% 10% 2	530
July	1st to 10th	Day	18% 10% 29% 28% 13% 3	525
	11th to 20th	Day	14% 6% 32% 30% 14% 3	524
	21st to end	Day	15% 9% 34% 29% 10% 2	576
August	1st to 10th	Day	13% 8% 30% 32% 12% 5%	526
	11th to 20th	Day	14% 8% 30% 28% 14% 5%	519
	21st to end	Day	12% 10% 30% 26% 16% 5%	583
September	1st to 10th	Day	19% 10% 25% 24% 17% 6%	502
	11th to 20th	Day	17% 11% 25% 24% 12% 10%	490
	21st to end	Day	21% 11% 31% 21% 10% 5%	490
October	1st to 10th	Day	28% 10% 25% 20% 12% 6%	490
	11th to 20th	Day	16% 17% 31% 26% 8% 3	473
	21st to end	Day	27% 15% 29% 19% 8% 2	429
November	1st to 10th	Day	19% 16% 30% 23% 10% 2	395
	11th to 20th	Day	29% 20% 27% 18% 7% 1	400
	21st to end	Day	26% 18% 31% 18% 7%	370
December	1st to 10th	Day	22% 25% 32% 16% 6%	200
	11th to 20th	Day	33% 24% 33% 10% 2	200
	21st to end	Day	25% 20% 33% 15% 5% 2	220
January	1st to 10th	Night	21% 20% 38% 19% 3	800
	11th to 20th	Night	28% 18% 31% 21% 2	800
	21st to end	Night	21% 18% 37% 21% 2	880
February	1st to 10th	Night	22% 18% 36% 22% 2	800
	11th to 20th	Night	24% 21% 38% 16% 1	783
	21st to end	Night	26% 18% 37% 17% 2	660
March	1st to 10th	Night	26% 19% 35% 18% 3	800
	11th to 20th	Night	23% 20% 35% 19% 3	800
	21st to end	Night	26% 18% 34% 19% 4	880
April	1st to 10th	Night	25% 18% 34% 19% 3	800
	11th to 20th	Night	24% 19% 34% 18% 4	791
	21st to end	Night	26% 19% 32% 19% 4	670
May	1st to 10th	Night	30% 15% 31% 21% 4	600
	11th to 20th	Night	22% 15% 39% 20% 5%	495
	21st to end	Night	21% 9% 31% 33% 6% 1	433
June	1st to 10th	Night	21% 6% 34% 33% 7%	400
	11th to 20th	Night	20% 9% 32% 32% 9%	400
	21st to end	Night	15% 5% 36% 36% 8% 1	400
July	1st to 10th	Night	14% 5% 28% 39% 14%	390
	11th to 20th	Night	15% 4 24% 41% 15% 1	388
	21st to end	Night	12% 5% 30% 41% 12%	438
August	1st to 10th	Night	12% 6% 26% 43% 11% 1	479
	11th to 20th	Night	14% 5% 23% 39% 17% 2	746
	21st to end	Night	14% 4 26% 37% 18% 1	880
September	1st to 10th	Night	17% 5% 29% 34% 14% 1	773
	11th to 20th	Night	17% 5% 25% 32% 18% 2	760
	21st to end	Night	19% 8% 28% 30% 14% 2	760
October	1st to 10th	Night	25% 8% 26% 27% 12% 2	760
	11th to 20th	Night	20% 10% 31% 28% 10% 1	760
	21st to end	Night	28% 13% 30% 23% 7%	836
November	1st to 10th	Night	17% 12% 34% 27% 9% 1	780
	11th to 20th	Night	24% 13% 34% 22% 6% 1	800
	21st to end	Night	24% 15% 33% 23% 5% 1	800
December	1st to 10th	Night	26% 14% 34% 19% 5% 1	800
	11th to 20th	Night	26% 16% 32% 23% 3	800
	21st to end	Night	27% 14% 35% 20% 3	880

Page generated at 2025-09-01 01:51:02. Version ac07

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