University of Connecticut Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the University of Connecticut Observatory clear sky chart from 2005-11-06 to 2024-01-16.

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% 3 5% 5% 3 4 5% 5% 7% 40%	74830
	Night	17% 7% 3 5% 5% 3 4 4 5% 7% 41%	74981
Transparency	Day	44% 6% 11% 21% 15% 2	74535
	Night	44% 6% 12% 22% 15% 2	74810
Seeing	Day	25% 17% 31% 18% 7% 2	13920
	Night	23% 15% 32% 22% 7% 1	23023

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	15% 6% 3 5% 5% 3 5% 5% 5% 6% 42%	1517
	11th to 20th	Day	18% 7% 4 6% 4 3 5% 5% 6% 7% 35%	1486
	21st to end	Day	14% 8% 3 5% 4 2 5% 5% 6% 7% 43%	1584
February	1st to 10th	Day	19% 7% 3 5% 5% 3 6% 5% 5% 7% 34%	1710
	11th to 20th	Day	14% 8% 2 5% 6% 3 5% 5% 4 7% 41%	1748
	21st to end	Day	20% 11% 3 6% 4 2 4 4 5% 5% 35%	1480
March	1st to 10th	Day	16% 7% 2 6% 5% 2 4 4 5% 7% 42%	1797
	11th to 20th	Day	17% 7% 4 6% 5% 4 5% 5% 6% 6% 37%	1858
	21st to end	Day	20% 7% 3 5% 5% 3 3 4 4 5% 40%	2353
April	1st to 10th	Day	19% 7% 2 5% 6% 3 4 5% 5% 6% 38%	2160
	11th to 20th	Day	18% 6% 2 4 4 3 4 4 5% 6% 42%	2134
	21st to end	Day	22% 6% 2 6% 6% 3 4 3 5% 6% 37%	2296
May	1st to 10th	Day	25% 6% 3 5% 5% 3 4 5% 5% 6% 32%	2340
	11th to 20th	Day	19% 7% 3 4 4 3 4 4 5% 7% 40%	2374
	21st to end	Day	15% 6% 2 5% 5% 3 4 4 5% 8% 42%	2725
June	1st to 10th	Day	19% 6% 2 5% 6% 3 5% 5% 6% 8% 34%	2520
	11th to 20th	Day	17% 6% 3 6% 5% 4 5% 6% 5% 9% 35%	2520
	21st to end	Day	16% 6% 3 6% 6% 3 5% 5% 6% 9% 35%	2520
July	1st to 10th	Day	14% 6% 2 5% 5% 4 5% 6% 6% 8% 40%	2520
	11th to 20th	Day	14% 5% 2 5% 6% 3 5% 6% 6% 9% 39%	2520
	21st to end	Day	13% 5% 3 5% 6% 4 5% 6% 7% 7% 38%	2744
August	1st to 10th	Day	12% 6% 3 6% 6% 4 5% 6% 8% 9% 36%	2391
	11th to 20th	Day	12% 7% 2 6% 5% 4 5% 5% 6% 9% 39%	2194
	21st to end	Day	12% 5% 2 6% 6% 4 5% 4 5% 8% 43%	2376
September	1st to 10th	Day	15% 7% 2 6% 5% 3 4 5% 5% 6% 44%	2160
	11th to 20th	Day	14% 6% 1 5% 4 2 3 4 5% 7% 50%	2012
	21st to end	Day	17% 6% 3 4 4 3 4 4 4 7% 44%	1980
October	1st to 10th	Day	23% 8% 3 4 3 2 3 3 5% 5% 40%	1980
	11th to 20th	Day	15% 7% 2 4 4 2 4 4 5% 7% 47%	1761
	21st to end	Day	19% 8% 2 6% 4 3 3 5% 5% 6% 39%	1782
November	1st to 10th	Day	14% 7% 2 6% 4 3 4 3 4 5% 48%	1671
	11th to 20th	Day	20% 5% 2 3 3 2 4 5% 5% 6% 45%	1710
	21st to end	Day	17% 6% 3 4 4 3 4 4 5% 6% 43%	1709
December	1st to 10th	Day	20% 7% 3 6% 5% 3 4 5% 5% 7% 36%	1520
	11th to 20th	Day	22% 7% 2 4 4 3 4 3 3 5% 44%	1906
	21st to end	Day	21% 10% 3 6% 6% 3 4 4 4 6% 34%	2772
January	1st to 10th	Night	15% 8% 3 5% 4 2 4 4 4 7% 43%	2649
	11th to 20th	Night	20% 8% 3 6% 4 3 4 4 4 6% 36%	2602
	21st to end	Night	13% 8% 4 5% 5% 3 4 4 5% 7% 43%	2771
February	1st to 10th	Night	17% 9% 4 6% 5% 3 3 4 4 6% 38%	2250
	11th to 20th	Night	16% 7% 4 5% 5% 3 5% 5% 5% 7% 38%	2098
	21st to end	Night	19% 8% 3 6% 5% 2 4 4 5% 6% 38%	1776
March	1st to 10th	Night	17% 7% 2 5% 5% 3 4 6% 5% 6% 41%	2150
	11th to 20th	Night	15% 7% 3 6% 7% 3 4 5% 5% 8% 38%	2101
	21st to end	Night	18% 8% 3 5% 4 2 4 4 5% 6% 40%	2000
April	1st to 10th	Night	19% 7% 3 5% 6% 3 4 5% 6% 6% 36%	1800
	11th to 20th	Night	19% 6% 3 6% 5% 2 4 5% 5% 6% 38%	1775
	21st to end	Night	23% 6% 3 6% 5% 2 3 3 4 7% 38%	1662
May	1st to 10th	Night	22% 7% 3 6% 6% 3 4 5% 6% 6% 32%	1620
	11th to 20th	Night	18% 7% 2 5% 5% 3 4 4 4 8% 37%	1534
	21st to end	Night	18% 7% 2 4 4 3 5% 4 5% 9% 39%	1550
June	1st to 10th	Night	18% 7% 3 6% 5% 3 5% 5% 5% 8% 38%	1440
	11th to 20th	Night	18% 6% 3 5% 5% 3 5% 4 5% 8% 38%	1440
	21st to end	Night	13% 6% 2 6% 5% 2 4 4 7% 10% 41%	1440
July	1st to 10th	Night	13% 5% 3 6% 5% 3 6% 6% 7% 8% 38%	1440
	11th to 20th	Night	15% 6% 3 5% 6% 3 5% 6% 6% 9% 37%	1440
	21st to end	Night	12% 5% 3 6% 6% 3 6% 5% 6% 7% 41%	1574
August	1st to 10th	Night	9% 6% 2 5% 6% 3 5% 5% 7% 9% 42%	1528
	11th to 20th	Night	12% 7% 2 6% 7% 3 6% 5% 6% 7% 39%	1703
	21st to end	Night	14% 5% 3 4 4 2 4 4 5% 7% 47%	1980
September	1st to 10th	Night	15% 6% 2 6% 5% 3 4 5% 6% 6% 41%	1800
	11th to 20th	Night	14% 6% 2 4 3 2 3 3 4 6% 52%	1948
	21st to end	Night	17% 6% 2 6% 4 2 4 4 5% 6% 44%	1980
October	1st to 10th	Night	20% 7% 3 5% 4 2 4 3 4 6% 41%	1980
	11th to 20th	Night	16% 5% 1 5% 4 3 4 4 6% 7% 45%	2197
	21st to end	Night	21% 8% 3 5% 4 3 3 4 4 6% 38%	2574
November	1st to 10th	Night	14% 6% 2 5% 4 2 4 4 5% 6% 49%	2407
	11th to 20th	Night	18% 7% 2 4 4 2 4 4 4 5% 44%	2470
	21st to end	Night	16% 6% 3 5% 4 3 4 4 4 6% 45%	2470
December	1st to 10th	Night	20% 8% 3 5% 5% 3 3 4 5% 6% 40%	2660
	11th to 20th	Night	20% 7% 2 4 4 3 4 5% 4 6% 41%	3288
	21st to end	Night	22% 10% 3 6% 4 2 3 3 4 5% 38%	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	41% 5% 11% 27% 13% 3	1518
	11th to 20th	Day	47% 7% 13% 19% 12% 2	1486
	21st to end	Day	40% 5% 13% 23% 15% 3	1584
February	1st to 10th	Day	48% 6% 13% 18% 13% 2	1710
	11th to 20th	Day	44% 6% 10% 22% 15% 3	1757
	21st to end	Day	50% 5% 9% 19% 14% 2	1480
March	1st to 10th	Day	43% 4 12% 22% 15% 4	1800
	11th to 20th	Day	46% 5% 10% 22% 13% 3	1858
	21st to end	Day	48% 4 9% 17% 17% 5%	2353
April	1st to 10th	Day	46% 6% 11% 19% 15% 4	2160
	11th to 20th	Day	41% 6% 11% 21% 19% 3	2134
	21st to end	Day	48% 5% 9% 19% 15% 3	2296
May	1st to 10th	Day	52% 5% 9% 19% 13% 2	2340
	11th to 20th	Day	43% 6% 9% 21% 18% 2	2378
	21st to end	Day	41% 7% 15% 22% 14% 2	2725
June	1st to 10th	Day	47% 7% 12% 22% 11% 1	2520
	11th to 20th	Day	45% 8% 12% 21% 13% 1	2520
	21st to end	Day	43% 10% 12% 21% 13% 1	2520
July	1st to 10th	Day	42% 7% 11% 22% 15% 2	2450
	11th to 20th	Day	41% 8% 13% 24% 11% 2	2445
	21st to end	Day	41% 9% 13% 20% 16% 3	2591
August	1st to 10th	Day	42% 9% 14% 22% 13% 1	2377
	11th to 20th	Day	43% 7% 13% 23% 13% 1	2194
	21st to end	Day	39% 7% 11% 23% 18% 2	2376
September	1st to 10th	Day	42% 6% 11% 21% 18% 2	2160
	11th to 20th	Day	35% 5% 11% 22% 24% 3	2012
	21st to end	Day	41% 4 10% 22% 22% 2	1980
October	1st to 10th	Day	47% 6% 8% 21% 15% 4	1980
	11th to 20th	Day	38% 5% 13% 26% 17% 1	1761
	21st to end	Day	46% 5% 11% 19% 15% 4	1782
November	1st to 10th	Day	40% 4 12% 22% 19% 2	1671
	11th to 20th	Day	39% 6% 12% 21% 20% 3	1710
	21st to end	Day	42% 5% 11% 26% 13% 3	1709
December	1st to 10th	Day	47% 7% 12% 17% 16% 1	1520
	11th to 20th	Day	44% 4 14% 18% 18% 2	1906
	21st to end	Day	52% 5% 11% 22% 10% 1	2772
January	1st to 10th	Night	42% 4 12% 23% 16% 3	2649
	11th to 20th	Night	49% 6% 12% 20% 10% 3	2602
	21st to end	Night	42% 4 12% 21% 19% 3	2771
February	1st to 10th	Night	48% 5% 12% 22% 12% 2	2250
	11th to 20th	Night	46% 5% 9% 21% 15% 4	2100
	21st to end	Night	47% 5% 8% 22% 14% 3	1776
March	1st to 10th	Night	42% 6% 11% 23% 15% 3	2160
	11th to 20th	Night	45% 7% 10% 20% 16% 3	2102
	21st to end	Night	44% 4 9% 18% 19% 5%	2000
April	1st to 10th	Night	47% 6% 11% 19% 15% 3	1800
	11th to 20th	Night	45% 6% 10% 19% 18% 2	1775
	21st to end	Night	47% 5% 10% 21% 13% 5%	1662
May	1st to 10th	Night	51% 6% 11% 17% 12% 3	1620
	11th to 20th	Night	46% 7% 11% 19% 15% 3	1535
	21st to end	Night	43% 9% 17% 18% 12% 1	1551
June	1st to 10th	Night	45% 8% 12% 20% 14% 1	1440
	11th to 20th	Night	44% 8% 14% 23% 11% 1	1440
	21st to end	Night	38% 9% 15% 24% 13%	1440
July	1st to 10th	Night	42% 9% 13% 21% 15% 1	1400
	11th to 20th	Night	43% 8% 15% 25% 8% 1	1392
	21st to end	Night	39% 8% 15% 22% 14% 2	1486
August	1st to 10th	Night	37% 9% 15% 25% 13% 2	1518
	11th to 20th	Night	42% 8% 14% 24% 11% 1	1703
	21st to end	Night	35% 6% 12% 27% 18% 1	1980
September	1st to 10th	Night	42% 7% 10% 24% 16% 1	1800
	11th to 20th	Night	35% 5% 11% 21% 26% 2	1948
	21st to end	Night	42% 5% 9% 21% 21% 3	1980
October	1st to 10th	Night	46% 5% 11% 21% 13% 3	1980
	11th to 20th	Night	38% 6% 13% 27% 15% 1	2197
	21st to end	Night	47% 6% 12% 20% 14% 1	2574
November	1st to 10th	Night	38% 5% 12% 22% 20% 4	2407
	11th to 20th	Night	42% 4 11% 21% 19% 3	2470
	21st to end	Night	40% 4 14% 27% 13% 2	2470
December	1st to 10th	Night	46% 5% 13% 21% 14% 1	2660
	11th to 20th	Night	45% 5% 13% 18% 17% 2	3288
	21st to end	Night	51% 4 9% 24% 12% 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	33% 22% 27% 16% 2	180
	11th to 20th	Day	35% 26% 27% 8% 4	172
	21st to end	Day	30% 26% 34% 7% 2 1	176
February	1st to 10th	Day	38% 25% 25% 12% 1	330
	11th to 20th	Day	33% 27% 31% 7% 2	416
	21st to end	Day	34% 25% 30% 8% 3	346
March	1st to 10th	Day	33% 20% 32% 14% 2	420
	11th to 20th	Day	32% 24% 30% 11% 3	420
	21st to end	Day	30% 23% 31% 10% 3 3	462
April	1st to 10th	Day	34% 23% 27% 11% 4 1	420
	11th to 20th	Day	31% 18% 32% 14% 4	412
	21st to end	Day	32% 20% 29% 13% 3 2	420
May	1st to 10th	Day	35% 19% 28% 12% 5% 1	423
	11th to 20th	Day	25% 16% 38% 15% 5% 1	447
	21st to end	Day	23% 12% 31% 24% 7% 4	485
June	1st to 10th	Day	22% 13% 32% 22% 9% 3	450
	11th to 20th	Day	19% 15% 37% 20% 7% 1	450
	21st to end	Day	17% 15% 35% 22% 8% 2	450
July	1st to 10th	Day	19% 8% 28% 28% 14% 3	445
	11th to 20th	Day	15% 9% 31% 29% 13% 4	444
	21st to end	Day	15% 10% 36% 28% 9% 2	488
August	1st to 10th	Day	15% 8% 33% 30% 12% 2	446
	11th to 20th	Day	16% 9% 31% 27% 12% 5%	439
	21st to end	Day	11% 12% 33% 23% 14% 6%	495
September	1st to 10th	Day	22% 8% 29% 21% 14% 6%	450
	11th to 20th	Day	19% 13% 30% 20% 11% 8%	450
	21st to end	Day	20% 14% 33% 19% 9% 5%	450
October	1st to 10th	Day	30% 13% 23% 19% 10% 4	450
	11th to 20th	Day	21% 17% 29% 22% 8% 2	426
	21st to end	Day	31% 16% 29% 16% 7% 1	396
November	1st to 10th	Day	20% 17% 34% 18% 9% 2	365
	11th to 20th	Day	31% 21% 28% 14% 5%	370
	21st to end	Day	27% 21% 28% 18% 5% 1	369
December	1st to 10th	Day	27% 29% 27% 14% 4	180
	11th to 20th	Day	36% 26% 31% 6% 1	180
	21st to end	Day	31% 28% 27% 9% 3 2	198
January	1st to 10th	Night	24% 21% 38% 15% 3	760
	11th to 20th	Night	30% 19% 32% 16% 2	744
	21st to end	Night	23% 23% 33% 18% 3	792
February	1st to 10th	Night	26% 20% 34% 18% 2	720
	11th to 20th	Night	27% 24% 37% 12% 1	703
	21st to end	Night	32% 20% 32% 14% 2	592
March	1st to 10th	Night	27% 21% 35% 15% 2	720
	11th to 20th	Night	24% 24% 33% 17% 3	720
	21st to end	Night	27% 21% 33% 16% 3	792
April	1st to 10th	Night	28% 19% 33% 18% 2	720
	11th to 20th	Night	23% 22% 31% 21% 3	711
	21st to end	Night	28% 17% 36% 15% 4	582
May	1st to 10th	Night	30% 16% 30% 19% 5%	540
	11th to 20th	Night	22% 16% 37% 21% 4	473
	21st to end	Night	21% 10% 32% 29% 6% 1	389
June	1st to 10th	Night	20% 8% 35% 28% 9% 1	360
	11th to 20th	Night	20% 8% 35% 31% 6%	360
	21st to end	Night	17% 5% 35% 35% 9%	360
July	1st to 10th	Night	15% 4 30% 35% 15%	350
	11th to 20th	Night	16% 2 27% 39% 16%	348
	21st to end	Night	14% 6% 34% 36% 9% 1	394
August	1st to 10th	Night	12% 4 33% 38% 11% 1	454
	11th to 20th	Night	14% 5% 28% 36% 17% 1	639
	21st to end	Night	15% 7% 25% 34% 19% 1	792
September	1st to 10th	Night	17% 8% 32% 29% 13% 1	720
	11th to 20th	Night	17% 8% 28% 29% 14% 3	720
	21st to end	Night	21% 7% 33% 25% 13% 2	720
October	1st to 10th	Night	26% 10% 28% 23% 10% 3	720
	11th to 20th	Night	20% 14% 32% 24% 9% 1	720
	21st to end	Night	29% 15% 31% 18% 6% 1	792
November	1st to 10th	Night	19% 14% 34% 24% 8% 1	740
	11th to 20th	Night	26% 18% 33% 17% 6% 1	760
	21st to end	Night	24% 18% 33% 19% 5%	760
December	1st to 10th	Night	30% 17% 32% 16% 5%	760
	11th to 20th	Night	27% 22% 32% 18% 2	760
	21st to end	Night	28% 19% 36% 15% 2	836

Page generated at 2024-01-13 19:28:28. Version ac07

Copyright 2024 Attilla Danko