Middlesex School Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Middlesex School 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% 6% 3 5% 5% 3 5% 4 5% 7% 40%	74839
	Night	17% 7% 3 5% 5% 3 4 4 5% 6% 41%	74962
Transparency	Day	43% 6% 11% 22% 15% 3	74544
	Night	43% 6% 11% 22% 16% 2	74791
Seeing	Day	23% 16% 33% 19% 8% 2	13920
	Night	22% 13% 33% 25% 7% 1	22944

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% 8% 3 6% 5% 3 4 4 4 6% 43%	1517
	11th to 20th	Day	19% 8% 2 5% 4 3 4 6% 5% 6% 38%	1486
	21st to end	Day	13% 7% 3 5% 5% 3 5% 4 5% 6% 44%	1584
February	1st to 10th	Day	20% 8% 3 5% 4 3 6% 4 6% 6% 34%	1670
	11th to 20th	Day	13% 7% 3 6% 6% 3 5% 5% 5% 7% 39%	1748
	21st to end	Day	19% 9% 3 6% 6% 4 4 4 3 5% 36%	1480
March	1st to 10th	Day	18% 7% 3 5% 4 4 4 5% 5% 6% 38%	1797
	11th to 20th	Day	17% 7% 3 5% 6% 3 5% 4 5% 6% 39%	1894
	21st to end	Day	19% 9% 3 6% 5% 3 3 3 4 6% 40%	2298
April	1st to 10th	Day	18% 9% 3 6% 5% 3 5% 3 5% 6% 38%	2160
	11th to 20th	Day	17% 6% 3 4 5% 3 5% 5% 5% 8% 38%	2147
	21st to end	Day	22% 6% 3 6% 6% 3 4 4 5% 6% 35%	2340
May	1st to 10th	Day	24% 9% 3 5% 4 3 5% 4 5% 7% 32%	2340
	11th to 20th	Day	18% 5% 2 6% 6% 3 4 4 4 7% 40%	2344
	21st to end	Day	14% 5% 2 5% 4 4 4 4 5% 8% 44%	2725
June	1st to 10th	Day	18% 6% 2 6% 5% 4 4 6% 7% 7% 34%	2520
	11th to 20th	Day	17% 5% 2 5% 5% 3 5% 6% 5% 9% 37%	2520
	21st to end	Day	18% 5% 2 6% 6% 3 5% 5% 6% 8% 35%	2520
July	1st to 10th	Day	14% 5% 3 5% 5% 3 5% 6% 6% 8% 41%	2520
	11th to 20th	Day	11% 5% 3 5% 6% 4 6% 5% 6% 10% 40%	2520
	21st to end	Day	11% 5% 2 6% 6% 2 5% 6% 7% 9% 41%	2744
August	1st to 10th	Day	9% 6% 2 5% 6% 4 6% 6% 6% 10% 39%	2362
	11th to 20th	Day	13% 5% 2 6% 5% 3 5% 6% 6% 9% 39%	2278
	21st to end	Day	12% 4 2 5% 5% 3 6% 5% 6% 8% 45%	2376
September	1st to 10th	Day	14% 6% 2 4 5% 3 4 5% 5% 7% 45%	2158
	11th to 20th	Day	16% 6% 2 4 3 2 3 3 4 6% 50%	1980
	21st to end	Day	17% 5% 3 5% 5% 2 4 3 4 7% 46%	1980
October	1st to 10th	Day	23% 6% 2 6% 4 3 4 4 5% 5% 40%	1980
	11th to 20th	Day	15% 6% 2 4 5% 2 4 4 5% 7% 46%	1756
	21st to end	Day	22% 7% 2 5% 4 3 3 5% 4 6% 39%	1782
November	1st to 10th	Day	16% 6% 3 5% 4 3 3 3 4 5% 49%	1671
	11th to 20th	Day	19% 6% 2 5% 3 2 4 4 4 5% 46%	1710
	21st to end	Day	19% 6% 3 5% 4 2 4 4 5% 5% 43%	1710
December	1st to 10th	Day	18% 8% 3 6% 5% 3 5% 3 5% 6% 39%	1544
	11th to 20th	Day	23% 5% 2 4 4 2 3 4 4 4 45%	1906
	21st to end	Day	22% 11% 3 6% 5% 3 4 4 4 6% 33%	2772
January	1st to 10th	Night	16% 9% 2 4 5% 3 4 4 4 5% 44%	2649
	11th to 20th	Night	18% 10% 3 6% 5% 3 4 4 5% 6% 36%	2602
	21st to end	Night	13% 9% 4 6% 5% 3 4 5% 5% 6% 41%	2772
February	1st to 10th	Night	17% 9% 2 6% 6% 3 5% 4 5% 5% 37%	2283
	11th to 20th	Night	15% 8% 4 5% 5% 3 4 4 5% 6% 40%	2098
	21st to end	Night	18% 9% 3 5% 4 2 4 4 4 7% 40%	1776
March	1st to 10th	Night	16% 8% 3 5% 5% 3 4 4 4 6% 41%	2150
	11th to 20th	Night	16% 8% 3 6% 6% 4 5% 5% 5% 5% 38%	2065
	21st to end	Night	18% 9% 3 5% 4 3 4 3 4 5% 41%	2056
April	1st to 10th	Night	18% 7% 3 6% 6% 4 4 4 5% 7% 37%	1800
	11th to 20th	Night	20% 6% 3 5% 5% 3 4 4 4 7% 40%	1762
	21st to end	Night	21% 7% 3 5% 5% 3 4 4 4 7% 38%	1620
May	1st to 10th	Night	24% 8% 3 5% 6% 3 4 5% 5% 5% 33%	1620
	11th to 20th	Night	19% 6% 2 5% 4 3 4 6% 6% 7% 37%	1564
	21st to end	Night	17% 6% 3 6% 5% 4 5% 5% 4 7% 39%	1550
June	1st to 10th	Night	19% 8% 3 6% 5% 3 3 3 6% 7% 37%	1440
	11th to 20th	Night	19% 7% 3 5% 5% 3 4 5% 5% 8% 36%	1440
	21st to end	Night	14% 5% 3 5% 5% 3 4 5% 7% 10% 39%	1440
July	1st to 10th	Night	14% 5% 3 5% 6% 3 5% 4 7% 8% 40%	1440
	11th to 20th	Night	12% 7% 4 5% 5% 4 4 5% 7% 9% 38%	1440
	21st to end	Night	12% 6% 2 5% 5% 3 5% 5% 6% 8% 44%	1574
August	1st to 10th	Night	9% 5% 2 4 5% 3 6% 5% 7% 8% 46%	1558
	11th to 20th	Night	11% 5% 2 6% 6% 4 6% 6% 7% 8% 38%	1619
	21st to end	Night	14% 4 2 4 4 3 4 4 5% 8% 47%	1980
September	1st to 10th	Night	13% 6% 2 5% 5% 2 3 4 5% 7% 47%	1800
	11th to 20th	Night	14% 5% 2 4 4 2 4 3 5% 6% 53%	1980
	21st to end	Night	16% 7% 2 4 4 2 3 5% 5% 6% 46%	1980
October	1st to 10th	Night	21% 6% 2 5% 4 3 4 3 4 5% 42%	1980
	11th to 20th	Night	16% 5% 2 4 4 3 4 5% 6% 7% 45%	2200
	21st to end	Night	22% 8% 3 5% 5% 2 4 4 4 6% 38%	2574
November	1st to 10th	Night	16% 5% 2 4 5% 3 3 4 5% 6% 48%	2407
	11th to 20th	Night	19% 6% 2 4 4 2 3 4 4 5% 46%	2470
	21st to end	Night	16% 7% 3 4 4 3 3 4 5% 7% 44%	2470
December	1st to 10th	Night	19% 8% 3 5% 5% 3 4 3 5% 6% 38%	2631
	11th to 20th	Night	19% 6% 3 5% 5% 2 5% 4 5% 6% 41%	3288
	21st to end	Night	21% 9% 3 6% 6% 3 4 4 5% 6% 34%	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	44% 3 9% 27% 14% 3	1518
	11th to 20th	Day	45% 7% 12% 20% 13% 3	1486
	21st to end	Day	40% 4 12% 25% 16% 3	1584
February	1st to 10th	Day	49% 5% 12% 19% 13% 3	1670
	11th to 20th	Day	45% 6% 11% 21% 14% 3	1757
	21st to end	Day	50% 4 8% 20% 14% 3	1480
March	1st to 10th	Day	45% 4 12% 22% 13% 4	1800
	11th to 20th	Day	45% 4 10% 21% 15% 4	1894
	21st to end	Day	48% 4 7% 18% 17% 7%	2298
April	1st to 10th	Day	48% 4 8% 19% 17% 4	2160
	11th to 20th	Day	44% 6% 10% 20% 17% 3	2147
	21st to end	Day	49% 5% 8% 17% 17% 4	2340
May	1st to 10th	Day	53% 4 8% 20% 13% 2	2340
	11th to 20th	Day	43% 7% 10% 18% 18% 4	2348
	21st to end	Day	39% 7% 13% 24% 15% 2	2725
June	1st to 10th	Day	46% 7% 13% 23% 10% 1	2520
	11th to 20th	Day	44% 8% 11% 19% 16% 2	2520
	21st to end	Day	45% 8% 12% 21% 13% 1	2520
July	1st to 10th	Day	41% 7% 10% 24% 16% 3	2450
	11th to 20th	Day	39% 7% 13% 26% 13% 2	2445
	21st to end	Day	38% 8% 13% 23% 14% 3	2591
August	1st to 10th	Day	40% 8% 14% 23% 14% 1	2348
	11th to 20th	Day	41% 8% 13% 24% 13% 1	2278
	21st to end	Day	36% 7% 10% 25% 19% 2	2376
September	1st to 10th	Day	37% 7% 11% 24% 19% 2	2158
	11th to 20th	Day	35% 4 9% 26% 23% 3	1980
	21st to end	Day	39% 5% 9% 22% 23% 3	1980
October	1st to 10th	Day	47% 4 9% 22% 14% 4	1980
	11th to 20th	Day	36% 6% 11% 30% 15% 2	1756
	21st to end	Day	46% 5% 12% 20% 15% 3	1782
November	1st to 10th	Day	40% 4 11% 25% 17% 2	1671
	11th to 20th	Day	41% 3 8% 24% 21% 2	1710
	21st to end	Day	42% 5% 15% 24% 13% 2	1710
December	1st to 10th	Day	47% 4 13% 17% 17% 1	1544
	11th to 20th	Day	44% 3 12% 19% 17% 5%	1906
	21st to end	Day	52% 5% 10% 21% 11% 1	2772
January	1st to 10th	Night	43% 4 11% 23% 16% 3	2649
	11th to 20th	Night	49% 6% 13% 20% 9% 3	2602
	21st to end	Night	44% 5% 10% 20% 19% 3	2772
February	1st to 10th	Night	48% 5% 12% 20% 13% 3	2283
	11th to 20th	Night	46% 4 9% 22% 15% 4	2100
	21st to end	Night	45% 4 7% 24% 16% 3	1776
March	1st to 10th	Night	44% 5% 8% 23% 15% 4	2160
	11th to 20th	Night	47% 5% 11% 17% 17% 4	2066
	21st to end	Night	46% 4 7% 18% 19% 6%	2056
April	1st to 10th	Night	47% 4 10% 19% 17% 3	1800
	11th to 20th	Night	45% 5% 9% 21% 18% 3	1762
	21st to end	Night	48% 4 10% 18% 15% 4	1620
May	1st to 10th	Night	53% 6% 8% 17% 13% 3	1620
	11th to 20th	Night	43% 7% 12% 18% 16% 4	1565
	21st to end	Night	45% 7% 14% 21% 13% 1	1551
June	1st to 10th	Night	47% 7% 11% 23% 11% 1	1440
	11th to 20th	Night	46% 6% 12% 20% 15% 2	1440
	21st to end	Night	37% 10% 14% 23% 14% 1	1440
July	1st to 10th	Night	39% 8% 13% 24% 15% 1	1400
	11th to 20th	Night	41% 8% 14% 26% 10% 1	1392
	21st to end	Night	37% 8% 13% 29% 11% 2	1486
August	1st to 10th	Night	34% 9% 15% 25% 16% 2	1548
	11th to 20th	Night	40% 9% 16% 22% 13% 1	1619
	21st to end	Night	36% 7% 10% 27% 20% 1	1980
September	1st to 10th	Night	37% 6% 11% 24% 19% 2	1800
	11th to 20th	Night	33% 6% 9% 22% 27% 2	1980
	21st to end	Night	38% 7% 8% 23% 22% 3	1980
October	1st to 10th	Night	45% 5% 11% 23% 12% 4	1980
	11th to 20th	Night	37% 7% 12% 28% 15% 1	2200
	21st to end	Night	48% 4 12% 20% 14% 1	2574
November	1st to 10th	Night	38% 5% 11% 22% 20% 3	2407
	11th to 20th	Night	42% 4 9% 22% 19% 3	2470
	21st to end	Night	41% 5% 13% 25% 14% 2	2470
December	1st to 10th	Night	47% 5% 12% 22% 13% 1	2631
	11th to 20th	Night	43% 5% 13% 18% 18% 2	3288
	21st to end	Night	51% 5% 7% 26% 11% 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	27% 23% 29% 16% 5%	180
	11th to 20th	Day	33% 19% 33% 12% 4	172
	21st to end	Day	23% 25% 36% 11% 5%	176
February	1st to 10th	Day	33% 18% 33% 14% 2	330
	11th to 20th	Day	26% 22% 37% 13% 2	416
	21st to end	Day	29% 18% 33% 17% 3	346
March	1st to 10th	Day	27% 18% 35% 18% 2	420
	11th to 20th	Day	28% 22% 32% 13% 4 1	420
	21st to end	Day	27% 20% 34% 9% 8% 2	462
April	1st to 10th	Day	31% 23% 25% 15% 5% 1	420
	11th to 20th	Day	28% 22% 33% 10% 6% 1	412
	21st to end	Day	30% 19% 32% 14% 3 2	420
May	1st to 10th	Day	33% 22% 28% 12% 4 2	423
	11th to 20th	Day	24% 17% 40% 14% 5% 1	447
	21st to end	Day	19% 13% 37% 22% 8% 2	485
June	1st to 10th	Day	19% 14% 40% 18% 8% 2	450
	11th to 20th	Day	16% 17% 39% 21% 5% 2	450
	21st to end	Day	18% 14% 35% 24% 8% 2	450
July	1st to 10th	Day	18% 10% 29% 27% 13% 2	445
	11th to 20th	Day	16% 7% 33% 30% 12% 3	444
	21st to end	Day	14% 8% 37% 31% 9% 1	488
August	1st to 10th	Day	12% 9% 32% 32% 12% 2	446
	11th to 20th	Day	17% 7% 32% 28% 13% 2	439
	21st to end	Day	14% 9% 34% 24% 14% 5%	495
September	1st to 10th	Day	21% 9% 28% 23% 16% 3	450
	11th to 20th	Day	20% 14% 28% 19% 12% 7%	450
	21st to end	Day	19% 15% 31% 22% 8% 5%	450
October	1st to 10th	Day	29% 13% 25% 20% 10% 4	450
	11th to 20th	Day	19% 16% 32% 22% 9% 2	401
	21st to end	Day	27% 15% 31% 18% 8% 1	396
November	1st to 10th	Day	22% 15% 32% 19% 10% 2	365
	11th to 20th	Day	30% 18% 29% 17% 5% 1	370
	21st to end	Day	28% 18% 30% 18% 6% 1	370
December	1st to 10th	Day	23% 25% 33% 15% 3	204
	11th to 20th	Day	33% 22% 33% 10% 2	180
	21st to end	Day	25% 23% 32% 15% 4 2	198
January	1st to 10th	Night	22% 18% 38% 19% 3	760
	11th to 20th	Night	28% 15% 35% 19% 2	744
	21st to end	Night	20% 18% 37% 21% 4	792
February	1st to 10th	Night	24% 17% 37% 20% 2	720
	11th to 20th	Night	23% 17% 40% 19% 1	703
	21st to end	Night	26% 18% 34% 19% 3 1	592
March	1st to 10th	Night	23% 17% 36% 20% 3	720
	11th to 20th	Night	22% 19% 37% 18% 4	720
	21st to end	Night	26% 16% 33% 20% 5%	792
April	1st to 10th	Night	24% 19% 32% 20% 4	720
	11th to 20th	Night	23% 17% 34% 19% 6%	698
	21st to end	Night	25% 16% 36% 20% 3	540
May	1st to 10th	Night	29% 14% 32% 22% 4	540
	11th to 20th	Night	22% 15% 38% 21% 4	503
	21st to end	Night	21% 8% 35% 29% 6% 1	389
June	1st to 10th	Night	22% 6% 35% 30% 6% 1	360
	11th to 20th	Night	18% 8% 36% 31% 6% 1	360
	21st to end	Night	16% 4 40% 34% 7%	360
July	1st to 10th	Night	15% 4 32% 38% 11% 1	350
	11th to 20th	Night	14% 4 28% 39% 14%	348
	21st to end	Night	13% 4 31% 42% 8% 1	394
August	1st to 10th	Night	11% 5% 31% 41% 11% 1	484
	11th to 20th	Night	11% 7% 30% 41% 11% 1	555
	21st to end	Night	15% 5% 24% 40% 14% 2	792
September	1st to 10th	Night	17% 5% 31% 33% 13%	720
	11th to 20th	Night	17% 8% 27% 29% 18% 2	720
	21st to end	Night	20% 8% 27% 31% 12% 2	720
October	1st to 10th	Night	25% 11% 25% 26% 12% 2	720
	11th to 20th	Night	20% 11% 32% 25% 11% 1	720
	21st to end	Night	28% 12% 32% 22% 5%	792
November	1st to 10th	Night	18% 14% 32% 26% 9% 1	740
	11th to 20th	Night	24% 16% 33% 20% 6% 1	760
	21st to end	Night	23% 19% 33% 20% 5% 1	760
December	1st to 10th	Night	26% 16% 34% 19% 4	760
	11th to 20th	Night	24% 17% 31% 24% 3	760
	21st to end	Night	25% 15% 36% 21% 3	836

Page generated at 2024-01-13 19:28:34. Version ac07

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