Jordan Hall Observatory
Clear Sky Chart
History

This sumarizes the distribution of each different forecast outcome for the Jordan Hall Observatory clear sky chart from 2005-11-06 to 2024-04-10.

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	15% 10% 5% 7% 6% 3 5% 5% 5% 7% 33%	83006
	Night	16% 11% 5% 7% 6% 3 4 4 5% 6% 32%	85820
Transparency	Day	51% 5% 11% 21% 11% 1	82708
	Night	53% 5% 11% 20% 11% 1	85639
Seeing	Day	20% 13% 29% 22% 12% 4	14866
	Night	21% 11% 29% 28% 11% 1	24896

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	15% 18% 11% 12% 7% 4 5% 4 5% 5% 16%	2527
	11th to 20th	Day	14% 16% 9% 13% 10% 5% 6% 5% 6% 6% 9%	2530
	21st to end	Day	26% 21% 8% 10% 6% 3 3 3 3 5% 11%	2783
February	1st to 10th	Day	16% 14% 7% 10% 8% 5% 6% 4 5% 5% 20%	1824
	11th to 20th	Day	15% 11% 5% 12% 10% 4 5% 5% 5% 6% 23%	1848
	21st to end	Day	17% 12% 8% 11% 6% 3 4 4 6% 5% 24%	1570
March	1st to 10th	Day	19% 13% 6% 7% 6% 3 3 4 4 6% 28%	3120
	11th to 20th	Day	12% 8% 7% 7% 6% 4 5% 5% 6% 9% 33%	3346
	21st to end	Day	19% 11% 7% 9% 8% 4 5% 5% 4 5% 23%	4005
April	1st to 10th	Day	17% 12% 5% 7% 5% 3 5% 4 5% 6% 31%	2269
	11th to 20th	Day	17% 10% 3 5% 5% 3 4 4 4 7% 37%	2138
	21st to end	Day	19% 9% 3 6% 5% 3 5% 4 5% 7% 34%	2340
May	1st to 10th	Day	17% 10% 4 6% 5% 3 5% 5% 5% 8% 33%	2340
	11th to 20th	Day	19% 9% 3 6% 4 2 5% 4 4 7% 36%	2301
	21st to end	Day	11% 6% 3 6% 6% 3 5% 5% 5% 9% 40%	2700
June	1st to 10th	Day	10% 5% 3 5% 6% 4 6% 5% 5% 8% 44%	2520
	11th to 20th	Day	10% 5% 2 6% 6% 4 5% 6% 5% 8% 43%	2520
	21st to end	Day	10% 5% 3 5% 5% 4 5% 6% 6% 9% 41%	2520
July	1st to 10th	Day	7% 5% 3 5% 5% 3 6% 5% 6% 9% 46%	2520
	11th to 20th	Day	9% 4 2 4 5% 4 5% 5% 7% 9% 46%	2520
	21st to end	Day	7% 4 2 4 6% 4 5% 6% 6% 10% 46%	2743
August	1st to 10th	Day	7% 4 2 5% 6% 4 5% 5% 6% 11% 44%	2341
	11th to 20th	Day	9% 5% 3 5% 5% 3 6% 6% 6% 9% 43%	2262
	21st to end	Day	9% 5% 2 5% 5% 3 5% 5% 6% 9% 47%	2376
September	1st to 10th	Day	10% 6% 2 6% 6% 3 5% 5% 6% 7% 44%	2147
	11th to 20th	Day	12% 5% 2 4 4 3 4 6% 7% 9% 45%	1980
	21st to end	Day	12% 5% 3 5% 6% 3 5% 4 5% 8% 42%	1980
October	1st to 10th	Day	14% 7% 3 6% 5% 4 4 4 5% 7% 42%	1980
	11th to 20th	Day	20% 8% 3 5% 5% 3 4 5% 6% 7% 33%	1780
	21st to end	Day	23% 10% 4 8% 7% 3 5% 5% 5% 6% 25%	1782
November	1st to 10th	Day	12% 11% 4 5% 6% 3 4 5% 5% 7% 38%	1672
	11th to 20th	Day	16% 16% 4 6% 8% 3 4 4 5% 5% 27%	1710
	21st to end	Day	21% 15% 5% 8% 6% 3 5% 4 3 5% 24%	1710
December	1st to 10th	Day	16% 15% 9% 11% 7% 4 4 4 3 5% 22%	1601
	11th to 20th	Day	13% 19% 10% 11% 7% 4 4 4 5% 4 19%	1918
	21st to end	Day	23% 21% 8% 13% 9% 3 4 3 3 4 10%	2783
January	1st to 10th	Night	17% 17% 8% 11% 7% 4 5% 4 5% 5% 17%	4428
	11th to 20th	Night	15% 19% 9% 13% 8% 4 4 6% 6% 5% 12%	4440
	21st to end	Night	25% 17% 11% 11% 7% 3 3 3 4 4 12%	4884
February	1st to 10th	Night	18% 14% 6% 8% 8% 4 4 4 5% 4 24%	2350
	11th to 20th	Night	14% 13% 6% 9% 8% 4 5% 4 5% 6% 26%	2218
	21st to end	Night	21% 12% 6% 10% 6% 4 5% 4 3 5% 25%	1884
March	1st to 10th	Night	17% 11% 5% 7% 6% 3 4 4 4 6% 33%	3733
	11th to 20th	Night	11% 7% 4 7% 6% 4 5% 5% 5% 6% 42%	3613
	21st to end	Night	19% 10% 6% 8% 5% 3 4 4 5% 6% 29%	3640
April	1st to 10th	Night	16% 10% 4 5% 6% 3 5% 5% 6% 7% 35%	1900
	11th to 20th	Night	18% 8% 3 6% 6% 3 5% 4 4 6% 37%	1769
	21st to end	Night	18% 8% 3 5% 5% 3 5% 5% 6% 6% 36%	1620
May	1st to 10th	Night	16% 9% 3 7% 6% 3 5% 5% 5% 7% 34%	1620
	11th to 20th	Night	18% 7% 3 5% 5% 3 4 4 5% 7% 39%	1592
	21st to end	Night	10% 6% 3 5% 6% 3 5% 5% 6% 8% 43%	1572
June	1st to 10th	Night	8% 4 3 4 4 3 5% 5% 6% 9% 49%	1440
	11th to 20th	Night	9% 4 3 5% 5% 3 5% 5% 6% 9% 45%	1440
	21st to end	Night	10% 4 2 5% 6% 3 4 6% 8% 11% 40%	1440
July	1st to 10th	Night	5% 4 2 4 4 4 5% 6% 6% 10% 51%	1440
	11th to 20th	Night	8% 4 2 3 5% 2 4 4 6% 10% 51%	1440
	21st to end	Night	6% 4 2 4 5% 3 6% 5% 7% 10% 50%	1575
August	1st to 10th	Night	7% 4 1 5% 4 3 4 6% 6% 10% 50%	1597
	11th to 20th	Night	8% 3 2 5% 4 4 4 5% 6% 9% 50%	1637
	21st to end	Night	9% 4 2 4 5% 3 4 5% 6% 8% 52%	1980
September	1st to 10th	Night	12% 5% 2 5% 4 3 4 5% 5% 7% 48%	1800
	11th to 20th	Night	11% 4 2 4 4 2 4 3 4 8% 54%	1975
	21st to end	Night	14% 6% 3 5% 4 2 3 4 5% 9% 45%	1980
October	1st to 10th	Night	15% 6% 3 4 5% 3 4 4 4 6% 46%	1980
	11th to 20th	Night	19% 10% 3 5% 6% 3 4 3 4 6% 37%	2176
	21st to end	Night	24% 10% 3 7% 5% 3 3 3 4 5% 33%	2574
November	1st to 10th	Night	14% 11% 4 6% 6% 2 4 4 4 6% 38%	2406
	11th to 20th	Night	14% 16% 4 8% 6% 4 5% 5% 5% 7% 28%	2470
	21st to end	Night	18% 16% 5% 8% 6% 3 3 4 4 6% 26%	2470
December	1st to 10th	Night	17% 16% 8% 9% 6% 3 5% 4 5% 6% 20%	2574
	11th to 20th	Night	16% 16% 8% 10% 9% 4 5% 5% 4 5% 19%	3279
	21st to end	Night	26% 19% 7% 8% 7% 3 4 4 4 5% 12%	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	71% 4 11% 11% 3	2528
	11th to 20th	Day	74% 4 8% 10% 4	2530
	21st to end	Day	78% 3 6% 10% 3	2783
February	1st to 10th	Day	66% 3 10% 13% 7% 1	1824
	11th to 20th	Day	61% 6% 8% 17% 7%	1858
	21st to end	Day	61% 3 12% 16% 7% 1	1570
March	1st to 10th	Day	58% 4 10% 20% 8%	3120
	11th to 20th	Day	47% 6% 12% 27% 7% 1	3346
	21st to end	Day	64% 4 11% 17% 4	4005
April	1st to 10th	Day	55% 3 10% 20% 12%	2269
	11th to 20th	Day	49% 4 10% 22% 13% 3	2138
	21st to end	Day	50% 4 12% 19% 12% 3	2340
May	1st to 10th	Day	50% 4 13% 23% 9% 1	2340
	11th to 20th	Day	49% 3 11% 22% 13% 2	2305
	21st to end	Day	41% 6% 12% 26% 14% 2	2700
June	1st to 10th	Day	37% 6% 11% 27% 17% 2	2520
	11th to 20th	Day	38% 5% 12% 24% 19% 3	2520
	21st to end	Day	39% 5% 15% 22% 16% 3	2520
July	1st to 10th	Day	35% 5% 11% 25% 19% 4	2451
	11th to 20th	Day	34% 5% 13% 28% 16% 3	2445
	21st to end	Day	34% 7% 12% 27% 17% 3	2591
August	1st to 10th	Day	34% 5% 16% 28% 16% 2	2327
	11th to 20th	Day	36% 6% 14% 27% 16% 2	2262
	21st to end	Day	34% 5% 12% 26% 18% 5%	2376
September	1st to 10th	Day	38% 5% 15% 21% 17% 4	2147
	11th to 20th	Day	35% 4 12% 30% 18% 2	1980
	21st to end	Day	41% 4 10% 26% 17% 2	1980
October	1st to 10th	Day	43% 4 11% 25% 16% 2	1980
	11th to 20th	Day	49% 4 11% 22% 13%	1780
	21st to end	Day	59% 4 11% 17% 8% 1	1782
November	1st to 10th	Day	45% 5% 12% 22% 15% 1	1672
	11th to 20th	Day	58% 5% 10% 18% 8% 1	1710
	21st to end	Day	63% 4 12% 17% 5%	1710
December	1st to 10th	Day	66% 3 11% 15% 5%	1601
	11th to 20th	Day	68% 4 11% 14% 2	1915
	21st to end	Day	80% 3 7% 7% 2	2783
January	1st to 10th	Night	69% 5% 9% 13% 4	4428
	11th to 20th	Night	73% 4 10% 10% 3	4440
	21st to end	Night	75% 4 8% 10% 3	4884
February	1st to 10th	Night	63% 4 9% 15% 8% 1	2350
	11th to 20th	Night	58% 4 8% 20% 8% 1	2219
	21st to end	Night	62% 4 9% 16% 8% 2	1884
March	1st to 10th	Night	53% 4 12% 24% 8%	3733
	11th to 20th	Night	42% 5% 11% 26% 14% 2	3613
	21st to end	Night	55% 5% 13% 19% 7% 1	3640
April	1st to 10th	Night	48% 5% 12% 21% 14% 1	1900
	11th to 20th	Night	49% 4 7% 21% 16% 3	1769
	21st to end	Night	49% 4 10% 21% 14% 2	1620
May	1st to 10th	Night	50% 4 12% 23% 10% 1	1620
	11th to 20th	Night	45% 5% 10% 21% 16% 2	1593
	21st to end	Night	39% 6% 15% 24% 14% 2	1573
June	1st to 10th	Night	31% 5% 12% 30% 19% 3	1440
	11th to 20th	Night	34% 6% 13% 26% 19% 2	1440
	21st to end	Night	36% 5% 17% 25% 15% 2	1440
July	1st to 10th	Night	30% 5% 15% 26% 19% 5%	1400
	11th to 20th	Night	29% 5% 14% 27% 22% 4	1392
	21st to end	Night	28% 8% 15% 24% 20% 5%	1487
August	1st to 10th	Night	29% 5% 13% 32% 19% 2	1587
	11th to 20th	Night	31% 6% 15% 27% 20% 2	1637
	21st to end	Night	31% 5% 11% 26% 22% 5%	1980
September	1st to 10th	Night	35% 6% 12% 24% 18% 5%	1800
	11th to 20th	Night	31% 4 8% 32% 24% 2	1975
	21st to end	Night	38% 4 10% 28% 19% 2	1980
October	1st to 10th	Night	39% 5% 8% 27% 19% 1	1980
	11th to 20th	Night	49% 3 9% 23% 15% 1	2176
	21st to end	Night	54% 4 10% 21% 10% 1	2574
November	1st to 10th	Night	46% 5% 9% 23% 15% 2	2406
	11th to 20th	Night	55% 5% 11% 19% 8% 1	2470
	21st to end	Night	60% 3 11% 18% 6%	2470
December	1st to 10th	Night	64% 4 10% 15% 6%	2574
	11th to 20th	Night	67% 4 11% 14% 4	3281
	21st to end	Night	75% 4 10% 8% 3	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% 23% 32% 17% 4 1	270
	11th to 20th	Day	30% 14% 38% 14% 3 1	270
	21st to end	Day	29% 18% 33% 18% 3 1	297
February	1st to 10th	Day	29% 22% 32% 12% 5%	270
	11th to 20th	Day	30% 22% 32% 14% 2	270
	21st to end	Day	32% 26% 33% 8% 1	225
March	1st to 10th	Day	26% 24% 35% 12% 3	270
	11th to 20th	Day	28% 15% 33% 19% 4 1	378
	21st to end	Day	33% 16% 28% 16% 6%	506
April	1st to 10th	Day	28% 15% 31% 19% 5% 1	457
	11th to 20th	Day	26% 22% 25% 15% 9% 2	412
	21st to end	Day	30% 15% 30% 16% 8% 2	420
May	1st to 10th	Day	29% 14% 30% 17% 8% 3	423
	11th to 20th	Day	25% 12% 29% 21% 9% 4	447
	21st to end	Day	14% 11% 32% 23% 14% 6%	654
June	1st to 10th	Day	13% 10% 31% 31% 11% 4	630
	11th to 20th	Day	14% 9% 30% 29% 15% 3	630
	21st to end	Day	15% 9% 33% 27% 12% 5%	630
July	1st to 10th	Day	10% 3 24% 30% 21% 12%	621
	11th to 20th	Day	11% 6% 31% 26% 17% 9%	618
	21st to end	Day	9% 7% 27% 33% 17% 7%	684
August	1st to 10th	Day	10% 2 20% 31% 22% 14%	460
	11th to 20th	Day	11% 5% 23% 31% 22% 8%	439
	21st to end	Day	10% 3 25% 29% 25% 7%	495
September	1st to 10th	Day	15% 8% 23% 32% 16% 7%	450
	11th to 20th	Day	10% 4 21% 28% 26% 10%	450
	21st to end	Day	17% 12% 22% 25% 17% 6%	450
October	1st to 10th	Day	19% 10% 26% 26% 16% 3	450
	11th to 20th	Day	27% 19% 28% 21% 5%	346
	21st to end	Day	34% 15% 38% 11% 2 1	297
November	1st to 10th	Day	17% 18% 38% 19% 7% 1	270
	11th to 20th	Day	30% 22% 27% 16% 5%	270
	21st to end	Day	28% 21% 35% 14% 2	270
December	1st to 10th	Day	24% 27% 32% 12% 4 1	270
	11th to 20th	Day	24% 27% 31% 14% 4	270
	21st to end	Day	30% 21% 34% 13% 1	297
January	1st to 10th	Night	25% 16% 33% 21% 5%	950
	11th to 20th	Night	25% 19% 33% 19% 3 1	950
	21st to end	Night	24% 14% 34% 24% 3	1045
February	1st to 10th	Night	28% 13% 35% 21% 3	777
	11th to 20th	Night	23% 17% 33% 24% 4	743
	21st to end	Night	29% 17% 34% 16% 4	628
March	1st to 10th	Night	22% 14% 34% 23% 6%	760
	11th to 20th	Night	19% 13% 37% 24% 7%	760
	21st to end	Night	26% 12% 32% 26% 4 1	736
April	1st to 10th	Night	21% 16% 32% 24% 6% 1	570
	11th to 20th	Night	23% 12% 32% 26% 6% 1	533
	21st to end	Night	22% 10% 32% 26% 9% 1	540
May	1st to 10th	Night	22% 8% 31% 31% 8% 1	540
	11th to 20th	Night	22% 5% 28% 33% 11%	531
	21st to end	Night	13% 5% 27% 36% 17% 1	584
June	1st to 10th	Night	9% 4 29% 42% 14% 1	540
	11th to 20th	Night	13% 4 29% 39% 15% 1	540
	21st to end	Night	12% 3 23% 40% 22% 1	540
July	1st to 10th	Night	8% 2 16% 42% 29% 4	525
	11th to 20th	Night	11% 2 21% 37% 25% 4	522
	21st to end	Night	7% 3 20% 41% 28% 1	591
August	1st to 10th	Night	8% 1 15% 46% 27% 3	537
	11th to 20th	Night	9% 1 15% 43% 27% 5%	534
	21st to end	Night	11% 2 17% 37% 30% 3	594
September	1st to 10th	Night	16% 3 18% 38% 22% 4	540
	11th to 20th	Night	11% 4 20% 37% 25% 4	715
	21st to end	Night	18% 7% 21% 32% 19% 3	720
October	1st to 10th	Night	18% 7% 27% 29% 18% 1	720
	11th to 20th	Night	24% 15% 29% 25% 6% 1	720
	21st to end	Night	29% 13% 31% 21% 6%	792
November	1st to 10th	Night	21% 15% 31% 24% 9% 1	740
	11th to 20th	Night	24% 17% 32% 22% 5% 1	760
	21st to end	Night	27% 12% 33% 22% 5%	760
December	1st to 10th	Night	27% 16% 34% 19% 4	864
	11th to 20th	Night	26% 15% 35% 20% 4	950
	21st to end	Night	34% 16% 30% 18% 2	1045

Page generated at 2024-04-07 09:28:10. Version ac07

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