Help with forecast details

[msacco]

Hello, since the weather is not very good lately, I've been searching for a good site to get some forecast, I've seen a lot of recommendations about meteoblue, but I'd like to get into details what each category means, and what is a good or bad weather.

So if someone will be able to just point out the important categories I need to look at, and what is a good/decent/bad value for that category, that would be very useful!

Thanks for the help!

[Davey-T]

Forecasts are exactly that, some ones best computer aided guess at what the weather might be like and not renowned for their accuracy here in the UK being stuck between Europe and the Atlantic.

You may have more luck in your country if your weather is more predictable, all of the forecasters use basically the same information fed into computers so they can all say something different depending on their algorithms.

I usually check the satellite images and look out of the window 

Dave 

[msacco]

15 minutes ago, Davey-T said:

Forecasts are exactly that, some ones best computer aided guess at what the weather might be like and not renowned for their accuracy here in the UK being stuck between Europe and the Atlantic.

You may have more luck in your country if your weather is more predictable, all of the forecasters use basically the same information fed into computers so they can all say something different depending on their algorithms.

I usually check the satellite images and look out of the window 

Dave 

My question was more about how to read that forecast information

[vlaiv]

Weather data or forecast?

Forecast is rather straight forward - look if cloudy or clear, what temperature and pressure, if using meteoblue astronomical seeing forecast is something that you want to note as well.

Not much to reading it really - the most important thing is cloud cover - you want it to be clear. Medium and low altitude clouds are usually different than high altitude clouds - you can easily spot them by eye. High altitude clouds are harder to see at night and often have same effect as fog / mist - making targets harder to see. If imaging they tend to produce star halos. Few percent low altitude and medium cloud cover is acceptable - you may get briefly interrupted in observing but you should be ok for most of the night. High altitude clouds of few percent in my experience usually mean more than few percent and these will be nuisance.

Temperature is important so you know what to wear - it is usually more cold than temperature alone would suggest - this is because observing is stationary activity and body is not producing enough heat - so wear worm clothes. It is also indicative of dew point (together with relative humidity and pressure) - how likely things are to dew up on you. Wind helps a bit with this - moving air keeps equipment from getting too cold and somewhat prevent dew from forming. High pressure means higher probability of fog and low ground level transparency if it is cold and humid enough (autumn, winter, sometimes early spring).

If imaging look at predicted temperature changes over the course of the night - this will give you hint how often you need to refocus. Wind as already said helps with dew but causes problems on lighter mounts when tracking long exposure or autoguiding. Larger and longer scopes are more susceptible to it because of larger surface and higher arm momentum.

Depending what you want out of forecast, you will read and interpret it differently. I've found that it is best to consult multiple sources. If conditions in the evening match that of forecast it is higher probability that it will be right during the course of the night. The further into future you are consulting forecast - less accurate it will be, so for best results limit yourself to half a day ahead - look at forecast in the afternoon / evening for that particular night.

Good sources of forecast:

sat24.com - "real time" satellite image of cloud cover (for night time use IR mode) - you can usually judge if there will be incoming clouds based on motion of cloud cover in past 1h.

jet stream forecast (can be found on couple websites) - this is important for good seeing - you don't want too much of a jet stream overhead.

general forecast (mostly cloud cover) - clearoutside.com, yr.no, meteoblue

astronomical seeing (together with jet stream forecast above - meteoblue

And finally something not often mentioned - aerosol optical depth - important for deep sky work, it represents "high altitude" transparency - general sky transparency in visible spectrum not related to local conditions (like fog or local air pollution)

https://atmosphere.copernicus.eu/catalogue#/

This forecast / chart in particular:

http://macc.copernicus-atmosphere.eu/d/services/gac/nrt/nrt_opticaldepth/

(for some reason currently reporting server error on link above)

 

 

[msacco]

14 minutes ago, vlaiv said:

Weather data or forecast?

Forecast is rather straight forward - look if cloudy or clear, what temperature and pressure, if using meteoblue astronomical seeing forecast is something that you want to note as well.

Not much to reading it really - the most important thing is cloud cover - you want it to be clear. Medium and low altitude clouds are usually different than high altitude clouds - you can easily spot them by eye. High altitude clouds are harder to see at night and often have same effect as fog / mist - making targets harder to see. If imaging they tend to produce star halos. Few percent low altitude and medium cloud cover is acceptable - you may get briefly interrupted in observing but you should be ok for most of the night. High altitude clouds of few percent in my experience usually mean more than few percent and these will be nuisance.

Temperature is important so you know what to wear - it is usually more cold than temperature alone would suggest - this is because observing is stationary activity and body is not producing enough heat - so wear worm clothes. It is also indicative of dew point (together with relative humidity and pressure) - how likely things are to dew up on you. Wind helps a bit with this - moving air keeps equipment from getting too cold and somewhat prevent dew from forming. High pressure means higher probability of fog and low ground level transparency if it is cold and humid enough (autumn, winter, sometimes early spring).

If imaging look at predicted temperature changes over the course of the night - this will give you hint how often you need to refocus. Wind as already said helps with dew but causes problems on lighter mounts when tracking long exposure or autoguiding. Larger and longer scopes are more susceptible to it because of larger surface and higher arm momentum.

Depending what you want out of forecast, you will read and interpret it differently. I've found that it is best to consult multiple sources. If conditions in the evening match that of forecast it is higher probability that it will be right during the course of the night. The further into future you are consulting forecast - less accurate it will be, so for best results limit yourself to half a day ahead - look at forecast in the afternoon / evening for that particular night.

Good sources of forecast:

sat24.com - "real time" satellite image of cloud cover (for night time use IR mode) - you can usually judge if there will be incoming clouds based on motion of cloud cover in past 1h.

jet stream forecast (can be found on couple websites) - this is important for good seeing - you don't want too much of a jet stream overhead.

general forecast (mostly cloud cover) - clearoutside.com, yr.no, meteoblue

astronomical seeing (together with jet stream forecast above - meteoblue

And finally something not often mentioned - aerosol optical depth - important for deep sky work, it represents "high altitude" transparency - general sky transparency in visible spectrum not related to local conditions (like fog or local air pollution)

https://atmosphere.copernicus.eu/catalogue#/

This forecast / chart in particular:

http://macc.copernicus-atmosphere.eu/d/services/gac/nrt/nrt_opticaldepth/

(for some reason currently reporting server error on link above)

 

 

Thanks for the reply, one thing I'd like to know is what do you mean by "you don't want too much of a jet stream overhead", or cloud cover, lets take meteoblue for example, we have Clouds(low, mid, high), what the numbers mean there? What is good clouds value, what is decent clouds value and what is high cloud value?

Same thing for Seeing indexes, jet stream as you mentioned and so on, the numbers simply doesn't tell me anything there, as I have nothing to compare it to. What I'd want to get is some reference to good/average/bad values for each section I need to focus on.

Thanks.

[vlaiv]

Ah, ok.

So cloud cover is usually divided into 3 segments: high altitude clouds, middle altitude clouds and low altitude clouds.

Clouds are divided by altitude because they behave and look differently like I explained above. Number next to each is sky cover in percentage.

So when it says 100% - that means total sky cloud cover - no stars are visible in any part of sky. To visualize cloud cover it useful to divide sky above you into 8 segments - like slices of orange. Main four quadrants being front / back and left/right combinations - then divide each into another half (slices of orange or slices of cake approach) - each carries 12.5% of sky cover. So 50% about half will be under clouds - 25% about one quarter. 10% will be less than one of mentioned slices and that is considered "clear" (but not perfectly clear - like no clouds). On a sunny day this means most places around you are sun lit, with occasional passing shade from small cloud. In observing it means - most of the time you will be able to observe target, but occasionally a cloud will cover it for couple of minutes. Again this is in relation to low and middle clouds - high clouds behave differently, but number is the same - expected percentage of sky covered with clouds.

With jet stream this graph sums it nicely:

You want to be without yellow /  red coverage in your region. Jet stream is expressed in speed / velocity units (so km/h, mp/h, m/s) and anything above about 6-8 m/s (or equivalent in other units) is starting to get progressively worse. It appears that perfectly "still" air or very slow jet stream is not good either. So good values are between 2 and 8 m/s. There needs to be some flow of air and it needs to be smooth and not turbulent - this is why it needs to be in this range.

Seeing index is meteoblue special thing - it is like seeing rating - 1 being very bad, 5 being excellent and other values in between just gradient between these two border cases (like school grades / marks). There is actual numerical measure of seeing, and it's forecast is also given on meteoblue - arc sec column. This is full width at half maximum of 2 second star image integration - value expressed in arc seconds.

Numerical value has more meaning for imaging than observing (if you take 2 second exposure star size will have FWHM of that much arc seconds for large telescope), although there is very good correlation between the two, but you need a sort of "table" to translate arc second seeing into visual seeing effects if you are not used to it. Here is a good comparison:

http://www.damianpeach.com/pickering.htm

(pickering scale animation)

https://www.handprint.com/ASTRO/seeing2.html

(Different quantification of seeing and their relation - look under "CCD Full Width Half Maximum" for explanation of arc second seeing)

[jinchuriki]

15 hours ago, vlaiv said:

Ah, ok.

So cloud cover is usually divided into 3 segments: high altitude clouds, middle altitude clouds and low altitude clouds.

Clouds are divided by altitude because they behave and look differently like I explained above. Number next to each is sky cover in percentage.

So when it says 100% - that means total sky cloud cover - no stars are visible in any part of sky. To visualize cloud cover it useful to divide sky above you into 8 segments - like slices of orange. Main four quadrants being front / back and left/right combinations - then divide each into another half (slices of orange or slices of cake approach) - each carries 12.5% of sky cover. So 50% about half will be under clouds - 25% about one quarter. 10% will be less than one of mentioned slices and that is considered "clear" (but not perfectly clear - like no clouds). On a sunny day this means most places around you are sun lit, with occasional passing shade from small cloud. In observing it means - most of the time you will be able to observe target, but occasionally a cloud will cover it for couple of minutes. Again this is in relation to low and middle clouds - high clouds behave differently, but number is the same - expected percentage of sky covered with clouds.

With jet stream this graph sums it nicely:

You want to be without yellow /  red coverage in your region. Jet stream is expressed in speed / velocity units (so km/h, mp/h, m/s) and anything above about 6-8 m/s (or equivalent in other units) is starting to get progressively worse. It appears that perfectly "still" air or very slow jet stream is not good either. So good values are between 2 and 8 m/s. There needs to be some flow of air and it needs to be smooth and not turbulent - this is why it needs to be in this range.

Seeing index is meteoblue special thing - it is like seeing rating - 1 being very bad, 5 being excellent and other values in between just gradient between these two border cases (like school grades / marks). There is actual numerical measure of seeing, and it's forecast is also given on meteoblue - arc sec column. This is full width at half maximum of 2 second star image integration - value expressed in arc seconds.

Numerical value has more meaning for imaging than observing (if you take 2 second exposure star size will have FWHM of that much arc seconds for large telescope), although there is very good correlation between the two, but you need a sort of "table" to translate arc second seeing into visual seeing effects if you are not used to it. Here is a good comparison:

http://www.damianpeach.com/pickering.htm

(pickering scale animation)

https://www.handprint.com/ASTRO/seeing2.html

(Different quantification of seeing and their relation - look under "CCD Full Width Half Maximum" for explanation of arc second seeing)

Wow, nice writing

[msacco]

15 hours ago, vlaiv said:

Ah, ok.

So cloud cover is usually divided into 3 segments: high altitude clouds, middle altitude clouds and low altitude clouds.

Clouds are divided by altitude because they behave and look differently like I explained above. Number next to each is sky cover in percentage.

So when it says 100% - that means total sky cloud cover - no stars are visible in any part of sky. To visualize cloud cover it useful to divide sky above you into 8 segments - like slices of orange. Main four quadrants being front / back and left/right combinations - then divide each into another half (slices of orange or slices of cake approach) - each carries 12.5% of sky cover. So 50% about half will be under clouds - 25% about one quarter. 10% will be less than one of mentioned slices and that is considered "clear" (but not perfectly clear - like no clouds). On a sunny day this means most places around you are sun lit, with occasional passing shade from small cloud. In observing it means - most of the time you will be able to observe target, but occasionally a cloud will cover it for couple of minutes. Again this is in relation to low and middle clouds - high clouds behave differently, but number is the same - expected percentage of sky covered with clouds.

With jet stream this graph sums it nicely:

You want to be without yellow /  red coverage in your region. Jet stream is expressed in speed / velocity units (so km/h, mp/h, m/s) and anything above about 6-8 m/s (or equivalent in other units) is starting to get progressively worse. It appears that perfectly "still" air or very slow jet stream is not good either. So good values are between 2 and 8 m/s. There needs to be some flow of air and it needs to be smooth and not turbulent - this is why it needs to be in this range.

Seeing index is meteoblue special thing - it is like seeing rating - 1 being very bad, 5 being excellent and other values in between just gradient between these two border cases (like school grades / marks). There is actual numerical measure of seeing, and it's forecast is also given on meteoblue - arc sec column. This is full width at half maximum of 2 second star image integration - value expressed in arc seconds.

Numerical value has more meaning for imaging than observing (if you take 2 second exposure star size will have FWHM of that much arc seconds for large telescope), although there is very good correlation between the two, but you need a sort of "table" to translate arc second seeing into visual seeing effects if you are not used to it. Here is a good comparison:

http://www.damianpeach.com/pickering.htm

(pickering scale animation)

https://www.handprint.com/ASTRO/seeing2.html

(Different quantification of seeing and their relation - look under "CCD Full Width Half Maximum" for explanation of arc second seeing)

Amazing explanation, thank you so much for taking your time to write it down, really appreciate it!!

[popeye85]

Fantastic exlanation-one to be pinned maybee?

Not heard of metroblue before-looks good although surprised how different the website is to the app! Anyone got any experience as to how accurate it is?