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Check the MetEd pages for extensive information on meteorology.

Displayed variables may change from one mountain region to another.

Meteogram at a glance

To get a quick overview of the situation, check the last panel first, precipitation. It tells you whether it is raining or snowing. If it shows orange bars (convective), it means storm activity.

Then check the solar radiation (2nd panel from the bottom). If there are uniform peaks reaching the maximum, skies are likely to be clear. When they are broken or flattened peaks, it is likely to be cloudy.

The freezing level (5th panel from the top) tells you where to expect ice or snow, but it will change depending on orientation and exposure.

The 2m temperature and dew point (6th panel from the top) tells you the temperature, reddish is warm bluish is cold, purple very cold. If the dew point line is close to temperature line it means high humidity, if it is far away, low humidity.

 

Wind levels

Wind speed in km/h is indicated at different levels for a seven day period. Levels are selected at 50 hPa (mb) intervals, to reflect the closest level to the summit and below. Pressure has a relative correspondence to altitude, depending on atmospheric conditions. The equivalent height level is given in bracket and represent the average over the chart period. The bottom chart is the geopotential at the given pressure level, this is the actual correspondence between altitude and pressure. The altitude of the wind speed at any level will be roughly parallel to the geopotential line.

The wind speed is marked red above 60 km/h. This is an arbitrary threshold. 60 km/h is rather windy at high altitude, it may be OK on easy ground but it can make progress impossible on more technical routes.

Meteogram

The model outputs are given at pressure level, that is why the Y-axis shows milibars (mb) or hectopascals (hPa). The height over sea level corresponding to a given pressure changes depending on latitude, surface temperature and atmospheric temperature profile. A correspondence for the standard atmosphere is given in the charts below.

pressure vs altitude
click to enlarge in a new window

pressure vs altitude
click to enlarge in a new window

Hovmoellers

Hovmoellers (or rather pseudo Hovmoellers). First panel indicates the evolution of several parameters in a vertical profile over a week period. indicated on the Y-axis is the pressure level (see above) and on the X-axis the date. Data are plotted at 3h intervals. Background colour is relative humidity (RH) from white (0%) to grey to green (100%). High relative humidity values (over 80% - 90%) might indicate condensation, and therefore cloud formation fog and likely snowfalls.

Colour contours indicate air temperature (°C). The 0°C isotherm is highlighted, which helps forecasting snow level. It is worth remembering that it can snow at higher temperatures, depending on relative humidity and temperature profile aloft. These contours, together with wind speed and direction can give a rough idea of air masses and fronts displacement.

Wind barbs indicate wind speed and direction. Convention is 10 knots per barb, 5 knots half a barb and 50 knots per triangle. For example, wind coming from the North West at 60 knots: wind barb NW 60K; or wind from the SW at 25 knots: wind barb SW 25K

CAPE Convective Available Potential Energy

CAPE is the maximum buoyant energy available to an ascending parcel of air. It is a measure of atmospheric instability, with high numbers indicating possible convective activity (and therefore thunderstorms)

TCC Total cloud Cover

The amount of cloud cover, 100% means completely overcast, 0% completely clear. The GFS model seems to overestimate the total cloud cover. Check the solar irradiation for a better estimate.

1000 - 500mb thickness

The 1000 - 500 thickess is the height difference between the 1000mb level and the 500mb level, measured in decameters (10m). As air expands with heat, a smaller thickness means colder air in between. A practical rule of thumb is that precipitation on the ground is usually as snow if the thickness is smaller tan 540 (loosely, 50% of the time will snow below 300m at thickness=5400m).

Totals - Totals and Lifted Index

These index are useful to determine atmospheric instability and thunderstorm risk.

Totals-Totals is calculated as TT = (T850 - T500) + (Td850 - T500),

where T is air temperature, Td is dew point and the numbers indicate the pressure level.

The lifted index is the temperature difference between ambient temperature at 500mb and te temperature of an air parcel lifted to the same level (dry-adiabatically to saturation and moist-adiabatically above that).

These tables reproduced from Ohio University give a practical idea of the index meaning:

Totals - totals
< 43Thunderstorms unlikely
43-44Isolated thunderstorms
45-46Scattered thunderstorms
47-48Scattered thunderstorms/ isolated severe
49-50Scattered t-storms/few severe/isolated tornadoes
51-52Scattered-numerous t-storms/few-scattered severe/isolated tornadoes
53-55Numerous thunderstorms/ scattered tornadoes
56+You don't want to know


Lifted Index
> 0Thunderstorms unlikely
0 - -2Thunderstorms possible - trigger needed
-3 - -5Thunderstorms probable
-5 - -7Strong/severe thunderstorms. Tornadoes possible
-7 - -9Move to Alaska
< -9Yikes!

SLP Sea Level Pressure

This is obtained by the theoretical reduction of the barometric pressure to sea level. If ground level is above sea level, it shows the extrapolated pressure to sea level. It is probably the most familiar figure in weather charts, low pressure is usually associated to unstable weather conditions while a high pressure indicates stability.

Wind speed at 10m

This is the estimated wind speed at 10m above ground surface. The GFS model has a resolution of 0.5°, or approximately 50 x 50 km. Therefore the terrain is smoothed and ground level may not correspont to actual ground level at a given point. Wind barbs indicate wind speed and direction as explained above.

Temperature and dew point temperature at 2m

Air temperature and dew point at 2m above ground level (see above comment on model resolution). Dew point is the temperature to which air must be cooled in order to reach saturation. The closer the dew point to the air temperature the higher the relative humidity, if they are equal it means saturated air.

Relative Humidity at 2m

Relative humidity at 2m above ground level. 100% indictes saturation and therefore likely condensation, fog or clouds.

Short wave irradiation at surface

This is useful to check wether is cloudy or clear and gives a better idea of cloud thickness than the total cloud cover chart. An approximate idea of the thickness of cloud cover can be estimated by comparison with clear skies irradiation values in the chart below. The chart is calculated at 1000m a.s.l. for the European spots and at 5000m a.s.l. for the Everest

 

Modelled solar radiation on a horizontal surface for clear skies

Precipitación

Indicates type and amount of accumulated precipitation over the last three hours. Note that precipitation smaller than 1mm/6h is normally unreliable. Precipitation amount is always given at the base of the mountain (the height of the grid cell point). In the summary table, the precipitation type is estimated for the summit conditions.
Total / Rain
Convective precipitation
Freezing rain
snow
Ice pellets

 

Wind Chill Index


It is being updated....

Conversion Table

1 meter
1 m/s
1 m/s
1 m/s
1 km/h
°C * 9/5 + 32
(°F - 32) * 5/9
0 °C
0 °F
1 mb
Pressure to height
=
=
=
=
=
=
=
=
=
=
 
3.28 feet
1.944 knots
3.6 km/h
2.237 mph
0.6214 mph
°F
°C
32 °F
-17.78 °C
1 hPa  

Speed equivalences

 

A note about resolution

Global forecasts are based on the GFS at 0.5° resolution, European Alps and Iberian Peninsula are based on the WRF at 0.1° and North America on the NAM (WRF) at 25km resolution. On the WRF regional model the initial and boundary conditions are from the global model GFS. This results in more accurate forecasts, which can show local differences from sites that are not far apart. Winds are more accurate, as at higher resolution the topography is better defined. At 0.1° resolution, the WRF is 25 times more detailed than the GFS (see the figure below).

GFS vs WRF resolution

The red square is a box of 0.5° by 0.5°, which is the area of a grid cell of the GFS model. The grey squares are boxes of 0.1° resolution, or the area of the grid cell of the WRF model, which is the model that we use for the high resolution forecasts.

The best way to forecast conditions at your local spot is developing a "feeling" on how the general conditions are modified locally, you will achieve this with a little bit of practice and careful observation. After all, meteorology is a science, but weather forecasting is an art.

 

Some References

Ahrens, C. D. (1994), Meteorology Today: An Introduction to Weather, Climate, and the Environment, 5th ed., West Pubishing Company, St Paul, MN.

Holton, J. R. (2004), An introduction to Dynamic Meteorology, Elsevier Academic Press, Burlington, MA.

Lackmann, G. (2011), Midlatitude Synoptic Meteorology: Dynamics, Analysis, and Forecasting, American Meteorological Society, Boston, MA.

Salby, M. L. (2012), Physics of the Atmosphere and Climate, Cambridge University Press, New York.

Stull, R. B. (2000), Meteorology For Scientists And Engineers, Brooks/Cole, Pacific Grove, CA.

Whiteman, C. D. (2000), Mountain Meteorology: Fundamentals and Applications, Oxford University Press, New York.

Weather and Forecasting (Journal)

Online resources