What do these indexes mean?

CAPE (Convectively Available Potential Energy)
This is a measure (in Joules per Kilogram of air (J/Kg)) of the amount of energy available/released by a rising parcel of air between 1000hpa and 500hpa. Technically it is calculated as the area between the environmental lapse rate and the parcels lapse rate, as shown on a tephigram, when the environmental lapse rate is on the left. It is commonly used to help calculate the likelihood of thunderstorms. Any value greater than 0 indicates a level of instability and can produce thunderstorms, though many other factors are obviously required. CAPE is generally accepted to need to exceed 1000J/Kg to produce severe thunderstorms.


LI (Lifted Index)
This is effectively a snapshot of the instability at the 500hpa level and is calculated by subtracting a parcels temperature from the environmental temperature (from a sounding), giving you the difference in ºC. If this value is negative it indicates the parcel is warmer than its surroundings and therefore has buoyancy. The more negative this value, the faster any updraught will be at 500hpa. Thunderstorms generally need to exceed the 500hpa level, so if LI is positive then thunderstorms are unlikely.


CIN (Convective Inhibition)
This is a measure (in Joules per Kilogram of air (J/Kg)) of the amount of energy required to force a parcel through a region of stability in the atmosphere. Technically it is calculated as the area between the environmental lapse rate and the parcels lapse rate, as shown on a tephigram, when the environmental lapse rate is on the right. This can be the friend or foe of convection... a smallish value possibly demonstrating a capping inversion. This should allow good insolation and allow temperatures to rise, possibly to the trigger temperature where convection is forced past the cap and released as free convection. Any CAPE above can then be realised. If the value is too low convection may clutter the sky and reduce insolation and therefore the temperature, preventing the best convection possible in the conditions from occurring. Alternatively, too high a value may result in the cap not being breached and the skies remaining clear.


TEMP Sig 99.5% (Potential Temperature)
This is purely a calculation of the probable temperature in ºC needed to allow convection, to 99.5%. This can be used to determine the probability that surface temperatures will be high enough to allow convection.


Surface Temperature
This is the forecast maximum temperature in ºC at 2 metres.


2 Metre Relative Humidity
This is the forecast relative humidity at 2 metres as a percentage of the maximum possible water vapour content of the atmosphere at the forecast temperature. High values would indicate mistiness or fog can be expected.


10 metre wind
This is the expected wind speed at the surface in knots.


Precipitation Rate
This is the forecast maximum precipitation rate to be expected in mm/h.


Precipitation 0-6hr
This is the total precipitation (in mm) expected in the 6 hours from the chart plot time.


MSL Pressure
This is the forecast atmospheric pressure reduced to sea level in millibars (mb).


1000-500hpa Thickness
Thickness is a useful indicator of the warmth of an air mass. The warmer the air is, the more it expands and hence the thicker it is. This is often used to forecast the probability of snowfall. Thicknesses below 535dam in dry air can support snowfall (other conditions being appropriate), whereas moist air can require thickness below 520dam before snowfall is possible.


Absolute Vorticity
This is used to indicate the force of dynamic lift, i.e. where there is a convergence of isobars, the air is effectively forced upwards as there is no escape downwards. This forced lift is generally expected along trough lines and frontal zones and can be another key factor to determine the amount of convection that can be expected. Low values where there is a divergence of the isobars tends to result in subsiding air which therefore squashes any convection.


Vertical Velocity
Similar to absolute vorticity, but this indicates the effective strength and direction of any forcing. A positive value, forecasting PVA (Positive Vorticity Advection) demonstrates the forcing will be upwards, a negative value showing NVA (Negative Vorticity Advection) demonstrating the forcing will be downwards. The larger the value, the stronger the forcing.


Temperature with Height
This is as it says and can be used as a good indicator of the general temperature of the air mass as a whole, as well as the gradient of temperature with height, which indicates the stability of that air mass. The temperature at altitude can also be used to indicate what precipitation can be expected. Temperatures below -5ºC at 850hpa often allows snowfall in an unstable atmosphere for example. Low temperatures in the upper reaches of the atmosphere can also be indicative of conditions where hail is likely and even lightning. Temperatures below -40ºC guarantee ice within the cloud and therefore an increased risk of hail and the separation of charge necessary for lightning.


Wind with Height
This can be used to indicate the position of the jet stream, as well as wind shear. Strong(ish) winds at altitude and a gradual change in direction with height can provide the ingredients for severe thunderstorms, as the updraughts and downdraughts are separated. This allows the convection to be stable and can allow large hail... even tornadoes.


Relative humidity with Height
Just as at the surface, the higher the relative humidity, the more likely the air will be saturated and cloud will form. This is therefore useful for forecasting the altitude at which clouds may form or how hazy the sky will be, assuming other factors fall in to line.