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THE 850 MB CHART

METEOROLOGIST JEFF HABY

What do I need to look for on this chart?:

**Chart is good for assessing low level warm air and cold air advection

1. Advection is a function of height contour spacing, the temperature gradient, and the angle isotherms cross height contours
2. Low level warm air advection contributes to synoptic scale rising air; Low level cold air advection contributes to synoptic scale sinking air

**Region of strong thermal gradient gives indication of 850 millibar front and regions of convergence

**Look for convergence, divergence, confluence, and diffluence

1. Air rises due to low level convergence and confluence

**Use dewpoint depression to determine if atmosphere is near saturation or dry at this level

**Determine intensity of highs and lows

1. Deep low (surrounded by several height contours)
2. Deep high (surrounded by several height contours) covering a large spatial area
3. Disregard highs and lows not surrounded by at least one isohypse
4. Several highs located near each other indicates one broad area of high pressure and not a scattering of individual highs


For locations close to sea level, the 850 mb chart represents the top (or close to the top) of the planetary boundary layer. In the PBL, friction and turbulent motion eddies are common. The wind blows at a more constant speed and direction above the 850 mb level. In high elevations areas such as the High Plains and Mountainous West, the 850 mb level will be near the surface or even below the surface. The 850 mb chart is used as a proxy for the surface chart at high elevation areas while the 700 mb chart is used as a proxy for the 850 mb chart.

The most intense thermal advections will be found in the low levels of the atmosphere. These thermal advections are termed either warm air advection (WAA) or cold air advection (CAA). The two isopleths displayed on the 850 chart are isotherms and height contours. The combination of these two isopleths determines the amount of thermal advection.

Thermal advection is a function of three factors: (1) the temperature gradient, (2) the height contour spacing and (3) the angle the isotherms make with the height contours. The temperature gradient is determined by how close the isotherms are to each other. Closely spaced isotherms increase the rate of thermal advection, especially if the wind is strong and blowing through the thermal gradient. The height contour spacing determines the strength of the wind. Closely spaced height contours will lead to stronger winds and the potential for a higher thermal advection. The third factor is the angle of isotherms to height contours. If isotherms are perpendicular to height contours then the advection potential is higher.

Thermal advection is maximized by the combination of:
1. Closely spaced isotherms
2. Closely spaced height contours
3. Isotherms perpendicular to height contours


Thermal advection is minimized by the combination of:
1. Widely spaced isotherms
2. Widely spaced height contours
3. Isotherms parallel to height contours



The next stage is to determine whether the advection is cold air or warm air advection. If isotherms are approaching your point of interest that are colder than the temperature at your point of interest, then it is cold air advection. If the isotherms are warmer, then it is warm air advection.

An important parameter to study on the 850 chart is the dewpoint depression. Each reporting station will give a value of the temperature and dewpoint depression in degrees Celsius. Suppose it is 17 Celsius with a 25 degree dewpoint depression. To find the dewpoint, subtract the dewpoint depression from the temperature. The dewpoint is 17 - 25 = -8 C. If the dewpoint depression is small at the surface and 850 mb, then the depth of near saturated conditions extends through the entire PBL, depending on elevation. Some 850 charts will show the temperature and dewpoint (not dewpoint depression). The dewpoint is always less than or equal to the temperature.

There will be a scattering of highs and lows across the 850 chart. The key is to eliminate the highs and lows which are not significant. Lows with several height contours surrounding them are significant while those without contours are not key weather players at the moment or represent only a slight relative minimum of pressure. High pressure covers a larger area than low pressure. At times you may see several high pressure over a broad region. These highs can be grouped together into one high pressure region.

Below is a list of items that can make you a better forecaster:

WATCH FOR:

Watch for return flow from the Gulf of Mexico. Often models have difficulty replacing a dry stable air mass with a warm and humid airmass in a quick time frame. When high pressure moves into the SE US, the clockwise flow will force Gulf air into the US. The same goes for lows transporting moisture out of the Gulf of Mexico. This rapid flux of moisture and warm air advection can bring unexpected precipitation the models did not pick up.

WATCH FOR:

Watch for height falls and height rises. Low pressure tends to develop toward the greater height falls. Height rises indicate low pressure is leaving or a ridge is building.


Location of 850 mb charts on the Internet given below

850 DIFAX

850 GRAPHICAL ANALYSIS

850 MODEL INITIALIZATION