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1

For practical (operational) analyses, thermal advection can be expressed as a function of three factors:

temperature gradient

height contour spacing

angle between isotherms and height contours

2

 Temperature Gradient

 It 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.

3

Height Contour Spacing

 It determines the strength of the wind. Closely spaced height contours will lead to stronger winds and the potential for ahigher thermal advection

4

Angle between Isotherms and Height Contours 

 If the isotherms are perpendicular to height contours then the advection potential is higher.

5

Thus, thermal advection is maximized by the combination of:

1. Closely spaced isotherms
2. Closely spaced height contours
3. Isotherms perpendicular to the height contours

6

show maximim thermal advection

7

Thermal advection is minimized by the combination of:

1. Widely spaced isotherms
2. Widely spaced height contours
3. Isotherms parallel to height contours

8

show minimum thermal advection

9

The next stage is to determine 

whether the advection is cold air or warm air advection.

10

 cold air advection

If isotherms are approaching your point of interest that are colder than the temperature at your point of interest

11

warm air advection

If the isotherms are warmer

12

Estimation of Temperature Advection on Weather Maps

step 1

Sketch a streamline that intersects your forecast station. This is the line or curve along which the distances will be marked off. On synoptic charts a reasonable distance to evaluate the temperature gradient to estimate the advection is 100 km.

13

Estimation of Temperature Advection on Weather Maps

Step-1: Sketch a streamline that intersects your forecast station. This is the line or curve along which the distances will be marked off. On synoptic charts a reasonable distance to evaluate the temperature gradient to estimate the advection is 100 km

step2

Highlight the portion of streamline that intersects the station back 100km

14

Estimation of Temperature Advection on Weather Maps

Step-1: Sketch a streamline that intersects your forecast station. This is the line or curve along which the distances will be marked off. On synoptic charts a reasonable distance to evaluate the temperature gradient to estimate the advection is 100 km.
Step-2: Highlight the portion of streamline that intersects the station back 100km.

Step-3

Step-3: The temperature gradient is evaluated using central finite differences as:

ΔT/Δs = T2-T1 /DISTANCE 

where T2 is the temperature furthest downwind on the segment and T1 is the temperature furthest upwind on the segment.
The temperatures are estimated by simply reading directly off of the isotherms.

15

Estimation of Temperature Advection on Weather Maps

Step-1: Sketch a streamline that intersects your forecast station. This is the line or curve along which the distances will be marked off. On synoptic charts a reasonable distance to evaluate the temperature gradient to estimate the advection is 100 km.
Step-2: Highlight the portion of streamline that intersects the station back 100km.

Step-3: The temperature gradient is evaluated using central finite differences as:

ΔT/Δs = T2-T1 /DISTANCE 

where T2 is the temperature furthest downwind on the segment and T1 is the temperature furthest upwind on the segment.
The temperatures are estimated by simply reading directly off of the isotherms.

Step-4: The temperature gradient calculated in the last step is multiplied by the average wind speed on the segment.
Average wind speed is obtained by reading the wind barbs on the chart and taking an average value for the region of the segment of streamline.