Surface Wind Modifiers

In previous blogs we have described how the surface winds we measure with wind instruments are created by large area pressure differences, synoptic winds, and by local temperature and pressure differences that create phenomena like sea breezes, thunderstorm winds and katabatic winds, local winds.   These winds are often modified by surface irregularities and obstacles that can significantly impact surface wind speed and direction.

As wind flows over irregular surfaces from forests, to buildings, to hills, and mountains both its speed and direction can be change by these surface wind modifiers.  Sailors know that wind flowing over a forest before reaching a body of water will reduce wind by up to ½ the wind speed in open water well away from the forested shoreline.  Mountain climbers know that wind speeds through mountain passes can often be much higher than surrounding wind speeds.  Pilots know that the wind speed measured at the top of a hangar may be significantly different than runway level wind as it is distorted by the uplifting effect of the building on the horizontal component of the wind.   Anyone that has walked the streets of Chicago when the wind blows off Lake Michigan knows that wind flowing between two buildings is squeezed into a smaller area with a consequent increase in velocity.  To properly site wind instruments each of these surface wind modifiers must be carefully considered.

Point to Ponder: What wind measurements are of importance at a rooftop helicopter landing pad, are these measurements useful to a weather forecaster?

Katabatic and Anabatic Winds:
Local Surface winds are sometimes more a function of Temperature Differences between mountain tops and lower elevations than overriding Synoptic winds.  These winds are sometimes called Mountain Winds as they occur most frequently in mountainous areas, meteorologist call them Katabatic or Anabatic Winds

Anabatic Winds are upslope winds driven by warmer surface temperatures on a mountain slope than the surrounding air column.  Katabatic winds are downslope winds created when the mountain surface is colder than the surrounding air and creates a down slope wind.   Katabatic wind may range over fairly large areas as in the case of the Santa Anna winds experienced throughout southern California during certain times of the year.  They can produce winds to 80 miles per hour and dominate local weather patterns for extended periods of time (weeks).  As shown in figure 2.4 below, they are initiated when cold air atop higher land masses begins to flow down hill (remember cold air is heavier than warm air) displacing the warm air below it and warming adiabatically and often gaining speed in the process.   When the lower elevations are hot desert areas the temperature differences can be quite substantial on the order of 60 to 70 degrees F.  The greater the temperature difference the stronger the wind.  They are often so well-known that they are given names like California’ Santa Anna as mentioned above, the Chinook of the pacific northwest or the  Fohn in Switzerland.

Figure 2.4 Katabatic Wind

As you can see wind can be derived from a number of different meteorological phenomena that are either caused by large scale synoptic pressure and temperature differences or by local temperature and pressure differences.  Once generated, however, there are many small scale surface structures that can modify the wind direction and speed and distort the accuracy of the observing instrumentation.  We call these wind modifiers and will talk about them in future blogs.

Point to Ponder: Why do hot air balloonists like sea/land breezes and katabatic wind flow?

Mature Thunderstorm Wind:
Thunderstorms are primarily local thermal weather phenomena (usually less than  5 miles to sometimes more than 30 miles in diameter), that are caused by either local surface heating , Air Mass Thunderstorms , or by weather systems such as fronts, converging winds, or troughs aloft that force upward motion of the surrounding air.  From a surface wind perspective, thunderstorms, regardless of their cause can quickly and substantially modify wind direction and speed.  As shown in figure 2.3 below, the wind outflow from the base of a thunderstorm tends to hit the ground a radiate axially from the storm center.  This out flow can and often does exceed 50 mph  and may contain gusts in front of the storm and opposing winds aloft that create wind shear (wind flowing in opposite directions) near the surface.  As thunderstorms move from their initial formation, through the mature stage (as shown) surface wind surrounding the storm changes from updrafts and inflow (at the initial stages) to down flow and outflow at the mature stage.  Local thunderstorm generated winds easily overcome most synoptic surface winds as the local temperature/pressure differences often are greater than the larger scale synoptic differences.

Figure 2.3 Mature Thunderstorm Wind

Points to Ponder: What happens to accuracy of wind measurement at an airport with a thunderstorm sitting over the middle of airport?  How do you measure wind shear?

Surface winds are often more a function of surface features or local thermal changes than the large area differences in barometric pressure that drive synoptic winds. Temperature differences between water and land and between mountain tops and valleys can cause the air to lift and descend and generate airflow parallel to the surface that will either add to or subtract from wind flow generated by overlying synoptic winds.  We will briefly consider several sources of local surface winds: Sea/Land Breezes, Thunderstorms and Mountain (katabatic/anabatic) winds, caused by geography differences and local thermal differences.

Sea/Land Breezes:
Sea/Land Breezes are formed as a result of temperature differences between large bodies of water and adjoining land masses, usually within a few miles of the coast.  Water will typically retain heat longer than dry land creating temperature differences during the day as the land warms faster than the water thereby warming the overlaying air and creating lift over the land.  The rising air decreases pressure over the land draws in the cooler air from the over the adjacent colder water causing a sea breeze.   At night the land adjacent to a body of water cools faster than the water causing the warmer air over the water to lift and draw the cooler air over the land toward the water, a land breeze.

As you can see from the drawing in figure 2.2 during the day when the sun warms the land faster than the water (sea or large lake) the air over the land is lifted (remember warm air rises)from the low pressure (less dense air at the surface and cools as it rises (adiabatic lifting).  Over the sea the warmer air aloft sinks and cools as it approaches the cool water, the surface wind is thereby caused to flow inland from the water to fill the low pressure area caused by the adiabatic lift of air over the land.

During the night when the air over the land is cooled to temperatures below the temperature of the adjacent water the opposite flow occurs and surface air flows from the land toward the sea.  This phenomenon is most noticeable in the summer time in the coastal areas and is often minimized or eliminated by strong synoptic winds flowing over the land, especially on the east coast of the U.S.

Figure 2.2 LAND AND SEA BREEZES
SEA BREEZE   (On shore in afternoon and evening)

Point to Ponder: If synoptic wind is flowing in the same direction as the upper level circulation of a  land or sea breeze does it increase the lower level , surface, wind flow or decrease it?

Surface winds are often dominated by high or low pressure systems that typically move from west to east across the United States.  In the Northern Hemisphere, these winds flow counter clockwise and inward toward the center of a Low Pressure System and clockwise and outward from a high pressure system as indicated by the arrows depicting wind speed and direction in the below NOAA charts (go to http://adds.aviationweather.noaa.gov for current charts).  Also you can see in the NOAA charts that the surface winds generally move in the direction from a high pressure system to a low pressure system.  The greater the pressure gradient (closer the constant pressure lines are together on the chart) of a high or low pressure system the greater the wind speed as indicated by the number and size of “barbs” on the wind speed arrows. The large barbs indicate 10 knots (multiply knots by 1.15 to get MPH) of wind speed and smaller barbs 5 knots of wind speed and are added together to get total wind speed.  For example: two large barbs and one small barb indicate 25 knots of wind.  Think of the barbs as feathers on an arrow that points in the direction of the wind.

Point to Ponder: Why don’t these charts show differences in wind speed and direction that should occur as a result of terrain variations like mountains and bodies of water interfering with synoptic winds created by large area atmospheric pressure differences?