Course 5 - Section 10 - Synoptic Features Flashcards
Two basic cloud forms1
Stratiform
appear in horizontal layers. They often have very small vertical motions associated with them and they can cover a large area
Cumuliform
Have a more prominent vertical formation and usually do not cover a very large area. They are the result of rising air currents and have significant updrafts and downdrafts
Cumuliform clouds are of particular interest to pilots and ATS personnel due to their impact on flying conditions.
Weather phenomena associated with cumuliform clouds can include precipitation, icing, high wind, turbulence, poor visibility, and runway contamination
weather associated with cumuliform clouds can even cause structural damage to aircraft and aviation infrastructure
List the four types of cumuliform clouds
Cumulus (CU)
Towering cumulus (TCU)
Cumulonimbus (CB)
Altocumulus castellanus (ACC)
Cumulus (CU) clouds
Develop vertically in the form of rising mounds, domes or towers
Have a bulging upper part
appear detached from other clouds
Are generally dense with sharp or ragged outlines
Are mostly brilliant white at the top and their bases are relatively dark and nearly horizontal
GET PROGRESSIVELY LARGER AND POTENTIALLY MORE SEVERE AS THE VERTICAL UPDRAFTS AND DOWNDRAFTS GROW
Towering Cumulus (TCU) clouds
Towering cumulus (TCU) clouds are also known as cumulus congestus
TCUs are made of a rapidly growing cumulus or individual dome shaped cloud. The height of TCU exceeds the width
THE DISTINCTIVE CAULIFLOWER TOP OF A TCU OFTEN MEANS SHOWERS BELOW
GET PROGRESSIVELY LARGER AND POTENTIALLY MORE SEVERE AS THE VERTICAL UPDRAFTS AND DOWNDRAFTS GROW
Cumulonimbus (CB) clouds
Cumulonimbus (CB) clouds are thunderclouds. THEY CONTAIN THUNDERSTORM ACTIVITY
They appear heave and dense with a considerable vertical extent in the form of a mountain or huge tower.
At least part of a CB’s upper portion is smooth, fibrous or striated, and nearly always flattened; this part often spreads out in the shape of an anvil or a vast plume
Low ragged clouds frequently occur under the base of CBs. These clouds may or may not merge with the base of the CB
CBs can attain heights of more than 70 000 feet, however, most CBs in Canada average about 39 000 feet. THESE CLOUDS RESULT IN SIGNIFICANT ICING AND TURBULENCE AND ARE OFTEN A FACTOR IN WEATHER RELATED AIRCRAFT ACCIDENTS
GET PROGRESSIVELY LARGER AND POTENTIALLY MORE SEVERE AS THE VERTICAL UPDRAFTS AND DOWNDRAFTS GROW
Altocumulus Castellanus (ACC) clouds
Altocumulus castellanus (ACC) clouds are white, grey or both white and grey. They contain patches, sheets, or layers of cloud generally with shading
ACCs are composed of laminae, rounded masses and rolls, which are sometimes partly fibrous or diffuse and which may or may not be merged
Most of the regularly arranged small elements have an apparent width of between one and five degrees
ACCs display vertical growth and are the result of mid-level instability which is defined as the up and down movement of air in the middle levels of the atmostphere.
ACCs ARE OFTEN A TELLTALE SIGN OF IMPENDING THUNDERSTORM DEVELOPMENT
Name the types of pressure systems
High pressure
Low pressure
troughs
ridges
High Pressure Areas
A high pressure area is a region where the atmospheric pressure is greater than its surrounding environment, with pressure values increasing towards the centre. It can cover hundreds to thousands of square km
A high pressure area may also be called a high or an anticyclone
On surface weather maps, highs are labelled H
AIR CIRCULATES CLOCKWISE AROUND THE CENTRE OF HIGH PRESSURE SYSTEMS
Low Pressure Areas
A low pressure area is a region where the atmospheric pressure is lower than the surrounding environment, with pressure values decreasing towards the centre. Its diameter can range from ten to hundreds of km
A low pressure area may also be called a low, a cyclone, or a depression
On surface weather maps, lows are labelled L
AIR CIRCULATES COUNTER CLOCKWISE AROUND THE CENTRE OF LOW-PRESSURE SYSTEMS
Troughs
Low pressure systems are often not a perfect circle. Their shape may include arms that extend from their centre. Those arms are called troughs
A trough is essentially an elongated region of relatively low atmospheric pressure, often associated with fronts
Ridges
high pressure systems can also have arms that extend from their centre. These are called ridges
A ridge is essentially an elongated region of relatively high atmospheric pressure
Isobars
Pressure systems are represented on weather maps using isobars
isobars are curved lines that connect points of equal pressure and show variations in pressure at any given time
Specifically, isobars join lines of equal mean sea level pressure (MSLP) and form pressure patterns that outline or enclose high and low pressure areas
- measured in hectopascals (hPa)
- analyzed on meteorological charts
- used to identify high and low pressure systems
- SPACED EVERY 4 hPa above and below 1000 hPa
Pressure Gradient
Pressure gradient is the rate of change of pressure with horizontal distance measured in kilometers
The speed of wind is directly proportional to the pressure gradient. The faster the pressure changes, the stronger the wind will be.
The difference in pressure between isobars is always 4 hPA, while the horizontal spacing or the distance in kilometers between the isobars varies
weak vs steep pressure gradient
How are pressure gradients calculated?
Pressure gradients are always calculated at 4 hPa in “X” kilometers, where “X” is the distance between two consecutive isobars
Steep Pressure Gradient
Isobars are close or crowded together
A low is termed “Deep”
A high is termed “Strong”
Weak pressure gradient
Isobars are far apart
A low is termed “shallow”
A high is termed “weak”
Coriolis force
The coriolis force is caused by the earth’s rotation. In the northern hemisphere, as the air moves from a high pressure area to a low pressure area, the coriolis force deflects moving air to the right.
This is because the earth rotates counter clockwise if you are standing on its surface in the northern hemisphere
The coriolis force does not cause wind; it affects the direction of air movement. Affected air flows parallel to the isobars. If the earth didn’t rotate, wind would flow directly from a high to a low pressure area
Buys Ballots Law
In the northern hemisphere, the pressure gradient force and coriolis force combine to cause air to flow parallel to the isobars, clockwise around a high, counter clock wise around a low
With the wind at your back in the northern hemisphere the area of low pressure lies to your left
veering and backing
veering: wind makes a clockwise change in direction
backing: wind makes a counter clockwise change in direction
Wind on surface maps
Arrows are used to represent wind information on surface maps
The arrow is always plotted to indicate the direction that the wind is blowing FROM
The arrow has barbs at one end that indicate wind speed. Each full barb representing 10 knots and each half barb representing 5 knots
In the image, a wind speed of 15 knots is indicated
Lifting Agents: Convection
With convection, air is heated from below by contact with the earth’s surface
Rising columns of air, known as thermals, are usually separated by areas of sinking air, known as subsidence
Lifting Agents: Orographic Lift
With orographic lift, air is forced up sloping terrain by the wind
The type of cloud that results depends on the stability f the lifted air
Lifting Agents: Frontal Lift
With frontal lift, air is forced to rise by a wedge of colder and denser air
Lifting Agents: Mechanical Turbulence
With mechanical turbulence, friction between the air and the ground causes the air to be stirred up into a series of swirling motion known as eddies
Mechanical turbulence depends on the strength of the wind and the roughness of the terrain
Lifting Agents: Convergence
Convergence occurs at the centre of low-pressure systems
The air converges at this point and is forced to rise, resulting in cloud and precipitation
Fronts
A front is a transition zone between two air masses
Fronts are often areas of intense weather significant to aviation
Since north america has four air mass structures (cA, mA, mP, mT) there are three frontal boundaries
Cold Front
Warm Front
Stationary Front
Front Symbology
On a weather map, different symbols are used to identify a cold front, warm front and stationary front
- An arrowhead is used to identify a COLD front
- A half circle is used to identify a warm front
- Alternating arrowheads and half-circles are used to identify a stationary front
Polar Front Theory
Surface wind change with cold fronts
Before a COLD FRONT arrives, the wind ahead of the front (in the warmer air mass) is typically out of the south-southwest. Once the front passes through, the wind will usually shift (VEER) around to the west-northwest (in the colder air mass)
Remember that air moves nearly parallel to the isobars but crosses isobars inwards AROUND A LOW because of surface friction.
The wind shift occurs at the frontal surface and is most significant in the lower levels. The wind shift associated with flight through a front is more abrupt at a cold front than at a warm front
Surface wind change with warm fronts
Before a WARM FRONT arrives, the wind ahead of the front in the cooler air mass is typically from the east, but once the front passes through, the wind usually shifts (VEERS) around to the south-southwest (in the warmer air mass)
Remember that air moves nearly parallel to the isobars but crosses isobars inwards AROUND A LOW because of surface friction.
The wind shift occurs at the frontal surface and is most significant in the lower levels. The wind shift associated with flight through a front is more abrupt at a cold front than at a warm front
Temperature and Warm fronts
The passage of a warm front will result in an increase in temperature that may begin BEFORE THE FRONT REACHES THE STATION
This is due to the proximity of the warm air to the surface as teh depth of cold air over teh station decreases, as illustrated below
Temperature and Cold fronts
The arrival of a cold front will generally result in a sudden decrease in surface temperature. The temperature change usually begins WHEN THE FRONT REACHES THE STATION
With the passage of a cold front, it is possible to see a small temperature increase for a short period of time after frontal passage, especially if there is no cloud cover to prevent the sun from heating the ground
The slight increase of temperature accompanying the passage of a cold front occurs only near the surface where the ground has been heated. We can therefore expect low-level convection to occur since the ground is heated unevenly
Air pressure and fronts
Fronts are generally associated with areas of low pressure
AS THE WARM FRONT APPROACHES a location, the PRESSURE WILL GENERALLY FALL. After the warm front passes, the fall in pressure becomes more gradual, or there may be a slight increase in pressure
Because the cold air is denser than warm air, ONCE THE COLD FRONT HAS PASSED A LOCATION, THE PRESSURE BEGINS TO RISE (sometimes quite steep)
Frontal Turbulence
Frontal turbulence is caused by
- Lifting of warm air by a frontal surface leading to instability
- abrupt shifts in wind direction and speed between the warm and cold air masses near the frontal surface
Frontal turbulence is the strongest when the lifted warm air is moist and unstable
The most severe cases of frontal turbulence are generally associated with fast moving cold fronts
In these cases, the mixture of the two air masses, as well as the difference in the windspeed and/or direction (wind shear) add to the intensity of the turbulence
Turbulence is more commonly associated with cold fronts but may also be present to a lesser degree in a warm front
Warm Fronts and Precipitation
Factors which determine precipitation
At a warm front, three factors determine the type of cloud and precipitation
- The moisture content of the overrunning warm air
- The stability of the overrunning warm air
- The degree of overrunning (the pressure gradient in the warm air and angle of warm air motion relative to the frontal surface)
