Instrument Ground School Basic Weather

Name: Instrument Ground School Basic Weather
Uploaded: 2017-08-15T04:12:00+00:00
Duration: PTM17S53
Channel: Angelique Sowell
Description: Instrument Ground School Basic Weather

Instrument Ground School Basic Weather
Authored By Lt Colonel Garrett L. Sager 30-Jan-2006 TX-129 Fort Worth Senior Squadron Modified by Lt Colonel Fred Blundell TX-129 Fort Worth Senior Squadron For Local Training Rev Jan-2014

This Training Slide Show is a project undertaken by Lt Colonel Fred Blundell of the TX-129 Fort Worth Senior Squadron, Fort Worth, TX for local use to assist those CAP Members interested in advancing their skills. The information contained herein is for CAP Member’s personal use and is not intended to replace or be a substitute for any of the CAP National Training Programs. Users should review the presentation’s Revision Number at the end of each file name to ensure that they have the most current publication.

Instrument Ground School Weather
Overview Hazards to Flight Causes & Types of Weather Sources of Information

Overview As pilots, we deal with weather every time we fly. As Instrument Pilots, our knowledge of weather must be more in depth than VFR pilots.

Hazards to Flight “Most Weather is Flyable” – Robert Buck There are 3 principle Weather Hazards can really get you into trouble: Low Visibility / Ceilings Thunderstorms Ice

Causes of Weather All weather processes are directly associated with the exchange of heat As an example, updrafts, caused by solar heating of the surface, cause cumulus clouds as the air raises and the moisture condenses, releasing the heat absorbed at the surface.

Causes of Weather Unequal heating of the Earth’s surface causes differences in temperature & pressure (and altimeter settings) Points of equal pressure are connected on weather maps using lines called “isobars” Wind flows from high-pressure to low pressure. Flow is perpendicular to isobars

Causes of Weather Coriollis force deflects wind to the right in Northern hemisphere. Earth rotates under the wind, causing an apparent change in wind direction. Coriollis force acts perpendicular to wind direction, and is proportional to wind speed It deflects wind so strongly that wind subsequently flows parallel to isobars.

Causes of Weather Coriollis force deflects wind to the right in Northern hemisphere. It’s greater at higher altitudes, since wind near the surface (below 2000 AGL) is slowed back friction. Since surface winds are slower, they are less effected by Coriollis force. Near the surface, the pressure gradient may be stronger than Coriollis force, causing the wind to flow across the isobars.

Layers of the Atmosphere Troposphere From the surface to the “tropopause”, the boundary between the troposphere & stratosphere. Most of our weather occurs in the troposphere Height of tropopause varies from about 25,000 feet near the poles, to about 37,000 feet in the mid-latitudes, to about 65,000 at the equator.

Layers of the Atmosphere

Layers of the Atmosphere Troposphere At the tropopause, temperature and wind vary greatly An abrupt change in the temperature lapse rate occurs at the tropopause Up to the tropopause, temperature decreases with altitude Above the tropopause, temperature is constant with altitude.

Layers of the Atmosphere Stratosphere Layer above the troposphere Low moisture, and few clouds Temperature remains relatively constant with altitude in the stratosphere. This creates a temperature inversion. Only the most powerful thunderstorms have sufficient energy to climb into the stratosphere.

Layers of the Atmosphere Jet Stream – a narrow, disjointed, wandering “river” of air. Flows from west to east, near the tropopause By definition, has speeds > 50 kt Weaker & farther north during summer Stronger & farther south during winter Moves with pressure ridges & troughs.

Air Masses Air Masses are large regions of air with uniform temperature and moisture properties. The boundary between air masses is called a “front”. A “warm front” occurs when a warmer air mass overtakes a cooler one A “cold front” occurs when a cooler air mass overtakes a warmer one.

Air Masses - Stability Lapse Rate – the change in temperature with altitude “Adiabatic” lapse rate is the temperature change due to expansional cooling. Dry adiabatic lapse rate is 3C per 1000 ft Adiabatic lapse rate varies from 1.1C to 2.8C, depending on moisture content Average adiabatic lapse rate is 2C per 1000 ft.

Air Masses - Stability Lapse rate can be used to gage atmospheric stability. Actual lapse rate is sometimes referred to as “ambient” lapse rate – average 2C per 1000 ft Lapse rate > 2C per 1000 ft, combined with high humidity indicates an unstable atmosphere, and thunderstorms are likely. Moist air is less stable than dry air, because it cools more slowly with altitude, so it must rise higher to cool.

Air Masses - Stability Cloud Formation due to lifting Determined by the stability of the air before lifting. Turbulence & cumuliform clouds (vertical development) are created when unstable air rises. Stable air lifted by mountain slopes creates stratiform clouds. Unstable air lifted by mountain slopes creates cumuliform clouds.

Air Masses - Stability Stable Air Stratiform Clouds & Fog Smooth Air Steady Precipitation Fair – poor visibility, due to haze & smoke

Air Masses - Stability UnStable Air Cumuliform Clouds Turbulent Air Showery Precipitation Good Visibility (outside of clouds)

Air Masses - Stability Cold air moving over warm surface results in turbulence, cumuliform clouds and good visibility This is due to the cold air being heated by surface, creating instability and lifting action Precipitation growth rate is enhanced by lifted, moisture laden air currents that condense, increasing droplet size.

Air Masses Fronts Wind changes across a front Wind shear may occur ahead of a warm front. Wind shear may occur just after a cold front passes Squall lines may develop ahead of a cold front Frontal waves and low-pressure cyclones usually from in slow moving cold fronts, or stationary fronts

Fronts

Temperature Inversions A temperature inversion is when temperature increases or remains constant with altitude, rather than decreases. Results in warm, stable air below the inversion May develop near the ground on cool, clear nights with light wind, due to terrestrial radiation. Stability results in smooth air, and haze, fog or low clouds.

Temperature & Dewpoint The ability of the air to hold moisture is directly related to it’s temperature. Warm air holds more moisture than cold air The temperature at which the air is 100% saturated is called the “dew point”. If the air temperature is within 3C (5F) of the dewpoint and is decreasing, expect low clouds and fog to form.

Temperature & Dewpoint Frost may form on a surface, if the surface temperature is below the dew point and the dew point is below freezing. Water vapor is visible when it condenses into clouds, fog or dew. Evaporation is when liquid water converts to vapor Sublimation is when ice converts directly to vapor.

Fog Radiation Fog occurs with clear skies, little wind, and small temperature – dew point spread over land – usually low flat areas Ground cools faster than air, cooling air very close to surface, resulting in a temperature inversion.

Fog Advection Fog forms when warm moist air, usually from over a large body of water, moves over cool land. Requires Wind Often happens in coastal areas Upslope Fog results from warm, moist air being forced up sloping terrain.

Fog Precipitation Induced Fog Caused by warm air dropping rain into cooler air beneath Evaporation from the precipitation saturates cooler air below, causing fog. Occurs easily in industrial areas, where pollution creates an abundance of nuclei for the moisture to condense onto.

Cloud Families High Clouds Ice crystals – little threat of aircraft icing. Middle Clouds Low Clouds Clouds with extensive vertical development. Cumulus, cumulonimbus

Cloud Families Standing Lenticular Altocumulus clouds (ACSL) are lens-shaped, and form on the downwind side of mountain ridges. Indicate very strong turbulence

Cumulus Clouds Cumulus clouds occur in unstable, moist air with a lifting action. Turbulence occurs at & below cloud level “Nimbus” means rain cloud Tower cumulus is an early sign of cumulonimbus Cumulonimbus Clouds (thunderstorms) contain the greatest turbulence.

Thunderstorms 3 stage life cycle Cumulus – the building stage. Towering Cumulus, with continuous updrafts Mature Greatest Intensity, both updrafts & downdrafts Dissipating Continuous downdrafts, raining out.

Thunderstorms

Thunderstorms Always has lightning, since lightning causes thunder. Produced by cumulonimbus clouds, and occur with Water Vapor Unstable Lapse Rate Lifting Action

Thunderstorms

Thunderstorms Produces wind shear turbulence If penetrating a thunderstorm, fly straight ahead, set power for turbulence penetration speed, and try to maintain level attitude. Do not turn around – turn increases load factor on airplane No not try to maintain altitude Severe Thunderstorms should be avoided by at least 20 miles. May cause turbulence and hail miles from the storm.

Thunderstorms “Squall Line” is a non-frontal, narrow band of thunderstorms forming ahead of a cold front Most severe conditions – hail, turbulence, tornados, etc. A “squall” is defined as a sudden increase in wind speed of at least 16 kts up to 22 kts, lasting for more than 1 minute.

Thunderstorms Embedded thunderstorms are storms that are obscured by cloudy conditions, haze layers, etc. Visual “see & avoid” may be inadequate “Spherics” or weather radar are essential to avoiding embedded thunderstorms. Radar detects precipitation, not instrument conditions.

Icing Tests indicate that frost, snow or ice with texture similar to coarse sandpaper reduces lift by up to 30%, and increased drag by up to 40%. “Freezing Level” is the altitude where freezing temperatures exist. Can be determined using the average lapse rate of 2C per 1000 ft.

Icing

Icing Requires 2 conditions Visible Moisture Temperature at or below freezing Freezing rain results in the fastest and greatest accumulation of ice. Indicates warmer temperatures above – temperature inversion.

Icing

Icing Ice pellets caused by rain freezing at higher altitude Heavy, wet snow means temperature is above freezing at your altitude. It formed above you, but is on the verge of melting Frost on the wings disrupts the flow over the wings, and should be removed before flight.

Weather Information Flight Service DUATS “The Weather Channel” Internet Sources NOAA / ADDS Intellicast FOUS

Reading: Aviation Weather Gleim Study Unit 8: Aviation Weather

Questions? Always Think Safety!

Instrument Ground School Basic Weather

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