Weather Elements

Weather Elements

Learning Outcomes • Define atmospheric pressure. • Understand physiological changes caused by changes in atmospheric pressure. • Define wind and the effects of wind chill. • - Define heat and methods of heat transfer. • - Explain what temperature is and how it can be expressed on scales. • Describe how a microburst can affect a plane’s flight. • Define the processes involved with cloud formation. • Describe and identify different cloud types. • Describe different types of weather fronts.

Important Terms • atmospheric pressure - the weight of all of the atmosphere's gases and molecules on the Earth's surface • conduction - heating by direct contact • convection - heat transfer by vertical motion • heat - the total energy of all molecules within a substance • microburst - a downdraft or down burst phenomenon that creates unstable air and thunderstorm turbulence • radiation - heat transferred by the Sun • temperature - a measure of molecular motion expressed on a man-made scale • wind - a body of air in motion • wind chill - temperature and wind speed used to explain how cold it feels • cloud– a visible mass of water or ice particles in the atmosphere from which rain and other forms of precipitation fall. • adiabatic lapse rate – the rate at which the temperature of the atmosphere falls as altitude increases. • front - a line along which one mass of air meets another that is different in temperature or density.

The Atmosphere http://www.srh.noaa.gov/srh/jetstream/index.htm

Pressure

This diagram shows two typical jet stream positions at the height of summer and of winter.

The Hydrologic Cycle

Our air is made up of gases. Each of these gases has molecules, and these molecules have weight. This weight, or push on the Earth's surface, is called atmospheric pressure. The weight, or atmospheric pressure, in a given space depends on the number of molecules occupying that space. • We notice pressure changes in our body, particularly our ears and sinuses. Our bodies have trouble adjusting to rapid decreases or increases in pressure. Airplanes or even elevators can make us physically uncomfortable. When an airplane is taking off, the outside pressure decreases so the pressure inside our ear is higher. Also, when a plane is landing, the outside pressure increases so the pressure inside our ear is lower. Normally, air can move through the ear and equalize the pressure. However, if you have a cold and your ears are blocked or you have blocked sinuses, the air can't equalize and you may feel some discomfort or pain.

Wind and Wind Chill We have all heard of the wind chill, but what exactly is it and how does it work? To determine wind chill, temperature and wind speed are used to explain how cold it feels. It may be 30° F outside, but feels like 9° F because of the combination of cold temperature and strong winds. Actually, heat is escaping from your body and warms the air next to you. If the wind is calm or almost calm, the warm air will stay next to your body. However, if the wind is blowing, it blows the warm air away from your body, and the faster it is blowing, the faster the heat is being carried away causing you to feel colder. Thus, the pysiological effect of wind chill on the body is important to maintain safe body temperature.

Why is wind important?

Temperature and Heat Transfer

Radiation • If you have stood in front of a fireplace or near a campfire, you have felt the heat transfer known as radiation. The side of you nearest the fire warms, while your other side remains unaffected by the heat. Although you are surrounded by air, the air has nothing to do with this transfer of heat. Heat lamps, that keep food warm, work in the same way. Radiation is the transfer of heat energy through space by electromagnetic radiation. • Most of the electromagnetic radiation that comes to the earth from the sun is in the form of visible light. Light is made of waves of different frequencies. The frequency is the number of instances that a repeated event occurs, over a set time. In electromagnetic radiation, the frequency is the number of times an electromagnetic wave moves past a point each second. • Our brains interpret these different frequencies into colors, including red, orange, yellow, green, blue, indigo, and violet. When the eye views all these different colors at the same time, it is interpreted as white. Waves from the sun which we cannot see are infrared, which have lower frequencies than red, and ultraviolet, which have higher frequencies than violet light. • Most of the solar radiation is absorbed by the atmosphere and much of what reaches the earth's surface is radiated back into the atmosphere to become heat energy. Dark colored objects such as asphalt absorb more of the radiant energy and warm faster that light colored objects. Dark objects also radiate their energy faster than lighter colored objects.

Conduction • Conduction is the transfer of heat energy from one substance to another or within a substance. Have you ever left a metal spoon in a pot of soup being heated on a stove? After a short time the handle of the spoon will become hot. • This is due to transfer of heat energy from molecule to molecule or from atom to atom. Also, when objects are welded together, the metal becomes hot (the orange-red glow) by the transfer of heat from an arc. This is called conduction and is a very effective method of heat transfer in metals. However, air conducts heat poorly.

Convection • Convection is the transfer of heat energy in a fluid. This type of heating is most commonly seen in the kitchen when you see liquid boiling. • Air in the atmosphere acts as a fluid. The sun's radiation strikes the ground, thus warming the rocks. As the rock's temperature rises due to conduction, heat energy is released into the atmosphere, forming a bubble of air which is warmer than the surrounding air. This bubble of air rises into the atmosphere. As it rises, the bubble cools with the heat contained in the bubble moving into the atmosphere. • As the hot air mass rises, the air is replaced by the surrounding cooler, more dense air, what we feel as wind. These movements of air masses can be small in a certain region, such as local cumulus clouds, or large cycles in the troposphere, covering large sections of the earth. Convection currents are responsible for many weather patterns in the troposphere.

Clouds As a bubble or parcel of air rises it moves into an area of lower pressure (pressure decreases with height). As this occurs the parcel expands. This requires energy, or work, which takes heat away from the parcel. So as air rises it cools. This is called an adiabatic process. • The rate at which the parcel cools with increasing elevation is called the "lapse rate". The lapse rate of unsaturated air (air with relative humidity <100%) is 5.4°F per 1000 feet. This is called the dry lapse rate. This means for each 1000 feet increase in elevation, the air temperature will decrease 5.4°F or 2°C.

The 4 Basic Cloud Types • Cirroform • Nimboform • Cumuloform • Stratoform

Cirro-form • High-level clouds which form above 20,000 feet (6,000 meters) and are usually composed of ice crystals. High-level clouds are typically thin and white in appearance, but can create an array of colors when the sun is low on the horizon. Cirrus generally occur in fair weather and point in the direction of air movement at their elevation.

Nimbo-form • Nimbus comes from the Latin word meaning "rain". These clouds typically form between 7,000 and 15,000 feet (2,100 to 4,600 meters) and bring steady precipitation. As the clouds thicken and precipitation begins to fall, the bases of the clouds tend to lower toward the ground.

Cumulo-form • Clouds look like white fluffy cotton balls or heaps and show the vertical motion or thermal uplift of air taking place in the atmosphere. The level at which condensation and cloud formation begins is indicated by a flat cloud base, and its height will depend upon the humidity of the rising air. The more humid the air, the lower the cloud base. The tops of these clouds can reach over 60,000 feet.

Strato-form • "Stratus" is Latin for layer or blanket. The clouds consist of a featureless low layer that can cover the entire sky like a blanket, bringing generally gray and dull weather. The cloud bases are usually only a few hundred feet above the ground. Over hills and mountains they can reach ground level when they may be called fog. Also, as fog "lifts" off the ground due to daytime heating, the fog forms a layer of low stratus clouds.

Fronts Fronts are the boundaries between two air masses. Fronts are classified as to which type of air mass (cold or warm) is replacing the other. For example, a cold front demarcates the leading edge of a cold air mass displacing a warmer air mass. A warm front is the leading edge of a warmer air mass replacing a colder air mass. If the front is essentially not moving (i.e. the air masses are not moving) it is called a stationary front.

Recognizing Fronts Fronts are usually detectable at the surface in a number of ways. Winds usually "converge" or come together at the fronts. Also, temperature differences can be quite noticeable from one side of the front to another. Finally, the pressure on either side of a front can vary significantly. Cold fronts typically move faster than warm fronts, so in time they "catch up" to warm fronts. As the two fronts merge, an occluded front forms. In the occluded front, the cold air undercuts the cooler air mass associated with the warm front, further lifting the already rising warm air.

Thunderstorm Link: Fronts are usually detectable at the surface in a number of ways. Winds usually "converge" or come together at the fronts. Also, temperature differences can be quite noticeable from one side of the front to another. Finally, the pressure on either side of a front can vary significantly. Fronts Link: http://www.canadiangeographic.ca/atlas/themes.aspx?id=weather&sub=weather_basics_begins&lang=En http://www.canadiangeographic.ca/atlas/themes.aspx?id=weather&sub=weather_phenomena_thunderstorm

Weather Elements