ST L1: Humidity/Thunderstorms Flashcards
Where do storms get their energy? What is the energy converted into?
from temperature and humidity in the air.
Energy converted into violent winds and storm circulations through pressure and buoyancy.
How do the storms increase their own longevity?
They draw in more heat and moisture.
What are thunderstorms?
Cumulonimbus clouds that have lightning and thunder.
What are cumulonimbus clouds?
Deep clouds with strong updrafts, with a cloud base near the ground and cloud top near the top of the troposphere (~11km above sea level).
Characteristic of mature thunderstorm?
anvil or mushroom top appearance that can spread large distances (hundreds of km) downwind if blown by strong winds aloft.
How wide is the main updraft?
~15km
How wide is the anvil?
100s of km
What is a CB?
Cumulonimbus cloud, aka thunderstorm.
Where is the anvil located?
Top of troposphere at location called the tropopause.
How to recognize thunderstorm?
Presence of their anvil. (May not see or hear lightning/thunder.) Cloudy air can overshoot above the anvil into the lower stratosphere, causing an “overshooting top.”
What hazards do thunderstorms cause to life and property?
Lightning, tornadoes, hail, downpours, local flooding, downburst, and gustfronts.
What does the weather radar do?
Transmits beam of microwaves into atmosphere, listens for the faint echo of microwave energy that bounces back off of raindrops.
Difference detected in weather radar between heavier and lighter rain?
Heavier rain causes stronger echo (red and yellow on radar displays). Lighter rain is displayed with blues and greens.
What can you see on visible satellite photos of thunderstorms?
The shadow of the thunderstorm anvil cast on the lower clouds or on the ground.
Describe movement of thunderstorms.
They usually move from southwest to northeast in the Northern Hemisphere.
Where is the best place to view thunderstorms?
Southeast of the storm. Good view, stay out of its path.
What clouds are sometimes visible on the underside of anvil clouds?
Mammatus clouds. They give NO clue as to intensity of the thunderstorm, or whether tornadoes are present.
Sometimes these clouds form on the underside of non-thunderstorm cloud
All of the energy that drives these violent motions comes from ____ in the air, in the form of _____ and _____ (stored in water vapour).
heat; sensible heat; latent heat
Describe air’s mixture of gases/
78% nitrogen, 21% oxygen, 0-4% water vapour, trace gases, liquid water droplets
What is humidity?
Amount of water vapour in the air
Sensible vs latent heat?
Sensible heat: can sense or feel; measured using temperature
Latent heat: hidden in water, measured using humidity
There is a transfer of energy between sensible and latent heat when water _____.
changes phase
Describe saturation. What is it important for?
max. amount of water vapour that air can carry at equilibrium.
Important in determining whether condensation occurs and latent heat is released into thunderstorm.
What is partial pressure? What is their sum called?
Pressure exerted by each individual gas. Altogether, called the total pressure, P, exerted by whole mixture
partial pressure for water vapour?
vapour pressure. symbol: e
What is special about water vapour?
it can easily condense into liquid.
What happens if both liquid and vapour are present near each other?
If both liquid and vapour are present near each other, then a constant exchange occurs as some vapour molecules hit the liquid surface and condense out of the air (thereby reducing the number of vapour molecules remaining in the air, which causes the air to become drier), while other water molecules evaporate from the liquid and re-enter the air (thereby increasing the humidity in the air)
For any given temperature of the liquid water, there is an equilibrium amount of vapour in the air for which the evaporation of liquid water into the air_____ the condensation of _____ out of the air. The vapour pressure corresponding to this equilibrium is called the _______ and is given symbol __.
balances ; vapour ; saturation vapour pressure ; eS
If there are too few vapour molecules in air, then there will be more ______ than ______, again gradually approaching the saturation or equilibrium value
evaporation; condensation
What can water vapour molecules condense on?
Liquid water surfaces and dust particles in the air.
How many dust particles in each cubic meter of air? What does this mean?
10^10. Actual air humidity rarely exceeds saturation value.
Describe humidity in locations where is insufficient liquid water to be evaporated?
Humidity can remain below the saturation value.
What does the saturation value mean?
maximum humidity that the air can hold
What is the cloud condensation nuclei (CNN)?
The dust on which humidity condenses to form cloud droplets
Which kind of air holds more water vapour at EQUILIBRIUM: warmer or colder?
warmer air
What do you call air containing max. amount of water vapour it can hold? What about air holding less?
Saturated. (cloudy or foggy)
Air holding less is called unsaturated. (not cloudy)
Besides water vapour pressure, what other ways to quantify humidity?
Mixing ratio, relative humidity, dew-point temperature.
What is mixing ratio?
Mass of vapour per mass of dry air. (a.k.a. mass of water vapour in mixture divided by mass of all the remaining gases) symbol: r.
Varies between [0, 0.04] kg per kg of air.
Mixing ratio always +ve. Units is usually: grams of water vapour/kg of air. [g/kg]
What is r? rs?
r tells us how much water vapour is actually contained in the air, while rs tells us the maximum amount that could be held.
What is Dew-Point Temperature? (Td)
Cooling unsaturated air at constant pressure until water vapour just begins to condense out, the temperature.
When cooling air, what happens to its vapour-holding capacity? What will eventually happen?
It reduces.
Cooling itself doesn’t change amount of water in air.
Eventually, the capacity is reduced to the point where the air can just hold that amount of water that it already does. Any cooler and not all the water can be retained as vapour at equilibrium, so some condenses onto the dust particles in the air to form cloud droplets.
For saturated air, what is the temperature equal to?
T=Td (dew-point temp.)
You don’t have to cool air at all, it already holds max. amount of water vapour possible.
What is dew-point depression?
Difference between actual air temperature and dew point temp. Dew-point depression=T-Td
So for saturated air, depression is 0. Larger difference for drier air.
What are hygrometers?
humidity sensors
what is one of the easiest and most accurate ways to measure humidity?
dew-point temperature.
just gradually cool a surface until dew starts to form.
What instrument measures the dew-point temperature of a surface with dew?
gradually cool a surface (such as a mirror) until dew just starts to form (which is easy to detect by looking at light reflect off the mirror). This temperature is the dew-point temperature, by definition. This instrument is called a dew-point hygrometer, or chilled-mirror hygrometer.
How to find all the humidity variables?
If you have dew-point temperature, actual air temperature measured with thermometer and barometer, all other humidity variables can be found.
What is the hygristor?
a sensor that measures how electrical resistance changes when a carbon-coated glass slide is exposed to humid air
What’s a humicap?
measures change in electrical capacitance of a very thin piece of plastic that is sandwiched between two thin metal meshes
What do microwave hygrometers and Lyman-alpha hygrometers do?
they send a beam of electromagnetic waves across a chamber or open path from a transmitter to a detector, and infer the humidity by the attenuation of the beam. These are relatively fast-response sensors.
Describe stratiform clouds.
Stratiform clouds are layered clouds. They have very large horizontal extent (10s to 1000s of km), but are often relatively thin (0.01 to 1 km). They are formed by mostly smooth, horizontal winds, and are named by their altitude
Types of stratiform clouds? Describe them.
cirrus, cirrostratus, cirrocumulus - high altitude (about 10 km), thin, made of ice crystals
altostratus, altocumulus - medium altitudes (about 5 km), made of water droplets
stratus, nimbostratus - low bases (0.1 to 2 km), thick, with widespread drizzle from the nimbostratus
Describe cirrus clouds.
Location: high (11 km)
They’re ice crystals.
Describe cirrostratus.
Located high. Made of ice, halo-looking.
Describe cirrocumulus
Located high. Made of ice, looks lumpy
Describe altostratus
Located mid (5 km). Liquid, corona (crown-like?)
Describe altocumulus
located mid. liquid.
Describe stratus
Located low. Widespread liquid drizzle.
Describe nimbostratus
Located lowest. Widespread liquid drizzle.
What are Cumuliform clouds? Describe them.
Cumuliform cloud tops look like popcorn, cotton balls, or cauliflower. They have significant vertical motion and turbulence, and are often formed from air parcels rising from near the ground under the cloud. They usually have flat bases, at altitudes very close to the theoretical LCL altitude. Often, their diameter is roughly equal to their thickness. Cumuliform clouds are named by their size:
cumulus humilis - fair-weather clouds, about 1 km in size
cumulus mediocris - medium size, about 4 km in size
cumulus congestus - towering cumulus, about 7 km in size
cumulonimbus - thunderstorms, about 11 km in size, with precipitation
Stratiform clouds classified by their_____.
Cumuliform clouds classified by their ______.
altitude;
thickness.
Cumulonimbus base features?
Beaver tail, wall cloud.
